An HPLC-CAD/fluorescence lipidomics platform using fluorescent fatty acids as metabolic tracers

Establishing the Role of Liver Fatty Acid-Binding Protein in Post-Golgi Very-Low-Density Lipoprotein Trafficking Using a Novel Fluorescence-Based Assay

The liver plays a crucial role in maintaining lipid homeostasis by converting toxic free fatty acids into VLDL, which the body uses for energy. Even minor changes in VLDL formation and secretion can result in serious health conditions such as atherosclerosis and non-alcoholic fatty liver disease. Despite the importance of VLDL, the proteins and signaling pathways involved in its regulation remain largely unknown. This study aims to develop a novel methodology to study intracellular VLDL transport events and explore the role of liver fatty acid-binding protein (LFABP) in VLDL transport and secretion. Current methods to study VLDL are often tedious, time-consuming, and expensive, underscoring the need for an alternative approach. We designed a new immunofluorescence-based assay to track the formation and secretion of VLDL in cells over time using fluorescently tagged TopFluor oleic acid. Confocal microscopy confirmed that TopFluor oleic acid enters hepatocytes and colocalizes with the ER, Golgi, and plasma membrane. Additionally, the collection of cell culture media revealed that TopFluor was incorporated into VLDL particles, as confirmed by fluorescence readings and ApoB100 immunoblots. This novel assay provides a valuable tool for further research into the mechanisms of VLDL regulation and the development of potential therapeutic targets for related diseases. Utilizing this assay, we identified LFABP as a key regulatory protein in post-Golgi VLDL trafficking. Our data suggest that LFABP plays a crucial role in this process, and its functional impairment leads to reduced VLDL secretion.

CD36 inhibition corrects lipid-FetuinA mediated insulin secretory defects by preventing intracellular lipid accumulation and inflammation in the pancreatic beta cells

CD36 is a multifunctional protein involved in long chain fatty acid uptake and immune modulation in different cells. Recently it was reported that increased expression of CD36 is evident in the islets of diabetic obese individuals. In this present study we investigated the role of CD36 in regulating intracellular lipid accumulation and inflammation in beta cells and its implication on secretory dysfunction. Additionally, we have elucidated the potential role of fetuinA, a circulatory glycoprotein and an endogenous ligand of TLR4, for aggravating lipid accumulation and insulin secretory defects in beta cells. MIN6 mouse insulinoma cells when incubated with palmitate and fetuinA together showed activation of TLR4-NFkB inflammatory cascade and increased uptake of palmitate, which was rescued by CD36 functional inhibition or knockdown. Moreover, glucose stimulated insulin secretion was restored with consequent downregulation of IL-1β secretion. TLR4 inhibition also decreased intracellular lipid content with a reduction of CD36, suggesting functional crosstalk between them. At physiological level, excess fetuinA in the islet milieu of HFD fed C57BL/6J mice or exogenous fetuinA administration (i.p.) promoted lipid accumulation in the islets resulting in decreased insulin secretion with increased CD36 expression. Interestingly, CD36 inhibition in HFD mice with a pharmacological inhibitor Salvianolic acid B attenuated inflammation, reduced intracellular lipid accumulation in beta cells and restored insulin secretory function. Therefore, our results suggest that inhibition of CD36 protects beta cells from the derogatory effects of lipid and fetuinA and restores secretory function and can be considered as a therapeutic target for obesity mediated beta cell dysfunction.

Zebrafish are resilient to the loss of major diacylglycerol acyltransferase enzymes

In zebrafish, maternally deposited yolk is the source of nutrients for embryogenesis prior to digestive system maturation. Yolk nutrients are processed and secreted to the growing organism by an extra-embryonic tissue, the yolk syncytial layer (YSL). The export of lipids from the YSL occurs through the production of triacylglycerol-rich lipoproteins. Here we report that mutations in the triacylglycerol synthesis enzyme, diacylglycerol acyltransferase-2 (Dgat2), cause yolk sac opacity due to aberrant accumulation of cytoplasmic lipid droplets in the YSL. Although triacylglycerol synthesis continues, it is not properly coupled to lipoprotein production as dgat2 mutants produce fewer, smaller, ApoB-containing lipoproteins. Unlike DGAT2-null mice, which are lipopenic and die soon after birth, zebrafish dgat2 mutants are viable, fertile, and exhibit normal mass and adiposity. Residual Dgat activity cannot be explained by the activity of other known Dgat isoenzymes, as dgat1a;dgat1b;dgat2 triple mutants continue to produce YSL lipid droplets and remain viable as adults. Further, the newly identified diacylglycerol acyltransferase, Tmem68, is also not responsible for the residual triacylglycerol synthesis activity. Unlike overexpression of Dgat1a and Dgat1b, monoacylglycerol acyltransferase-3 (Mogat3b) overexpression does not rescue yolk opacity, suggesting it does not possess Dgat activity in the YSL. However, mogat3b;dgat2 double mutants exhibit increased yolk opacity and often have structural alterations of the yolk extension. Quadruple mogat3b;dgat1a;dgat1b;dgat2 mutants either have severely reduced viability and stunted growth or do not survive past 3 days post fertilization, depending on the dgat2 mutant allele present. Our study highlights the remarkable ability of vertebrates to synthesize triacylglycerol through multiple biosynthetic pathways.

Gestational Diabetes-like Fuels Impair Mitochondrial Function and Long-Chain Fatty Acid Uptake in Human Trophoblasts

In the parent, gestational diabetes mellitus (GDM) causes both hyperglycemia and hyperlipidemia. Despite excess lipid availability, infants exposed to GDM are at risk for essential long-chain polyunsaturated fatty acid (LCPUFA) deficiency. Isotope studies have confirmed less LCPUFA transfer from the parent to the fetus, but how diabetic fuels impact placental fatty acid (FA) uptake and lipid droplet partitioning is not well-understood. We evaluated the effects of high glucose conditions, high lipid conditions, and their combination on trophoblast growth, viability, mitochondrial bioenergetics, BODIPY-labeled fatty acid (FA) uptake, and lipid droplet dynamics. The addition of four carbons or one double bond to FA acyl chains dramatically affected the uptake in both BeWo and primary isolated cytotrophoblasts (CTBs). The uptake was further impacted by media exposure. The combination-exposed trophoblasts had more mitochondrial protein (p = 0.01), but impaired maximal and spare respiratory capacities (p < 0.001 and p < 0.0001), as well as lower viability (p = 0.004), due to apoptosis. The combination-exposed trophoblasts had unimpaired uptake of BODIPY C12 but had significantly less whole-cell and lipid droplet uptake of BODIPY C16, with an altered lipid droplet count, area, and subcellular localization, whereas these differences were not seen with individual high glucose or lipid exposure. These findings bring us closer to understanding how GDM perturbs active FA transport to increase the risk of adverse outcomes from placental and neonatal lipid accumulation alongside LCPUFA deficiency.

A high-cholesterol zebrafish diet promotes hypercholesterolemia and fasting-associated liver steatosis

Zebrafish are an ideal model organism to study lipid metabolism and to elucidate the molecular underpinnings of human lipid-associated disorders. Unlike murine models, to which various standardized high lipid diets such as a high-cholesterol diet (HCD) are available, there has yet to be a uniformly adopted zebrafish HCD protocol. In this study, we have developed an improved HCD protocol and thoroughly tested its impact on zebrafish lipid deposition and lipoprotein regulation in a dose- and time-dependent manner. The diet stability, reproducibility, and fish palatability were also validated. Fish fed HCD developed hypercholesterolemia as indicated by significantly elevated ApoB-containing lipoproteins (ApoB-LPs) and increased plasma levels of cholesterol and cholesterol esters. Feeding of the HCD to larvae for 8 days produced hepatic steatosis that became more stable and sever after 1 day of fasting and was associated with an opaque liver phenotype (dark under transmitted light). Unlike larvae, adult fish fed HCD for 14 days followed by a 3-day fast did not develop a stable fatty liver phenotype, though the fish had higher ApoB-LP levels in plasma and an upregulated lipogenesis gene fasn in adipose tissue. In conclusion, our HCD zebrafish protocol represents an effective and reliable approach for studying the temporal characteristics of the physiological and biochemical responses to high levels of dietary cholesterol and provides insights into the mechanisms that may underlie fatty liver disease.

Protocol for monitoring fatty acid trafficking from lipid droplets to mitochondria in cultured cells

Intracellular trafficking of fatty acids (FAs) between organelles is critical for cells to adjust their metabolism in response to stimuli such as exercise, fasting, and cold exposure. Here, we describe a protocol to monitor trafficking of FAs from lipid droplets to mitochondria. We describe the labeling of organelles in cultured C2C12 myoblasts with transfection and dyes. We detail a pulse-chase labeling paradigm using a fluorescent FA analog, live-cell imaging to visualize trafficking of FAs, and steps to quantify FA trafficking. For complete details on the use and execution of this protocol, please refer to Miner et al.1.

Lipid conjugate dissociation analysis improves the in vivo understanding of lipid-based nanomedicine

Lipid conjugates have advanced the field of lipid-based nanomedicine by promoting active-targeting (ligand, peptide, antibody), stability (PEGylation), controlled release (lipoid prodrug), and probe-based tracking (fluorophore). Recent findings indicate lipid conjugates dissociating from nanomedicine upon encountering a biological environment. Yet, implications for (pre)clinical outcomes remain unclear. In this study, using the zebrafish model (Danio rerio), we investigated the fate of liposome-incorporated lipid fluorophore conjugates (LFCs) after intravenous (IV) administration. LFCs having a bilayer mismatch and relatively polar fluorophore revealed counter-predictive outcomes for Caelyx/Doxil (clearance vs. circulating) and AmBisome-like liposomes (scavenger endothelial cell vs. macrophage uptake). Findings on LFC (mis)match for Caelyx/Doxil-like liposomes were supported by translational intravital imaging studies in mice. Importantly, contradicting observations suggest to originate from LFC dissociation in vivo, which was investigated by Asymmetric Flow Field-Flow Fractionation (AF4) upon liposome-serum incubation in situ. Our data suggests that LFCs matching with the liposome bilayer composition - that did not dissociate upon serum incubation - revealed improved predictive outcomes for liposome biodistribution profiles. Altogether, this study highlights the critical importance of fatty acid tail length and headgroup moiety when selecting lipid conjugates for lipid-based nanomedicine.

Zebrafish ApoB-Containing Lipoprotein Metabolism: A Closer Look

Zebrafish have become a powerful model of mammalian lipoprotein metabolism and lipid cell biology. Most key proteins involved in lipid metabolism, including cholesteryl ester transfer protein, are conserved in zebrafish. Consequently, zebrafish exhibit a human-like lipoprotein profile. Zebrafish with mutations in genes linked to human metabolic diseases often mimic the human phenotype. Zebrafish larvae develop rapidly and externally around the maternally deposited yolk. Recent work revealed that any disturbance of lipoprotein formation leads to the accumulation of cytoplasmic lipid droplets and an opaque yolk, providing a visible phenotype to investigate disturbances of the lipoprotein pathway, already leading to discoveries in MTTP (microsomal triglyceride transfer protein) and ApoB (apolipoprotein B). By 5 days of development, the digestive system is functional, making it possible to study fluorescently labeled lipid uptake in the transparent larvae. These and other approaches enabled the first in vivo description of the STAB (stabilin) receptors, showing lipoprotein uptake in endothelial cells. Various zebrafish models have been developed to mimic human diseases by mutating genes known to influence lipoproteins (eg, ldlra, apoC2). This review aims to discuss the most recent research in the zebrafish ApoB-containing lipoprotein and lipid metabolism field. We also summarize new insights into lipid processing within the yolk cell and how changes in lipid flux alter yolk opacity. This curious new finding, coupled with the development of several techniques, can be deployed to identify new players in lipoprotein research directly relevant to human disease.

A high-cholesterol zebrafish diet promotes hypercholesterolemia and fasting-associated liver triglycerides accumulation

Zebrafish are an ideal model organism to study lipid metabolism and to elucidate the molecular underpinnings of human lipid-associated disorders. In this study, we provide an improved protocol to assay the impact of a high-cholesterol diet (HCD) on zebrafish lipid deposition and lipoprotein regulation. Fish fed HCD developed hypercholesterolemia as indicated by significantly elevated ApoB-containing lipoproteins (ApoB-LP) and increased plasma levels of cholesterol and cholesterol esters. Feeding of the HCD to larvae (8 days followed by a 1 day fast) and adult female fish (2 weeks, followed by 3 days of fasting) was also associated with a fatty liver phenotype that presented as severe hepatic steatosis. The HCD feeding paradigm doubled the levels of liver triacylglycerol (TG), which was striking because our HCD was only supplemented with cholesterol. The accumulated liver TG was unlikely due to increased de novo lipogenesis or inhibited β-oxidation since no differentially expressed genes in these pathways were found between the livers of fish fed the HCD versus control diets. However, fasted HCD fish had significantly increased lipogenesis gene fasn in adipose tissue and higher free fatty acids (FFA) in plasma. This suggested that elevated dietary cholesterol resulted in lipid accumulation in adipocytes, which supplied more FFA during fasting, promoting hepatic steatosis. In conclusion, our HCD zebrafish protocol represents an effective and reliable approach for studying the temporal characteristics of the physiological and biochemical responses to high levels of dietary cholesterol and provides insights into the mechanisms that may underlie fatty liver disease.

Zebrafish facilitate non-alcoholic fatty liver disease research: Tools, models and applications

Non-alcoholic fatty liver disease (NAFLD) has become an increasingly epidemic metabolic disease worldwide. NAFLD can gradually deteriorate from simple liver steatosis, inflammation and fibrosis to liver cirrhosis and/or hepatocellular carcinoma. Zebrafish are vertebrate animal models that are genetically and metabolically conserved with mammals and have unique advantages such as high fecundity, rapid development ex utero and optical transparency. These features have rendered zebrafish an emerging model system for liver diseases and metabolic diseases favoured by many researchers in recent years. In the present review, we summarize a series of tools for zebrafish NAFLD research and the models established through different dietary feeding, hepatotoxic chemical treatments and genetic manipulations via transgenic or genome editing technologies. We also discuss how zebrafish models facilitate NAFLD studies by providing novel insights into NAFLD pathogenesis, toxicology research, and drug evaluation and discovery.

Autolysosomal exocytosis of lipids protect neurons from ferroptosis

During oxidative stress neurons release lipids that are internalized by glia. Defects in this coordinated process play an important role in several neurodegenerative diseases. Yet, the mechanisms of lipid release and its consequences on neuronal health are unclear. Here, we demonstrate that lipid-protein particle release by autolysosome exocytosis protects neurons from ferroptosis, a form of cell death driven by lipid peroxidation. We show that during oxidative stress, peroxidated lipids and iron are released from neurons by autolysosomal exocytosis which requires the exocytic machinery VAMP7 and syntaxin 4. We observe membrane-bound lipid-protein particles by TEM and demonstrate that these particles are released from neurons using cryoEM. Failure to release these lipid-protein particles causes lipid hydroperoxide and iron accumulation and sensitizes neurons to ferroptosis. Our results reveal how neurons protect themselves from peroxidated lipids. Given the number of brain pathologies that involve ferroptosis, defects in this pathway likely play a key role in the pathophysiology of neurodegenerative disease.

From worms to humans: Understanding intestinal lipid metabolism via model organisms

The intestine is responsible for efficient absorption and packaging of dietary lipids before they enter the circulatory system. This review provides a comprehensive overview of how intestinal enterocytes from diverse model organisms absorb dietary lipid and subsequently secrete the largest class of lipoproteins (chylomicrons) to meet the unique needs of each animal. We discuss the putative relationship between diet and metabolic disease progression, specifically Type 2 Diabetes Mellitus. Understanding the molecular response of intestinal cells to dietary lipid has the potential to undercover novel therapies to combat metabolic syndrome.

Zebra-Sphinx: Modeling Sphingolipidoses in Zebrafish

Sphingolipidoses are inborn errors of metabolism due to the pathogenic mutation of genes that encode for lysosomal enzymes, transporters, or enzyme cofactors that participate in the sphingolipid catabolism. They represent a subgroup of lysosomal storage diseases characterized by the gradual lysosomal accumulation of the substrate(s) of the defective proteins. The clinical presentation of patients affected by sphingolipid storage disorders ranges from a mild progression for some juvenile- or adult-onset forms to severe/fatal infantile forms. Despite significant therapeutic achievements, novel strategies are required at basic, clinical, and translational levels to improve patient outcomes. On these bases, the development of in vivo models is crucial for a better understanding of the pathogenesis of sphingolipidoses and for the development of efficacious therapeutic strategies. The teleost zebrafish (Danio rerio) has emerged as a useful platform to model several human genetic diseases owing to the high grade of genome conservation between human and zebrafish, combined with precise genome editing and the ease of manipulation. In addition, lipidomic studies have allowed the identification in zebrafish of all of the main classes of lipids present in mammals, supporting the possibility to model diseases of the lipidic metabolism in this animal species with the advantage of using mammalian lipid databases for data processing. This review highlights the use of zebrafish as an innovative model system to gain novel insights into the pathogenesis of sphingolipidoses, with possible implications for the identification of more efficacious therapeutic approaches.

ISG15 Is a Novel Regulator of Lipid Metabolism during Vaccinia Virus Infection

Interferon-stimulated gene 15 (ISG15) is a 15-kDa ubiquitin-like modifier that binds to target proteins in a process termed ISGylation. ISG15, first described as an antiviral molecule against many viruses, participates in numerous cellular processes, from immune modulation to the regulation of genome stability. Interestingly, the role of ISG15 as a regulator of cell metabolism has recently gained strength. We previously described ISG15 as a regulator of mitochondrial functions in bone marrow-derived macrophages (BMDMs) in the context of Vaccinia virus (VACV) infection. Here, we demonstrate that ISG15 regulates lipid metabolism in BMDMs and that ISG15 is necessary to modulate the impact of VACV infection on lipid metabolism. We show that Isg15-/- BMDMs demonstrate alterations in the levels of several key proteins of lipid metabolism that result in differences in the lipid profile compared with Isg15+/+ (wild-type [WT]) BMDMs. Specifically, Isg15-/- BMDMs present reduced levels of neutral lipids, reflected by decreased lipid droplet number. These alterations are linked to increased levels of lipases and are independent of enhanced fatty acid oxidation (FAO). Moreover, we demonstrate that VACV causes a dysregulation in the proteomes of BMDMs and alterations in the lipid content of these cells, which appear exacerbated in Isg15-/- BMDMs. Such metabolic changes are likely caused by increased expression of the metabolic regulators peroxisome proliferator-activated receptor-γ (PPARγ) and PPARγ coactivator-1α (PGC-1α). In summary, our results highlight that ISG15 controls BMDM lipid metabolism during viral infections, suggesting that ISG15 is an important host factor to restrain VACV impact on cell metabolism. IMPORTANCE The functions of ISG15 are continuously expanding, and growing evidence supports its role as a relevant modulator of cell metabolism. In this work, we highlight how the absence of ISG15 impacts macrophage lipid metabolism in the context of viral infections and how poxviruses modulate metabolism to ensure successful replication. Our results open the door to new advances in the comprehension of macrophage immunometabolism and the interaction between VACV and the host.

Genetic therapy in a mitochondrial disease model suggests a critical role for liver dysfunction in mortality

The clinical and largely unpredictable heterogeneity of phenotypes in patients with mitochondrial disorders demonstrates the ongoing challenges in the understanding of this semi-autonomous organelle in biology and disease. Previously, we used the gene-breaking transposon to create 1200 transgenic zebrafish strains tagging protein-coding genes (Ichino et al., 2020), including the lrpprc locus. Here, we present and characterize a new genetic revertible animal model that recapitulates components of Leigh Syndrome French Canadian Type (LSFC), a mitochondrial disorder that includes diagnostic liver dysfunction. LSFC is caused by allelic variations in the LRPPRC gene, involved in mitochondrial mRNA polyadenylation and translation. lrpprc zebrafish homozygous mutants displayed biochemical and mitochondrial phenotypes similar to clinical manifestations observed in patients, including dysfunction in lipid homeostasis. We were able to rescue these phenotypes in the disease model using a liver-specific genetic model therapy, functionally demonstrating a previously under-recognized critical role for the liver in the pathophysiology of this disease.

BODIPY-Labeled Estrogens for Fluorescence Analysis of Environmental Microbial Degradation

Biodegradation of estrogen hormone micropollutants is a well-established approach toward their remediation. Fluorescently labeled substrates are used extensively for rapid, near-real-time analysis of biological processes and are a potential tool for studying biodegradation processes faster and more efficiently than conventional approaches. However, it is important to understand how the fluorescently tagged surrogates compare with the natural substrate in terms of chemical analysis and the intended application. We derivatized three natural estrogens with BODIPY fluorophores by azide-alkyne cycloaddition click reaction and developed an analytical workflow based on simple liquid-liquid extraction and HPLC-PDA analysis. The developed methods allow for concurrent analysis of both fluorescent and natural estrogens with comparable recovery, accuracy, and precision. We then evaluated the use of BODIPY-labeled estrogens as surrogate substrates for studying biodegradation using a model bacterium for estrogen metabolism. The developed analytical methods were successfully employed to compare the biological transformation of 17β-estradiol (E2), with and without the BODIPY fluorescent tag. Through measuring the complete degradation of E2 and the transformation of BODIPY-estradiol to BODIPY-estrone in the presence of a co-substrate, we found that BODIPY-labeled estrogens are biologically viable surrogates for investigating biodegradation in environmental bacteria.

Lipid Droplets' Role in the Regulation of β-Cell Function and β-Cell Demise in Type 2 Diabetes

During development of type 2 diabetes (T2D), excessive nutritional load is thought to expose pancreatic islets to toxic effects of lipids and reduce β-cell function and mass. However, lipids also play a positive role in cellular metabolism and function. Thus, proper trafficking of lipids is critical for β cells to maximize the beneficial effects of these molecules while preventing their toxic effects. Lipid droplets (LDs) are organelles that play an important role in the storage and trafficking of lipids. In this review, we summarize the discovery of LDs in pancreatic β cells, LD lifecycle, and the effect of LD catabolism on β-cell insulin secretion. We discuss factors affecting LD formation such as age, cell type, species, and nutrient availability. We then outline published studies targeting critical LD regulators, primarily in rat and human β-cell models, to understand the molecular effect of LD formation and degradation on β-cell function and health. Furthermore, based on the abnormal LD accumulation observed in human T2D islets, we discuss the possible role of LDs during the development of β-cell failure in T2D. Current knowledge indicates that proper formation and clearance of LDs are critical to normal insulin secretion, endoplasmic reticulum homeostasis, and mitochondrial integrity in β cells. However, it remains unclear whether LDs positively or negatively affect human β-cell demise in T2D. Thus, we discuss possible research directions to address the knowledge gap regarding the role of LDs in β-cell failure.

Role of CD36 in Palmitic Acid Lipotoxicity in Neuro-2a Neuroblastoma Cells

Elevated level of palmitic acid (PA), a long-chain saturated fatty acid (SFA), is lipotoxic to many different types of cells including Neuro-2a (N2a) neuroblastoma cells. CD36 is a multifunctional membrane glycoprotein that acts as a fatty acid translocase (FAT) facilitating the transport of long-chain free fatty acids (FFAs) into cells, serves a fatty acid (FA) sensing function in areas including taste buds and the proximal gut, and acts as a scavenger receptor that binds to many ligands, including FAs, collagen, oxidized low-density lipoproteins, and anionic phospholipids. However, the involvement of CD36 in FA uptake and PA lipotoxicity in N2a cells remains unclear. In this study, we examined FA uptake in BSA- and PA-treated N2a cells and investigated the involvement of CD36 in FA uptake and PA lipotoxicity in N2a cells. Our data showed that PA treatment promoted FA uptake in N2a cells, and that treatment with sulfo-N-succinimidyl oleate (SSO), a CD36 inhibitor, significantly decreased FA uptake in BSA- and PA-treated N2a cells, and ameliorated PA-induced decrease of cell viability, decrease of diploid cells, and increase of tetraploid cells. We also found that CD36 knockdown significantly decreased FA uptake in both BSA- and PA-treated cells as compared to their corresponding wild-type controls, and dramatically attenuated PA-induced cell cycle defects in N2a cells. Our data suggest that CD36 may play a critical role in FA uptake and PA lipotoxicity in N2a cells. CD36 may therefore represent a regulatory target against pathologies caused by excess FAs.

Palmitic Acid Lipotoxicity in Microglia Cells Is Ameliorated by Unsaturated Fatty Acids

Obesity and metabolic syndrome are associated with cognitive decline and dementia. Palmitic acid (PA) is increased in the cerebrospinal fluid of obese patients with cognitive impairment. This study was therefore designed to examine fatty acid (FA) lipotoxicity in BV2 microglia cells. We found that PA induced time- and dose-dependent decrease in cell viability and increase in cell death without affecting the cell cycle profile and that PA lipotoxicity did not depend on cell surface free fatty acid receptors but rather on FA uptake. Treatment with sulfosuccinimidyl oleate (SSO), an irreversible inhibitor of fatty acid translocase CD36, significantly inhibited FA uptake in BSA- and PA-treated cells and blocked PA-induced decrease in cell viability. Inhibition of ER stress or treatment with N-acetylcysteine was not able to rescue PA lipotoxicity. Our study also showed that unsaturated fatty acids (UFAs), such as linoleic acid (LA), oleic acid (OA), α-linolenic acid (ALA), and docosahexaenoic acid (DHA), were not lipotoxic but instead protected microglia against PA-induced decrease in cell viability. Co-treatment of PA with LA, OA, and DHA significantly inhibited FA uptake in PA-treated cells. All UFAs tested induced the incorporation of FAs into and the amount of neutral lipids, while PA did not significantly affect the amount of neutral lipids compared with BSA control.

Imaging cytoplasmic lipid droplets in vivo with fluorescent perilipin 2 and perilipin 3 knock-in zebrafish

Cytoplasmic lipid droplets are highly dynamic storage organelles that are critical for cellular lipid homeostasis. While the molecular details of lipid droplet dynamics are a very active area of investigation, this work has been primarily performed in cultured cells. Taking advantage of the powerful transgenic and in vivo imaging opportunities available in zebrafish, we built a suite of tools to study lipid droplets in real time from the subcellular to the whole organism level. Fluorescently tagging the lipid droplet-associated proteins, perilipin 2 and perilipin 3, in the endogenous loci permits visualization of lipid droplets in the intestine, liver, and adipose tissue. Using these tools, we found that perilipin 3 is rapidly loaded on intestinal lipid droplets following a high-fat meal and later replaced by perilipin 2. These powerful new tools will facilitate studies on the role of lipid droplets in different tissues, under different genetic and physiological manipulations, and in a variety of human disease models.

Perilipin 2 downregulation in β cells impairs insulin secretion under nutritional stress and damages mitochondria

Perilipin 2 (PLIN2) is a lipid droplet (LD) protein in β cells that increases under nutritional stress. Downregulation of PLIN2 is often sufficient to reduce LD accumulation. To determine whether PLIN2 positively or negatively affects β cell function under nutritional stress, PLIN2 was downregulated in mouse β cells, INS1 cells, and human islet cells. β Cell–specific deletion of PLIN2 in mice on a high-fat diet reduced glucose-stimulated insulin secretion (GSIS) in vivo and in vitro. Downregulation of PLIN2 in INS1 cells blunted GSIS after 24-hour incubation with 0.2 mM palmitic acid. Downregulation of PLIN2 in human pseudoislets cultured at 5.6 mM glucose impaired both phases of GSIS, indicating that PLIN2 is critical for GSIS. Downregulation of PLIN2 decreased specific OXPHOS proteins in all 3 models and reduced oxygen consumption rates in INS1 cells and mouse islets. Moreover, we found that PLIN2-deficient INS1 cells increased the distribution of a fluorescent oleic acid analog to mitochondria and showed signs of mitochondrial stress, as indicated by susceptibility to fragmentation and alterations of acyl-carnitines and glucose metabolites. Collectively, PLIN2 in β cells has an important role in preserving insulin secretion, β cell metabolism, and mitochondrial function under nutritional stress.

Untargeted lipidomics analysis of Mori Fructus polysaccharide on acute alcoholic liver injury in mice using ultra performance liquid chromatography-quadrupole-orbitrap-high resolution mass spectrometry

ETHNOPHARMACOLOGICAL RELEVANCE

Mori Fructus (MF) is a traditional Chinese medicine with long application history, which has protective effects on liver diseases. Mori Fructus polysaccharide (MFP) is one of the active ingredients of MF and possesses therapeutic effects against acute alcoholic liver injury. Recent researches have reported that the therapeutic effects of MFP might be related to the regulation of several lipid metabolic pathways. However, the mechanism of lipid metabolism was still unclear.

AIM OF THE STUDY

This study aimed to investigate the regulatory effect of MFP on lipid metabolism in mice with acute alcoholic liver injury, and to enrich the application of its hepatoprotective effect.

MATERIALS AND METHODS

Fifty Specific Pathogen Free (SPF) Kunming (KM) male mice were divided into five groups randomly, including normal control group, model group, bifendate positive group (220 mg/ kg), MFPA1 group (50 mg/ kg) and MFPB1 group (50 mg/ kg). A model of acute alcoholic liver injury was established by treating the mice with ethanol. The liver sections were processed and histopathological changes was observed with hematoxylin and eosin staining. The secretion levels of biochemical indexes in the liver and serum were assayed by ELISA. The untargeted lipidomics analysis was performed on a Q Exactive Focus quadrupole orbitrap high resolution mass spectrometry coupled with a Dionex Ultimate 3000 RSLC (HPG) ultra-performance liquid chromatography (UPLC-Q-Orbitrap-HRMS) system (Thermo Fisher Scientific), with a HESI ionization source.

RESULTS

It was observed that abnormal glutamic-pyruvic transaminase enzyme (ALT), glutamic-oxaloacetic aminotransferase (AST), triglyceride (TG), superoxide dismutase (SOD), and malondialdehyde (MDA) concentrations were ameliorated after irrigation of MFP or bifendate. Histopathology examination showed that intragastric infusion of MFP can be helpful in the repair of damaged liver in mice. The multivariate analysis of hepatic lipids showed segregation of ethanol-fed groups from the normal controls. After the comparison of mass spectra, 10 lipids were found to have different content in the normal control group and the model group. Differential lipids that were increased by ethanol exposure included fatty acids, phosphatidylcholine, and lyso-phosphatidylethanolamine, whereas lyso-phosphatidylcholine decreased. Among them, 4 lipids almost returned to the level of normal mice after MFP treatment.

CONCLUSION

The therapeutic effect of MFP on acute alcoholic liver injury may be achieved by regulating a variety of metabolic pathways, including linoleic acid metabolism, α - linolenic acid metabolism, and glycerolphospholipid metabolism. These results revealed that MFP efficiently exerted hepatoprotective benefits, and its potential effect was associated with the amelioration of lipid metabolism.

Electrochemical degradation and saponification of porcine adipose tissue

Body contouring achieved via subcutaneous adipose tissue reduction has notably advanced over the past century, from suction assisted lipectomy to techniques with reduced degrees of invasiveness including laser, radiofrequency, high frequency focused ultrasound, cryolipolysis, and drug-based injection approaches. These costly techniques have focused on damaging adipocyte cell membranes, hydrolyzing triglycerides (TGs), or inducing apoptosis. Here, we present a simple, low-cost technique, termed electrochemical lipolysis (ECLL). During ECLL, saline is injected into the subcutaneous adipose tissue, followed by insertion of needle electrodes and application of an electrical potential. Electrolysis of saline creates localized pH gradients that drive adipocyte death and saponification of TGs. Using pH mapping, various optical imaging techniques, and biochemical assays, we demonstrate the ability of ECLL to induce acid and base injury, cell death, and the saponification of triglycerides in ex vivo porcine adipose tissue. We define ECLL's potential role as a minimally-invasive, ultra-low-cost technology for reducing and contouring adipose tissue, and present ECLL as a potential new application of an emerging electrochemical redox based treatment modality.

Adipose Triglyceride Lipase Is a Key Lipase for the Mobilization of Lipid Droplets in Human β-Cells and Critical for the Maintenance of Syntaxin 1a Levels in β-Cells

Lipid droplets (LDs) are frequently increased when excessive lipid accumulation leads to cellular dysfunction. Distinct from mouse β-cells, LDs are prominent in human β-cells. However, the regulation of LD mobilization (lipolysis) in human β-cells remains unclear. We found that glucose increases lipolysis in nondiabetic human islets but not in islets in patients with type 2 diabetes (T2D), indicating dysregulation of lipolysis in T2D islets. Silencing adipose triglyceride lipase (ATGL) in human pseudoislets with shRNA targeting ATGL (shATGL) increased triglycerides (TGs) and the number and size of LDs, indicating that ATGL is the principal lipase in human β-cells. In shATGL pseudoislets, biphasic glucose-stimulated insulin secretion (GSIS), and insulin secretion to 3-isobutyl-1-methylxanthine and KCl were all reduced without altering oxygen consumption rate compared with scramble control. Like human islets, INS1 cells showed visible LDs, glucose-responsive lipolysis, and impairment of GSIS after ATGL silencing. ATGL-deficient INS1 cells and human pseudoislets showed reduced SNARE protein syntaxin 1a (STX1A), a key SNARE component. Proteasomal degradation of Stx1a was accelerated likely through reduced palmitoylation in ATGL-deficient INS1 cells. Therefore, ATGL is responsible for LD mobilization in human β-cells and supports insulin secretion by stabilizing STX1A. The dysregulated lipolysis may contribute to LD accumulation and β-cell dysfunction in T2D islets.

Modeling Lysosomal Storage Diseases in the Zebrafish

Lysosomal storage diseases (LSDs) are a family of 70 metabolic disorders characterized by mutations in lysosomal proteins that lead to storage material accumulation, multiple-organ pathologies that often involve neurodegeneration, and early mortality in a significant number of patients. Along with the necessity for more effective therapies, there exists an unmet need for further understanding of disease etiology, which could uncover novel pathways and drug targets. Over the past few decades, the growth in knowledge of disease-associated pathways has been facilitated by studies in model organisms, as advancements in mutagenesis techniques markedly improved the efficiency of model generation in mammalian and non-mammalian systems. In this review we highlight non-mammalian models of LSDs, focusing specifically on the zebrafish, a vertebrate model organism that shares remarkable genetic and metabolic similarities with mammals while also conferring unique advantages such as optical transparency and amenability toward high-throughput applications. We examine published zebrafish LSD models and their reported phenotypes, address organism-specific advantages and limitations, and discuss recent technological innovations that could provide potential solutions.

Fermentation Products of Commensal Bacteria Alter Enterocyte Lipid Metabolism

Despite the recognized capacity of the gut microbiota to regulate intestinal lipid metabolism, the role of specific commensal species remains undefined. Here, we aimed to understand the bacterial effectors and molecular mechanisms by which Lactobacillus paracasei and Escherichia coli regulate lipid metabolism in enterocytes. We show that L-lactate produced by L. paracasei inhibits chylomicron secretion from enterocytes and promotes lipid storage by a mechanism involving L-lactate absorption by enterocytes, its conversion to malonyl-CoA, and the subsequent inhibition of lipid beta-oxidation. In contrast, acetate produced by E. coli also inhibits chylomicron secretion by enterocytes but promotes lipid oxidation by a mechanism involving acetate absorption by enterocytes, its metabolism to acetyl-CoA and AMP, and the subsequent upregulation of the AMPK/PGC-1α/PPARα pathway. Our study opens perspectives for developing specific bacteria- and metabolite-based therapeutic interventions against obesity, atherosclerosis, and malnutrition by targeting lipid metabolism in enterocytes.

Parallel Reaction Monitoring reveals structure-specific ceramide alterations in the zebrafish

Extensive characterisations of the zebrafish genome and proteome have established a foundation for the use of the zebrafish as a model organism; however, characterisation of the zebrafish lipidome has not been as comprehensive. In an effort to expand current knowledge of the zebrafish sphingolipidome, a Parallel Reaction Monitoring (PRM)-based liquid chromatography-mass spectrometry (LC-MS) method was developed to comprehensively quantify zebrafish ceramides. Comparison between zebrafish and a human cell line demonstrated remarkable overlap in ceramide composition, but also revealed a surprising lack of most sphingadiene-containing ceramides in the zebrafish. PRM analysis of zebrafish embryogenesis identified developmental stage-specific ceramide changes based on long chain base (LCB) length. A CRISPR-Cas9-generated zebrafish model of Farber disease exhibited reduced size, early mortality, and severe ceramide accumulation where the amplitude of ceramide change depended on both acyl chain and LCB lengths. Our method adds an additional level of detail to current understanding of the zebrafish lipidome, and could aid in the elucidation of structure-function associations in the context of lipid-related diseases.

Novel nanoarchitecture of arginine-glycine-aspartate conjugated gold nanoparticles: a sensitive and selective platform for detecting arachidonic acid

A novel electrochemical approach for determination of arachidonic acid (ARA) was developed based on the linear arginine-glycine-aspartic-Au (RGD-Au) nanomaterial modified on glassy carbon electrode (GCE). The prepared material was characterized by transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), ultraviolet-visible spectroscopy (UV-vis), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The electrochemical signal was obtained from the reduction of 1,4-naphthoquinone and ARA served as a proton source. Under the optimum experimental conditions, the RGD-Au-based electrode was used to analyze ARA. Meanwhile, the electrochemical characteristics were also studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and linear sweep voltammetry (LSV). The sensor showed a wider linear range from 0.5 to 100 μM and the linear fitting equation was I_p (μA) = 0.0721 c + 2.4583 (R² = 0.9987) with a detection limit of 80 nM. The application of the sensor in real samples was tested and compared with that of LC-MS/MS. This sensor would be a promising platform for detection of ARA in blood plasma. Graphical abstract.

A novel system to quantify intestinal lipid digestion and transport

The zebrafish larva is a powerful tool for the study of dietary triglyceride (TG) digestion and how fatty acids (FA) derived from dietary lipids are absorbed, metabolized and distributed to the body. While fluorescent FA analogues have enabled visualization of FA metabolism, methods for specifically assaying TG digestion are badly needed. Here we present a novel High Performance Liquid Chromatography (HPLC) method that quantitatively differentiates TG and phospholipid (PL) molecules with one or two fluorescent FA analogues. We show how this tool may be used to discriminate between undigested and digested TG or phosphatidylcholine (PC), and also the products of TG or PC that have been digested, absorbed and re-synthesized into new lipid molecules. Using this approach, we explored the dietary requirement of zebrafish larvae for phospholipids. Here we demonstrate that dietary TG is digested and absorbed in the intestinal epithelium, but without dietary PC, TG accumulates and is not transported out of the enterocytes. Consequently, intestinal ER stress increases and the ingested lipid is not available support the energy and metabolic needs of other tissues. In TG diets with PC, TG is readily transported from the intestine and subsequently metabolized.

Lipid Uptake, Metabolism, and Transport in the Larval Zebrafish

The developing zebrafish is a well-established model system for studies of energy metabolism, and is amenable to genetic, physiological, and biochemical approaches. For the first 5 days of life, nutrients are absorbed from its endogenous maternally deposited yolk. At 5 days post-fertilization, the yolk is exhausted and the larva has a functional digestive system including intestine, liver, gallbladder, pancreas, and intestinal microbiota. The transparency of the larval zebrafish, and the genetic and physiological similarity of its digestive system to that of mammals make it a promising system in which to address questions of energy homeostasis relevant to human health. For example, apolipoprotein expression and function is similar in zebrafish and mammals, and transgenic animals may be used to examine both the transport of lipid from yolk to body in the embryo, and the trafficking of dietary lipids in the larva. Additionally, despite the identification of many fatty acid and lipid transport proteins expressed by vertebrates, the cell biological processes that mediate the transport of dietary lipids from the intestinal lumen to the interior of enterocytes remain to be elucidated. Genetic tractability and amenability to live imaging and a range of biochemical methods make the larval zebrafish an ideal model in which to address open questions in the field of lipid transport, energy homeostasis, and nutrient metabolism.