Nuclear Lipid Droplets Research Articles

Nuclear lipid droplets in Caco2 cells originate from nascent precursors and in situ at the nuclear envelope

Intestinal epithelial cells convert excess fatty acids into triglyceride (TAG) for storage in cytoplasmic lipid droplets and secretion in chylomicrons. Nuclear lipid droplets (nLDs) are present in intestinal cells but their origin and relationship to cytoplasmic TAG synthesis and secretion is unknown. nLDs and related lipid-associated promyelocytic leukemia structures (LAPS) were abundant in oleate-treated Caco2 but less frequent in other human colorectal cancer cell lines and mouse intestinal organoids. nLDs and LAPS in undifferentiated oleate-treated Caco2 cells harbored the phosphatidate phosphatase Lipin1, its product diacylglycerol, and CTP:phosphocholine cytidylyltransferase (CCT)α. CCTα knockout Caco2 cells had fewer but larger nLDs, indicating a reliance on de novo PC synthesis for assembly. Differentiation of Caco2 cells caused large nLDs and LAPS to form regardless of oleate treatment or CCTα expression. nLDs and LAPS in Caco2 cells did not associate with apoCIII and apoAI and formed dependently of microsomal triglyceride transfer protein expression and activity, indicating they are not derived from endoplasmic reticulum luminal LDs precursors. Instead, undifferentiated Caco2 cells harbored a constitutive pool of nLDs and LAPS in proximity to the nuclear envelope that expanded in size and number with oleate treatment. Inhibition of TAG synthesis did affect the number of nascent nLDs and LAPS but prevented their association with promyelocytic leukemia protein, Lipin1α, and diacylglycerol, which instead accumulated on the nuclear membranes. Thus, nLD and LAPS biogenesis in Caco2 cells is not linked to lipoprotein secretion but involves biogenesis and/or expansion of nascent nLDs by de novo lipid synthesis.

Abstract 1624 Regulation of phosphatidylcholine synthesis at the inner nuclear membrane and nuclear lipid droplets

Abstract 1624 Regulation of phosphatidylcholine synthesis at the inner nuclear membrane and nuclear lipid droplets

Lipid- and phospho-regulation of CTP:Phosphocholine Cytidylyltransferase αassociation with nuclear lipid droplets.

Fatty acids stored in triacylglycerol-rich lipid droplets are assembled with a surface monolayer composed primarily of phosphatidylcholine (PC). Fatty acids stimulate PC synthesis by translocating CTP:phosphocholine cytidylyltransferase (CCT) α to the inner nuclear membrane, nuclear lipid droplets (nLD) and lipid associated promyelocytic leukemia (PML) structures (LAPS). Huh7 cells were used to identify how CCTαtranslocation onto these nuclear structures are regulated by fatty acids and phosphorylation of its serine-rich P-domain. Oleate treatment of Huh7 cells increased nLDs and LAPS that became progressively enriched in CCTα. In cells expressing the phosphatidic acid phosphatase Lipin1α or 1β, the expanded pool of nLDs and LAPS had a proportional increase in associated CCTα. In contrast, palmitate induced few nLDs and LAPS and inhibited the oleate-dependent translocation of CCTα without affecting total nLDs. Phospho-memetic or phospho-null mutations in the P-domain revealed that a 70% phosphorylation threshold, rather than site-specific phosphorylation, regulated CCTα association with nLDs and LAPS. In vitro candidate kinase and inhibitor studies in Huh7 cells identified cyclin-dependent kinase (CDK) 1 and 2 as putative P-domain kinases. In conclusion, CCTα translocation onto nLDs and LAPS is dependent on available surface area and fatty acid composition, as well as threshold phosphorylation of the P-domain potentially involving CDKs.

Ultrabright organic fluorescent probe for quantifying the dynamics of cytosolic/nuclear lipid droplets

Lipid droplets (LDs) are extremely active organelles that play a crucial role in energy metabolism, membrane formation, and the production of lipid-derived signaling molecules by regulating lipid storage and release. Nevertheless, directly limited by the lack of superior fluorescent probes, studies of LDs dynamic motion velocity have been rarely reported, especially for nuclear LDs. Herein, a novel organic fluorescent probe Lipi-Bright has been rationally developed based on bridged cyclization of distyrylbenzene. The fully ring-fused molecule structure endows the probe with high photostability. Moreover, this new fluorescent probe displays the features of excellent LDs staining specificity as well as ultrahigh fluorescence brightness. Lipi-Bright labeled LDs was dozens of times brighter than representative probes BODIPY 493/503 or Nile Red. Consequently, by in-situ time-lapse fluorescence imaging, the dynamics of LDs have been quantitatively studied. For instance, the velocities of cytosolic LDs (37 ± 15 nm/s) are found to be obviously faster than those of nuclear LDs (24 ± 4 nm/s), and both the cytosolic LDs and the nuclear LDs would be moved faster or slower depend on the various stimulations. Overall, this work providing plentiful information on LDs dynamics will greatly facilitate the in-depth investigation of lipid metabolism.

Identification of novel proteins regulating lipid droplet biogenesis in filamentous fungi.

Lipid droplets (LDs) are storage organelles for neutral lipids which are critical for lipid homeostasis. Current knowledge of fungal LD biogenesis is largely limited to budding yeast, while LD regulation in multinucleated filamentous fungi which exhibit considerable metabolic activity remains unexplored. In this study, 19 LD-associated proteins were identified in the multinucleated species Aspergillus oryzae using a colocalization screening of a previously established enhanced green fluorescent protein (EGFP) fusion library. Functional screening identified 12 lipid droplet-regulating (LDR) proteins whose loss of function resulted in irregular LD biogenesis, particularly in terms of LD number and size. Bioinformatics analysis, targeted mutagenesis, and microscopy revealed four LDR proteins that localize to LD via the putative amphipathic helices (AHs). Further analysis revealed that LdrA with an Opi1 domain is essential for cytoplasmic and nuclear LD biogenesis involving a novel AH. Phylogenetic analysis demonstrated that the patterns of gene evolution were predominantly based on gene duplication. Our study identified a set of novel proteins involved in the regulation of LD biogenesis, providing unique molecular and evolutionary insights into fungal lipid storage.

Distinct features of two lipid droplets types in cell nuclei from patients with liver diseases

Lipid droplets (LDs) have been observed in the nuclei of hepatocytes; however, their significance in liver disease remains unresolved. Our purpose was to explore the pathophysiological features of intranuclear LDs in liver diseases. We included 80 patients who underwent liver biopsies; the specimens were dissected and fixed for electron microscopy analysis. Depending on the presence of adjacent cytoplasmic invagination of the nuclear membrane, LDs in the nuclei were classified into two types: nucleoplasmic LDs (nLDs) and cytoplasmic LD invagination with nucleoplasmic reticulum (cLDs in NR). nLDs were found in 69% liver samples and cLDs in NR were found in 32%; no correlation was observed between the frequencies of the two LD types. nLDs were frequently found in hepatocytes of patients with nonalcoholic steatohepatitis, whereas cLDs in NR were absent from the livers of such patients. Further, cLDs in NR were often found in hepatocytes of patients with lower plasma cholesterol level. This indicates that nLDs do not directly reflect cytoplasmic lipid accumulation and that formation of cLDs in NR is inversely correlated to the secretion of very low-density lipoproteins. Positive correlations were found between the frequencies of nLDs and endoplasmic reticulum (ER) luminal expansion, suggesting that nLDs are formed in the nucleus upon ER stress. This study unveiled the presence of two distinct nuclear LDs in various liver diseases.

SIR telomere silencing depends on nuclear envelope lipids and modulates sensitivity to a lysolipid.

The nuclear envelope (NE) is important in maintaining genome organization. The role of lipids in communication between the NE and telomere regulation was investigated, including how changes in lipid composition impact gene expression and overall nuclear architecture. Yeast was treated with the non-metabolizable lysophosphatidylcholine analog edelfosine, known to accumulate at the perinuclear ER. Edelfosine induced NE deformation and disrupted telomere clustering but not anchoring. Additionally, the association of Sir4 at telomeres decreased. RNA-seq analysis showed altered expression of Sir-dependent genes located at sub-telomeric (0-10 kb) regions, consistent with Sir4 dispersion. Transcriptomic analysis revealed that two lipid metabolic circuits were activated in response to edelfosine, one mediated by the membrane sensing transcription factors, Spt23/Mga2, and the other by a transcriptional repressor, Opi1. Activation of these transcriptional programs resulted in higher levels of unsaturated fatty acids and the formation of nuclear lipid droplets. Interestingly, cells lacking Sir proteins displayed resistance to unsaturated-fatty acids and edelfosine, and this phenotype was connected to Rap1.

Issue Information

The FEBS JournalVolume 290, Issue 7 p. 1667-1669 Issue Cover & InformationFree Access Issue Information First published: 05 April 2023 https://doi.org/10.1111/febs.16490AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Graphical Abstract Cover Illustration Representative confocal immunofluorescence image showing the colocalization of promyelocytic leukaemia (PML) nuclear bodies with nuclear lipid droplets in a glioblastoma cell line. See the Original Article by Umaru et al. in this issue. Volume290, Issue7April 2023Pages 1667-1669 RelatedInformation

Solvatochromic Near-Infrared Aggregation-Induced Emission-Active Acrylonitriles by Acceptor Modulation for Low-Power Stimulated Emission Depletion Nanoscopy

Stimulated emission depletion (STED) nanoscopy has broadened our horizons to unravel mysterious functions of cellular structures on an unparalleled nanometer scale. Nevertheless, an intense depletion laser power is a general prerequisite for STED super-resolution imaging with a satisfactory resolution, inevitably leading to severe photobleaching of fluorophores, irreparable photodamage to biosamples, and impaired imaging quality. Herein, by modulating distinct acceptor units, a series of donor–acceptor–acceptor structured fluorescent acrylonitriles featured with aggregation-induced emission (AIE) for STED super-resolution imaging at a low depletion power were systematically developed. These AIE luminogens (AIEgens) exhibited tunable near-infrared emissions (650–733 nm) and high fluorescence quantum yields (QYs of up to 26.8%) in the solid state, significant Stokes shifts, and large two-photon absorption cross-sections. They also exhibited a typical solvatochromic effect and ultrahigh QYs of up to 98.4% in low-polarity solvents. Furthermore, these lipophilic solvatochromic acrylonitriles specifically lit up lipid droplets (LDs) with exceptionally high photostability in a wash-free manner. By taking TPA-BT-ANBI as an example, the STED super-resolution imaging of LDs with excellent resolution of 62 and 80 nm for cytosolic and nuclear LDs, respectively, at a low saturation depletion power of 0.83 MW/cm2 and extended time-lapse imaging of LD dynamics was achieved. Subsequent use of TPA-BT-ANBI for the optical discrimination of fatty liver tissues and two-photon deep-tissue imaging was also demonstrated. This study opens new avenues for versatile photostable materials at low depletion powers for target-specific STED super-resolution imaging.

The Effect of Oxidized Fish Oil on Blood Parameters and Liver Histology of Nile Tilapia (Oreochromis niloticus)

Fish oil is a virtually unique source of natural LC omega-3 fatty acids EPA, DPA, and DHA, as an important source of lipid in aquafeeds. However during processing and storage, fish oil is prone to oxidation. The oxidized oil has many negative effects on the growth and health of farmed fish, therefore this study was conducted to investigate the effect of oxidized anchovy oil on blood parameters and liver histology of Nile tilapia (Oreochromis niloticus). Nile tilapia fingerlings with an initial average body weight of (15.07±0.58 g) were stocked in nine fiberglass tanks (with a tank capacity of 120 liters) in three groups in triplicates at a density of 50 fish/tank. Fish were fed with three test diets containing 35% crude protein and 8% crude fat (POV: 4.85, 50.10, and 100.80 meq/kg-1) twice a day at a feeding rate of 3% of their body weight per day for 12 weeks. During the experiment fish blood parameters analysis, and liver histology tests were performed on 3 fish taken randomly from each tank at the beginning of the experiment and monthly. The results of blood chemistry analysis showed better results in the group fed with feed that contained fresh oil (A) compared to groups fed with feeds that contained oxidized oils (B and C), but no significant differences (P > 0.05) between feed groups. Whereas the results of serum biochemical analysis showed significant differences (P > 0.05) between feed groups during the experiment period, oxidized fish oil increased serum ALT, AST activity, and TCHO level (P < 0.05), and decreased ALP activity. The liver histology test showed the best results in group A compared to oxidized oil groups (B, C), oxidized fish oil caused damage to the liver structure, and hepatocyte nuclear migration and lipid droplets were observed in liver samples from the oxidized oil groups. In conclusion, the present study demonstrates that oxidized fish oil may have a negative effect on the liver histology and blood parameters of Nile tilapia fingerlings.

Hepatocytes Deficient in Nuclear Envelope Protein Lamina-associated Polypeptide 1 are an Ideal Mammalian System to Study Intranuclear Lipid Droplets

Lipid droplets (LDs) are generally considered to be synthesized in the ER and utilized in the cytoplasm. However, LDs have been observed inside nuclei in some cells, although recent research on nuclear LDs has focused on cultured cell lines. To better understand nuclear LDs that occur in vivo, here we examined LDs in primary hepatocytes from mice following depletion of the nuclear envelope protein lamina-associated polypeptide 1 (LAP1). Microscopic image analysis showed that LAP1-depleted hepatocytes contain frequent nuclear LDs, which differ from cytoplasmic LDs in their associated proteins. We found type 1 nucleoplasmic reticula, which are invaginations of the inner nuclear membrane, are often associated with nuclear LDs in these hepatocytes. Furthermore, in vivo depletion of the nuclear envelope proteins lamin A and C from mouse hepatocytes led to severely abnormal nuclear morphology, but significantly fewer nuclear LDs than were observed upon depletion of LAP1. In addition, we show both high-fat diet feeding and fasting of mice increased cytoplasmic lipids in LAP1-depleted hepatocytes but reduced nuclear LDs, demonstrating a relationship of LD formation with nutritional state. Finally, depletion of microsomal triglyceride transfer protein did not change the frequency of nuclear LDs in LAP1-depleted hepatocytes, suggesting that it is not required for the biogenesis of nuclear LDs in these cells. Together, these data show that LAP1-depleted hepatocytes represent an ideal mammalian system to investigate the biogenesis of nuclear LDs and their partitioning between the nucleus and cytoplasm in response to changes in nutritional state and cellular metabolism in vivo.

Structure Rigidification Promoted Ultrabright Solvatochromic Fluorescent Probes for Super-Resolution Imaging of Cytosolic and Nuclear Lipid Droplets.

Lipid droplets (LDs) containing cytosolic and nuclear LDs have recently received increasing attention because of their diverse biological roles in living systems. However, developing fluorescent probes for super-resolution visualization of these subcellular LDs still remains challenging due to insufficient fluorescence brightness and poor nuclear membrane permeability. Herein, we rationally synthesized a series of ultrabright solvatochromic fluorescent probes based on benzoboranils (BBAs) for LD-specific super-resolution imaging using structured illumination microscopy (SIM). The rigidly structured probes exhibit ultrahigh fluorescence quantum yields of up to 99.9% in low-polar solvents. They also show more significant fluorescence enhancements in lipid environments than commercial LD probes. Owing to these excellent merits, our lipophilic fluorescent probes can specifically light up subcellular LDs at ultralow concentrations down to 10 nM. Further use of BBA-CF3 for super-resolution SIM imaging of cytosolic and nuclear LDs and their fusion process was successfully achieved. The unprecedented spatial resolution for nuclear LDs with an FWHM value of 142 nm was also acquired. Collectively, our ultrabright fluorescent probes hold tremendous potential to unveil the mysterious roles of cytosolic and nuclear LDs in biological research using SIM.

CTP:phosphocholine cytidylyltransferase alpha regulates nLD biogenesis in Caco2 cells

Recent investigations reveal that nuclear lipid droplets (nLDs) stimulate lipid metabolic enzymes in response to excess fatty acid loading of cells. For example, in oleate‐treated hepatocytes the rate‐limiting enzyme in the CDP‐choline pathway, CTP:phosphocholine cytidylyltransferase alpha (CCT alpha), is recruited to the surface of nLDs and stimulates the synthesis of phosphatidylcholine (PC). Hepatocyte nLDs derive from ApoB‐free LDs in the lumen of the endoplasmic reticulum that migrate into the nucleoplasm through type I nucleoplasmic reticulum invaginations of the inner nuclear membrane (INM). Once formed inside the nucleus, nLDs recruit proteins CCT alpha, Lipin1 and promyelocytic leukemia (PML) to their surfaces. Endothelial cells of the colon also express ApoB lipoproteins for chylomicron assembly and secretion, so we hypothesized that these cells might also form nLDs in response to excess lipid loading. Indeed, using immunofluorescence confocal microscopy we found that oleate‐treated Caco2 cells formed between 5 to 10 nLDs per cell that recruited both CCT alpha and PML to their surface. CRISPR/Cas9‐mediated CCT alpha knockout Caco2 (CCT alphaKO) cells had significantly fewer but larger cLDs and nLDs compared to wild‐type cells. The percentage of PML‐positive nLDs increased in CCT alphaKO cells, suggesting that CCT alpha and PML compete for access to the nLD surface following oleate treatment. To determine if phosphorylation of CCT alpha regulates translocation to nLDs, phospho‐mimetic and phospho‐null S315/S319 and Y359/S362 mutants were expressed in CCT alphaKO cells and localization to nLDs was measured. In vitrophosphorylation assays of recombinant, dephosphorylated CCT alpha indicated that AMP‐dependent kinase phosphorylated CCT alpha S319, which could be involved in regulating CCT alpha activity on nLDs. In summary, we have demonstrated that oleate treatment induces nLD biogenesis in Caco2 cells and that CCT alpha activity regulates the biogenesis of both nLDs and cLDs.

The rate‐limiting enzyme in the CDP‐choline pathway is regulated by phosphorylation‐domain charge density

Eukaryotic cell membranes are primarily composed of phospholipids, the most abundant of which is phosphatidylcholine (PC). De novo synthesis of PC by the CDP‐choline pathway is under the control of the rate‐limiting enzyme CTP:phosphocholine cytidylyltransferase (CCT). The nuclear CCTa isoform is regulated by translocation to the inner nuclear membrane (INM) and nuclear lipid droplets (nLD) in response to lipid activators, such as oleate, or deficiency in PC content. CCTa association with the INM is mediated by a positively‐charged membrane‐binding M‐domain and a phosphorylated P‐domain containing 16 serine phospho‐sites. Mutation of all P‐domain serine residues to alanine (dephosphorylated mimic) enhanced association with nLDs in oleate‐treated U2OS cells, while mutation to glutamate (phosphorylated mimic) or mutation of M‐domain lysine residues ablated nLD association. However, it is unknown if phosphorylation regulation of CCTa association with the INM and nLDs involves specific serine residues or the net charge of the P‐domain. To address this question, we mutated 4 sets of 2‐3 serine residues in the P‐domain to alanine or aspartate. CCTa cDNAs expressing different combinations of these mutations were expressed in oleate‐treated U2OS cells and the frequency of nLD association was measured. The mutants were also expressed in CHO cells with a temperature‐sensitive CCTα isoform and INM translocation in response to oleate was determined. The greatest inhibitory effect on INM and nLD association was observed when all 4 sets of serine residues were mutated to aspartate and not by any one site, suggesting that net charge density of the P‐domain regulates affinity for the NE and nLD. Additionally, CCTa serine‐to‐alanine mutants expressed in U2OS cells were unstable compared to the corresponding aspartate mutants. We conclude that CCTa association with the INM and nLD is inhibited as overall P‐domain phosphorylation increases. In its active membrane‐associated, dephosphorylated state CCTa protein has decreased stability, a potently negative feedback mechanism to control PC synthesis. These results provide novel insight into how PC synthesis is regulated that will be essential for identifying kinases and phosphatases that act on CCTα.

Proteome analysis of nuclear Lipid‐Associated Promyelocytic Leukemia (PML) Structures (LAPS) Using Biotin Proximity Labelling

Lipid droplets (LD) are organelles composed of a phospholipid monolayer surrounding a core of and triacylglycerol and cholesterol ester. LD have a primary role in energy homeostasis and lipotoxic stress through the regulated storage and release of fatty acids. While LD are cytoplasmic, lipoprotein secreting cells harbor nuclear LD (nLD) that associate with PML, the structural protein of PML nuclear bodies (PML‐NB) that regulate DNA repair and stress responses. Knockout studies in U2OS cells indicated that the 50% of nLDs contain PML isoform II (PML‐II) and have a unique lipid and protein composition marked by the lack of canonical PML‐NB‐associated proteins, such as DAXX, SP100 and SUMO. Owing to their unique composition, we term these subnuclear lipid:protein domains, lipid‐associated PML structures (LAPS). To gain insight into LAPS function, we conducted proximity labeling and mass spectrometry (MS) studies using PML‐I and PML‐II modified by fusion to the ascorbate peroxidase APEX2 that uses biotin‐phenol to biotinylate proteins. U2OS and Huh7 cells expressing PML‐APEX2 fusion proteins were treated with and without oleate, and biotinylated proteins were isolated and analyzed by LC‐MS/MS. Using standard filtering techniques (CRAPome, SAINT, Bayesian false discovery rate) we identified several high confidence interacting proteins associated with the COPII coatomer complex, including Sec24A, Sec23IP, Sec24B, and Sec23B. These Sec23/24 proteins were associated with PML‐I and, to a lesser extent, PML‐II under oleate‐treated conditions. Sec23B was observed to associate with both PML‐NBs as well as LAPS by immunofluorescence microscopy in U2OS cells expressing PML‐II‐APEX2. The COPII pathway has previously been implicated in protein delivery to cytoplasmic LD setting a precedence for the interaction we observed with LAPS. Thus, given our preliminary data showing interaction between Sec 23/24 heterodimer, PML‐NBs and LAPS we are now pursuing the hypothesis that lipid membrane deformation and/or cargo transport activity in the nucleus by COPII proteins could play a role in lipid homeostasis.

Nuclear Lipid Droplet Birth during Replicative Stress.

The nuclear membrane defines the boundaries that confine, protect and shape the genome. As such, its blebbing, ruptures and deformations are known to compromise the integrity of genetic material. Yet, drastic transitions of the nuclear membrane such as its invagination towards the nucleoplasm or its capacity to emit nuclear lipid droplets (nLD) have not been evaluated with respect to their impact on genome dynamics. To begin assessing this, in this work we used Saccharomyces cerevisiae as a model to ask whether a selection of genotoxins can trigger the formation of nLD. We report that nLD formation is not a general feature of all genotoxins, but of those engendering replication stress. Exacerbation of endogenous replication stress by genetic tools also elicited nLD formation. When exploring the lipid features of the nuclear membrane at the base of this emission, we revealed a link with the unsaturation profile of its phospholipids and, for the first time, of its sterol content. We propose that stressed replication forks may stimulate nLD birth by anchoring to the inner nuclear membrane, provided that the lipid context is adequate. Further, we point to a transcriptional feed-back process that counteracts the membrane’s proneness to emit nLD. With nLD representing platforms onto which genome-modifying reactions can occur, our findings highlight them as important players in the response to replication stress.

Nuclear lipid droplets - how are they different from their cytoplasmic siblings?

Lipid droplets (LDs) in the cytoplasm are formed in the endoplasmic reticulum (ER) and are connected with various organelles, both structurally and functionally. This is in contrast to LDs in the nucleus, which are separated from organelles in the cytoplasm. How nuclear lipid droplets form and what function they have were not known for many years. Recent results have revealed that nuclear LDs in hepatocytes are derived from lipoprotein precursors in the ER lumen, whereas those in non-hepatocytes and budding yeast newly form in the inner nuclear membrane. Although nuclear LDs are far fewer in number than cytoplasmic LDs, the unique location appears to bestow upon them specific functions, which are potentially linked to nuclear biology. This Review will provide an overview of our current understanding of nuclear LDs, discuss how they are different from cytoplasmic LDs and highlight knowledge gaps that need to be filled in future studies.

Reprogrammed lipid metabolism protects inner nuclear membrane against unsaturated fat.

SummaryThe cell nucleus is surrounded by a double membrane. The lipid packing and viscosity of membranes is critical for their function and is tightly controlled by lipid saturation. Circuits regulating the lipid saturation of the outer nuclear membrane (ONM) and contiguous endoplasmic reticulum (ER) are known. However, how lipid saturation is controlled in the inner nuclear membrane (INM) has remained enigmatic. Using INM biosensors and targeted genetic manipulations, we show that increased lipid unsaturation causes a reprogramming of lipid storage metabolism across the nuclear envelope (NE). Cells induce lipid droplet (LD) formation specifically from the distant ONM/ER, whereas LD formation at the INM is suppressed. In doing so, unsaturated fatty acids are shifted away from the INM. We identify the transcription circuits that topologically reprogram LD synthesis and identify seipin and phosphatidic acid as critical effectors. Our study suggests a detoxification mechanism protecting the INM from excess lipid unsaturation.

The disassembly of lipid droplets in Chlamydomonas.

Lipid droplets (LDs) are ubiquitous and specialized organelles in eukaryotic cells. Consisting of a triacylglycerol core surrounded by a monolayer of membrane lipids, LDs are decorated with proteins and have myriad functions, from carbon/energy storage to membrane lipid remodeling and signal transduction. The biogenesis and turnover of LDs are therefore tightly coordinated with cellular metabolic needs in a fluctuating environment. Lipid droplet turnover requires remodeling of the protein coat, lipolysis, autophagy and fatty acid β-oxidation. Several key components of these processes have been identified in Chlamydomonas (Chlamydomonas reinhardtii), including the major lipid droplet protein, a CXC-domain containing regulatory protein, the phosphatidylethanolamine-binding DTH1 (DELAYED IN TAG HYDROLYSIS1), two lipases and two enzymes involved in fatty acid β-oxidation. Here, we review LD turnover and discuss its physiological significance in Chlamydomonas, a major model green microalga in research on algal oil.

Nuclear lipid droplets and nuclear damage in Caenorhabditis elegans.

Fat stored in the form of lipid droplets has long been considered a defining characteristic of cytoplasm. However, recent studies have shown that nuclear lipid droplets occur in multiple cells and tissues, including in human patients with fatty liver disease. The function(s) of stored fat in the nucleus has not been determined, and it is possible that nuclear fat is beneficial in some situations. Conversely, nuclear lipid droplets might instead be deleterious by disrupting nuclear organization or triggering aggregation of hydrophobic proteins. We show here that nuclear lipid droplets occur normally in C. elegans intestinal cells and germ cells, but appear to be associated with damage only in the intestine. Lipid droplets in intestinal nuclei can be associated with novel bundles of microfilaments (nuclear actin) and membrane tubules that might have roles in damage repair. To increase the normal, low frequency of nuclear lipid droplets in wild-type animals, we used a forward genetic screen to isolate mutants with abnormally large or abundant nuclear lipid droplets. Genetic analysis and cloning of three such mutants showed that the genes encode the lipid regulator SEIP-1/seipin, the inner nuclear membrane protein NEMP-1/Nemp1/TMEM194A, and a component of COPI vesicles called COPA-1/α-COP. We present several lines of evidence that the nuclear lipid droplet phenotype of copa-1 mutants results from a defect in retrieving mislocalized membrane proteins that normally reside in the endoplasmic reticulum. The seip-1 mutant causes most germ cells to have nuclear lipid droplets, the largest of which occupy more than a third of the nuclear volume. Nevertheless, the nuclear lipid droplets do not trigger apoptosis, and the germ cells differentiate into gametes that produce viable, healthy progeny. Thus, our results suggest that nuclear lipid droplets are detrimental to intestinal nuclei, but have no obvious deleterious effect on germ nuclei.