Proteome of skeletal muscle lipid droplet reveals association with mitochondria and apolipoprotein a-I

Ethanol disrupts hepatocellular lipophagy by altering Rab5-centric LD-lysosome trafficking

BACKGROUND

Previous reports suggest that lipid droplets (LDs) in the hepatocyte can be catabolized by a direct engulfment from nearby endolysosomes (microlipophagy). Further, it is likely that this process is compromised by chronic ethanol (EtOH) exposure leading to hepatic steatosis. This study investigates the hepatocellular machinery supporting microlipophagy and EtOH-induced alterations in this process with a focus on the small, endosome-associated, GTPase Rab5.

METHODS AND RESULTS

Here we report that this small Ras-related GTPase is a resident component of LDs, and its activity is important for hepatocellular LD-lysosome proximity and physical interactions. We find that Rab5 siRNA knockdown causes an accumulation of LDs in hepatocytes by inhibiting lysosome dependent LD catabolism. Importantly, Rab5 appears to support this process by mediating the recruitment of early endosomal and or multivesicular body compartments to the LD surface before lysosome fusion. Interestingly, while wild-type or a constituently active GTPase form (Q79L) of Rab5 supports LD-lysosome transport, this process is markedly reduced in cells expressing a GTPase dead (S34N) Rab5 protein or in hepatocytes exposed to chronic EtOH.

CONCLUSIONS

These findings support the novel premise of an early endosomal/multivesicular body intermediate compartment on the LD surface that provides a "docking" site for lysosomal trafficking, not unlike the process that occurs during the hepatocellular degradation of endocytosed ligands that is also known to be compromised by EtOH exposure.

Co-immunoprecipitation for identifying protein-protein interaction on lipid droplets

The lipid droplet (LD) is a conserved organelle that exists in almost all organisms, ranging from bacteria to mammals. Dysfunctions in LDs are linked to a range of human metabolic syndromes. The formation of protein complexes on LDs is crucial for maintaining their function. Investigating how proteins interact on LDs is essential for understanding the role of LDs. We have developed an effective method to uncover protein-protein interactions and protein complexes specifically on LDs. In this method, we conduct co-immunoprecipitation (co-IP) experiments using LD proteins extracted directly from isolated LDs, rather than utilizing proteins from cell lysates. To elaborate, we begin by purifying LDs with high-quality and extracting LD-associated proteins. Subsequently, the co-IP experiment is performed on these LD-associated proteins directly, which would enhance the co-IP experiment specificity of LD-associated proteins. This method enables researchers to directly unveil protein complexes on LDs and gain deeper insights into the functional roles of proteins associated with LDs.

A missense variant in MYOF is associated with ARVC and sudden cardiac death

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is rare autosomal dominant genetic disorder that leads to severe arrhythmia and sudden cardiac death. Although previous studies in clinical, pathological and genetics of ARVC established consensus diagnostic criteria and expanded the spectrum of pathogenic genes, there is still a proportion of patients with unclear causative factors. Here, whole-exome sequencing was employed to investigate the genetic etiology of a 15-year-old sudden cardiac death female caused by ARVC. A novel variant of MYOF (NM_013451.3: c.4723G > C: p.D1575H) was identified, which is a member of the Ferlin family of proteins is associated with cardiomyopathy. And the bioinformatics analysis predicted the pathogenicity of this variant. We report the first variant of MYOF in ARVC, which imply a vital role of MYOF in cardiomyopathy.

Reorganization of mitochondria-organelle interactions during postnatal development in skeletal muscle

Skeletal muscle cellular development requires the integrated assembly of mitochondria and other organelles adjacent to the sarcomere in support of muscle contractile performance. However, it remains unclear how interactions among organelles and with the sarcomere relates to the development of muscle cell function. Here, we combine 3D volume electron microscopy, proteomic analyses, and live cell functional imaging to investigate the postnatal reorganization of mitochondria-organelle interactions in skeletal muscle. We show that while mitochondrial networks are disorganized and loosely associated with the contractile apparatus at birth, contact sites among mitochondria, lipid droplets and the sarcoplasmic reticulum are highly abundant in neonatal muscles. The maturation process is characterized by a transition to highly organized mitochondrial networks wrapped tightly around the muscle sarcomere but also to less frequent interactions with both lipid droplets and the sarcoplasmic reticulum. Concomitantly, expression of proteins involved in mitochondria-organelle membrane contact sites decreases during postnatal development in tandem with a decrease in abundance of proteins associated with sarcomere assembly despite an overall increase in contractile protein abundance. Functionally, parallel measures of mitochondrial membrane potential, NADH redox status, and NADH flux within intact cells revealed that mitochondria in adult skeletal muscle fibres maintain a more activated electron transport chain compared with neonatal muscle mitochondria. These data demonstrate a developmental redesign reflecting a shift from muscle cell assembly and frequent inter-organelle communication toward a muscle fibre with mitochondrial structure, interactions, composition and function specialized to support contractile function. KEY POINTS: Mitochondrial network organization is remodelled during skeletal muscle postnatal development. The mitochondrial outer membrane is in frequent contact with other organelles at birth and transitions to more close associations with the contractile apparatus in mature muscles. Mitochondrial energy metabolism becomes more activated during postnatal development. Understanding the developmental redesign process within skeletal muscle cells may help pinpoint specific areas of deficit in muscles with developmental disorders.

An ICT-switched fluorescent probe for visualizing lipid and HClO in lipid droplets during ferroptosis

Herein, we have designed and synthesized a fluorescent probe, LDs-ClO, which can detect hypochlorous acid and lipid accumulation simultaneously in lipid droplets of live RAW 264.7 cells. Cell ferroptosis was discovered to lead to an increase in HClO levels, and to possibly further stimulate accumulation of lipid. We expect the results of this work with LDs-ClO to promote the study of physiological and pathological processes related to lipid droplets and hypochlorite.

Isolation and Lipidomic Profiling of Neuronal Lipid Droplets: Unveiling the Lipid Landscape for insights into Neurodegenerative Disorders

Recent advances have expanded the role of lipid droplets (LDs) beyond passive lipid storage, implicating their involvement in various metabolic processes across mammalian tissues. Neuronal LDs, long debated in existence, have been identified in several neural structures, raising questions about their contribution to neurodegenerative disorders. Elucidating the specific chemical makeup of these organelles within neurons is critical for understanding their implication in neural pathologies. This study outlines an improved methodology to stimulate and isolate mature LDs from cultured primary neurons, offering insights into their unique lipid-protein composition. Integrating this method with high-throughput techniques may unveil disease-specific alterations in lipid metabolism, providing avenues for potential therapeutic interventions.

New insights into the function of lipid droplet-related proteins and lipid metabolism of salt-stimulated porcine biceps femoris: label-free quantitative phosphoproteomics, morphometry and bioinformatics

BACKGROUND

Lipid droplets (LDs) are important multifunctional organelles responsible for lipid metabolism of postmortem muscle. However, the dynamics in their building blocks (cores and layers) and phosphorylation of lipid droplet-related proteins (LDRPs) regulating meat lipolysis remain unknown at salt-stimulated conditions.

RESULTS

LDRPs extracted from cured porcine biceps femoris (1% and 3% salt) were subjected to label-free quantitative phosphoproteomic analysis and LDs morphological validation. Results indicated that 3% salt curing significantly decreased triglyceride (TG) content with increase in glycerol and decrease in LDs fluorescence compared to 1% salt curing. Comparative phosphoproteomics showed that there were significant changes in phosphorylation at 386 sites on 174 LDRPs between assayed groups (P < 0.05). These differential proteins were mainly involved in lipid and carbohydrate metabolism. Curing of 3% salt induced more site-specific phosphorylation of perilipin 1 (PLIN1, at Ser81) and adipose triglyceride lipase (ATGL, at Ser399) than 1%, whereas the phosphorylation (at Ser600) of hormone-sensitive lipase (HSL) was up-regulated. Ultrastructure imaging showed that LDs were mostly associated with mitochondria, and the average diameter of LDs decreased from 2.34 μm (1% salt) to 1.73 μm (3% salt).

CONCLUSION

Phosphoproteomics unraveled salt-stimulated LDRPs phosphorylation of cured porcine meat provoked intensified lipolysis. Curing of 3% salt allowed an enhanced lipolysis than 1% by up-regulating the phosphorylation sites of LDRPs and recruited lipases. The visible splitting of LDs, together with sarcoplasmic disorganization, supported the lipolysis robustness following 3% salt curing. The finding provides optimization ideas for high-quality production of cured meat products. © 2023 Society of Chemical Industry.

Regulation of lipolysis by 14-3-3 proteins on human adipocyte lipid droplets

Adipocyte lipid droplets (LDs) play a crucial role in systemic lipid metabolism by storing and releasing lipids to meet the organism's energy needs. Hormonal signals such as catecholamines and insulin act on adipocyte LDs, and impaired responsiveness to these signals can lead to uncontrolled lipolysis, lipotoxicity, and metabolic disease. To investigate the mechanisms that control LD function in human adipocytes, we applied proximity labeling mediated by enhanced ascorbate peroxidase (APEX2) to identify the interactome of PLIN1 in adipocytes differentiated from human mesenchymal progenitor cells. We identified 70 proteins that interact specifically with PLIN1, including PNPLA2 and LIPE, which are the primary effectors of regulated triglyceride hydrolysis, and 4 members of the 14-3-3 protein family (YWHAB, YWHAE, YWHAZ, and YWHAG), which are known to regulate diverse signaling pathways. Functional studies showed that YWHAB is required for maximum cyclic adenosine monophosphate (cAMP)-stimulated lipolysis, as its CRISPR-Cas9-mediated knockout mitigates lipolysis through a mechanism independent of insulin signaling. These findings reveal a new regulatory mechanism operating in human adipocytes that can impact lipolysis and potentially systemic metabolism.

ATG2A-mediated bridge-like lipid transport regulates lipid droplet accumulation

ATG2 proteins facilitate bulk lipid transport between membranes. ATG2 is an essential autophagy protein, but ATG2 also localizes to lipid droplets (LDs), and genetic depletion of ATG2 increases LD numbers while impairing fatty acid transport from LDs to mitochondria. How ATG2 supports LD homeostasis and whether lipid transport regulates this homeostasis remains unknown. Here we demonstrate that ATG2 is preferentially recruited to phospholipid monolayers such as those surrounding LDs rather than to phospholipid bilayers. In vitro, ATG2 can drive phospholipid transport from artificial LDs with rates that correlate with the binding affinities, such that phospholipids are moved much more efficiently when one of the ATG2-interacting structures is an artificial LD. ATG2 is thought to exhibit 'bridge-like" lipid transport, with lipids flowing across the protein between membranes. We mutated key amino acids within the bridge to form a transport-dead ATG2 mutant (TD-ATG2A) which we show specifically blocks bridge-like, but not shuttle-like, lipid transport in vitro. TD-ATG2A still localizes to LDs, but is unable to rescue LD accumulation in ATG2 knockout cells. Thus, ATG2 has a natural affinity for, and an enhanced activity upon LD surfaces and uses bridge-like lipid transport to support LD dynamics in cells.

Mammalian lipid droplets: structural, pathological, immunological and anti-toxicological roles

Mammalian lipid droplets (LDs) are specialized cytosolic organelles consisting of a neutral lipid core surrounded by a membrane made up of a phospholipid monolayer and a specific population of proteins that varies according to the location and function of each LD. Over the past decade, there have been significant advances in the understanding of LD biogenesis and functions. LDs are now recognized as dynamic organelles that participate in many aspects of cellular homeostasis plus other vital functions. LD biogenesis is a complex, highly-regulated process with assembly occurring on the endoplasmic reticulum although aspects of the underpinning molecular mechanisms remain elusive. For example, it is unclear how many enzymes participate in the biosynthesis of the neutral lipid components of LDs and how this process is coordinated in response to different metabolic cues to promote or suppress LD formation and turnover. In addition to enzymes involved in the biosynthesis of neutral lipids, various scaffolding proteins play roles in coordinating LD formation. Despite their lack of ultrastructural diversity, LDs in different mammalian cell types are involved in a wide range of biological functions. These include roles in membrane homeostasis, regulation of hypoxia, neoplastic inflammatory responses, cellular oxidative status, lipid peroxidation, and protection against potentially toxic intracellular fatty acids and lipophilic xenobiotics. Herein, the roles of mammalian LDs and their associated proteins are reviewed with a particular focus on their roles in pathological, immunological and anti-toxicological processes.

Isolation and proteomic study of fish liver lipid droplets

Lipid droplets (LDs) are a neutral lipid storage organelle that is conserved in almost all species. Excessive storage of neutral lipids in LDs is directly associated with many metabolic syndromes. Zebrafish is a better model animal for the study of LD biology due to its transparent embryonic stage compared to other organisms. However, the study of LDs in fish has been difficult due to the lack of specific LD marker proteins and the limitation of purification technology. In this paper, the purification and proteomic analysis of liver LDs of fish including zebrafish and Carassius auratus were performed for the first time. 259 and 267 proteins were identified respectively. Besides most of the identified proteins were reported in previous LD proteomes of mammals, indicating the similarity between mammal and fish LDs. We also identified many unique proteins of liver LDs in fish that are involved in the regulation of LD dynamics. Through morphological and biochemical analysis, we found that the marker protein Plin2 of zebrafish LD was located on LDs in Huh7 cells. These results will facilitate further study of LDs in fish and liver metabolic diseases using fish as a model animal.

Cellular communication through extracellular vesicles and lipid droplets

Cellular communication is essential for effective coordination of biological processes. One major form of intercellular communication occurs via the release of extracellular vesicles (EVs). These vesicles mediate intercellular communication through the transfer of their cargo and are actively explored for their role in various diseases and their potential therapeutic and diagnostic applications. Conversely, lipid droplets (LDs) are vesicles that transfer cargo within cells. Lipid droplets play roles in various diseases and evidence for their ability to transfer cargo between cells is emerging. To date, there has been little interdisciplinary research looking at the similarities and interactions between these two classes of small lipid vesicles. This review will compare the commonalities and differences between EVs and LDs including their biogenesis and secretion, isolation and characterisation methodologies, composition, and general heterogeneity and discuss challenges and opportunities in both fields.

Intramuscular lipid utilization during exercise: a systematic review, meta-analysis, and meta-regression

Intramuscular lipid (IMCL) utilization during exercise was controversial as numerous studies did not observe a decline in IMCL content post-exercise when assessed in muscle biopsies using biochemical techniques. Contemporary techniques including immunofluorescence microscopy and ¹H-magnetic resonance spectroscopy (¹H-MRS) offer advantages over biochemical techniques. The primary aim of this systematic review, meta-analysis, and meta-regression was to examine the net degradation of IMCL in response to an acute bout of cycling exercise in humans, as assessed with different analytical approaches. A secondary aim was to explore the factors influencing IMCL degradation including feeding status, exercise variables, and participant characteristics. A total of 44 studies met the inclusion criteria using biochemical, immunofluorescence, and ¹H-MRS techniques. A meta-analysis was completed using a random effects model and percentage change in IMCL content calculated from the standardized mean difference. Cycling exercise resulted in a net degradation of IMCL regardless of technique (total effect -23.7%, 95% CI = -28.7 to -18.7%) and there was no difference when comparing fasted versus fed-state exercise (P > 0.05). IMCL degradation using immunofluorescence techniques detected larger effects in type I fibers compared with whole muscle using biochemical techniques (P = 0.003) and in type I fibers compared with type II fibers (P < 0.001). Although IMCL degradation was associated with exercise duration, V̇o_2max, and BMI, none of these factors independently related to the change in IMCL content. These findings provide strong evidence that the analytical approach can influence the assessment of IMCL degradation in human skeletal muscle in response to exercise.

Lipid-droplet associated mitochondria promote fatty-acid oxidation through a distinct bioenergetic pattern in male Wistar rats

Mitochondria empower the liver to regulate lipid homeostasis by enabling fatty acid oxidation during starvation and lipogenesis during nutrient-rich conditions. It is unknown if mitochondria can seamlessly regulate these two distinct processes or if two discrete populations of mitochondria achieve these two functions in the liver. For the first time in the liver, we report the isolation of two distinct populations of mitochondria from male Wistar rats on an ad-libitum diet: cytoplasmic mitochondria and lipid droplet-associated mitochondria. Our studies show that while lipid droplet mitochondria exhibit higher fatty acid oxidation and are marked by enhanced levels of pACC2, MFN2, and CPT1 activity, cytoplasmic mitochondria are associated with higher respiration capacity. Notably, lipid droplet-associated mitochondria isolated from a non-alcoholic fatty liver disease (NAFLD) rat model are compromised for fatty acid oxidation. We demonstrate the importance of functional segregation of mitochondria as any aberration in lipid droplet-associated mitochondria may lead to NAFLD.

Identification of Key Genes and Biological Pathways Associated with Skeletal Muscle Maturation and Hypertrophy in Bos taurus, Ovis aries, and Sus scrofa

The aim of the current study was to identify the major genes and pathways involved in the process of hypertrophy and skeletal muscle maturation that is common for Bos taurus, Ovis aries, and Sus scrofa species. Gene expression profiles related to Bos taurus, Ovis aries, and Sus scrofa muscle, with accession numbers GSE44030, GSE23563, and GSE38518, respectively, were downloaded from the GEO database. Differentially expressed genes (DEGs) were screened out using the Limma package of R software. Genes with Fold Change > 2 and an adjusted p-value < 0.05 were identified as significantly different between two treatments in each species. Subsequently, gene ontology and pathway enrichment analyses were performed. Moreover, hub genes were detected by creating a protein−protein interaction network (PPI). The results of the analysis in Bos taurus showed that in the period of 280 dpc−3-months old, a total of 1839 genes showed a significant difference. In Ovis aries, however, during the period of 135dpc−2-months old, a total of 486 genes were significantly different. Additionally, in the 91 dpc−adult period, a total of 2949 genes were significantly different in Sus scrofa. The results of the KEGG pathway enrichment analysis and GO function annotation in each species separately revealed that in Bos taurus, DEGs were mainly enriched through skeletal muscle fiber development and skeletal muscle contraction, and the positive regulation of fibroblast proliferation, positive regulation of skeletal muscle fiber development, PPAR signaling pathway, and HIF-1 signaling pathway. In Ovis aries, DEGs were mainly enriched through regulating cell growth, skeletal muscle fiber development, the positive regulation of fibroblast proliferation, skeletal muscle cell differentiation, and the PI3K-Akt signaling, HIF-1 signaling, and Rap1 signaling pathways. In Sus scrofa, DEGs were mainly enriched through regulating striated muscle tissue development, the negative regulation of fibroblast proliferation and myoblast differentiation, and the HIF-1 signaling, AMPK signaling, and PI3K-Akt signaling pathways. Using a Venn diagram, 36 common DEGs were identified between Bos taurus, Ovis aries, and Sus scrofa. A biological pathways analysis of 36 common DEGs in Bos taurus, Ovis aries, and Sus scrofa allowed for the identification of common pathways/biological processes, such as myoblast differentiation, the regulation of muscle cell differentiation, and positive regulation of skeletal muscle fiber development, that orchestrated the development and maturation of skeletal muscle. As a result, hub genes were identified, including PPARGC1A, MYOD1, EPAS1, IGF2, CXCR4, and APOA1, in all examined species. This study provided a better understanding of the relationships between genes and their biological pathways in the skeletal muscle maturation process.

Tandem mass tag labeled quantitative proteomic analysis of differential protein expression on total alkaloid of Aconitum flavum Hand.-Mazz. against melophagus ovinus

Melophagus ovinus disease is a common ectoparasitosis, which can lead to a decrease in animal production performance, product quality, and even death. Aconitum flavum Hand.-Mazz. has many pharmacological activities including insecticidal, heat-clearing, analgesic, and dehumidifying. However, there are few researches focused on the effects and related mechanism of Aconitum flavum Hand.-Mazz. in killing Melophagus ovinus. In this study, 11 alkaloids of Aconitum flavum Hand.-Mazz. were detected, and its total alkaloid activity was determined. The results showed when the total alkaloid concentration was 64 mg/ml and the treatment time was 16 h, the killing rate of Melophagus ovinus reached 100%. Through the observation of the differences in the surface of Melophagus ovinus in each experimental group, it was found that the morphology of the posterior end of the female Melophagus ovinus in the alkaloid treatment group was significantly different from that of the blank and positive control groups, and most of the epidermal tissue was obsessive and missing. Moreover, the enzyme activity determination results of 64 mg/ml group were significantly different when compared with the normal control group, while there was no significant difference in other groups. Then, the Melophagus ovinus gene library was established by the unreferenced genome transcriptome sequencing, the proteomic comparison was performed using tandem mass tag labeled protein detection technology, and finally, the samples were quantitatively analyzed by liquid chromatography-mass spectrometry tandem and bioinformatics methods. Based on the above experimental results, it was speculated that Aconitum flavum Hand.-Mazz. total alkaloids may cause the imbalance of protein disulfide isomerase expressions by affecting the regulation of Hsp40 cellular protein homeostasis and the oxidation of protein disulfide isomerase and related proteins. This would affect the selective recognition of signal sequence, the targeted transport of Sec 61, and the correct folding of the three-dimensional structure of amino acid chain, weakening the clearance of amino acid chains that cannot be correctly folded and eventually resulting in the killing of Melophagus ovinus. This study preliminarily revealed the mechanism of Aconitum flavum Hand.-Mazz. total alkaloids against Melophagus ovinus and provided a theoretical basis for the screening of Melophagus ovinus action targets and the development of new veterinary drugs.

Aggregation caused quenching to aggregation induced emission transformation: a precise tuning based on BN-doped polycyclic aromatic hydrocarbons toward subcellular organelle specific imaging

Polycyclic aromatic hydrocarbons (PAHs) with boron–nitrogen (BN) moieties have attracted tremendous interest due to their intriguing electronic and optoelectronic properties. However, most of the BN-fused π-systems reported to date are difficult to modify and exhibit traditional aggregation-caused quenching (ACQ) characteristics. This phenomenon greatly limits their scope of application. Thus, continuing efforts to seek novel, structurally distinct and functionally diverse structures are highly desirable. Herein, we proposed a one-stone-two-birds strategy including simultaneous exploration of reactivity and tuning of the optical and electronic properties for BN-containing π-skeletons through flexible regioselective functionalization engineering. In this way, three novel functionalized BN luminogens (DPA-BN-BFT, MeO-DPA-BN-BFT and DMA-DPA-BN-BFT) with similar structures were obtained. Intriguingly, DPA-BN-BFT, MeO-DPA-BN-BFT and DMA-DPA-BN-BFT exhibit completely different emission behaviors. Fluorogens DPA-BN-BFT and MeO-DPA-BN-BFT exhibit a typical ACQ effect; in sharp contrast, DMA-DPA-BN-BFT possesses a prominent aggregation induced emission (AIE) effect. To the best of our knowledge, this is the first report to integrate ACQ and AIE properties into one BN aromatic backbone with subtle modified structures. Comprehensive analysis of the crystal structure and theoretical calculations reveal that relatively large twisting angles, multiple intermolecular interactions and tight crystal packing modes endow DMA-DPA-BN-BFT with strong AIE behavior. More importantly, cell imaging demonstrated that luminescent materials DPA-BN-BFT and DMA-DPA-BN-BFT can highly selectively and sensitively detect lipid droplets (LDs) in living MCF-7 cells. Overall, this work provides a new viewpoint of the rational design and synthesis of advanced BN–polycyclic aromatics with AIE features and triggers the discovery of new functions and properties of azaborine chemistry.

A one-stone-two-birds strategy including simultaneous exploration of reactivity and tuning of the optical and electronic properties for BN-fused polycyclic aromatics through flexible regioselective functionalization engineering is presented.

Lipophagy at a glance

Lipophagy is a central cellular process for providing the cell with a readily utilized, high energy source of neutral lipids. Since its discovery over a decade ago, we are just starting to understand the molecular components that drive lipophagy, how it is activated in response to nutrient availability, and its potential as a therapeutic target in disease. In this Cell Science at a Glance article and the accompanying poster, we first provide a brief overview of the different structural and enzymatic proteins that comprise the lipid droplet (LD) proteome and reside within the limiting phospholipid monolayer of this complex organelle. We then highlight key players in the catabolic breakdown of LDs during the functionally linked lipolysis and lipophagy processes. Finally, we discuss what is currently known about macro- and micro-lipophagy based on findings in yeast, mammalian and other model systems, and how impairment of these important functions can lead to disease states.

Artificial Lipid Droplets: Novel Effective Biomaterials to Protect Cells against Oxidative Stress and Lipotoxicity

Lipid droplets (LDs) play an important role in the regulation of cellular stress. This suggests LDs can be applied as safe and effective biomaterials to alleviate cellular stress and lipotoxicity. Here, we constructed a convenient method to generate stable and pure artificial lipid droplets (aLDs). aLDs can maintain their biological function by incubating LD-associated proteins or organelles in vitro. It was validated that perilipin-coated aLDs could be uptaken by cells, significantly reducing hydrogen peroxide-induced reactive oxidative species (ROS) and alleviating cellular lipotoxicity caused by excess fatty acid. Our work demonstrated a direct role of LDs in regulating cellular stress levels, providing methods and potential value for future research and medical applications of LDs.

Lipid droplets in skeletal muscle during grass snake (Natrix natrix L.) development

Lipid droplets (LDs) are common organelles observed in Eucaryota. They are multifunctional organelles (involved in lipid storage, metabolism, and trafficking) that originate from endoplasmic reticulum (ER). LDs consist of a neutral lipid core, made up of diacyl- and triacylglycerols (DAGs and TAGs) and cholesterol esters (CEs), surrounded by a phospholipid monolayer and proteins, which are necessary for their structure and dynamics. Here, we report the protein and lipid composition as well as characterization and dynamics of grass snake (Natrix natrix) skeletal muscle LDs at different developmental stages. In the present study, we used detailed morphometric, LC-MS, quantitative lipidomic analyses of LDs isolated from the skeletal muscles of the snake embryos, immunofluorescence, and TEM. Our study also provides a valuable insight concerning the LDs' multifunctionality and ability to interact with a variety of organelles. These LD features are reflected in their proteome composition, which contains scaffold proteins, metabolic enzymes signalling polypeptides, proteins necessary for the formation of docking sites, and many others. We also provide insights into the biogenesis and growth of muscle LDs goes beyond the conventional mechanism based on the synthesis and incorporation of TAGs and LD fusion. We assume that the formation and functioning of grass snake muscle LDs are based on additional mechanisms that have not yet been identified, which could be related to the unique features of reptiles that are manifested in the after-hatching period of life, such as a reptile-specific strategy for energy saving during hibernation.

Practicable Applications of Aggregation-Induced Emission with Biomedical Perspective

Considerable efforts have been made into developing aggregation-induced emission fluorogens (AIEgens)-containing nano-therapeutic systems due to the excellent properties of AIEgens. Compared to other fluorescent molecules, AIEgens have advantages including low background, high signal-to-noise ratio, good sensitivity, and resistance to photobleaching, in addition to being exempt from concentration quenching or aggregation-caused quenching effects. The present review outlines the major developments in the biomedical applications of AIEgens-containing systems. From a literature survey, the recent AIE works are reviewed and the reasons why AIEgens are chosen in various biomedical applications are highlighted. The research activities on AIEgens-containing systems are increasing rapidly, therefore, the present review is timely.

Lipid droplets contribute myogenic differentiation in C2C12 by promoting the remodeling of the acstin-filament

Lipid droplet (LD), a multi-functional organelle, is found in most eukaryotic cells. LDs participate in the regulation of many cellular processes including proliferation, stress, and apoptosis. Previous studies showed the athlete's paradox that trained athletes accumulate LDs in their skeletal muscle. However, the impact of LDs on skeletal muscle and myogenesis is not clear. We discovered that C2C12 myoblast cells containing more LDs formed more multinucleated muscle fibers. We also discovered that LDs promoted cell migration and fusion by promoting actin-filaments remodeling. Mechanistically, two LD-proteins, Acyl-CoA synthetase long chain family member 3 (ACSL3) and lysophosphatidylcholine acyltransferase 1 (LPCAT1), medicated the recruitment of actinin proteins which contributed to actin-filaments formation on the surface of LDs. During remodeling, the actinin proteins on LDs surface translocated to actin-filaments via ARF1/COPI vesicles. Our study demonstrate LDs contribute to cell differentiation, which lead to new insight into the LD function.

Lipid droplets diversity and functions in inflammation and immune response

INTRODUCTION

Lipid droplets (LDs) are dynamic and evolutionary conserved lipid-enriched organelles composed of a core of neutral lipids surrounded by a monolayer of phospholipids associated with a diverse array of proteins that are cell- and stimulus-regulated. Far beyond being simply a deposit of neutral lipids, accumulating evidence demonstrate that LDs act as spatial and temporal local for lipid and protein compartmentalization and signaling organization.

AREAS COVERED

This review focuses on the progress in our understanding of LD protein diversity and LD functions in the context of cell signaling and immune responses, highlighting the relationship between LD composition with the multiple roles of this organelle in immunometabolism, inflammation and host-response to infection.

EXPERT OPINION

LDs are essential platforms for various cellular processes, including metabolic regulation, cell signaling, and immune responses. The functions of LD in infection and inflammatory disease are associated with the dynamic and complexity of their proteome. Our contemporary view place LDs as critical regulators of different inflammatory and infectious diseases and key markers of leukocyte activation.

Quantitative Structure-Activity Relationship Enables the Rational Design of Lipid Droplet-Targeting Carbon Dots for Visualizing Bisphenol A-Induced Nonalcoholic Fatty Liver Disease-like Changes

Lipid droplets (LDs) play indispensable roles in numerous physiological processes; hence, the visualization of the dynamic behavior of LDs in living cells is of great importance in physiological and pathological research. In this article, the quantitative structure-activity relationship (QSAR) theory was employed as an effective design strategy for the development of organelle-targeting carbon dots (CDs). The lipid-water partition coefficient (Log P) of the QSAR was adopted as a key parameter to predict the cellular uptake and subcellular localization of CDs in live cells. By carefully adjusting the molecular structure and lipophilicity of the precursors, p-phenylenediamine-derivatized nucleolus-targeting hydrophilic CDs were converted to lipophilic CDs [4-piperidinoaniline (PA) CDs] with inherent LD-targeting performance. The PA CDs were able to indicate the dynamic behavior of LDs and visualize the changes of bisphenol A-induced nonalcoholic fatty liver disease-like changes in a cellular model. The QSAR strategy of CDs demonstrated here is expected to be increasingly exploited as a powerful design tool for developing various organelle-targeting CDs.

Tunable NIR AIE-active optical materials for lipid droplet imaging in typical model organisms and photodynamic therapy

Near infrared (NIR) luminescent materials with aggregation-induced emission (AIE) features have attracted enormous attention in the areas of medical imaging and diagnostic therapeutics because of their low background fluorescence and strong tissue penetration. This study reports a series of easily synthesized AIEgen molecules that feature NIR emission. These molecules have a donor-donor-π-acceptor (D1-D2-π-A) structure with intramolecular charge transfer (ICT) character. The nature of charge transfer transition can be modified by different structures of D2, i.e. phenyl, thiophene, and furan ring. These AIEgens have high selectivity towards lipid droplets (LDs) in vitro and in vivo, such as zebrafish, Caenorhabditis elegans, and oil crop tissue. In addition, the effect of photodynamic therapy (PDT) on SMMC-7721 cells was investigated, and the results indicate that these AIEgens have potential application for PDT on cancer cells with white light illumination. This study reveals that these triphenylamine (TPA)-based AIEgens have great potential for biological imaging and preclinical applications of PDT.

Rab18 binds PLIN2 and ACSL3 to mediate lipid droplet dynamics

Lipid droplet (LD) is a vital organelle governing lipid homeostasis and Rab18 has been linked to lipid metabolism. However, the mechanisms of Rab18-mediated LD dynamics in myoblast cells remain elusive. Here, we report that Rab18 plays an important role in oleic acid (OA)-induced LD accumulation in mouse myoblast C2C12 cells. Rab18 was translocated from the endoplasmic reticulum (ER) to LDs during LD accumulation, which was regulated by perilipin 2 (PLIN2), a major LD protein. LD-associated Rab18 bound with the C terminus of PLIN2 and the LD localization of Rab18 was diminished when PLIN2 was depleted. Moreover, loss of function of Rab18 led to reduced triacylglycerol (TAG) level and fewer but larger LDs. In contrast, overexpression of Rab18 resulted in elevated TAG content and LD number. Furthermore, LD-associated Rab18 interacted with acyl-CoA synthetase long-chain family member 3 (ACSL3), which in turn promoted the LD localization of this protein. These data show that Rab18 interacts with PLIN2 and forms a complex with PLIN2 and ACSL3, which plays a critical role in LD accumulation and dynamics of myoblast cells.

Comparative proteomics reveals that lipid droplet-anchored mitochondria are more sensitive to cold in brown adipocytes

Brown adipose tissue (BAT) is specialized for uncoupled heat production through mitochondrion fueled majorly from fatty acids (FAs) of lipid droplets (LDs). How the interaction between the two organelles contributes the generation of heat remains elusive. Here, we report that LD-anchored mitochondria (LDAM) were observed in the BAT of mice raised at three different temperatures, 30 °C, 23 °C, and 6 °C. The biochemical analyses including Western blotting of electron transport chain subunits showed that LDAM were functional. Comparative proteomics analysis was conducted, which revealed differential expressions of proteins between LDAM and cytoplasmic mitochondria (CM) at different temperatures. Higher expressions of proteins at low temperature were observed for i) FA β-oxidation in LDAM including FA synthesis and uncoupling, ii) pseudo-futile cycle in CM, and iii) two shuttle systems: glycerol 3-phosphate in both CM and LDAM and citrate malate in CM. Together, these results suggest that LDs and LDAM form a preorganized and functional organelle complex that permits the rapid response to cold.

The ménage à trois of autophagy, lipid droplets and liver disease

Autophagic pathways cross with lipid homeostasis and thus provide energy and essential building blocks that are indispensable for liver functions. Energy deficiencies are compensated by breaking down lipid droplets (LDs), intracellular organelles that store neutral lipids, in part by a selective type of autophagy, referred to as lipophagy. The process of lipophagy does not appear to be properly regulated in fatty liver diseases (FLDs), an important risk factor for the development of hepatocellular carcinomas (HCC). Here we provide an overview on our current knowledge of the biogenesis and functions of LDs, and the mechanisms underlying their lysosomal turnover by autophagic processes. This review also focuses on nonalcoholic steatohepatitis (NASH), a specific type of FLD characterized by steatosis, chronic inflammation and cell death. Particular attention is paid to the role of macroautophagy and macrolipophagy in relation to the parenchymal and non-parenchymal cells of the liver in NASH, as this disease has been associated with inappropriate lipophagy in various cell types of the liver.Abbreviations: ACAT: acetyl-CoA acetyltransferase; ACAC/ACC: acetyl-CoA carboxylase; AKT: AKT serine/threonine kinase; ATG: autophagy related; AUP1: AUP1 lipid droplet regulating VLDL assembly factor; BECN1/Vps30/Atg6: beclin 1; BSCL2/seipin: BSCL2 lipid droplet biogenesis associated, seipin; CMA: chaperone-mediated autophagy; CREB1/CREB: cAMP responsive element binding protein 1; CXCR3: C-X-C motif chemokine receptor 3; DAGs: diacylglycerols; DAMPs: danger/damage-associated molecular patterns; DEN: diethylnitrosamine; DGAT: diacylglycerol O-acyltransferase; DNL: de novo lipogenesis; EHBP1/NACSIN (EH domain binding protein 1); EHD2/PAST2: EH domain containing 2; CoA: coenzyme A; CCL/chemokines: chemokine ligands; CCl_4: carbon tetrachloride; ER: endoplasmic reticulum; ESCRT: endosomal sorting complexes required for transport; FA: fatty acid; FFAs: free fatty acids; FFC: high saturated fats, fructose and cholesterol; FGF21: fibroblast growth factor 21; FITM/FIT: fat storage inducing transmembrane protein; FLD: fatty liver diseases; FOXO: forkhead box O; GABARAP: GABA type A receptor-associated protein; GPAT: glycerol-3-phosphate acyltransferase; HCC: hepatocellular carcinoma; HDAC6: histone deacetylase 6; HECT: homologous to E6-AP C-terminus; HFCD: high fat, choline deficient; HFD: high-fat diet; HSCs: hepatic stellate cells; HSPA8/HSC70: heat shock protein family A (Hsp70) member 8; ITCH/AIP4: itchy E3 ubiquitin protein ligase; KCs: Kupffer cells; LAMP2A: lysosomal associated membrane protein 2A; LDs: lipid droplets; LDL: low density lipoprotein; LEP/OB: leptin; LEPR/OBR: leptin receptor; LIPA/LAL: lipase A, lysosomal acid type; LIPE/HSL: lipase E, hormone sensitive type; LIR: LC3-interacting region; LPS: lipopolysaccharide; LSECs: liver sinusoidal endothelial cells; MAGs: monoacylglycerols; MAPK: mitogen-activated protein kinase; MAP3K5/ASK1: mitogen-activated protein kinase kinase kinase 5; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MCD: methionine-choline deficient; MGLL/MGL: monoglyceride lipase; MLXIPL/ChREBP: MLX interacting protein like; MTORC1: mechanistic target of rapamycin kinase complex 1; NAFLD: nonalcoholic fatty liver disease; NAS: NAFLD activity score; NASH: nonalcoholic steatohepatitis; NPC: NPC intracellular cholesterol transporter; NR1H3/LXRα: nuclear receptor subfamily 1 group H member 3; NR1H4/FXR: nuclear receptor subfamily 1 group H member 4; PDGF: platelet derived growth factor; PIK3C3/VPS34: phosphatidylinositol 3-kinase catalytic subunit type 3; PLIN: perilipin; PNPLA: patatin like phospholipase domain containing; PNPLA2/ATGL: patatin like phospholipase domain containing 2; PNPLA3/adiponutrin: patatin like phospholipase domain containing 3; PPAR: peroxisome proliferator activated receptor; PPARA/PPARα: peroxisome proliferator activated receptor alpha; PPARD/PPARδ: peroxisome proliferator activated receptor delta; PPARG/PPARγ: peroxisome proliferator activated receptor gamma; PPARGC1A/PGC1α: PPARG coactivator 1 alpha; PRKAA/AMPK: protein kinase AMP-activated catalytic subunit; PtdIns3K: class III phosphatidylinositol 3-kinase; PtdIns3P: phosphatidylinositol-3-phosphate; PTEN: phosphatase and tensin homolog; ROS: reactive oxygen species; SE: sterol esters; SIRT1: sirtuin 1; SPART/SPG20: spartin; SQSTM1/p62: sequestosome 1; SREBF1/SREBP1c: sterol regulatory element binding transcription factor 1; TAGs: triacylglycerols; TFE3: transcription factor binding to IGHM enhancer 3; TFEB: transcription factor EB; TGFB1/TGFβ: transforming growth factor beta 1; Ub: ubiquitin; UBE2G2/UBC7: ubiquitin conjugating enzyme E2 G2; ULK1/Atg1: unc-51 like autophagy activating kinase 1; USF1: upstream transcription factor 1; VLDL: very-low density lipoprotein; VPS: vacuolar protein sorting; WIPI: WD-repeat domain, phosphoinositide interacting; WDR: WD repeat domain.

Perilipin 4 Protein: an Impending Target for Amyotrophic Lateral Sclerosis

The pathogenesis of amyotrophic lateral sclerosis (ALS) might exist some relationships with the abnormal lipidomic metabolisms. Therefore, we observed and analyzed the alteration of perilipin 4 (PLIN 4) distribution in the anterior horns (AH); the central canals (CC) and its surrounding gray matter; the posterior horns (PH); and the anterior, lateral, and posterior funiculus (AF, LF, and PF) of the cervical, thoracic, and lumbar segments, as well as the alteration of PLIN 4 expression in the entire spinal cords at the pre-onset, onset, and progression stages of Tg(SOD1*G93A)1Gur (TG) mice and the same period of wild-type(WT) by fluorescent immunohistochemistry, the Western blot, and the image analysis. Results showed that the PLIN 4 distributions in the spinal AH, CC and its surrounding gray matter, PH, AF, and PF of the cervical, thoracic, and lumbar segments in the TG mice at the pre-onset, onset, and progression stages significantly increased compared with those at the same periods of WT mice; the gray matter was especially significant. No significant changes were detected in the LF. PLIN 4 extensively distributed in the neurons and the proliferation neural cells. The PLIN 4 distributions significantly gradually increased from the pre-onset to onset to progression stages, and significantly correlated with the gradual increase death of neural cells. Total PLIN 4 expression in the spinal cords of TG mice significantly increased from the pre-onset, to onset, and to progression stages compared with that in the WT mice. Our data suggested that the PLIN 4 distribution and expression alterations might participate in the death of neural cells in the pathogenesis of ALS through modulating the lipidomic metabolisms and the neural cell proliferation.

Nuclear-lipid-droplet proteome: carboxylesterase as a nuclear lipase involved in lipid-droplet homeostasis

Nuclear-lipid droplets (nLD)—a dynamic cellular organelle that stores neutral lipids, within the nucleus of eukaryotic cells—consists of a hydrophobic triacylglycerol –cholesterol-ester core enriched in oleic acid (OA) surrounded by a monolayer of polar lipids, cholesterol, and proteins. nLD are probably involved in nuclear-lipid homeostasis serving as an endonuclear buffer that provides or incorporates lipids and proteins participating in signaling pathways, as transcription factors and enzymes of lipid metabolism and nuclear processes. In the present work, we analyzed the nLD proteome and hypothesized that nLD-monolayer proteins could be involved in processes similar as the ones occurring in the cLD including lipid metabolism and other cellular functions. We evaluated the rat-liver–nLD proteome under physiological and nonpathological conditions by GeLC-MS². Since isolated nLD are highly diluted, a protein-concentrating isolation protocol was designed. Thirty-five proteins were identified within the functional categories: cytoskeleton and structural, transcription and translation, histones, protein-folding and posttranslational modification, cellular proliferation and/or cancer, lipid metabolism, and transport. Purified nLD contained an enzyme from the lipid-metabolism pathway, carboxylesterase 1d (Ces1d/Ces3). Nuclear Carboxylesterase localization was confirmed by Western blotting. By in-silico analyses rat Ces1d/Ces3 secondary and tertiary structure predicted would be equivalent to human CES1. These results—the first nLD proteome—demonstrate that a tandem-GeLC-MS²-analysis protocol facilitates studies like these on rat-liver nuclei. A diversity of cellular-protein function was identified indicating the direct or indirect nLD participation and involving Ces1d/Ces3 in the LD-population homeostasis.

Lipid droplet membrane proteome remodeling parallels ethanol-induced hepatic steatosis and its resolution

Lipid droplets (LDs) are composed of neutral lipids enclosed in a phospholipid monolayer, which harbors membrane-associated proteins that regulate LD functions. Despite the crucial role of LDs in lipid metabolism, remodeling of LD protein composition in disease contexts, such as steatosis, remains poorly understood. We hypothesized that chronic ethanol consumption, subsequent abstinence from ethanol, or fasting differentially affects the LD membrane proteome content and that these changes influence how LDs interact with other intracellular organelles. Here, male Wistar rats were pair-fed liquid control or ethanol diets for 6 weeks, and then, randomly chosen animals from both groups were either refed a control diet for 7 days or fasted for 48 h before euthanizing. From all groups, LD membrane proteins from purified liver LDs were analyzed immunochemically and by MS proteomics. Liver LD numbers and sizes were greater in ethanol-fed rats than in pair-fed control, 7-day refed, or fasted rats. Compared with control rats, ethanol feeding markedly altered the LD membrane proteome, enriching LD structural perilipins and proteins involved in lipid biosynthesis, while lowering LD lipase levels. Ethanol feeding also lowered LD-associated mitochondrial and lysosomal proteins. In 7-day refed (i.e., ethanol-abstained) or fasted-ethanol-fed rats, we detected distinct remodeling of the LD proteome, as judged by lower levels of lipid biosynthetic proteins, and enhanced LD interaction with mitochondria and lysosomes. Our study reveals evidence of significant remodeling of the LD membrane proteome that regulates ethanol-induced steatosis, its resolution after withdrawal and abstinence, and changes in LD interactions with other intracellular organelles.

Adaptive Thermogenesis Driving Catch-Up Fat Is Associated With Increased Muscle Type 3 and Decreased Hepatic Type 1 Iodothyronine Deiodinase Activities: A Functional and Proteomic Study

Refeeding after caloric restriction induces weight regain and a disproportionate recovering of fat mass rather than lean mass (catch-up fat) that, in humans, associates with higher risks to develop chronic dysmetabolism. Studies in a well-established rat model of semistarvation-refeeding have reported that catch-up fat associates with hyperinsulinemia, glucose redistribution from skeletal muscle to white adipose tissue and suppressed adaptive thermogenesis sustaining a high efficiency for fat deposition. The skeletal muscle of catch-up fat animals exhibits reduced insulin-stimulated glucose utilization, mitochondrial dysfunction, delayed in vivo contraction-relaxation kinetics, increased proportion of slow fibers and altered local thyroid hormone metabolism, with suggestions of a role for iodothyronine deiodinases. To obtain novel insights into the skeletal muscle response during catch-up fat in this rat model, the functional proteomes of tibialis anterior and soleus muscles, harvested after 2 weeks of caloric restriction and 1 week of refeeding, were studied. Furthermore, to assess the implication of thyroid hormone metabolism in catch-up fat, circulatory thyroid hormones as well as liver type 1 (D1) and liver and skeletal muscle type 3 (D3) iodothyronine deiodinase activities were evaluated. The proteomic profiling of both skeletal muscles indicated catch-up fat-induced alterations, reflecting metabolic and contractile adjustments in soleus muscle and changes in glucose utilization and oxidative stress in tibialis anterior muscle. In response to caloric restriction, D3 activity increased in both liver and skeletal muscle, and persisted only in skeletal muscle upon refeeding. In parallel, liver D1 activity decreased during caloric restriction, and persisted during catch-up fat at a time-point when circulating levels of T4, T3 and rT3 were all restored to those of controls. Thus, during catch-up fat, a local hypothyroidism may occur in liver and skeletal muscle despite systemic euthyroidism. The resulting reduced tissue thyroid hormone bioavailability, likely D1- and D3-dependent in liver and skeletal muscle, respectively, may be part of the adaptive thermogenesis sustaining catch-up fat. These results open new perspectives in understanding the metabolic processes associated with the high efficiency of body fat recovery after caloric restriction, revealing new implications for iodothyronine deiodinases as putative biological brakes contributing in suppressed thermogenesis driving catch-up fat during weight regain.

Two Types of Contact Between Lipid Droplets and Mitochondria

Lipid droplets (LDs) and mitochondria are essential organelles involved in cellular lipid metabolism and energy homeostasis. Accumulated studies have revealed that the physical contact between these two organelles is important for their functions. Current understanding of the contact between cellular organelles is highly dynamic, fitting a "kiss-and-run" model. The same pattern of contact between LDs and mitochondria has been reported and several proteins are found to mediate this contact, such as perilipin1 (PLIN1) and PLIN5. Another format of the contact has also been found and termed anchoring. LD-anchored mitochondria (LDAM) are identified in oxidative tissues including brown adipose tissue (BAT), skeletal muscle, and heart muscle, and this anchoring between these two organelles is conserved from mouse to monkey. Moreover, this anchoring is generated during the brown/beige adipocyte differentiation. In this review, we will summarize previous studies on the interaction between LDs and mitochondria, categorize the types of the contacts into dynamic and stable/anchored, present their similarities and differences, discuss their potential distinct molecular mechanism, and finally propose a working hypothesis that may explain why and how cells use two patterns of contact between LDs and mitochondria.

TBC1D1 interacting proteins, VPS13A and VPS13C, regulate GLUT4 homeostasis in C2C12 myotubes

Proteins involved in the spaciotemporal regulation of GLUT4 trafficking represent potential therapeutic targets for the treatment of insulin resistance and type 2 diabetes. A key regulator of insulin- and exercise-stimulated glucose uptake and GLUT4 trafficking is TBC1D1. This study aimed to identify proteins that regulate GLUT4 trafficking and homeostasis via TBC1D1. Using an unbiased quantitative proteomics approach, we identified proteins that interact with TBC1D1 in C2C12 myotubes including VPS13A and VPS13C, the Rab binding proteins EHBP1L1 and MICAL1, and the calcium pump SERCA1. These proteins associate with TBC1D1 via its phosphotyrosine binding (PTB) domains and their interactions with TBC1D1 were unaffected by AMPK activation, distinguishing them from the AMPK regulated interaction between TBC1D1 and AMPKα1 complexes. Depletion of VPS13A or VPS13C caused a post-transcriptional increase in cellular GLUT4 protein and enhanced cell surface GLUT4 levels in response to AMPK activation. The phenomenon was specific to GLUT4 because other recycling proteins were unaffected. Our results provide further support for a role of the TBC1D1 PTB domains as a scaffold for a range of Rab regulators, and also the VPS13 family of proteins which have been previously linked to fasting glycaemic traits and insulin resistance in genome wide association studies.

Identification and analysis of lipid droplet-related proteome in the adipose tissue of grass carp (Ctenopharyngodon idella) under fed and starved conditions

Fat accumulation in the mesenteric adipose tissue is a serious problem in grass carp (Ctenopharyngodon idella) culture. Lipid droplet-related proteins (LDRPs) are involved in the formation, degradation, and biological functions of lipid droplets. In this study, we aimed to provide reference proteomics data to study lipid droplet regulation in fish. We isolated LDRPs from the mesenteric adipose tissue of grass carp (1-year-old) after normal feeding and 7 days of starvation, and identified and analysed them using isobaric tags for relative and absolute quantitation (iTRAQ) technology. Short-term starvation had no significant effect on the body weight, condition factor, visceral index, hepatopancreas index, intraperitoneal fat index, adipose tissue triglyceride content, and adipocyte size of grass carp. Nine hundred and fifty proteins were identified and annotated using the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases; they are involved in a variety of metabolic and signalling pathways, including amino acid, lipid, and carbohydrate metabolism, and the PI3K-Akt signalling pathway. There were 296 differentially expressed proteins (DEPs), with 143 up-regulated and 153 down-regulated proteins. Three proteins involved in triglyceride and fatty acid syntheses and two proteins involved in autophagy were up-regulated, and six proteins involved in lipid catabolism were down-regulated. These results indicate that under short-term starvation, lipid droplets in the adipose tissue of grass carp may maintain their shape by promoting fat production and inhibiting lipolysis, and autophagy may be one of the main strategies for coping with short-term energy deprivation.

The biology of lipid droplet-bound mitochondria

Proper regulation of cellular lipid storage and oxidation is indispensable for the maintenance of cellular energy homeostasis and health. Mitochondrial function has been shown to be a main determinant of functional lipid storage and oxidation, which is of particular interest for the adipose tissue, as it is the main site of triacylglyceride storage in lipid droplets (LDs). Recent studies have identified a subpopulation of mitochondria attached to LDs, peridroplet mitochondria (PDM) that can be separated from cytoplasmic mitochondria (CM) by centrifugation. PDM have distinct bioenergetics, proteome, cristae organization and dynamics that support LD build-up, however their role in adipose tissue biology remains largely unexplored. Therefore, understanding the molecular basis of LD homeostasis and their relationship to mitochondrial function and attachment in adipocytes is of major importance.

Mitofusin 2 in Mature Adipocytes Controls Adiposity and Body Weight

We found that exposure of adult animals to caloriedense foods rapidly abolished expression of mitofusin 2 (Mfn2), a gene promoting mitochondrial fusion and mitochondrion-endoplasmic reticulum interactions, in white and brown fat. Mfn2 mRN was also robustly lower in obese human subjects compared with lean controls. Adipocyte-specific knockdown of Mfn2 in adult mice led to increased food intake, adiposity, and impaired glucose metabolism on standard chow as well as on a diet with high calorie content. The body weight and adiposity of mature adipocyte-specific Mfn2 knockout mice on a standard diet were similar to those of control mice on a high-fat diet. The transcriptional profile of the adipose tissue in adipocyte-specific Mfn2 knockout mice was consistent with adipocyte proliferation, increased lipogenesis at the tissue level, and decreased glucose utilization at the systemic level. These observations suggest a possible crucial role for mitochondrial dynamics in adipocytes in initiating systemic metabolic dysregulation.

Mancini et al. find that the mitochondrial fusion protein Mfn2 is lower in adipose tissue of mice on a high-fat diet and that of obese humans and that this protein in the fat is important for systemic control of metabolism.

Skeletal muscle lipid droplets are resynthesized before being coated with perilipin proteins following prolonged exercise in elite male triathletes

Intramuscular triglycerides (IMTG) are a key substrate during prolonged exercise, but little is known about the rate of IMTG resynthesis in the postexercise period. We investigated the hypothesis that the distribution of the lipid droplet (LD)-associated perilipin (PLIN) proteins is linked to IMTG storage following exercise. Fourteen elite male triathletes (27 ± 1 yr, 66.5 ± 1.3 mL·kg^-1·min^-1) completed 4 h of moderate-intensity cycling. During the first 4 h of recovery, subjects received either carbohydrate or H₂O, after which both groups received carbohydrate. Muscle biopsies collected pre- and postexercise and 4 and 24 h postexercise were analyzed using confocal immunofluorescence microscopy for fiber type-specific IMTG content and PLIN distribution with LDs. Exercise reduced IMTG content in type I fibers (-53%, P = 0.002), with no change in type IIa fibers. During the first 4 h of recovery, IMTG content increased in type I fibers (P = 0.014), but was not increased more after 24 h, where it was similar to baseline levels in both conditions. During recovery the number of LDs labeled with PLIN2 (70%), PLIN3 (63%), and PLIN5 (62%; all P < 0.05) all increased in type I fibers. Importantly, the increase in LDs labeled with PLIN proteins only occurred at 24 h postexercise. In conclusion, IMTG resynthesis occurs rapidly in type I fibers following prolonged exercise in highly trained individuals. Furthermore, increases in IMTG content following exercise preceded an increase in the number of LDs labeled with PLIN proteins. These data, therefore, suggest that the PLIN proteins do not play a key role in postexercise IMTG resynthesis.

Cells Undergo Major Changes in the Quantity of Cytoplasmic Organelles after Uptake of Gold Nanoparticles with Biologically Relevant Surface Coatings

Here, we use cryo soft X-ray tomography (cryo-SXT), which delivers 3D ultrastructural volumes of intact cells without chemical fixation or staining, to gain insight about nanoparticle uptake for nanomedicine. We initially used dendritic polyglycerol sulfate (dPGS) with potential diagnostic and therapeutic applications in inflammation. Although dPGS-coated gold nanoparticle (dPGS-AuNP) uptake followed a conventional endocytic/degradative pathway in human lung epithelial cell lines (A549), with cryo-SXT, we detected ∼5% of dPGS-AuNPs in the cytoplasm, a level undetectable by confocal light microscopy. We also observed ∼5% of dPGS-AuNPs in a rarely identified subcellular site, namely, lipid droplets, which are important for cellular energy metabolism. Finally, we also found substantial changes in the quantity of cytoplasmic organelles upon dPGS-AuNP uptake over the 1-6 h incubation period; the number of small vesicles and mitochondria significantly increased, and the number of multivesicular bodies and the number and volume of lipid droplets significantly decreased. Although nearly all organelle numbers at 6 h were still significantly different from controls, most appeared to be returning to normal levels. To test for generality, we also examined cells after uptake of gold nanoparticles coated with a different agent, polyethylenimine (PEI), used for nucleic acid delivery. PEI nanoparticles did not enter lipid droplets, but they induced similar, albeit less pronounced, changes in the quantity of cytoplasmic organelles. We confirmed these changes in organelle quantities for both nanoparticle coatings by confocal fluorescence microscopy. We suggest this cytoplasmic remodeling could reflect a more common cellular response to coated gold nanoparticle uptake.

The Puzzling Conservation and Diversification of Lipid Droplets from Bacteria to Eukaryotes

Membrane compartments are amongst the most fascinating markers of cell evolution from prokaryotes to eukaryotes, some being conserved and the others having emerged via a series of primary and secondary endosymbiosis events. Membrane compartments comprise the system limiting cells (one or two membranes in bacteria, a unique plasma membrane in eukaryotes) and a variety of internal vesicular, subspherical, tubular, or reticulated organelles. In eukaryotes, the internal membranes comprise on the one hand the general endomembrane system, a dynamic network including organelles like the endoplasmic reticulum, the Golgi apparatus, the nuclear envelope, etc. and also the plasma membrane, which are linked via direct lateral connectivity (e.g. between the endoplasmic reticulum and the nuclear outer envelope membrane) or indirectly via vesicular trafficking. On the other hand, semi-autonomous organelles, i.e. mitochondria and chloroplasts, are disconnected from the endomembrane system and request vertical transmission following cell division. Membranes are organized as lipid bilayers in which proteins are embedded. The budding of some of these membranes, leading to the formation of the so-called lipid droplets (LDs) loaded with hydrophobic molecules, most notably triacylglycerol, is conserved in all clades. The evolution of eukaryotes is marked by the acquisition of mitochondria and simple plastids from Gram-positive bacteria by primary endosymbiosis events and the emergence of extremely complex plastids, collectively called secondary plastids, bounded by three to four membranes, following multiple and independent secondary endosymbiosis events. There is currently no consensus view of the evolution of LDs in the Tree of Life. Some features are conserved; others show a striking level of diversification. Here, we summarize the current knowledge on the architecture, dynamics, and multitude of functions of the lipid droplets in prokaryotes and in eukaryotes deriving from primary and secondary endosymbiosis events.

Lipid droplet-associated kinase STK25 regulates peroxisomal activity and metabolic stress response in steatotic liver

Nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH) are emerging as leading causes of liver disease worldwide and have been recognized as one of the major unmet medical needs of the 21st century. Our recent translational studies in mouse models, human cell lines, and well-characterized patient cohorts have identified serine/threonine kinase (STK)25 as a protein that coats intrahepatocellular lipid droplets (LDs) and critically regulates liver lipid homeostasis and progression of NAFLD/NASH. Here, we studied the mechanism-of-action of STK25 in steatotic liver by relative quantification of the hepatic LD-associated phosphoproteome from high-fat diet-fed Stk25 knockout mice compared with their wild-type littermates. We observed a total of 131 proteins and 60 phosphoproteins that were differentially represented in STK25-deficient livers. Most notably, a number of proteins involved in peroxisomal function, ubiquitination-mediated proteolysis, and antioxidant defense were coordinately regulated in Stk25 ^-/- versus wild-type livers. We confirmed attenuated peroxisomal biogenesis and protection against oxidative and ER stress in STK25-deficient human liver cells, demonstrating the hepatocyte-autonomous manner of STK25's action. In summary, our results suggest that regulation of peroxisomal function and metabolic stress response may be important molecular mechanisms by which STK25 controls the development and progression of NAFLD/NASH.

PSMD1 and PSMD2 regulate HepG2 cell proliferation and apoptosis via modulating cellular lipid droplet metabolism

Background

Obesity and nonalcoholic steatohepatitis (NASH) are well-known risk factors of hepatocellular carcinoma (HCC). The lipid-rich environment enhances the proliferation and metastasis abilities of tumor cells. Previous studies showed the effect of the ubiquitin–proteasome system (UPS) on tumor cell proliferation. However, the underlying mechanism of UPS in regulating the proliferation of lipid-rich tumor cells is not totally clear.

Results

Here, we identify two proteasome 26S subunits, non-ATPase 1 and 2 (PSMD1 and PSMD2), which regulate HepG2 cells proliferation via modulating cellular lipid metabolism. Briefly, the knockdown of PSMD1 and/or PSMD2 decreases the formation of cellular lipid droplets, the provider of the energy and membrane components for tumor cell proliferation. Mechanically, PSMD1 and PSMD2 regulate the expression of genes related to de novo lipid synthesis via p38-JNK and AKT signaling. Moreover, the high expression of PSMD1 and PSMD2 is significantly correlated with poor prognosis of HCC.

Conclusion

We demonstrate that PSMD1 and PSMD2 promote the proliferation of HepG2 cells via facilitating cellular lipid droplet accumulation. This study provides a potential therapeutic strategy for the treatment of lipid-rich tumors.

Lipotoxicity in Kidney, Heart, and Skeletal Muscle Dysfunction

Dyslipidemia is a common nutritional and metabolic disorder in patients with chronic kidney disease. Accumulating evidence supports the hypothesis that prolonged metabolic imbalance of lipids leads to ectopic fat distribution in the peripheral organs (lipotoxicity), including the kidney, heart, and skeletal muscle, which accelerates peripheral inflammation and afflictions. Thus, lipotoxicity may partly explain progression of renal dysfunction and even extrarenal complications, including renal anemia, heart failure, and sarcopenia. Additionally, endoplasmic reticulum stress activated by the unfolded protein response pathway plays a pivotal role in lipotoxicity by modulating the expression of key enzymes in lipid synthesis and oxidation. Here, we review the molecular mechanisms underlying lipid deposition and resultant tissue damage in the kidney, heart, and skeletal muscle, with the goal of illuminating the nutritional aspects of these pathologies.

Protein kinase MST3 modulates lipid homeostasis in hepatocytes and correlates with nonalcoholic steatohepatitis in humans

Ectopic lipid storage in the liver is considered the main risk factor for nonalcoholic steatohepatitis (NASH). Understanding the molecular networks controlling hepatocellular lipid deposition is therefore essential for developing new strategies to effectively prevent and treat this complex disease. Here, we describe a new regulator of lipid partitioning in human hepatocytes: mammalian sterile 20-like (MST) 3. We found that MST3 protein coats lipid droplets in mouse and human liver cells. Knockdown of MST3 attenuated lipid accumulation in human hepatocytes by stimulating β-oxidation and triacylglycerol secretion while inhibiting fatty acid influx and lipid synthesis. We also observed that lipogenic gene expression and acetyl-coenzyme A carboxylase protein abundance were reduced in MST3-deficient hepatocytes, providing insight into the molecular mechanisms underlying the decreased lipid storage. Furthermore, MST3 expression was positively correlated with key features of NASH (i.e., hepatic lipid content, lobular inflammation, and hepatocellular ballooning) in human liver biopsies. In summary, our results reveal a role of MST3 in controlling the dynamic metabolic balance of liver lipid catabolism vs. lipid anabolism. Our findings highlight MST3 as a potential drug target for the prevention and treatment of NASH and related complex metabolic diseases.-Cansby, E., Kulkarni, N. M., Magnusson, E., Kurhe, Y., Amrutkar, M., Nerstedt, A., Ståhlman, M., Sihlbom, C., Marschall, H.-U., Borén, J., Blüher, M., Mahlapuu, M. Protein kinase MST3 modulates lipid homeostasis in hepatocytes and correlates with nonalcoholic steatohepatitis in humans.

Fat, fight, and beyond: The multiple roles of lipid droplets in infections and inflammation

Increased accumulation of cytoplasmic lipid droplets (LDs) in host nonadipose cells is commonly observed in response to numerous infectious diseases, including bacterial, parasite, and fungal infections. LDs are lipid-enriched, dynamic organelles composed of a core of neutral lipids surrounded by a monolayer of phospholipids associated with a diverse array of proteins that are cell and stimulus regulated. Far beyond being simply a deposit of neutral lipids, LDs have come to be seen as an essential platform for various cellular processes, including metabolic regulation, cell signaling, and the immune response. LD participation in the immune response occurs as sites for compartmentalization of several immunometabolic signaling pathways, production of inflammatory lipid mediators, and regulation of antigen presentation. Infection-driven LD biogenesis is a complexly regulated process that involves innate immune receptors, transcriptional and posttranscriptional regulation, increased lipid uptake, and new lipid synthesis. Accumulating evidence demonstrates that intracellular pathogens are able to exploit LDs as an energy source, a replication site, and/or a mechanism of immune response evasion. Nevertheless, LDs can also act in favor of the host as part of the immune and inflammatory response to pathogens. Here, we review recent findings that explored the new roles of LDs in the context of host-pathogen interactions.