Seipin is required for converting nascent to mature lipid droplets

Cytoskeleton regulates lipid droplet fusion and lipid storage by controlling lipid droplet movement

Lipid droplets (LDs) are highly dynamic organelles that maintain cellular lipid homeostasis through size and number control. In adipose tissue, CIDEC plays a crucial role in LD fusion and lipid homeostasis. However, the regulatory factors and mechanisms of LD fusion remain largely unknown. Here, we established a high-throughput LD phenotypic screen on a compound library consisting of 2010 small molecules, and identified 11 cytoskeleton inhibitors that negatively regulate LD size. Using specific inhibitors against each of the three types of cytoskeleton, our data showed that the disruption of microtubules and microfilaments but not intermediate filaments limits CIDEC-mediated LD fusion and growth by reducing LD movement and LD-LD contact. The collective effect of microtubule inhibitors results in a small LD phenotype which favors lipolysis upon activation of cAMP-PKA pathway in adipocytes. Our findings demonstrate that cytoskeleton is involved in the process of LD fusion and growth, indicating their role in lipid storage metabolism. One-Sentence Summary: Cytoskeleton regulates lipid droplet fusion and lipid storage.

Tools to Dissect Lipid Droplet Regulation, Players, and Mechanisms

Spurred by the authors' own recent discovery of reactive metabolite-regulated nexuses involving lipid droplets (LDs), this perspective discusses the latest knowledge and multifaceted approaches toward deconstructing the function of these dynamic organelles, LD-associated localized signaling networks, and protein players. Despite accumulating knowledge surrounding protein families and pathways of conserved importance for LD homeostasis surveillance and maintenance across taxa, much remains to be understood at the molecular level. In particular, metabolic stress-triggered contextual changes in LD-proteins' localized functions, crosstalk with other organelles, and feedback signaling loops and how these are specifically rewired in disease states remain to be illuminated with spatiotemporal precision. We hope this perspective promotes an increased interest in these essential organelles and innovations of new tools and strategies to better understand context-specific LD regulation critical for organismal health.

Packaging "vegetable oils": Insights into plant lipid droplet proteins

Plant neutral lipids, also known as "vegetable oils", are synthesized within the endoplasmic reticulum (ER) membrane and packaged into subcellular compartments called lipid droplets (LDs) for stable storage in the cytoplasm. The biogenesis, modulation, and degradation of cytoplasmic LDs in plant cells are orchestrated by a variety of proteins localized to the ER, LDs, and peroxisomes. Recent studies of these LD-related proteins have greatly advanced our understanding of LDs not only as steady oil depots in seeds but also as dynamic cell organelles involved in numerous physiological processes in different tissues and developmental stages of plants. In the past 2 decades, technology advances in proteomics, transcriptomics, genome sequencing, cellular imaging and protein structural modeling have markedly expanded the inventory of LD-related proteins, provided unprecedented structural and functional insights into the protein machinery modulating LDs in plant cells, and shed new light on the functions of LDs in nonseed plant tissues as well as in unicellular algae. Here, we review critical advances in revealing new LD proteins in various plant tissues, point out structural and mechanistic insights into key proteins in LD biogenesis and dynamic modulation, and discuss future perspectives on bridging our knowledge gaps in plant LD biology.

Alteration of Lipid Metabolism in Hypoxic Cancer Cells

Due to uncontrolled cell proliferation and disrupted vascularization, many cancer cells in solid tumors have limited oxygen supply. The hypoxic microenvironments of tumors lead to metabolic reprogramming of cancer cells, contributing to therapy resistance and metastasis. To identify better targets for the effective removal of hypoxia-adaptive cancer cells, it is crucial to understand how cancer cells alter their metabolism in hypoxic conditions. Here, we studied lipid metabolic changes in cancer cells under hypoxia using coherent Raman scattering (CRS) microscopy. We discovered the accumulation of lipid droplets (LDs) in the endoplasmic reticulum (ER) in hypoxia. Time-lapse CRS microscopy revealed the release of old LDs and the reaccumulated LDs in the ER during hypoxia exposure. Additionally, we explored the impact of carbon sources on LD formation and found that MIA PaCa2 cells preferred fatty acid uptake for LD formation, while glucose was essential to alleviate lipotoxicity. Hyperspectral-stimulated Raman scattering (SRS) microscopy revealed a reduction in cholesteryl ester content and a decrease in lipid saturation levels of LDs in hypoxic MIA PaCa2 cancer cells. This alteration in LD content is linked to reduced efficacy of treatments targeting cholesteryl ester formation. This study unveils important lipid metabolic changes in hypoxic cancer cells, providing insights that could lead to better treatment strategies for hypoxia-resistant cancer cells.

Lipid droplets in central nervous system and functional profiles of brain cells containing lipid droplets in various diseases

Lipid droplets (LDs), serving as the convergence point of energy metabolism and multiple signaling pathways, have garnered increasing attention in recent years. Different cell types within the central nervous system (CNS) can regulate energy metabolism to generate or degrade LDs in response to diverse pathological stimuli. This article provides a comprehensive review on the composition of LDs in CNS, their generation and degradation processes, their interaction mechanisms with mitochondria, the distribution among different cell types, and the roles played by these cells-particularly microglia and astrocytes-in various prevalent neurological disorders. Additionally, we also emphasize the paradoxical role of LDs in post-cerebral ischemia inflammation and explore potential underlying mechanisms, aiming to identify novel therapeutic targets for this disease.

Dynamical modelling of lipid droplet formation suggests a key function of membrane phospholipids

Cells store triacylglycerol (TAG) within lipid droplets (LDs). A dynamic model describing complete LD formation at the endoplasmic reticulum (ER) membrane does not yet exist. A biochemical-biophysical model of LD synthesis is proposed. It describes the time-dependent accumulation of TAG in the ER membrane as the formation of a potential LD (pLD) bounded by spherical caps of the inner and outer monolayers of the membrane. The expansion rate of the pLD depends on the TAG supply, the elastic properties of the ER membrane, and the recruitment of phospholipids (PLs) to the cap-covering monolayers. Model simulations provided the following insights: (a) Marginal differences in the surface tension of the cap monolayers are sufficient to fully drive the expansion of the pLD towards the cytosol or lumen. (b) Selective reduction of PL supply to the luminal monolayer ensures stable formation of cytosolic LDs, irrespective of variations in the elasto-mechanical properties of the ER membrane. (c) The rate of TAG supply to the cytosolic monolayer has a major effect on the size and maturation time of LDs but has no significant effect on the TAG export per individual LD. The recruitment of additional PLs to the cap monolayers of pLDs critically controls the budding direction, size, and maturation time of LDs. The ability of cells to acquire additional LD initiation sites appears to be key to coping with acutely high levels of potentially toxic free fatty acids.

Diet-Induced Obesity Modulates Close-Packing of Triacylglycerols in Lipid Droplets of Adipose Tissue

Adipose-derived lipid droplets (LDs) are rich in triacylglycerols (TAGs), which regulate essential cellular processes, such as energy storage. Although TAG accumulation and LD expansion in adipocytes occur during obesity, how LDs dynamically package TAGs in response to excessive nutrients remains elusive. Here, we found that LD lipidomes display a remarkable increase in TAG acyl chain saturation under calorie-dense diets, turning them conducive to close-packing. Using high-resolution X-ray diffraction, solid-state NMR, and imaging, we show that beyond size expansion LDs from mice under varied obesogenic diets govern fat accumulation by packing TAGs in different crystalline polymorphs. Consistently, LDs and tissue stiffen for high-calorie-fed mice with more than a 2-fold increase in elastic moduli compared to normal diet. Our data suggest that in addition to expanding, adipocyte LDs undergo structural remodeling by close-packing rigid and highly saturated TAGs in response to caloric overload, as opposed to liquid TAGs in a low-calorie diet. This work provides insights into how lipid packing within LDs can allow for the rapid and optimal expansion of fat during the initial stages of obesity.

Targeted modulation of pennycress lipid droplet proteins impacts droplet morphology and seed oil content

Lipid droplets (LDs) are unusual organelles that have a phospholipid monolayer surface and a hydrophobic matrix. In oilseeds, this matrix is nearly always composed of triacylglycerols (TGs) for efficient storage of carbon and energy. Various proteins play a role in their assembly, stability and turnover, and even though the major structural oleosin proteins in seed LDs have been known for decades, the factors influencing LD formation and dynamics are still being uncovered mostly in the "model oilseed" Arabidopsis. Here we identified several key LD biogenesis proteins in the seeds of pennycress, a potential biofuel crop, that were correlated previously with seed oil content and characterized here for their participation in LD formation in transient expression assays and stable transgenics. One pennycress protein, the lipid droplet associated protein-interacting protein (LDIP), was able to functionally complement the Arabidopsis ldip mutant, emphasizing the close conservation of lipid storage among these two Brassicas. Moreover, loss-of-function ldip mutants in pennycress exhibited increased seed oil content without compromising plant growth, raising the possibility that LDIP or other LD biogenesis factors may be suitable targets for improving yields in oilseed crops more broadly.

Involvement of a battery of investigated genes in lipid droplet pathophysiology and associated comorbidities

Lipid droplets (LDs) are highly specialized energy storage organelles involved in the maintenance of lipid homoeostasis by regulating lipid flux within white adipose tissue (WAT). The physiological function of adipocytes and LDs can be compromised by mutations in several genes, leading to NEFA-induced lipotoxicity, which ultimately manifests as metabolic complications, predominantly in the form of dyslipidemia, ectopic fat accumulation, and insulin resistance. In this review, we delineate the effects of mutations and deficiencies in genes - CIDEC, PPARG, BSCL2, AGPAT2, PLIN1, LIPE, LMNA, CAV1, CEACAM1, and INSR - involved in lipid droplet metabolism and their associated pathophysiological impairments, highlighting their roles in the development of lipodystrophies and metabolic dysfunction.

Nuclear membrane: A key potential therapeutic target for lipid metabolism

Lipid homeostasis plays a pivotal role in cellular growth, necessitating the engagement of numerous lipid metabolism genes and the cohesive functioning of organelles. While the nucleus is traditionally recognized for its genetic roles, emerging evidence highlights its significant contribution to lipid homeostasis maintenance. Certain nuclear membrane proteins or associated proteins have the capacity to directly catalyze lipid synthesis or modification processes. Mutations in the genes encoding these proteins can lead to disrupted lipid metabolism, contributing to a spectrum of metabolic disorders. This article provides a comprehensive reviews of the investigations exploring the interplay between nuclear membrane proteins and lipid metabolism. Additionally, it delves into the heterogeneity of the nuclear membrane, positioning it as a novel therapeutic target for managing metabolic disorders and mitigating adverse drug reactions.

Molecular mechanisms of lipid droplets-mitochondria coupling in obesity and metabolic syndrome: insights and pharmacological implications

Abnormal lipid accumulation is a fundamental contributor to obesity and metabolic disorders. Lipid droplets (LDs) and mitochondria (MT) serve as organelle chaperones in lipid metabolism and energy balance. LDs play a crucial role in lipid storage and mobilization, working in conjunction with MT to regulate lipid metabolism within the liver, brown adipose tissue, and skeletal muscle, thereby maintaining metabolic homeostasis. The novelty of our review is the comprehensive description of LD and MT interaction mechanisms. We also focus on the current drugs that target this metabolism, which provide novel approaches for obesity and related metabolism disorder treatment.

CLSTN3B promotes lipid droplet maturation and lipid storage in mouse adipocytes

Interorganelle contacts facilitate material exchanges and sustain the structural and functional integrity of organelles. Lipid droplets (LDs) of adipocytes are responsible for energy storage and mobilization responding to body needs. LD biogenesis defects compromise the lipid-storing capacity of adipocytes, resulting in ectopic lipid deposition and metabolic disorders, yet how the uniquely large LDs in adipocytes attain structural and functional maturation is incompletely understood. Here we show that the mammalian adipocyte-specific protein CLSTN3B is crucial for adipocyte LD maturation. CLSTN3B employs an arginine-rich segment to promote extensive contact and hemifusion-like structure formation between the endoplasmic reticulum (ER) and LD, allowing ER-to-LD phospholipid diffusion during LD expansion. CLSTN3B ablation results in reduced LD surface phospholipid density, increased turnover of LD-surface proteins, and impaired LD functions. Our results establish the central role of CLSTN3B in the adipocyte-specific LD maturation pathway that enhances lipid storage and maintenance of metabolic health under caloric overload in mice of both sexes.

The evolving landscape of ER-LD contact sites

Lipid droplets (LDs) are evolutionarily conserved dynamic organelles that play an important role in cellular physiology. Growing evidence suggests that LD biogenesis occurs at discrete endoplasmic reticulum (ER) subdomains demarcated by the lipodystrophy protein, Seipin, lack of which impairs adipogenesis. However, the mechanisms of how these domains are selected is not completely known. These ER sites undergo ordered assembly of proteins and lipids to initiate LD biogenesis and facilitate establishment of ER-LD contact sites, a prerequisite for proper growth and maturation of droplets. LDs retain both physical and functional association with the ER throughout their lifecycle to facilitate bi-directional communication, such as exchange of proteins and lipids between the two organelles at these ER-LD contact sites. In recent years several molecular tethers have been identified that bridge ER and LDs together including few proteins that are found exclusively at these ER-LD contact interface. Here, we discuss recent advances in understanding the role of factors that ensure functionality of ER-LD contact site machinery for LD homeostasis.

Perilipin2 depletion causes lipid droplet enlargement in the ovarian corpus luteum in mice

Lipid droplets (LDs) are endoplasmic reticulum-derived organelles that store neutral lipids (mostly triglycerides and cholesterol esters) within a phospholipid monolayer and appear in most eukaryotic cells. Perilipins (PLINs, comprising PLIN1-5) are abundant LD-associated proteins with highly variable expression levels among tissues. Although PLINs are expressed in the mammalian ovaries, little is known about their subcellular localization and physiological functions. In this study, we investigated the localization of PLIN1-3 and their relationship with LD synthesis using mCherry-HPos reporter mice, thereby enabling the visualization of LD biogenesis in vivo. PLIN2 and PLIN3 were localized as puncta in granulosa cells with low levels of LD synthesis in developing follicles. This localization pattern was quite different from that of PLIN1, which was mainly localized in the theca and interstitial cells with high levels of LD synthesis. In the corpus luteum, where LD synthesis is highly induced, PLIN2 and PLIN3 are abundant in the particulate structures, whereas PLIN1 is poorly distributed. We also generated global Plin2-deficient mice using the CRSPR/Cas9 system and demonstrated that the lack of PLIN2 did not alter the distribution of PLIN1 and PLIN3 but unexpectedly induced LD enlargement in the corpus luteum. Collectively, our results suggest that the localization of PLIN1-3 is spatiotemporally regulated and that PLIN2 deficiency influences LD mobilization in the corpus luteum within the ovaries.

Lipid Droplets Big and Small: Basic Mechanisms That Make Them All

Lipid droplets (LDs) are dynamic storage organelles with central roles in lipid and energy metabolism. They consist of a core of neutral lipids, such as triacylglycerol, which is surrounded by a monolayer of phospholipids and specialized surface proteins. The surface composition determines many of the LD properties, such as size, subcellular distribution, and interaction with partner organelles. Considering the diverse energetic and metabolic demands of various cell types, it is not surprising that LDs are highly heterogeneous within and between cell types. Despite their diversity, all LDs share a common biogenesis mechanism. However, adipocytes have evolved specific adaptations of these basic mechanisms, enabling the regulation of lipid and energy metabolism at both the cellular and organismal levels. Here, we discuss recent advances in the understanding of both the general mechanisms of LD biogenesis and the adipocyte-specific adaptations controlling these fascinating organelles.

Lipid droplets sequester palmitic acid to disrupt endothelial ciliation and exacerbate atherosclerosis in male mice

Disruption of ciliary homeostasis in vascular endothelial cells has been implicated in the development of atherosclerosis. However, the molecular basis for the regulation of endothelial cilia during atherosclerosis remains poorly understood. Herein, we provide evidence in male mice that the accumulation of lipid droplets in vascular endothelial cells induces ciliary loss and contributes to atherosclerosis. Triglyceride accumulation in vascular endothelial cells differentially affects the abundance of free fatty acid species in the cytosol, leading to stimulated lipid droplet formation and suppressed protein S-palmitoylation. Reduced S-palmitoylation of ciliary proteins, including ADP ribosylation factor like GTPase 13B, results in the loss of cilia. Restoring palmitic acid availability, either through pharmacological inhibition of stearoyl-CoA desaturase 1 or a palmitic acid-enriched diet, significantly restores endothelial cilia and mitigates the progression of atherosclerosis. These findings thus uncover a previously unrecognized role of lipid droplets in regulating ciliary homeostasis and provide a feasible intervention strategy for preventing and treating atherosclerosis.

Imaging and proteomics toolkits for studying organelle contact sites

Organelle contact sites are regions where two heterologous membranes are juxtaposed by molecular tethering complexes. These contact sites are important in inter-organelle communication and cellular functional integration. However, visualizing these minute foci and identifying contact site proteomes have been challenging. In recent years, fluorescence-based methods have been developed to visualize the dynamic physical interaction of organelles while proximity labeling approaches facilitate the profiling of proteomes at contact sites. In this review, we explain the design principle for these contact site reporters: a dual-organelle interaction mechanism based on how endogenous tethers and/or tethering complexes localize to contact sites. We classify the contact site reporters into three categories: (i) single-protein systems, (ii) two-component systems with activated reporter signal upon organelle proximity, and (iii) reporters for contact site proteomes. We also highlight advanced imaging analysis with high temporal-spatial resolution and the use of machine-learning algorithms for detecting contact sites.

Neuronal lipid droplets play a conserved and sex-biased role in maintaining whole-body energy homeostasis

Lipids are essential for neuron development and physiology. Yet, the central hubs that coordinate lipid supply and demand in neurons remain unclear. Here, we combine invertebrate and vertebrate models to establish the presence and functional significance of neuronal lipid droplets (LD) in vivo. We find that LD are normally present in neurons in a non-uniform distribution across the brain, and demonstrate triglyceride metabolism enzymes and lipid droplet-associated proteins control neuronal LD formation through both canonical and recently-discovered pathways. Appropriate LD regulation in neurons has conserved and male-biased effects on whole-body energy homeostasis across flies and mice, specifically neurons that couple environmental cues with energy homeostasis. Mechanistically, LD-derived lipids support neuron function by providing phospholipids to sustain mitochondrial and endoplasmic reticulum homeostasis. Together, our work identifies a conserved role for LD as the organelle that coordinates lipid management in neurons, with implications for our understanding of mechanisms that preserve neuronal lipid homeostasis and function in health and disease.

PMP22 duplication dysregulates lipid homeostasis and plasma membrane organization in developing human Schwann cells

Charcot-Marie-Tooth disease type 1A (CMT1A) is the most common inherited peripheral neuropathy caused by a 1.5 Mb tandem duplication of chromosome 17 harbouring the PMP22 gene. This dose-dependent overexpression of PMP22 results in disrupted Schwann cell myelination of peripheral nerves. To obtain better insights into the underlying pathogenic mechanisms in CMT1A, we investigated the role of PMP22 duplication in cellular homeostasis in CMT1A mouse models and in patient-derived induced pluripotent stem cells differentiated into Schwann cell precursors (iPSC-SCPs). We performed lipidomic profiling and bulk RNA sequencing (RNA-seq) on sciatic nerves of two developing CMT1A mouse models and on CMT1A patient-derived iPSC-SCPs. For the sciatic nerves of the CMT1A mice, cholesterol and lipid metabolism was downregulated in a dose-dependent manner throughout development. For the CMT1A iPSC-SCPs, transcriptional analysis unveiled a strong suppression of genes related to autophagy and lipid metabolism. Gene ontology enrichment analysis identified disturbances in pathways related to plasma membrane components and cell receptor signalling. Lipidomic analysis confirmed the severe dysregulation in plasma membrane lipids, particularly sphingolipids, in CMT1A iPSC-SCPs. Furthermore, we identified reduced lipid raft dynamics, disturbed plasma membrane fluidity and impaired cholesterol incorporation and storage, all of which could result from altered lipid storage homeostasis in the patient-derived CMT1A iPSC-SCPs. Importantly, this phenotype could be rescued by stimulating autophagy and lipolysis. We conclude that PMP22 duplication disturbs intracellular lipid storage and leads to a more disordered plasma membrane owing to an alteration in the lipid composition, which might ultimately lead to impaired axo-glial interactions. Moreover, targeting lipid handling and metabolism could hold promise for the treatment of patients with CMT1A.

A fluorogenic complementation tool kit for interrogating lipid droplet-organelle interaction

Contact sites between lipid droplets and other organelles are essential for cellular lipid and energy homeostasis upon metabolic demands. Detection of these contact sites at the nanometer scale over time in living cells is challenging. We developed a tool kit for detecting contact sites based on fluorogen-activated bimolecular complementation at CONtact sites, FABCON, using a reversible, low-affinity split fluorescent protein, splitFAST. FABCON labels contact sites with minimal perturbation to organelle interaction. Via FABCON, we quantitatively demonstrated that endoplasmic reticulum (ER)- and mitochondria (mito)-lipid droplet contact sites are dynamic foci in distinct metabolic conditions, such as during lipid droplet biogenesis and consumption. An automated analysis pipeline further classified individual contact sites into distinct subgroups based on size, likely reflecting differential regulation and function. Moreover, FABCON is generalizable to visualize a repertoire of organelle contact sites including ER-mito. Altogether, FABCON reveals insights into the dynamic regulation of lipid droplet-organelle contact sites and generates new hypotheses for further mechanistical interrogation during metabolic regulation.

Interplay between lipid metabolism, lipid droplets and RNA virus replication

Lipids play essential roles in the cell as components of cellular membranes, signaling molecules, and energy storage sources. Lipid droplets are cellular organelles composed of neutral lipids, such as triglycerides and cholesterol esters, and are also considered as cellular energy reserves, yet new functions have been recently associated with these structures, such as regulators of oxidative stress and cellular lipotoxicity, as well as modulators of pathogen infection through immune regulation. Lipid metabolism and lipid droplets participate in the infection process of many RNA viruses and control their replication and assembly, among others. Here, we review and discuss the contribution of lipid metabolism and lipid droplets over the replication cycle of RNA viruses, altogether pointing out potentially new pharmacological antiviral targets associated with lipid metabolism.

The triglyceride-synthesizing enzyme diacylglycerol acyltransferase 2 modulates the formation of the hepatitis C virus replication organelle

The replication organelle of hepatitis C virus (HCV), called membranous web, is derived from the endoplasmic reticulum (ER) and mainly comprises double membrane vesicles (DMVs) that concentrate the viral replication complexes. It also tightly associates with lipid droplets (LDs), which are essential for virion morphogenesis. In particular acyl-CoA:diacylglycerol acyltransferase 1 (DGAT1), a rate-limiting enzyme in triglyceride synthesis, promotes early steps of virus assembly. The close proximity between ER membranes, DMVs and LDs therefore permits the efficient coordination of the HCV replication cycle. Here, we demonstrate that exaggerated LD accumulation due to the excessive expression of the DGAT1 isozyme, DGAT2, dramatically impairs the formation of the HCV membranous web. This effect depended on the enzymatic activity and ER association of DGAT2, whereas the mere LD accumulation was not sufficient to hamper HCV RNA replication. Our lipidomics data indicate that both HCV infection and DGAT2 overexpression induced membrane lipid biogenesis and markedly increased phospholipids with long chain polyunsaturated fatty acids, suggesting a dual use of these lipids and their possible competition for LD and DMV biogenesis. On the other hand, overexpression of DGAT2 depleted specific phospholipids, particularly oleyl fatty acyl chain-containing phosphatidylcholines, which, in contrast, are increased in HCV-infected cells and likely essential for viral infection. In conclusion, our results indicate that lipid exchanges occurring during LD biogenesis regulate the composition of intracellular membranes and thereby affect the formation of the HCV replication organelle. The potent antiviral effect observed in our DGAT2 overexpression system unveils lipid flux that may be relevant in the context of steatohepatitis, a hallmark of HCV infection, but also in physiological conditions, locally in specific subdomains of the ER membrane. Thus, LD formation mediated by DGAT1 and DGAT2 might participate in the spatial compartmentalization of HCV replication and assembly factories within the membranous web.

The lipid droplet assembly complex consists of seipin and four accessory factors in budding yeast

Seipin, a crucial protein for cellular lipid droplet (LD) assembly, oligomerizes at the interface between the endoplasmic reticulum and LDs to facilitate neutral lipid packaging. Using proximity labeling, we identified four proteins-Ldo45, Ldo16, Tgl4, and Pln1-that are recruited to the vicinity of yeast seipin, the Sei1-Ldb16 complex, exclusively when seipin function is intact, hence termed seipin accessory factors. Localization studies identified Tgl4 at the endoplasmic reticulum-LD contact site, in contrast to Ldo45, Ldo16, and Pln1 at the LD surface. Cells with compromised seipin function resulted in uneven distribution of these proteins with aberrant LDs, supporting a central role of seipin in orchestrating their association with the LD. Overexpression of any seipin accessory factor causes LD aggregation and affects a subset of LD protein distribution, highlighting the importance of their stoichiometry. Although single factor mutations show minor LD morphology changes, the combined mutations have additive effects. Lastly, we present evidence that seipin accessory factors assemble and interact with seipin in the absence of neutral lipids and undergo dynamical rearrangements during LD formation induction, with Ldo45 acting as a central hub recruiting other factors to interact with the seipin complex.

Adipogenin Dictates Adipose Tissue Expansion by Facilitating the Assembly of a Dodecameric Seipin Complex

Adipogenin (Adig) is an evolutionarily conserved microprotein and is highly expressed in adipose tissues and testis. Here, we identify Adig as a critical regulator for lipid droplet formation in adipocytes. We determine that Adig interacts directly with seipin, leading to the formation of a rigid complex. We solve the structure of the seipin/Adig complex by Cryo-EM at 2.98Å overall resolution. Surprisingly, seipin can form two unique oligomers, undecamers and dodecamers. Adig selectively binds to the dodecameric seipin complex. We further find that Adig promotes seipin assembly by stabilizing and bridging adjacent seipin subunits. Functionally, Adig plays a key role in generating lipid droplets in adipocytes. In mice, inducible overexpression of Adig in adipocytes substantially increases fat mass, with enlarged lipid droplets. It also elevates thermogenesis during cold exposure. In contrast, inducible adipocyte-specific Adig knockout mice manifest aberrant lipid droplet formation in brown adipose tissues and impaired cold tolerance.

Identification of genetic suppressors for a BSCL2 lipodystrophy pathogenic variant in Caenorhabditis elegans

Seipin (BSCL2), a conserved endoplasmic reticulum protein, plays a critical role in lipid droplet (LD) biogenesis and in regulating LD morphology, pathogenic variants of which are associated with Berardinelli-Seip congenital generalized lipodystrophy type 2 (BSCL2). To model BSCL2 disease, we generated an orthologous BSCL2 variant, seip-1(A185P), in Caenorhabditis elegans. In this study, we conducted an unbiased chemical mutagenesis screen to identify genetic suppressors that restore embryonic viability in the seip-1(A185P) mutant background. A total of five suppressor lines were isolated and recovered from the screen. The defective phenotypes of seip-1(A185P), including embryonic lethality and impaired eggshell formation, were significantly suppressed in each suppressor line. Two of the five suppressor lines also alleviated the enlarged LDs in the oocytes. We then mapped a suppressor candidate gene, lmbr-1, which is an ortholog of human limb development membrane protein 1 (LMBR1). The CRISPR/Cas9 edited lmbr-1 suppressor alleles, lmbr-1(S647F) and lmbr-1(P314L), both significantly suppressed embryonic lethality and defective eggshell formation in the seip-1(A185P) background. The newly identified suppressor lines offer valuable insights into potential genetic interactors and pathways that may regulate seipin in the lipodystrophy model.

Rab18 maintains homeostasis of subcutaneous adipose tissue to prevent obesity-induced metabolic disorders

Metabolically healthy obesity refers to obese individuals who do not develop metabolic disorders. These people store fat in subcutaneous adipose tissue (SAT) rather than in visceral adipose tissue (VAT). However, the molecules participating in this specific scenario remain elusive. Rab18, a lipid droplet (LD)-associated protein, mediates the contact between the endoplasmic reticulum (ER) and LDs to facilitate LD growth and maturation. In the present study, we show that the protein level of Rab18 is specifically upregulated in the SAT of obese people and mice. Rab18 adipocyte-specific knockout (Rab18 AKO) mice had a decreased volume ratio of SAT to VAT compared with wildtype mice. When subjected to high-fat diet (HFD), Rab18 AKO mice had increased ER stress and inflammation, reduced adiponectin, and decreased triacylglycerol (TAG) accumulation in SAT. In contrast, TAG accumulation in VAT, brown adipose tissue (BAT) or liver of Rab18 AKO mice had a moderate increase without ER stress stimulation. Rab18 AKO mice developed insulin resistance and systematic inflammation. Rab18 AKO mice maintained body temperature in response to acute and chronic cold induction with a thermogenic SAT, similar to the counterpart mice. Furthermore, Rab18-deficient 3T3-L1 adipocytes were more prone to palmitate-induced ER stress, indicating the involvement of Rab18 in alleviating lipid toxicity. Rab18 AKO mice provide a good animal model to investigate metabolic disorders such as impaired SAT. In conclusion, our studies reveal that Rab18 is a key and specific regulator that maintains the proper functions of SAT by alleviating lipid-induced ER stress.

Impact of Seipin in cholesterol mediated lipid droplet maturation; status of endoplasmic reticulum stress and lipophagy

The global prevalence of nonalcoholic fatty liver disease (NAFLD) defined by the increased number of lipid droplets (LDs) in hepatocytes, have risen continuously in parallel with the obesity. LDs and related proteins are known to affect cellular metabolism and signaling. Seipin, one of the most important LD-related proteins, plays a critical role in LD biogenesis. Although the role of adipose tissue-specific Seipin silencing is known, hepatocyte-specific silencing upon cholesterol-mediated lipid accumulation has not been investigated. In our study, we investigated the effect of Seipin on endoplasmic reticulum (ER) stress and lipophagy in cholesterol accumulated mouse hepatocyte cells. In this direction, cholesterol accumulation was induced by cholesterol-containing liposome, while Seipin mRNA and protein levels were reduced by siRNA. Our findings show that cholesterol containing liposome administration in hepatocytes increases both Seipin protein and number of large LDs. However Seipin silencing reduced the increase of cholesterol mediated large LDs and Glucose-regulated protein 78 (GRP78) mRNA. Additionally, lysosome-LD colocalization increased only in cells treated with cholesterol containing liposome, while the siRNA against Seipin did not lead any significant difference. According to our findings, we hypothesize that Seipin silencing in hepatocytes reduced cholesterol mediated LD maturation as well as GRP78 levels, but not lipophagy.

Rab1b facilitates lipid droplet growth by ER-to-lipid droplet targeting of DGAT2

Lipid droplets (LDs) comprise a triglyceride core surrounded by a lipid monolayer enriched with proteins, many of which function in LD homeostasis. How proteins are targeted to the growing LD is still unclear. Rab1b, a GTPase regulating secretory transport, was recently associated with targeting proteins to LDs in a Drosophila RNAi screen. LD formation was prevented in human hepatoma cells overexpressing dominant-negative Rab1b. We thus hypothesized that Rab1b recruits lipid-synthesizing enzymes, facilitating LD growth. Here, FRET between diacylglycerol acyltransferase 2 (DGAT2) and Rab1b and activity mutants of the latter demonstrated that Rab1b promotes DGAT2 ER to the LD surface redistribution. Last, alterations in LD metabolism and DGAT2 redistribution, consistent with Rab1b activity, were caused by mutations in the Rab1b-GTPase activating protein TBC1D20 in Warburg Micro syndrome (WARBM) model mice fibroblasts. These data contribute to our understanding of the mechanism of Rab1b in LD homeostasis and WARBM, a devastating autosomal-recessive disorder caused by mutations in TBC1D20.

Seipin deficiency-induced lipid dysregulation leads to hypomyelination-associated cognitive deficits via compromising oligodendrocyte precursor cell differentiation

Seipin is one key mediator of lipid metabolism that is highly expressed in adipose tissues as well as in the brain. Lack of Seipin gene, Bscl2, leads to not only severe lipid metabolic disorders but also cognitive impairments and motor disabilities. Myelin, composed mainly of lipids, facilitates nerve transmission and is important for motor coordination and learning. Whether Seipin deficiency-leaded defects in learning and motor coordination is underlined by lipid dysregulation and its consequent myelin abnormalities remains to be elucidated. In the present study, we verified the expression of Seipin in oligodendrocytes (OLs) and their precursors, oligodendrocyte precursor cells (OPCs), and demonstrated that Seipin deficiency compromised OPC differentiation, which led to decreased OL numbers, myelin protein, myelinated fiber proportion and thickness of myelin. Deficiency of Seipin resulted in impaired spatial cognition and motor coordination in mice. Mechanistically, Seipin deficiency suppressed sphingolipid metabolism-related genes in OPCs and caused morphological abnormalities in lipid droplets (LDs), which markedly impeded OPC differentiation. Importantly, rosiglitazone, one agonist of PPAR-gamma, substantially restored phenotypes resulting from Seipin deficiency, such as aberrant LDs, reduced sphingolipids, obstructed OPC differentiation, and neurobehavioral defects. Collectively, the present study elucidated how Seipin deficiency-induced lipid dysregulation leads to neurobehavioral deficits via impairing myelination, which may pave the way for developing novel intervention strategy for treating metabolism-involved neurological disorders.

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.

Molecular dynamics simulations of intracellular lipid droplets: a new tool in the toolbox

Lipid droplets (LDs) are ubiquitous intracellular organelles with a central role in multiple lipid metabolic pathways. However, identifying correlations between their structural properties and their biological activity has proved challenging, owing to their unique physicochemical properties as compared with other cellular membranes. In recent years, molecular dynamics (MD) simulations, a computational methodology allowing the accurate description of molecular assemblies down to their individual components, have been demonstrated to be a useful and powerful approach for studying LD structural and dynamical properties. In this short review, we attempt to highlight, as comprehensively as possible, how MD simulations have contributed to our current understanding of multiple molecular mechanisms involved in LD biology.

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.

A unifying mechanism for seipin-mediated lipid droplet formation

Lipid droplets (LDs) are dynamic organelles essential for cellular lipid homeostasis. Assembly of LDs occurs in the endoplasmic reticulum (ER), and the conserved ER membrane protein seipin emerged as a key player in this process. Here, we review recent advances provided by structural, biochemical, and in silico analysis that revealed mechanistic insights into the molecular role of the seipin complexes and led to an updated model for LD biogenesis. We further discuss how other ER components cooperate with seipin during LD biogenesis. Understanding the molecular mechanisms underlying seipin-mediated LD assembly is important to uncover the fundamental aspects of lipid homeostasis and organelle biogenesis and to provide hints on the pathogenesis of lipid storage disorders.

Emerging functions of the mitochondria-ER-lipid droplet three-way junction in coordinating lipid transfer, metabolism, and storage in cells

Over the past two decades, we have witnessed a growing appreciation for the importance of membrane contact sites (CS) in facilitating direct communication between organelles. CS are tiny regions where the membranes of two organelles meet but do not fuse and allow the transfer of metabolites between organelles, playing crucial roles in the coordination of cellular metabolic activities. The significant advancements in imaging techniques and molecular and cell biology research have revealed that CS are more complex than what originally thought, and as they are extremely dynamic, they can remodel their shape, composition, and functions in accordance with metabolic and environmental changes and can occur between more than two organelles. Here, we describe how recent studies led to the identification of a three-way mitochondria-ER-lipid droplet CS and discuss the emerging functions of these contacts in maintaining lipid storage, homeostasis, and balance. We also summarize the properties and functions of key protein components localized at the mitochondria-ER-lipid droplet interface, with a special focus on lipid transfer proteins. Understanding tripartite CS is essential for unraveling the complexities of inter-organelle communication and cooperation within cells.

Zooming into lipid droplet biology through the lens of electron microscopy

Electron microscopy (EM), in its various flavors, has significantly contributed to our understanding of lipid droplets (LD) as central organelles in cellular metabolism. For example, EM has illuminated that LDs, in contrast to all other cellular organelles, are uniquely enclosed by a single phospholipid monolayer, revealed the architecture of LD contact sites with different organelles, and provided near-atomic resolution maps of key enzymes that regulate neutral lipid biosynthesis and LD biogenesis. In this review, we first provide a brief history of pivotal findings in LD biology unveiled through the lens of an electron microscope. We describe the main EM techniques used in the context of LD research and discuss their current capabilities and limitations, thereby providing a foundation for utilizing suitable EM methodology to address LD-related questions with sufficient level of structural preservation, detail, and resolution. Finally, we highlight examples where EM has recently been and is expected to be instrumental in expanding the frontiers of LD biology.

An Automated Imaging-Based Screen for Genetic Modulators of ER Organisation in Cultured Human Cells

Hereditary spastic paraplegias (HSPs) are a heterogeneous group of mono-genetic inherited neurological disorders, whose primary manifestation is the disruption of the pyramidal system, observed as a progressive impaired gait and leg spasticity in patients. Despite the large list of genes linked to this group, which exceeds 80 loci, the number of cellular functions which the gene products engage is relatively limited, among which endoplasmic reticulum (ER) morphogenesis appears central. Mutations in genes encoding ER-shaping proteins are the most common cause of HSP, highlighting the importance of correct ER organisation for long motor neuron survival. However, a major bottleneck in the study of ER morphology is the current lack of quantitative methods, with most studies to date reporting, instead, on qualitative changes. Here, we describe and apply a quantitative image-based screen to identify genetic modifiers of ER organisation using a mammalian cell culture system. An analysis reveals significant quantitative changes in tubular ER and dense sheet ER organisation caused by the siRNA-mediated knockdown of HSP-causing genes ATL1 and RTN2. This screen constitutes the first attempt to examine ER distribution in cells in an automated and high-content manner and to detect genes which impact ER organisation.

A metabolically controlled contact site between vacuoles and lipid droplets in yeast

The lipid droplet (LD) organization proteins Ldo16 and Ldo45 affect multiple aspects of LD biology in yeast. They are linked to the LD biogenesis machinery seipin, and their loss causes defects in LD positioning, protein targeting, and breakdown. However, their molecular roles remained enigmatic. Here, we report that Ldo16/45 form a tether complex with Vac8 to create vacuole lipid droplet (vCLIP) contact sites, which can form in the absence of seipin. The phosphatidylinositol transfer protein (PITP) Pdr16 is a further vCLIP-resident recruited specifically by Ldo45. While only an LD subpopulation is engaged in vCLIPs at glucose-replete conditions, nutrient deprivation results in vCLIP expansion, and vCLIP defects impair lipophagy upon prolonged starvation. In summary, Ldo16/45 are multifunctional proteins that control the formation of a metabolically regulated contact site. Our studies suggest a link between LD biogenesis and breakdown and contribute to a deeper understanding of how lipid homeostasis is maintained during metabolic challenges.

Lipid Droplets: Formation, Degradation, and Their Role in Cellular Responses to Flavivirus Infections

Lipid droplets (LDs) are cellular organelles derived from the endoplasmic reticulum (ER), serving as lipid storage sites crucial for maintaining cellular lipid homeostasis. Recent attention has been drawn to their roles in viral replication and their interactions with viruses. However, the precise biological functions of LDs in viral replication and pathogenesis remain incompletely understood. To elucidate the interaction between LDs and viruses, it is imperative to comprehend the biogenesis of LDs and their dynamic interactions with other organelles. In this review, we explore the intricate pathways involved in LD biogenies within the cytoplasm, encompassing the uptake of fatty acid from nutrients facilitated by CD36-mediated membranous protein (FABP/FATP)-FA complexes, and FA synthesis via glycolysis in the cytoplasm and the TCL cycle in mitochondria. While LD biogenesis primarily occurs in the ER, matured LDs are intricately linked to multiple organelles. Viral infections can lead to diverse consequences in terms of LD status within cells post-infection, potentially involving the breakdown of LDs through the activation of lipophagy. However, the exact mechanisms underlying LD destruction or accumulation by viruses remain elusive. The significance of LDs in viral replication renders them effective targets for developing broad-spectrum antivirals. Moreover, considering that reducing neutral lipids in LDs is a strategy for anti-obesity treatment, LD depletion may not pose harm to cells. This presents LDs as promising antiviral targets for developing therapeutics that are minimally or non-toxic to the host.

Natural variation in Fatty Acid 9 is a determinant of fatty acid and protein content

Soybean is one of the most economically important crops worldwide and an important source of unsaturated fatty acids and protein for the human diet. Consumer demand for healthy fats and oils is increasing, and the global demand for vegetable oil is expected to double by 2050. Identification of key genes that regulate seed fatty acid content can facilitate molecular breeding of high-quality soybean varieties with enhanced fatty acid profiles. Here, we analysed the genetic architecture underlying variations in soybean seed fatty acid content using 547 accessions, including mainly landraces and cultivars from northeastern China. Through fatty acid profiling, genome re-sequencing, population genomics analyses, and GWAS, we identified a SEIPIN homologue at the FA9 locus as an important contributor to seed fatty acid content. Transgenic and multiomics analyses confirmed that FA9 was a key regulator of seed fatty acid content with pleiotropic effects on seed protein and seed size. We identified two major FA9 haplotypes in 1295 resequenced soybean accessions and assessed their phenotypic effects in a field planting of 424 accessions. Soybean accessions carrying FA9H2 had significantly higher total fatty acid contents and lower protein contents than those carrying FA9H1 . FA9H2 was absent in wild soybeans but present in 13% of landraces and 26% of cultivars, suggesting that it may have been selected during soybean post-domestication improvement. FA9 therefore represents a useful genetic resource for molecular breeding of high-quality soybean varieties with specific seed storage profiles.

Lipid droplets and cellular lipid flux

Lipid droplets are dynamic organelles that store neutral lipids, serve the metabolic needs of cells, and sequester lipids to prevent lipotoxicity and membrane damage. Here we review the current understanding of the mechanisms of lipid droplet biogenesis and turnover, the transfer of lipids and metabolites at membrane contact sites, and the role of lipid droplets in regulating fatty acid flux in lipotoxicity and cell death.

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.

CLSTN3B enhances adipocyte lipid droplet structure and function via endoplasmic reticulum contact

Interorganelle contacts facilitate material exchanges and sustain the structural and functional integrity of organelles. Lipid droplets (LDs) of adipocytes are responsible for energy storage and mobilization responding to body needs. LD biogenesis defects compromise the lipid-storing capacity of adipocytes, resulting in ectopic lipid deposition and metabolic disorders, yet how the uniquely large LDs in adipocytes attain structural and functional maturation is incompletely understood. Here we show that the mammalian adipocyte-specific protein CLSTN3B is crucial for adipocyte LD maturation. CLSTN3B employs an arginine-rich segment to promote extensive contact and hemifusion-like structure formation between the endoplasmic reticulum (ER) and LD, allowing ER-to-LD phospholipid diffusion during LD expansion. CLSTN3B ablation results in reduced LD surface phospholipid density, increased turnover of LD-surface proteins, and impaired LD functions. Our results establish the central role of CLSTN3B in the adipocyte-specific LD maturation pathway that enhances lipid storage and maintenance of metabolic health under caloric overload.

ER-LD Membrane Contact Sites: A Budding Area in the Pathogen Survival Strategy

The endoplasmic reticulum (ER) and lipid droplets (LDs) are essential organelles involved in lipid synthesis, storage, and transport. Physical membrane contacts between the ER and LDs facilitate lipid and protein exchange and thus play a critical role in regulating cellular lipid homeostasis. Recent research has revealed that ER-LD membrane contact sites are targeted by pathogens seeking to exploit host lipid metabolic processes. Both viruses and bacteria manipulate ER-LD membrane contact sites to enhance their replication and survival within the host. This review discusses the research advancements elucidating the mechanisms by which pathogens manipulate the ER-LD contacts through protein molecular mimicry and host cell protein manipulation, thereby hijacking host lipid metabolic processes to facilitate pathogenesis. Understanding the crosstalk between ER and LDs during infection provides deeper insight into host lipid regulation and uncovers potential therapeutic targets for treating infectious diseases.

Tld1 is a regulator of triglyceride lipolysis that demarcates a lipid droplet subpopulation

Cells store lipids in the form of triglyceride (TG) and sterol ester (SE) in lipid droplets (LDs). Distinct pools of LDs exist, but a pervasive question is how proteins localize to and convey functions to LD subsets. Here, we show that the yeast protein YDR275W/Tld1 (for TG-associated LD protein 1) localizes to a subset of TG-containing LDs and reveal it negatively regulates lipolysis. Mechanistically, Tld1 LD targeting requires TG, and it is mediated by two distinct hydrophobic regions (HRs). Molecular dynamics simulations reveal that Tld1's HRs interact with TG on LDs and adopt specific conformations on TG-rich LDs versus SE-rich LDs in yeast and human cells. Tld1-deficient yeast display no defect in LD biogenesis but exhibit elevated TG lipolysis dependent on lipase Tgl3. Remarkably, overexpression of Tld1, but not LD protein Pln1/Pet10, promotes TG accumulation without altering SE pools. Finally, we find that Tld1-deficient cells display altered LD mobilization during extended yeast starvation. We propose that Tld1 senses TG-rich LDs and regulates lipolysis on LD subpopulations.

Lipid droplet biogenesis in the ovary

Background

Lipid droplets (LDs) are organelles consisting of a central core of neutral lipids covered by a single layer of phospholipids and are found in most eukaryotic cells. Accumulating evidence suggests that LDs not only store neutral lipids but also coordinate with other organelles for lipid metabolism within cells.

Methods

This review focuses on the synthesis of LDs during follicular development and highlights the factors involved in the regulation of LD biogenesis within the ovary.

Main Findings

In the mammalian ovary, the presence of LDs has long been recognized mainly by morphological analysis. However, their distribution in the ovary varies according to the region and cell type; for example, LDs are abundant in the medulla, which has a rich blood vessel network, in interstitial cells, which are the site of steroid production, and surrounding growing follicles, while they are poor in granulosa cells within follicles. LDs are also enriched in the corpus luteum after ovulation and massively accumulate in atretic follicles during follicular growth. Furthermore, LD synthesis is synchronized with angiogenesis during follicular development.

Conclusion

Addressing the functional link between LD biogenesis and angiogenesis is essential for understanding the molecular basis underlying LD biology, as well as the ovarian dysfunction with metabolic disorders.

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.

Lipid content evaluation of Drosophila tumour associated haemocytes through Third Harmonic Generation measurements

Non-linear microscopy is a powerful imaging tool to examine structural properties and subcellular processes of various biological samples. The competence of Third Harmonic Generation (THG) includes the label free imaging with diffraction-limited resolution and three-dimensional visualization with negligible phototoxicity effects. In this study, THG records and quantifies the lipid content of Drosophila haemocytes, upon encountering normal or tumorigenic neural cells, in correlation with their shape or their state. We show that the lipid accumulations of adult haemocytes are similar before and after encountering normal cells. In contrast, adult haemocytes prior to their interaction with cancer cells have a low lipid index, which increases while they are actively engaged in phagocytosis only to decrease again when haemocytes become exhausted. This dynamic change in the lipid accrual of haemocytes upon encountering tumour cells could potentially be a useful tool to assess the phagocytic capacity or activation state of tumour-associated haemocytes.

Genetically encoded multimeric tags for subcellular protein localization in cryo-EM

Cryo-electron tomography (cryo-ET) allows for label-free high-resolution imaging of macromolecular assemblies in their native cellular context. However, the localization of macromolecules of interest in tomographic volumes can be challenging. Here we present a ligand-inducible labeling strategy for intracellular proteins based on fluorescent, 25-nm-sized, genetically encoded multimeric particles (GEMs). The particles exhibit recognizable structural signatures, enabling their automated detection in cryo-ET data by convolutional neural networks. The coupling of GEMs to green fluorescent protein-tagged macromolecules of interest is triggered by addition of a small-molecule ligand, allowing for time-controlled labeling to minimize disturbance to native protein function. We demonstrate the applicability of GEMs for subcellular-level localization of endogenous and overexpressed proteins across different organelles in human cells using cryo-correlative fluorescence and cryo-ET imaging. We describe means for quantifying labeling specificity and efficiency, and for systematic optimization for rare and abundant protein targets, with emphasis on assessing the potential effects of labeling on protein function.

Concept of lipid droplet biogenesis

Lipid droplets (LD) are functionally conserved fat storage organelles found in all cell types. LDs have a unique structure comprising of a hydrophobic core of neutral lipids (fat), triacylglycerol (TAG) and cholesterol esters (CE) surrounded by a phospholipid monolayer. LD surface is decorated by a multitude of proteins and enzymes rendering this compartment functional. Accumulating evidence suggests that LDs originate from discrete ER-subdomains, demarcated by the lipodystrophy protein seipin, however, the mechanisms of which are not well understood. LD biogenesis factors together with biophysical properties of the ER membrane orchestrate spatiotemporal regulation of LD nucleation and growth at specific ER subdomains in response to metabolic cues. Defects in LD formation manifests in several human pathologies, including obesity, lipodystrophy, ectopic fat accumulation, and insulin resistance. Here, we review recent advances in understanding the molecular events during initial stages of eukaryotic LD assembly and discuss the critical role of factors that ensure fidelity of this process.

A fluorogenic complementation tool kit for interrogating lipid droplet-organelle interaction

Contact sites between lipid droplets and other organelles are essential for cellular lipid and energy homeostasis. Detection of these contact sites at nanometer scale over time in living cells is challenging. Here, we developed a tool kit for detecting contact sites based on Fluorogen-Activated Bimolecular complementation at CONtact sites, FABCON, using a reversible, low affinity split fluorescent protein, splitFAST. FABCON labels contact sites with minimal perturbation to organelle interaction. Via FABCON, we quantitatively demonstrated that endoplasmic reticulum (ER)- and mitochondria (mito)-lipid droplet contact sites are dynamic foci in distinct metabolic conditions, such as during lipid droplet biogenesis and consumption. An automated analysis pipeline further classified individual contact sites into distinct subgroups based on size, likely reflecting differential regulation and function. Moreover, FABCON is generalizable to visualize a repertoire of organelle contact sites including ER-mito. Altogether, FABCON reveals insights into the dynamic regulation of lipid droplet-organelle contact sites and generates new hypotheses for further mechanistical interrogation during metabolic switch.