The FATP1-DGAT2 complex facilitates lipid droplet expansion at the ER-lipid droplet interface

LET-767 determines lipid droplet protein targeting and lipid homeostasis

Lipid droplets (LDs) are composed of a core of neutral lipids wrapped by a phospholipid (PL) monolayer containing several hundred proteins that vary between different cells or organisms. How LD proteins target to LDs is still largely unknown. Here, we show that RNAi knockdown or gene mutation of let-767, encoding a member of hydroxysteroid dehydrogenase (HSD), displaced the LD localization of three well-known LD proteins: DHS-3 (dehydrogenase/reductase), PLIN-1 (perilipin), and DGAT-2 (diacylglycerol O-acyltransferase 2), and also prevented LD growth in Caenorhabditis elegans. LET-767 interacts with ARF-1 (ADP-ribosylation factor 1) to prevent ARF-1 LD translocation for appropriate LD protein targeting and lipid homeostasis. Deficiency of LET-767 leads to the release of ARF-1, which further recruits and promotes translocation of ATGL-1 (adipose triglyceride lipase) to LDs for lipolysis. The displacement of LD proteins caused by LET-767 deficiency could be reversed by inhibition of either ARF-1 or ATGL-1. Our work uncovers a unique LET-767 for determining LD protein targeting and maintaining lipid homeostasis.

Newsights of endoplasmic reticulum in hypoxia

The endoplasmic reticulum (ER) is important to cells because of its essential functions, including synthesizing three major nutrients and ion transport. When cellular homeostasis is disrupted, ER quality control (ERQC) system is activated effectively to remove misfolded and unfolded proteins through ER-phagy, ER-related degradation (ERAD), and molecular chaperones. When unfolded protein response (UPR) and ER stress are activated, the cell may be suffering a huge blow, and the most probable consequence is apoptosis. The membrane contact points between the ER and sub-organelles contribute to communication between the organelles. The decrease in oxygen concentration affects the morphology and structure of the ER, thereby affecting its function and further disrupting the stable state of cells, leading to the occurrence of disease. In this study, we describe the functions of ER-, ERQC-, and ER-related membrane contact points and their changes under hypoxia, which will help us further understand ER and treat ER-related diseases.

Potential Role of Lauric Acid in Milk Fat Synthesis in Chinese Holstein Cows Based on Integrated Analysis of Ruminal Microbiome and Metabolome

The composition and metabolic profile of the ruminal microbiome have an impact on milk composition. To unravel the ruminal microbiome and metabolome affecting milk fat synthesis in dairy cows, 16S rRNA and internal transcribed spacer (ITS) gene sequencing, as well as ultra-high performance liquid chromatography-mass spectrometry (UHPLC-MS/MS) methods were used to investigate the significant differences in ruminal bacterial and fungal communities as well as metabolome among Chinese Holstein cows with contrasting milk fat contents under the same diet (H-MF 5.82 ± 0.41% vs. L-MF 3.60 ± 0.12%). Another objective was to culture bovine mammary epithelial cells (BMECs) to assess the effect of metabolites on lipid metabolism. Results showed that the acetate-to-propionate ratio and xylanase activity in ruminal fluid were both higher in H-MF. Microbiome sequencing identified 10 types of bacteria and four types of fungi differently abundant at the genus level. Metabolomics analysis indicated 11 different ruminal metabolites between the two groups, the majority of which were lipids and organic acids. Among these, lauric acid (LA) was enriched in fatty acid biosynthesis with its concentration in milk fat of H-MF cows being greater (217 vs. 156 mg per 100 g milk), thus, it was selected for an in vitro study with BMECs. Exogenous LA led to a marked increase in intracellular triglyceride (TG) content and lipid droplet formation, and it upregulated the mRNA abundance of fatty acid uptake and activation (CD36 and ACSL1), TG synthesis (DGAT1, DGAT2 and GPAM), and transcriptional regulation (SREBP1) genes. Taken together, the greater relative abundance of xylan-fermenting bacteria and fungi, and lower abundance of bacteria suppressing short-chain fatty acid-producing bacteria or participating in fatty acid hydrogenation altered lipids and organic acids in the rumen of dairy cows. In BMECs, LA altered the expression of genes involved in lipid metabolism in mammary cells, ultimately promoting milk fat synthesis. Thus, it appears that this fatty acid plays a key role in milk fat synthesis.

ER-organelle contacts: A signaling hub for neurological diseases

Neuronal health is closely linked to the homeostasis of intracellular organelles, and organelle dysfunction affects the pathological progression of neurological diseases. In contrast to isolated cellular compartments, a growing number of studies have found that organelles are largely interdependent structures capable of communicating through membrane contact sites (MCSs). MCSs have been identified as key pathways mediating inter-organelle communication crosstalk in neurons, and their alterations have been linked to neurological disease pathology. The endoplasmic reticulum (ER) is a membrane-bound organelle capable of forming an extensive network of pools and tubules with important physiological functions within neurons. There are multiple MCSs between the ER and other organelles and the plasma membrane (PM), which regulate a variety of cellular processes. In this review, we focus on ER-organelle MCSs and their role in a variety of neurological diseases. We compared the biological effects between different tethering proteins and the effects of their respective disease counterparts. We also discuss how altered ER-organelle contacts may affect disease pathogenesis. Therefore, understanding the molecular mechanisms of ER-organelle MCSs in neuronal homeostasis will lay the foundation for the development of new therapies targeting ER-organelle contacts.

CIDE proteins and their regulatory mechanisms in lipid droplet fusion and growth

The cell death-inducing DFF45-like effector (CIDE) proteins, including Cidea, Cideb, and Cidec/Fsp27, regulate various aspects of lipid homeostasis, including lipid storage, lipolysis, and lipid secretion. This review focuses on the physiological roles of CIDE proteins based on studies on knockout mouse models and human patients bearing CIDE mutations. The primary cellular function of CIDE proteins is to localize to lipid droplets (LDs) and to control LD fusion and growth across different cell types. We propose a four-step process of LD fusion, characterized by (a) the recruitment of CIDE proteins to the LD surface and CIDE movement, (b) the enrichment and condensate formation of CIDE proteins to form LD fusion plates at LD-LD contact sites, (c) lipid transfer through lipid-permeable passageways within the fusion plates, and (d) the completion of LD fusion. Lastly, we outline CIDE-interacting proteins as regulatory factors, as well as their contribution in LD fusion.

Kombucha Tea-associated microbes remodel host metabolic pathways to suppress lipid accumulation

The popularity of the ancient, probiotic-rich beverage Kombucha Tea (KT) has surged in part due to its purported health benefits, which include protection against metabolic diseases; however, these claims have not been rigorously tested and the mechanisms underlying host response to the probiotics in KT are unknown. Here, we establish a reproducible method to maintain C. elegans on a diet exclusively consisting of Kombucha Tea-associated microbes (KTM), which mirrors the microbial community found in the fermenting culture. KT microbes robustly colonize the gut of KTM-fed animals and confer normal development and fecundity. Intriguingly, animals consuming KTMs display a marked reduction in total lipid stores and lipid droplet size. We find that the reduced fat accumulation phenotype is not due to impaired nutrient absorption, but rather it is sustained by a programed metabolic response in the intestine of the host. KTM consumption triggers widespread transcriptional changes within core lipid metabolism pathways, including upregulation of a suite of lysosomal lipase genes that are induced during lipophagy. The elevated lysosomal lipase activity, coupled with a decrease in lipid droplet biogenesis, is partially required for the reduction in host lipid content. We propose that KTM consumption stimulates a fasting-like response in the C. elegans intestine by rewiring transcriptional programs to promote lipid utilization. Our results provide mechanistic insight into how the probiotics in Kombucha Tea reshape host metabolism and how this popular beverage may impact human metabolism.

Stay in touch with the endoplasmic reticulum

The endoplasmic reticulum (ER), which is composed of a continuous network of tubules and sheets, forms the most widely distributed membrane system in eukaryotic cells. As a result, it engages a variety of organelles by establishing membrane contact sites (MCSs). These contacts regulate organelle positioning and remodeling, including fusion and fission, facilitate precise lipid exchange, and couple vital signaling events. Here, we systematically review recent advances and converging themes on ER-involved organellar contact. The molecular basis, cellular influence, and potential physiological functions for ER/nuclear envelope contacts with mitochondria, Golgi, endosomes, lysosomes, lipid droplets, autophagosomes, and plasma membrane are summarized.

Acyl-CoA binding protein is required for lipid droplet degradation in the diatom Phaeodactylum tricornutum

Diatoms (Bacillariophyceae) accumulate neutral storage lipids in lipid droplets during stress conditions, which can be rapidly degraded and recycled when optimal conditions resume. Since nutrient and light availability fluctuate in marine environments, storage lipid turnover is essential for diatom dominance of marine ecosystems. Diatoms have garnered attention for their potential to provide a sustainable source of omega-3 fatty acids. Several independent proteomic studies of lipid droplets isolated from the model oleaginous pennate diatom Phaeodactylum tricornutum have identified a previously uncharacterized protein with an acyl-CoA binding (ACB) domain, Phatrdraft_48778, here referred to as Phaeodactylum tricornutum acyl-CoA binding protein (PtACBP). We report the phenotypic effects of CRISPR-Cas9 targeted genome editing of PtACBP. ptacbp mutants were defective in lipid droplet and triacylglycerol degradation, as well as lipid and eicosapentaenoic acid synthesis, during recovery from nitrogen starvation. Transcription of genes responsible for peroxisomal β-oxidation, triacylglycerol lipolysis, and eicosapentaenoic acid synthesis was inhibited. A lipid-binding assay using a synthetic ACB domain from PtACBP indicated preferential binding specificity toward certain polar lipids. PtACBP fused to eGFP displayed an endomembrane-like pattern, which surrounded the periphery of lipid droplets. PtACBP is likely responsible for intracellular acyl transport, affecting cell division, development, photosynthesis, and stress response. A deeper understanding of the molecular mechanisms governing storage lipid turnover will be crucial for developing diatoms and other microalgae as biotechnological cell factories.

Early-life stress triggers long-lasting organismal resilience and longevity via tetraspanin

Early-life stress experiences can produce lasting impacts on organismal adaptation and fitness. How transient stress elicits memory-like physiological effects is largely unknown. Here, we show that early-life thermal stress strongly up-regulates tsp-1, a gene encoding the conserved transmembrane tetraspanin in C. elegans. TSP-1 forms prominent multimers and stable web-like structures critical for membrane barrier functions in adults and during aging. Increased TSP-1 abundance persists even after transient early-life heat stress. Such regulation requires CBP-1, a histone acetyltransferase that facilitates initial tsp-1 transcription. Tetraspanin webs form regular membrane structures and mediate resilience-promoting effects of early-life thermal stress. Gain-of-function TSP-1 confers marked C. elegans longevity extension and thermal resilience in human cells. Together, our results reveal a cellular mechanism by which early-life thermal stress produces long-lasting memory-like impact on organismal resilience and longevity.

Neurotrophic factor MANF regulates autophagy and lysosome function to promote proteostasis in C. elegans

The conserved mesencephalic astrocyte-derived neurotrophic factor (MANF) protects dopaminergic neurons but also functions in several other tissues. Previously, we showed that Caenorhabditis elegans manf-1 null mutants have increased ER stress, dopaminergic neurodegeneration, protein aggregation, slower growth, and a reduced lifespan. The multiple requirements of MANF in different systems suggest its essential role in regulating cellular processes. However, how intracellular and extracellular MANF regulates broader cellular function remains unknown. Here, we report a novel mechanism of action for manf-1 that involves the autophagy transcription factor HLH-30/TFEB-mediated signaling to regulate lysosomal function and aging. We generated multiple transgenic strains overexpressing MANF-1 and found that animals had extended lifespan, reduced protein aggregation, and improved neuronal health. Using a fluorescently tagged MANF-1, we observed different tissue localization of MANF-1 depending on the ER retention signal. Further subcellular analysis showed that MANF-1 localizes within cells to the lysosomes. These findings were consistent with our transcriptomic studies and, together with analysis of autophagy regulators, demonstrate that MANF-1 regulates protein homeostasis through increased autophagy and lysosomal activity. Collectively, our findings establish MANF as a critical regulator of the stress response, proteostasis, and aging.

Mitochondrial dysfunction abrogates dietary lipid processing in enterocytes

Digested dietary fats are taken up by enterocytes where they are assembled into pre-chylomicrons in the endoplasmic reticulum followed by transport to the Golgi for maturation and subsequent secretion to the circulation1. The role of mitochondria in dietary lipid processing is unclear. Here we show that mitochondrial dysfunction in enterocytes inhibits chylomicron production and the transport of dietary lipids to peripheral organs. Mice with specific ablation of the mitochondrial aspartyl-tRNA synthetase DARS2 (ref. 2), the respiratory chain subunit SDHA3 or the assembly factor COX10 (ref. 4) in intestinal epithelial cells showed accumulation of large lipid droplets (LDs) in enterocytes of the proximal small intestine and failed to thrive. Feeding a fat-free diet suppressed the build-up of LDs in DARS2-deficient enterocytes, which shows that the accumulating lipids derive mostly from digested fat. Furthermore, metabolic tracing studies revealed an impaired transport of dietary lipids to peripheral organs in mice lacking DARS2 in intestinal epithelial cells. DARS2 deficiency caused a distinct lack of mature chylomicrons concomitant with a progressive dispersal of the Golgi apparatus in proximal enterocytes. This finding suggests that mitochondrial dysfunction results in impaired trafficking of chylomicrons from the endoplasmic reticulum to the Golgi, which in turn leads to storage of dietary lipids in large cytoplasmic LDs. Taken together, these results reveal a role for mitochondria in dietary lipid transport in enterocytes, which might be relevant for understanding the intestinal defects observed in patients with mitochondrial disorders5.

Liposome-Mediated Anti-Viral Drug Delivery Across Blood-Brain Barrier: Can Lipid Droplet Target Be Game Changers?

Lipid droplets (LDs) are subcellular organelles secreted from the endoplasmic reticulum (ER) that play a major role in lipid homeostasis. Recent research elucidates additional roles of LDs in cellular bioenergetics and innate immunity. LDs activate signaling cascades for interferon response and secretion of pro-inflammatory cytokines. Since balanced lipid homeostasis is critical for neuronal health, LDs play a crucial role in neurodegenerative diseases. RNA viruses enhance the secretion of LDs to support various phases of their life cycle in neurons which further leads to neurodegeneration. Targeting the excess LD formation in the brain could give us a new arsenal of antiviral therapeutics against neuroviruses. Liposomes are a suitable drug delivery system that could be used for drug delivery in the brain by crossing the Blood-Brain Barrier. Utilizing this, various pharmacological inhibitors and non-coding RNAs can be delivered that could inhibit the biogenesis of LDs or reduce their sizes, reversing the excess lipid-related imbalance in neurons. Liposome-Mediated Antiviral Drug Delivery Across Blood-Brain Barrier. Developing effective antiviral drug is challenging and it doubles against neuroviruses that needs delivery across the Blood-Brain Barrier (BBB). Lipid Droplets (LDs) are interesting targets for developing antivirals, hence targeting LD formation by drugs delivered using Liposomes can be game changers.

Early-life stress triggers long-lasting organismal resilience and longevity via tetraspanin

Early-life stress experiences can produce lasting impacts on organismal adaptation and fitness. How transient stress elicits memory-like physiological effects is largely unknown. Here we show that early-life thermal stress strongly up-regulates tsp-1, a gene encoding the conserved transmembrane tetraspanin in C. elegans. TSP-1 forms prominent multimers and stable web-like structures critical for membrane barrier functions in adults and during aging. The up-regulation of TSP-1 persists even after transient early-life stress. Such regulation requires CBP-1, a histone acetyl-transferase that facilitates initial tsp-1 transcription. Tetraspanin webs form regular membrane structures and mediate resilience-promoting effects of early-life thermal stress. Gain-of-function TSP-1 confers marked C. elegans longevity extension and thermal resilience in human cells. Together, our results reveal a cellular mechanism by which early-life thermal stress produces long-lasting memory-like impact on organismal resilience and longevity.

The metabolic response of human trophoblasts derived from term placentas to metformin

AIMS/HYPOTHESIS

Metformin is increasingly used therapeutically during pregnancy worldwide, particularly in the treatment of gestational diabetes, which affects a substantial proportion of pregnant women globally. However, the impact on placental metabolism remains unclear. In view of the association between metformin use in pregnancy and decreased birthweight, it is essential to understand how metformin modulates the bioenergetic and anabolic functions of the placenta.

METHODS

A cohort of 55 placentas delivered by elective Caesarean section at term was collected from consenting participants. Trophoblasts were isolated from the placental samples and treated in vitro with clinically relevant doses of metformin (0.01 mmol/l or 0.1 mmol/l) or vehicle. Respiratory function was assayed using high-resolution respirometry to measure oxygen concentration and calculated [Formula: see text]. Glycolytic rate and glycolytic stress assays were performed using Agilent Seahorse XF assays. Fatty acid uptake and oxidation measurements were conducted using radioisotope-labelled assays. Lipidomic analysis was conducted using LC-MS. Gene expression and protein analysis were performed using RT-PCR and western blotting, respectively.

RESULTS

Complex I-supported oxidative phosphorylation was lower in metformin-treated trophoblasts (0.01 mmol/l metformin, 61.7% of control, p<0.05; 0.1 mmol/l metformin, 43.1% of control, p<0.001). The proton efflux rate arising from glycolysis under physiological conditions was increased following metformin treatment, up to 23±5% above control conditions following treatment with 0.1 mmol/l metformin (p<0.01). There was a significant increase in triglyceride concentrations in trophoblasts treated with 0.1 mmol/l metformin (p<0.05), particularly those of esters of long-chain polyunsaturated fatty acids. Fatty acid oxidation was reduced by ~50% in trophoblasts treated with 0.1 mmol/l metformin compared with controls (p<0.001), with no difference in uptake between treatment groups.

CONCLUSIONS/INTERPRETATION

In primary trophoblasts derived from term placentas metformin treatment caused a reduction in oxidative phosphorylation through partial inactivation of complex I and potentially by other mechanisms. Metformin-treated trophoblasts accumulate lipids, particularly long- and very-long-chain polyunsaturated fatty acids. Our findings raise clinically important questions about the balance of risk of metformin use during pregnancy, particularly in situations where the benefits are not clear-cut and alternative therapies are available.

ACS-20/FATP4 mediates the anti-ageing effect of dietary restriction in C. elegans

Dietary restriction is an effective anti-ageing intervention across species. However, the molecular mechanisms from the metabolic aspects of view are still underexplored. Here we show ACS-20 as a key mediator of dietary restriction on healthy ageing from a genetic screen of the C. elegans acyl-CoA synthetase family. ACS-20 functions in the epidermis during development to regulate dietary restriction-induced longevity. Functional transcriptomics studies reveal that elevated expression of PTR-8/Patched is responsible for the proteostasis and lifespan defects of acs-20. Furthermore, the conserved NHR-23 nuclear receptor serves as a transcriptional repressor of ptr-8 and a key regulator of dietary restriction-induced longevity. Mechanistically, a specific region in the ptr-8 promoter plays a key role in mediating the transcription regulation and lifespan extension under dietary restriction. Altogether, these findings identify a highly conserved lipid metabolism enzyme as a key mediator of dietary restriction-induced lifespan and healthspan extension and reveal the downstream transcriptional regulation mechanisms.

Mechanisms of Lipid Droplet Accumulation in Steatotic Liver Diseases

The steatotic diseases of metabolic dysfunction-associated steatotic liver disease (MASLD), alcohol-associated liver disease (ALD), and chronic hepatitis C (HCV) account for the majority of liver disease prevalence, morbidity, and mortality worldwide. While these diseases have distinct pathogenic and clinical features, dysregulated lipid droplet (LD) organelle biology represents a convergence of pathogenesis in all three. With increasing understanding of hepatocyte LD biology, we now understand the roles of LD proteins involved in these diseases but also how genetics modulate LD biology to either exacerbate or protect against the phenotypes associated with steatotic liver diseases. Here, we review the history of the LD organelle and its biogenesis and catabolism. We also review how this organelle is critical not only for the steatotic phenotype of liver diseases but also for their advanced phenotypes. Finally, we summarize the latest attempts and challenges of leveraging LD biology for therapeutic gain in steatotic diseases. In conclusion, the study of dysregulated LD biology may lead to novel therapeutics for the prevention of disease progression in the highly prevalent steatotic liver diseases of MASLD, ALD, and HCV.

A membrane-sensing mechanism links lipid metabolism to protein degradation at the nuclear envelope

Lipid composition determines organelle identity; however, whether the lipid composition of the inner nuclear membrane (INM) domain of the ER contributes to its identity is not known. Here, we show that the INM lipid environment of animal cells is under local control by CTDNEP1, the master regulator of the phosphatidic acid phosphatase lipin 1. Loss of CTDNEP1 reduces association of an INM-specific diacylglycerol (DAG) biosensor and results in a decreased percentage of polyunsaturated containing DAG species. Alterations in DAG metabolism impact the levels of the resident INM protein Sun2, which is under local proteasomal regulation. We identify a lipid-binding amphipathic helix (AH) in the nucleoplasmic domain of Sun2 that prefers membrane packing defects. INM dissociation of the Sun2 AH is linked to its proteasomal degradation. We suggest that direct lipid-protein interactions contribute to sculpting the INM proteome and that INM identity is adaptable to lipid metabolism, which has broad implications on disease mechanisms associated with the nuclear envelope.

Arf1 coordinates fatty acid metabolism and mitochondrial homeostasis

Lipid mobilization through fatty acid β-oxidation is a central process essential for energy production during nutrient shortage. In yeast, this catabolic process starts in the peroxisome from where β-oxidation products enter mitochondria and fuel the tricarboxylic acid cycle. Little is known about the physical and metabolic cooperation between these organelles. Here we found that expression of fatty acid transporters and of the rate-limiting enzyme involved in β-oxidation is decreased in cells expressing a hyperactive mutant of the small GTPase Arf1, leading to an accumulation of fatty acids in lipid droplets. Consequently, mitochondria became fragmented and ATP synthesis decreased. Genetic and pharmacological depletion of fatty acids phenocopied the arf1 mutant mitochondrial phenotype. Although β-oxidation occurs in both mitochondria and peroxisomes in mammals, Arf1's role in fatty acid metabolism is conserved. Together, our results indicate that Arf1 integrates metabolism into energy production by regulating fatty acid storage and utilization, and presumably organelle contact sites.

Lipid droplet-associated lncRNA LIPTER preserves cardiac lipid metabolism

Lipid droplets (LDs) are cellular organelles critical for lipid homeostasis, with intramyocyte LD accumulation implicated in metabolic disorder-associated heart diseases. Here we identify a human long non-coding RNA, Lipid-Droplet Transporter (LIPTER), essential for LD transport in human cardiomyocytes. LIPTER binds phosphatidic acid and phosphatidylinositol 4-phosphate on LD surface membranes and the MYH10 protein, connecting LDs to the MYH10-ACTIN cytoskeleton and facilitating LD transport. LIPTER and MYH10 deficiencies impair LD trafficking, mitochondrial function and survival of human induced pluripotent stem cell-derived cardiomyocytes. Conditional Myh10 deletion in mouse cardiomyocytes leads to LD accumulation, reduced fatty acid oxidation and compromised cardiac function. We identify NKX2.5 as the primary regulator of cardiomyocyte-specific LIPTER transcription. Notably, LIPTER transgenic expression mitigates cardiac lipotoxicity, preserves cardiac function and alleviates cardiomyopathies in high-fat-diet-fed and Leprdb/db mice. Our findings unveil a molecular connector role of LIPTER in intramyocyte LD transport, crucial for lipid metabolism of the human heart, and hold significant clinical implications for treating metabolic syndrome-associated heart diseases.

Saccharomyces cerevisiae Δ9-desaturase Ole1 forms a supercomplex with Slc1 and Dga1

Biosynthesis of the various lipid species that compose cellular membranes and lipid droplets depends on the activity of multiple enzymes functioning in coordinated pathways. The flux of intermediates through lipid biosynthetic pathways is regulated to respond to nutritional and environmental demands placed on the cell necessitating that there be extensive flexibility in pathway activity and organization. This flexibility can in part be achieved through the organization of biosynthetic enzymes into metabolon supercomplexes. However, the composition and organization of such supercomplexes remains unclear. Here, we identified protein-protein interactions between acyltransferases Sct1, Gpt2, Slc1, Dga1 and the Δ9 acyl-CoA desaturase Ole1 in Saccharomyces cerevisiae. We further determined that a subset of these acyltransferases interact with each other without Ole1 acting as a scaffold. We show that truncated versions of Dga1 lacking the carboxyl-terminal 20 amino acid residues are non-functional and unable to bind Ole1. Furthermore, charged-to-alanine scanning mutagenesis revealed that a cluster of charged residues near the carboxyl-terminus were required for the interaction with Ole1. Mutation of these charged residues disrupted the interaction between Dga1 and Ole1, but allowed Dga1 to retain catalytic activity and to induce lipid droplet formation. These data support the formation of a complex of acyltransferases involved in lipid biosynthesis that interacts with Ole1, the sole acyl-CoA desaturase in S. cerevisiae, that can channel unsaturated acyl-chains toward phospholipid or triacylglycerol synthesis. This desaturasome complex may provide the architecture that allows for the necessary flux of de novo synthesized unsaturated acyl-CoA to phospholipid or triacylglycerol synthesis as demanded by cellular requirements.

Structure and function of lipid droplet assembly complexes

Cells store lipids as a reservoir of metabolic energy and membrane component precursors in organelles called lipid droplets (LDs). LD formation occurs in the endoplasmic reticulum (ER) at LD assembly complexes (LDAC), consisting of an oligomeric core of seipin and accessory proteins. LDACs determine the sites of LD formation and are required for this process to occur normally. Seipin oligomers form a cage-like structure in the membrane that may serve to facilitate the phase transition of neutral lipids in the membrane to form an oil droplet within the LDAC. Modeling suggests that, as the LD grows, seipin anchors it to the ER bilayer and conformational shifts of seipin transmembrane segments open the LDAC dome toward the cytoplasm, enabling the emerging LD to egress from the ER.

Lipid droplet biogenesis and functions in health and disease

Ubiquitous yet unique, lipid droplets are intracellular organelles that are increasingly being recognized for their versatility beyond energy storage. Advances uncovering the intricacies of their biogenesis and the diversity of their physiological and pathological roles have yielded new insights into lipid droplet biology. Despite these insights, the mechanisms governing the biogenesis and functions of lipid droplets remain incompletely understood. Moreover, the causal relationship between the biogenesis and function of lipid droplets and human diseases is poorly resolved. Here, we provide an update on the current understanding of the biogenesis and functions of lipid droplets in health and disease, highlighting a key role for lipid droplet biogenesis in alleviating cellular stresses. We also discuss therapeutic strategies of targeting lipid droplet biogenesis, growth or degradation that could be applied in the future to common diseases, such as cancer, hepatic steatosis and viral infection.

The adipocyte supersystem of insulin and cAMP signaling

Adipose tissue signals to brain, liver, and muscles to control whole body metabolism through secreted lipid and protein factors as well as neurotransmission, but the mechanisms involved are incompletely understood. Adipocytes sequester triglyceride (TG) in fed conditions stimulated by insulin, while in fasting catecholamines trigger TG hydrolysis, releasing glycerol and fatty acids (FAs). These antagonistic hormone actions result in part from insulin's ability to inhibit cAMP levels generated through such G-protein-coupled receptors as catecholamine-activated β-adrenergic receptors. Consistent with these antagonistic signaling modes, acute actions of catecholamines cause insulin resistance. Yet, paradoxically, chronically activating adipocytes by catecholamines cause increased glucose tolerance, as does insulin. Recent results have helped to unravel this conundrum by revealing enhanced complexities of these hormones' signaling networks, including identification of unexpected common signaling nodes between these canonically antagonistic hormones.

Application of Caenorhabditis elegans in Lipid Metabolism Research

Over the last decade, the development and prevalence of obesity have posed a serious public health risk, which has prompted studies on the regulation of adiposity. With the ease of genetic manipulation, the diversity of the methods for characterizing body fat levels, and the observability of feeding behavior, Caenorhabditis elegans (C. elegans) is considered an excellent model for exploring energy homeostasis and the regulation of the cellular fat storage. In addition, the homology with mammals in the genes related to the lipid metabolism allows many aspects of lipid modulation by the regulators of the central nervous system to be conserved in this ideal model organism. In recent years, as the complex network of genes that maintain an energy balance has been gradually expanded and refined, the regulatory mechanisms of lipid storage have become clearer. Furthermore, the development of methods and devices to assess the lipid levels has become a powerful tool for studies in lipid droplet biology and the regulation of the nematode lipid metabolism. Herein, based on the rapid progress of C. elegans lipid metabolism-related studies, this review outlined the lipid metabolic processes, the major signaling pathways of fat storage regulation, and the primary experimental methods to assess the lipid content in nematodes. Therefore, this model system holds great promise for facilitating the understanding, management, and therapies of human obesity and other metabolism-related diseases.

Inhibition of Diacylglycerol Acyltransferase 2 Versus Diacylglycerol Acyltransferase 1: Potential Therapeutic Implications of Pharmacology

PURPOSE

Hepatic steatosis due to altered lipid metabolism and accumulation of hepatic triglycerides is a hallmark of nonalcoholic fatty liver disease (NAFLD). Diacylglycerol acyltransferase (DGAT) enzymes, DGAT1 and DGAT2, catalyze the terminal reaction in triglyceride synthesis, making them attractive targets for pharmacologic intervention. There is a common misconception that these enzymes are related; however, despite their similar names, DGAT1 and DGAT2 differ significantly on multiple levels. As we look ahead to future clinical studies of DGAT2 inhibitors in patients with NAFLD and nonalcoholic steatohepatitis (NASH), we review key differences and include evidence to highlight and support DGAT2 inhibitor (DGAT2i) pharmacology.

METHODS

Three Phase I, randomized, double-blind, placebo-controlled trials assessed the safety, tolerability, and pharmacokinetic properties of the DGAT2i ervogastat (PF-06865571) in healthy adult participants (Single Dose Study to Assess the Safety, Tolerability and Pharmacokinetics of PF-06865571 [study C2541001] and Study to Assess the Safety, Tolerability, and Pharmacokinetics of Multiple Doses of PF-06865571 in Healthy, Including Overweight and Obese, Adult Subjects [study C2541002]) or participants with NAFLD (2-Week Study in People With Nonalcoholic Fatty Liver Disease [study C2541005]). Data from 2 Phase I, randomized, double-blind, placebo-controlled trials of the DGAT1i PF-04620110 in healthy participants (A Single Dose Study of PF-04620110 in Overweight and Obese, Otherwise Healthy Volunteers [study B0961001] and A Multiple Dose Study of PF-04620110 in Overweight and Obese, Otherwise Healthy Volunteers [study B0961002]) were included for comparison. Safety outcomes were the primary end point in all studies, except in study C2541005, in which safety was the secondary end point, with relative change from baseline in whole liver fat at day 15 assessed as the primary end point. Safety data were analyzed across studies by total daily dose of ervogastat (5, 15, 50, 100, 150, 500, 600, 1000, and 1500 mg) or PF-04620110 (0.3, 1, 3, 5, 7, 10, 14, and 21 mg), with placebo data pooled separately across ervogastat and PF-04620110 studies.

FINDINGS

Published data indicate that DGAT1 and DGAT2 differ in multiple dimensions, including gene family, subcellular localization, substrate preference, and specificity, with unrelated pharmacologic inhibition properties and differing safety profiles. Although initial nonclinical studies suggested a potentially attractive therapeutic profile with DGAT1 inhibition, genetic and pharmacologic data suggest otherwise, with common gastrointestinal adverse events, including nausea, vomiting, and diarrhea, limiting further clinical development. Conversely, DGAT2 inhibition, although initially not pursued as aggressively as a potential target for pharmacologic intervention, has consistent efficacy in nonclinical studies, with reduced triglyceride synthesis accompanied by reduced expression of genes essential for de novo lipogenesis. In addition, early clinical data indicate antisteatotic effects with DGAT2i ervogastat, in participants with NAFLD, accompanied by a well-tolerated safety profile.

IMPLICATIONS

Although pharmacologic DGAT1is are limited by an adverse safety profile, data support use of DGAT2i as an effective and well-tolerated therapeutic strategy for patients with NAFLD, NASH, and NASH with liver fibrosis.

CLINICALTRIALS

gov identifiers: NCT03092232, NCT03230383, NCT03513588, NCT00799006, and NCT00959426.

The Contribution of Lipotoxicity to Diabetic Kidney Disease

Lipotoxicity is a fundamental pathophysiologic mechanism in diabetes and non-alcoholic fatty liver disease and is now increasingly recognized in diabetic kidney disease (DKD) pathogenesis. This review highlights lipotoxicity pathways in the podocyte and proximal tubule cell, which are arguably the two most critical sites in the nephron for DKD. The discussion focuses on membrane transporters and lipid droplets, which represent potential therapeutic targets, as well as current and developing pharmacologic approaches to reduce renal lipotoxicity.

Dysregulation of organelle membrane contact sites in neurological diseases

The defining evolutionary feature of eukaryotic cells is the emergence of membrane-bound organelles. Compartmentalization allows each organelle to maintain a spatially, physically, and chemically distinct environment, which greatly bolsters individual organelle function. However, the activities of each organelle must be balanced and are interdependent for cellular homeostasis. Therefore, properly regulated interactions between organelles, either physically or functionally, remain critical for overall cellular health and behavior. In particular, neuronal homeostasis depends heavily on the proper regulation of organelle function and cross talk, and deficits in these functions are frequently associated with diseases. In this review, we examine the emerging role of organelle contacts in neurological diseases and discuss how the disruption of contacts contributes to disease pathogenesis. Understanding the molecular mechanisms underlying the formation and regulation of organelle contacts will broaden our knowledge of their role in health and disease, laying the groundwork for the development of new therapies targeting interorganelle cross talk and function.

Acyl-CoA:diacylglycerol acyltransferase: Properties, physiological roles, metabolic engineering and intentional control

Acyl-CoA:diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) catalyzes the last reaction in the acyl-CoA-dependent biosynthesis of triacylglycerol (TAG). DGAT activity resides mainly in membrane-bound DGAT1 and DGAT2 in eukaryotes and bifunctional wax ester synthase-diacylglycerol acyltransferase (WSD) in bacteria, which are all membrane-bound proteins but exhibit no sequence homology to each other. Recent studies also identified other DGAT enzymes such as the soluble DGAT3 and diacylglycerol acetyltransferase (EaDAcT), as well as enzymes with DGAT activities including defective in cuticular ridges (DCR) and steryl and phytyl ester synthases (PESs). This review comprehensively discusses research advances on DGATs in prokaryotes and eukaryotes with a focus on their biochemical properties, physiological roles, and biotechnological and therapeutic applications. The review begins with a discussion of DGAT assay methods, followed by a systematic discussion of TAG biosynthesis and the properties and physiological role of DGATs. Thereafter, the review discusses the three-dimensional structure and insights into mechanism of action of human DGAT1, and the modeled DGAT1 from Brassica napus. The review then examines metabolic engineering strategies involving manipulation of DGAT, followed by a discussion of its therapeutic applications. DGAT in relation to improvement of livestock traits is also discussed along with DGATs in various other eukaryotic organisms.

Insights Into the Biogenesis and Emerging Functions of Lipid Droplets From Unbiased Molecular Profiling Approaches

Lipid droplets (LDs) are spherical, single sheet phospholipid-bound organelles that store neutral lipids in all eukaryotes and some prokaryotes. Initially conceived as relatively inert depots for energy and lipid precursors, these highly dynamic structures play active roles in homeostatic functions beyond metabolism, such as proteostasis and protein turnover, innate immunity and defense. A major share of the knowledge behind this paradigm shift has been enabled by the use of systematic molecular profiling approaches, capable of revealing and describing these non-intuitive systems-level relationships. Here, we discuss these advances and some of the challenges they entail, and highlight standing questions in the field.

Surveying Low-Cost Methods to Measure Lifespan and Healthspan in Caenorhabditis elegans

The discovery and development of Caenorhabditis elegans as a model organism was influential in biology, particularly in the field of aging. Many historical and contemporary studies have identified thousands of lifespan-altering paradigms, including genetic mutations, transgenic gene expression, and hormesis, a beneficial, low-grade exposure to stress. With its many advantages, including a short lifespan, easy and low-cost maintenance, and fully sequenced genome with homology to almost two-thirds of all human genes, C. elegans has quickly been adopted as an outstanding model for stress and aging biology. Here, several standardized methods are surveyed for measuring lifespan and healthspan that can be easily adapted into almost any research environment, especially those with limited equipment and funds. The incredible utility of C. elegans is featured, highlighting the capacity to perform powerful genetic analyses in aging biology without the necessity of extensive infrastructure. Finally, the limitations of each analysis and alternative approaches are discussed for consideration.

A Flexible Network of Lipid Droplet Associated Proteins Support Embryonic Integrity of C. elegans

In addition to coordinating the storage and mobilization of neutral fat, lipid droplets (LDs) are conserved organelles that can accommodate additional cargos in order to support animal development. However, it is unclear if each type of cargo is matched with a specific subset of LDs. Here, we report that SEIP-1/seipin defines a subset of oocyte LDs that are required for proper eggshell formation in C. elegans. Using a photoconvertible fluorescent protein-based imaging assay, we found that SEIP-1 positive LDs were selectively depleted after fertilization, coincident of the formation of a lipid-rich permeability barrier of the eggshell. Loss of SEIP-1 function caused impenetrant embryonic arrest, which could be worsened by FAT-3/fatty acyl-CoA desaturase deficiency or suppressed by PLIN-1/Perilipin deficiency. The embryonic development of seip-1; plin-1 mutant in turn depended on the recruitment of RAB-18/Rab18 to LDs, which was not observed in wild type embryos. We propose that SEIP-1 dependent and independent mechanisms act in parallel to ensure the packaging and export of lipid-rich permeability barrier constituents, which involve LDs. The identity of these LDs, as defined by their associated proteins, exhibits unexpected plasticity that ultimately ensures the survival of embryos ex utero.

Seipin collaborates with the ER membrane to control the sites of lipid droplet formation

Most cells store metabolic energy in lipid droplets (LDs). LDs are composed of a hydrophobic core, covered by a phospholipid monolayer, and functionalized by a specific set of proteins. Formation of LDs takes place in the endoplasmic reticulum (ER), where neutral lipid biosynthetic enzymes are located. Recent evidence indicate that this process is confined to specific ER subdomains, where proteins meet to initiate LD assembly. The lipodystrophy protein Seipin, is emerging as a major coordinator of LD biogenesis. Seipin forms a large oligomeric toroidal structure, which traps neutral lipids to promote LD nucleation. Here, we discuss the role of LD biogenesis factors that associate with Seipin to assemble functional LDs.

Dietary S. maltophilia induces supersized lipid droplets by enhancing lipogenesis and ER-LD contacts in C. elegans

Dietary and symbiotic bacteria can exert powerful influence on metazoan lipid metabolism. Recent studies have emerged that microbiota have a role in animal obesity and related health disorders, but the mechanisms by which bacteria influence lipid storage in their host are unknown. To reduce the complexity of the relationship between gut microbiota and the host, Caenorhabditis elegans (C. elegans) has been chosen as a model organism to study interspecies interaction. Here, we demonstrate that feeding C. elegans with an opportunistic pathogenic bacterium Stenotrophomonas maltophilia (S. maltophilia) retards growth and promotes excessive neutral lipid storage. Gene expression analysis reveals that dietary S. maltophilia induces a lipogenic transcriptional response that includes the SREBP ortholog SBP-1, and fatty acid desaturases FAT-6 and FAT-7. Live imaging and ultrastructural analysis suggest that excess neutral lipid is stored in greatly expanded lipid droplets (LDs), as a result of enhanced endoplasmic reticulum (ER)-LD interaction. We also report that loss of function mutations in dpy-9 in C. elegans confers resistance to S. maltophilia. Dietary S. maltophilia induces supersized LDs by enhancing lipogenesis and ER-LD contacts in C. elegans. This work delineates a new model for understanding microbial regulation of metazoan physiology.

Circulating Mitochondrial DNA and Inter-Organelle Contact Sites in Aging and Associated Conditions

Mitochondria are primarily involved in cell bioenergetics, regulation of redox homeostasis, and cell death/survival signaling. An immunostimulatory property of mitochondria has also been recognized which is deployed through the extracellular release of entire or portioned organelle and/or mitochondrial DNA (mtDNA) unloading. Dynamic homo- and heterotypic interactions involving mitochondria have been described. Each type of connection has functional implications that eventually optimize mitochondrial activity according to the bioenergetic demands of a specific cell/tissue. Inter-organelle communications may also serve as molecular platforms for the extracellular release of mitochondrial components and subsequent ignition of systemic inflammation. Age-related chronic inflammation (inflamm-aging) has been associated with mitochondrial dysfunction and increased extracellular release of mitochondrial components—in particular, cell-free mtDNA. The close relationship between mitochondrial dysfunction and cellular senescence further supports the central role of mitochondria in the aging process and its related conditions. Here, we provide an overview of (1) the mitochondrial genetic system and the potential routes for generating and releasing mtDNA intermediates; (2) the pro-inflammatory pathways elicited by circulating mtDNA; (3) the participation of inter-organelle contacts to mtDNA homeostasis; and (4) the link of these processes with senescence and age-associated conditions.

A Bayesian hierarchically structured prior for gene-based association testing with multiple traits in genome-wide association studies

Although genome-wide association studies (GWAS) often collect data on multiple correlated traits for complex diseases, conventional gene-based analysis is usually univariate, and therefore, treating traits as uncorrelated. Multivariate analysis of multiple correlated traits can potentially increase the power to detect genes that affect some or all of these traits. In this study, we propose the multivariate hierarchically structured variable selection (HSVS-M) model, a flexible Bayesian model that tests the association of a gene with multiple correlated traits. With only summary statistics, HSVS-M can account for the correlations among genetic variants and among traits simultaneously and can also estimate the various directions and magnitudes of associations between a gene and multiple traits. Simulation studies show that HSVS-M substantially outperforms competing methods in various scenarios, particularly when variants in a gene are associated with a trait in similar directions and magnitudes. We applied HSVS-M to the summary statistics of a meta-analysis GWAS on four lipid traits from the Global Lipids Genetics Consortium and identified 15 genes that have also been confirmed as risk factors in previous studies.

Targeting EDEM protects against ER stress and improves development and survival in C. elegans

EDEM-1, EDEM-2 and EDEM-3 are key players for the quality control of newly synthesized proteins in the endoplasmic reticulum (ER) by accelerating disposal and degradation of misfolded proteins through ER Associated Degradation (ERAD). Although many previous studies reported the role of individual ERAD components especially in cell-based systems, still little is known about the consequences of ERAD dysfunction under physiological and ER stress conditions in the context of a multicellular organism. Here we report the first individual and combined characterization and functional interplay of EDEM proteins in Caenorhabditis elegans using single, double, and triple mutant combinations. We found that EDEM-2 has a major role in the clearance of misfolded proteins from ER under physiological conditions, whereas EDEM-1 and EDEM-3 roles become prominent under acute ER stress. In contrast to SEL-1 loss, the loss of EDEMs in an intact organism induces only a modest ER stress under physiological conditions. In addition, chronic impairment of EDEM functioning attenuated both XBP-1 activation and up-regulation of the stress chaperone GRP78/BiP, in response to acute ER stress. We also show that pre-conditioning to EDEM loss in acute ER stress restores ER homeostasis and promotes survival by activating ER hormesis. We propose a novel role for EDEM in fine-tuning the ER stress responsiveness that affects ER homeostasis and survival.

ER stress and UPR^ER malfunctions have been implicated in the pathogenesis of neurodegeneration, metabolic and inflammatory diseases as well as tumor progression and diabetes, whereby disturbed ER homeostasis negatively influences the pathology of the disease. Under ER stress conditions, the cells either activate UPR^ER-dependent cytoprotective mechanisms when ER stress is at subtoxic levels or, in case of an excessive ER stress, the cytotoxic response stimulates cell death. Here, we used Caenorhabditis elegans to study the cellular responses to ER stress at organismal level. We show that EDEMs respond differently to ER stress stimuli, and moreover, EDEMs deficiencies activate an XBP-1 independent adaptive program to promote organism survival under acute ER stress. Corroborated with the fact that loss of EDEM-2 and EDEM-3 induces resistance to acute ER stress in an intact organism, our data implicate EDEM proteins in a broader response to ER stress than previously established, which opens a new avenue for understanding the regulation of ER stress with implications for clinical and therapeutic investigations.

Abnormal accumulation of lipid droplets in neurons induces the conversion of alpha-Synuclein to proteolytic resistant forms in a Drosophila model of Parkinson's disease

Parkinson’s disease (PD) is a neurodegenerative disorder characterized by alpha-synuclein (αSyn) aggregation and associated with abnormalities in lipid metabolism. The accumulation of lipids in cytoplasmic organelles called lipid droplets (LDs) was observed in cellular models of PD. To investigate the pathophysiological consequences of interactions between αSyn and proteins that regulate the homeostasis of LDs, we used a transgenic Drosophila model of PD, in which human αSyn is specifically expressed in photoreceptor neurons. We first found that overexpression of the LD-coating proteins Perilipin 1 or 2 (dPlin1/2), which limit the access of lipases to LDs, markedly increased triacylglyclerol (TG) loaded LDs in neurons. However, dPlin-induced-LDs in neurons are independent of lipid anabolic (diacylglycerol acyltransferase 1/midway, fatty acid transport protein/dFatp) and catabolic (brummer TG lipase) enzymes, indicating that alternative mechanisms regulate neuronal LD homeostasis. Interestingly, the accumulation of LDs induced by various LD proteins (dPlin1, dPlin2, CG7900 or Klarsicht^LD-BD) was synergistically amplified by the co-expression of αSyn, which localized to LDs in both Drosophila photoreceptor neurons and in human neuroblastoma cells. Finally, the accumulation of LDs increased the resistance of αSyn to proteolytic digestion, a characteristic of αSyn aggregation in human neurons. We propose that αSyn cooperates with LD proteins to inhibit lipolysis and that binding of αSyn to LDs contributes to the pathogenic misfolding and aggregation of αSyn in neurons.

Parkinson’s disease (PD) is a neurodegenerative disease characterized by the neurotoxic aggregation of the alpha-synuclein (αSyn) protein. Cellular models of the disease are also associated with an abnormal fat storage in the form of lipid droplets (LDs). However, in which cells, neuron or glial cells, LDs accumulate in the organism remains unknown. To understand the relationship between αSyn and the accumulation of LDs, we used a Drosophila (fruit fly) model of PD. We found that, in the presence of a protein that coats LDs, perilipin, LDs accumulate in photoreceptor neurons of the fly. Interestingly, the accumulation of LDs induced by perilipin or other LD-coating proteins was enhanced in the presence of αSyn. Using human neuronal cell lines and the fly, we could show that LD-coating and αSyn proteins localize at the surface of LDs. Finally, we observed that the process of αSyn aggregation was enhanced in the presence of LDs by using a biochemical approach. We thus propose that the association of αSyn with LDs could contribute to αSyn aggregation and progression of the pathology.

Hello from the other side: Membrane contact of lipid droplets with other organelles and subsequent functional implications

Lipid droplets (LDs) are ubiquitous organelles that play crucial roles in response to physiological and environmental cues. The identification of several neutral lipid synthesizing and regulatory protein complexes have propelled significant advance on the mechanisms of LD biogenesis in the endoplasmic reticulum (ER). Increasing evidence suggests that distinct proteins and regulatory factors, which localize to membrane contact sites (MCS), are involved not only in interorganellar lipid exchange and transport, but also function in other important cellular processes, including autophagy, mitochondrial dynamics and inheritance, ion signaling and inter-regulation of these MCS. More and more tethers and molecular determinants are associated to MCS and to a diversity of cellular and pathophysiological processes, demonstrating the dynamics and importance of these junctions in health and disease. The conjugation of lipids with proteins in supramolecular complexes is known to be paramount for many biological processes, namely membrane biosynthesis, cell homeostasis, regulation of organelle division and biogenesis, and cell growth. Ultimately, this physical organization allows the contact sites to function as crucial metabolic hubs that control the occurrence of chemical reactions. This leads to biochemical and metabolite compartmentalization for the purposes of energetic efficiency and cellular homeostasis. In this review, we will focus on the structural and functional aspects of LD-organelle interactions and how they ensure signaling exchange and metabolites transfer between organelles.

Endocrine pheromones couple fat rationing to dauer diapause through HNF4α nuclear receptors

Developmental diapause is a widespread strategy for animals to survive seasonal starvation and environmental harshness. Diapaused animals often ration body fat to generate a basal level of energy for enduring survival. How diapause and fat rationing are coupled, however, is poorly understood. The nematode Caenorhabditis elegans excretes pheromones to the environment to induce a diapause form called dauer larva. Through saturated forward genetic screens and CRISPR knockout, we found that dauer pheromones feed back to repress the transcription of ACOX-3, MAOC-1, DHS-28, DAF-22 (peroxisomal β-oxidation enzymes dually involved in pheromone synthesis and fat burning), ALH-4 (aldehyde dehydrogenase for pheromone synthesis), PRX-10 and PRX-11 (peroxisome assembly and proliferation factors). Dysfunction of these pheromone enzymes and factors relieves the repression. Surprisingly, transcription is repressed not by pheromones excreted but by pheromones endogenous to each animal. The endogenous pheromones regulate the nuclear translocation of HNF4α family nuclear receptor NHR-79 and its co-receptor NHR-49, and, repress transcription through the two receptors. The feedback repression maintains pheromone homeostasis, increases fat storage, decreases fat burning, and prolongs dauer lifespan. Thus, the exocrine dauer pheromones possess an unexpected endocrine function to mediate a peroxisome-nucleus crosstalk, coupling dauer diapause to fat rationing.

mmBCFA C17iso ensures endoplasmic reticulum integrity for lipid droplet growth

In eukaryote cells, lipid droplets (LDs) are key intracellular organelles that dynamically regulate cellular energy homeostasis. LDs originate from the ER and continuously contact the ER during their growth. How the ER affects LD growth is largely unknown. Here, we show that RNAi knockdown of acs-1, encoding an acyl-CoA synthetase required for the biosynthesis of monomethyl branched-chain fatty acids C15iso and C17iso, remarkably prevented LD growth in Caenorhabditis elegans. Dietary C17iso, or complex lipids with C17iso including phosphatidylcholine, phosphatidylethanolamine, and triacylglycerol, could fully restore the LD growth in the acs-1RNAi worms. Mechanistically, C17iso may incorporate into phospholipids to ensure the membrane integrity of the ER so as to maintain the function of ER-resident enzymes such as SCD/stearoyl-CoA desaturase and DGAT2/diacylglycerol acyltransferase for appropriate lipid synthesis and LD growth. Collectively, our work uncovers a unique fatty acid, C17iso, as the side chain of phospholipids for determining the ER homeostasis for LD growth in an intact organism, C. elegans.

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.

Fatty acid desaturation is essential for C. elegans longevity at high temperature

Metabolic reprogramming is crucial for the adaptation to environmental temperature stress. It is generally accepted that fatty acid (FA) desaturation is suppressed at high temperature, which decreases the ratio of unsaturated FAs to saturated FAs (UFAs/SFAs) to maintain the fluidity of cell membranes and favor cellular survival. Here by working in C. elegans, we found that FA desaturation is essential for longevity in response to temperature upshift at the organismal level, opposite to its role in cellular survival. High temperature unexpectedly increases the contents of total fat and multiple UFA species. Specifically, monounsaturated oleic acid (OA) is required for animal survival at high temperature. Mechanistic study showed that OA acts through HSF-1, which in turn promotes histone acetylation as well as the expression of defense genes that are crucial for longevity at high temperature. Together, our findings reveal an unprecedented role for FA desaturation in organismal fitness to temperature upshift, and implicate divergent metabolic requirements between cellular and organismal survival upon temperature stress.

Retinal pigment epithelium 65 kDa protein (RPE65): An update

Vertebrate vision critically depends on an 11-cis-retinoid renewal system known as the visual cycle. At the heart of this metabolic pathway is an enzyme known as retinal pigment epithelium 65 kDa protein (RPE65), which catalyzes an unusual, possibly biochemically unique, reaction consisting of a coupled all-trans-retinyl ester hydrolysis and alkene geometric isomerization to produce 11-cis-retinol. Early work on this isomerohydrolase demonstrated its membership to the carotenoid cleavage dioxygenase superfamily and its essentiality for 11-cis-retinal production in the vertebrate retina. Three independent studies published in 2005 established RPE65 as the actual isomerohydrolase instead of a retinoid-binding protein as previously believed. Since the last devoted review of RPE65 enzymology appeared in this journal, major advances have been made in a number of areas including our understanding of the mechanistic details of RPE65 isomerohydrolase activity, its phylogenetic origins, the relationship of its membrane binding affinity to its catalytic activity, its role in visual chromophore production for rods and cones, its modulation by macromolecules and small molecules, and the involvement of RPE65 mutations in the development of retinal diseases. In this article, I will review these areas of progress with the goal of integrating results from the varied experimental approaches to provide a comprehensive picture of RPE65 biochemistry. Key outstanding questions that may prove to be fruitful future research pursuits will also be highlighted.

Lipid Droplet-Organelle Contact Sites as Hubs for Fatty Acid Metabolism, Trafficking, and Metabolic Channeling

Cells prepare for fluctuations in nutrient availability by storing energy in the form of neutral lipids in organelles called Lipid Droplets (LDs). Upon starvation, fatty acids (FAs) released from LDs are trafficked to different cellular compartments to be utilized for membrane biogenesis or as a source of energy. Despite the biochemical pathways being known in detail, the spatio-temporal regulation of FA synthesis, storage, release, and breakdown is not completely understood. Recent studies suggest that FA trafficking and metabolism are facilitated by inter-organelle contact sites that form between LDs and other cellular compartments such as the Endoplasmic Reticulum (ER), mitochondria, peroxisomes, and lysosomes. LD-LD contact sites are also sites where FAs are transferred in a directional manner to support LD growth and expansion. As the storage site of neutral lipids, LDs play a central role in FA homeostasis. In this mini review, we highlight the role of LD contact sites with other organelles in FA trafficking, channeling, and metabolism and discuss the implications for these pathways on cellular lipid and energy homeostasis.

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

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

Nuclear receptors NHR-49 and NHR-79 promote peroxisome proliferation to compensate for aldehyde dehydrogenase deficiency in C. elegans

The intracellular level of fatty aldehydes is tightly regulated by aldehyde dehydrogenases to minimize the formation of toxic lipid and protein adducts. Importantly, the dysregulation of aldehyde dehydrogenases has been implicated in neurologic disorder and cancer in humans. However, cellular responses to unresolved, elevated fatty aldehyde levels are poorly understood. Here, we report that ALH-4 is a C. elegans aldehyde dehydrogenase that specifically associates with the endoplasmic reticulum, mitochondria and peroxisomes. Based on lipidomic and imaging analysis, we show that the loss of ALH-4 increases fatty aldehyde levels and reduces fat storage. ALH-4 deficiency in the intestine, cell-nonautonomously induces NHR-49/NHR-79-dependent hypodermal peroxisome proliferation. This is accompanied by the upregulation of catalases and fatty acid catabolic enzymes, as indicated by RNA sequencing. Such a response is required to counteract ALH-4 deficiency since alh-4; nhr-49 double mutant animals are sterile. Our work reveals unexpected inter-tissue communication of fatty aldehyde levels and suggests pharmacological modulation of peroxisome proliferation as a therapeutic strategy to tackle pathology related to excess fatty aldehydes.

Fatty aldehydes are generated during the turnover of membrane lipids and when cells are under oxidative stress. Because excess fatty aldehydes form toxic adducts with proteins and lipids, their levels are tightly controlled by a family of aldehyde dehydrogenases whose dysfunction has been implicated in genetic disease and cancer in humans. Here, we characterize mutant C. elegans that lack a conserved, membrane-associated aldehyde dehydrogenase ALH-4. Despite elevated levels of fatty aldehydes, these mutant worms survive by increasing the abundance of peroxisomes, which are important organelles for lipid metabolism. Such peroxisome proliferative response depends on the activation of transcription factors NHR-49 and NHR-79, via putative endocrine signals. Accordingly, the fertility of alh-4 mutant worms relies on NHR-49. Our work suggests a latent mechanism that may be activated during aldehyde dehydrogenase deficiency.

The Mitochondrial PHB Complex Determines Lipid Composition and Interacts With the Endoplasmic Reticulum to Regulate Ageing

Metabolic disorders are frequently associated with physiological changes that occur during ageing. The mitochondrial prohibitin complex (PHB) is an evolutionary conserved context-dependent modulator of longevity, which has been linked to alterations in lipid metabolism but which biochemical function remains elusive. In this work we aimed at elucidating the molecular mechanism by which depletion of mitochondrial PHB shortens the lifespan of wild type animals while it extends that of insulin signaling receptor (daf-2) mutants. A liquid chromatography coupled with mass spectrometry approach was used to characterize the worm lipidome of wild type and insulin deficient animals upon PHB depletion. Toward a mechanistic interpretation of the insights coming from this analysis, we used a combination of biochemical, microscopic, and lifespan analyses. We show that PHB depletion perturbed glycerophospholipids and glycerolipids pools differently in short- versus long-lived animals. Interestingly, PHB depletion in otherwise wild type animals induced the endoplasmic reticulum (ER) unfolded protein response (UPR), which was mitigated in daf-2 mutants. Moreover, depletion of DNJ-21, which functionally interacts with PHB in mitochondria, mimicked the effect of PHB deficiency on the UPR^ER and on the lifespan of wild type and insulin signaling deficient mutants. Our work shows that PHB differentially modulates lipid metabolism depending on the worm’s metabolic status and provides evidences for a new link between PHB and ER homeostasis in ageing regulation.

DGAT2 stability is increased in response to DGAT1 inhibition in gene edited HepG2 cells

In eukaryotic organisms, two unrelated acyl-CoA:diacylglycerol acyltransferase (DGAT) enzymes, DGAT1 and DGAT2, catalyze the final step of the triacylglycerol biosynthetic pathway. Both enzymes are highly expressed in lipogenic tissues, such as adipose tissue, small intestine and the liver. DGAT2 has a prominent role in hepatocyte lipid metabolism synthesizing triacylglycerols that are utilized for very low-density lipoprotein assembly. However, due to the lack of useful antibodies to detect endogenous DGAT2 protein, it has been difficult to determine how this enzyme functions at the cellular level. We have unsuccessfully tested many commercial antibodies as well as our own "in-house" antibodies. There is currently no evidence that DGAT2 undergoes processing such that antigenic epitopes to these antibodies are removed. As an alternative, many studies have utilized epitope tagged versions of DGAT2 overexpressed in cells. These approaches can provide valuable information about a protein, but can be subject to artifacts, such as mislocalization, misregulation, protein aggregation and abnormal protein-protein interactions. In this study, we used gene editing with CRISPR/Cas9 to add three consecutive FLAG epitopes to the C-terminus of endogenous DGAT2 in HepG2 cells. HepG2 cells, derived from a human hepatocellular carcinoma, have been routinely used as a cell model to study human hepatocyte lipid and lipoprotein metabolism. Using this system allowed us to successfully detect DGAT2 expressed from its endogenous locus in HepG2 cells by immunoblotting with anti-FLAG antibodies.

Nuclear lipid droplets form in the inner nuclear membrane in a seipin-independent manner

Nuclear lipid droplets (LDs) in hepatocytes are derived from precursors of very-low-density lipoprotein in the ER lumen, but it is not known how cells lacking the lipoprotein secretory function form nuclear LDs. Here, we show that the inner nuclear membrane (INM) of U2OS cells harbors triglyceride synthesis enzymes, including ACSL3, AGPAT2, GPAT3/GPAT4, and DGAT1/DGAT2, and generates nuclear LDs in situ. mTOR inhibition increases nuclear LDs by inducing the nuclear translocation of lipin-1 phosphatidic acid (PA) phosphatase. Seipin, a protein essential for normal cytoplasmic LD formation in the ER, is absent in the INM. Knockdown of seipin increases nuclear LDs and PA in the nucleus, whereas seipin overexpression decreases these. Seipin knockdown also up-regulates lipin-1β expression, and lipin-1 knockdown decreases the effect of seipin knockdown on nuclear LDs without affecting PA redistribution. These results indicate that seipin is not directly involved in nuclear LD formation but instead restrains it by affecting lipin-1 expression and intracellular PA distribution.