Structural and functional assessment of perilipin 2 lipid binding domain(s)

Anti-adipogenic and anti-obesity effects of morroniside in vitro and in vivo

Obesity is a multifaceted medical condition characterized by the pathological accumulation of excessive lipids in the body. We investigated the effects of morroniside, a bioactive compound derived from Cornus officinalis, on adipogenesis. We used a preadipocyte 3T3-L1 stable cell line and primary cultured adipose-derived stem cells (ADSCs) in vitro and ovariectomized (OVX) and a high-fat diet (HFD)-fed obese mouse model in vivo. Preadipocyte 3T3-L1 cells and ADSCs incubated with morroniside during adipocyte differentiation and obese mice subjected to OVX and HFD received oral morroniside treatment for 12 weeks. Morroniside treatment significantly reduced adipocyte differentiation and fatty acid accumulation and downregulated adipogenesis-related gene expression, concomitant with a decrease in triglyceride content and an increase in glycerol release in cells. The results of the in vivo study showed that morroniside ameliorated obesity-related phenotypes by reducing body weight gain, hepatic steatosis, and adipose tissue in obese mice. These findings suggest that morroniside is a promising compound for preventing and treating obesity.

Molecular mechanisms of perilipin protein function in lipid droplet metabolism

Perilipins are abundant lipid droplet (LD) proteins present in all metazoans and also in Amoebozoa and fungi. Humans express five perilipins, which share a similar domain organization: an amino-terminal PAT domain and an 11-mer repeat region, which can fold into amphipathic helices that interact with LDs, followed by a structured carboxy-terminal domain. Variations of this organization that arose during vertebrate evolution allow for functional specialization between perilipins in relation to the metabolic needs of different tissues. We discuss how different features of perilipins influence their interaction with LDs and their cellular targeting. PLIN1 and PLIN5 play a direct role in lipolysis by regulating the recruitment of lipases to LDs and LD interaction with mitochondria. Other perilipins, particularly PLIN2, appear to protect LDs from lipolysis, but the molecular mechanism is not clear. PLIN4 stands out with its long repetitive region, whereas PLIN3 is most widely expressed and is used as a nascent LD marker. Finally, we discuss the genetic variability in perilipins in connection with metabolic disease, prominent for PLIN1 and PLIN4, underlying the importance of understanding the molecular function of perilipins.

Binding of perilipin 3 to membranes containing diacylglycerol is mediated by conserved residues within its PAT domain

Perilipins (PLINs) constitute an evolutionarily conserved family of proteins that specifically associate with the surface of lipid droplets (LDs). These proteins function in LD biogenesis and lipolysis and help to stabilize the surface of LDs. PLINs are typically composed of three different protein domains. They share an N-terminal PAT domain of unknown structure and function, a central region containing 11-mer repeats that form amphipathic helices, and a C-terminal domain that adopts a 4-helix bundle structure. How exactly these three distinct domains contribute to PLIN function remains to be determined. Here, we show that the N-terminal PAT domain of PLIN3 binds diacylglycerol (DAG), the precursor to triacylglycerol, a major storage lipid of LDs. PLIN3 and its PAT domain alone bind liposomes with micromolar affinity and PLIN3 binds artificial LDs containing low concentrations of DAG with nanomolar affinity. The PAT domain of PLIN3 is predicted to adopt an amphipathic triangular shaped structure. In silico ligand docking indicates that DAG binds to one of the highly curved regions within this domain. A conserved aspartic acid residue in the PAT domain, E86, is predicted to interact with DAG, and we found that its substitution abrogates high affinity binding of DAG as well as DAG-stimulated association with liposome and artificial LDs. These results indicate that the PAT domain of PLINs harbor specific lipid-binding properties that are important for targeting these proteins to the surface of LDs and to ER membrane domains enriched in DAG to promote LD formation.

Functional Pdgfra fibroblast heterogeneity in normal and fibrotic mouse lung

Aberrant fibroblast function plays a key role in the pathogenesis of idiopathic pulmonary fibrosis, a devastating disease of unrelenting extracellular matrix deposition in response to lung injury. Platelet-derived growth factor α-positive (Pdgfra+) lipofibroblasts (LipoFBs) are essential for lung injury response and maintenance of a functional alveolar stem cell niche. Little is known about the effects of lung injury on LipoFB function. Here, we used single-cell RNA-Seq (scRNA-Seq) technology and PdgfraGFP lineage tracing to generate a transcriptomic profile of Pdgfra+ fibroblasts in normal and injured mouse lungs 14 days after bleomycin exposure, generating 11 unique transcriptomic clusters that segregated according to treatment. While normal and injured LipoFBs shared a common gene signature, injured LipoFBs acquired fibrogenic pathway activity with an attenuation of lipogenic pathways. In a 3D organoid model, injured Pdgfra+ fibroblast-supported organoids were morphologically distinct from those cultured with normal fibroblasts, and scRNA-Seq analysis suggested distinct transcriptomic changes in alveolar epithelia supported by injured Pdgfra+ fibroblasts. In summary, while LipoFBs in injured lung have not migrated from their niche and retain their lipogenic identity, they acquire a potentially reversible fibrogenic profile, which may alter the kinetics of epithelial regeneration and potentially contribute to dysregulated repair, leading to fibrosis.

Regulation of lipid droplets and cholesterol metabolism in adrenal cortical cells

The adrenal gland is composed of two distinctly different endocrine moieties. The interior medulla consists of neuroendocrine chromaffin cells that secrete catecholamines like adrenaline and noradrenaline, while the exterior cortex consists of steroidogenic cortical cells that produce steroid hormones, such as mineralocorticoids (aldosterone), glucocorticoids (cortisone and cortisol) and androgens. Synthesis of steroid hormones in cortical cells requires substantial amounts of cholesterol, which is the common precursor for steroidogenesis. Cortical cells may acquire cholesterol from de novo synthesis and uptake from circulating low- and high-density lipoprotein particles (LDL and HDL). As cholesterol is part of the plasma membrane in all mammalian cells and an important regulator of membrane fluidity, cellular levels of free cholesterol are tightly regulated. To ensure a robust supply of cholesterol for steroidogenesis and to avoid cholesterol toxicity, cortical cells store large amounts of cholesterol as cholesteryl esters in intracellular lipid droplets. Cortical steroidogenesis relies on both mobilization of cholesterol from lipid droplets and constant uptake of circulating cholesterol to replenish lipid droplet stores. This chapter will describe mechanisms involved in cholesterol uptake, cholesteryl ester synthesis, lipid droplet formation, hydrolysis of stored cholesteryl esters, as well as their impact on steroidogenesis. Additionally, animal models and human diseases characterized by altered cortical cholesteryl ester storage, with or without abnormal steroidogenesis, will be discussed.

Structural insights into perilipin 3 membrane association in response to diacylglycerol accumulation

Lipid droplets (LDs) are dynamic organelles that contain an oil core mainly composed of triglycerides (TAG) that is surrounded by a phospholipid monolayer and LD-associated proteins called perilipins (PLINs). During LD biogenesis, perilipin 3 (PLIN3) is recruited to nascent LDs as they emerge from the endoplasmic reticulum. Here, we analyze how lipid composition affects PLIN3 recruitment to membrane bilayers and LDs, and the structural changes that occur upon membrane binding. We find that the TAG precursors phosphatidic acid and diacylglycerol (DAG) recruit PLIN3 to membrane bilayers and define an expanded Perilipin-ADRP-Tip47 (PAT) domain that preferentially binds DAG-enriched membranes. Membrane binding induces a disorder to order transition of alpha helices within the PAT domain and 11-mer repeats, with intramolecular distance measurements consistent with the expanded PAT domain adopting a folded but dynamic structure upon membrane binding. In cells, PLIN3 is recruited to DAG-enriched ER membranes, and this requires both the PAT domain and 11-mer repeats. This provides molecular details of PLIN3 recruitment to nascent LDs and identifies a function of the PAT domain of PLIN3 in DAG binding.

Inhibitory Effects of Loganin on Adipogenesis In Vitro and In Vivo

Obesity is characterized by the excessive accumulation of mature adipocytes that store surplus energy in the form of lipids. In this study, we investigated the inhibitory effects of loganin on adipogenesis in mouse preadipocyte 3T3-L1 cells and primary cultured adipose-derived stem cells (ADSCs) in vitro and in mice with ovariectomy (OVX)- and high-fat diet (HFD)-induced obesity in vivo. For an in vitro study, loganin was co-incubated during adipogenesis in both 3T3-L1 cells and ADSCs, lipid droplets were evaluated by oil red O staining, and adipogenesis-related factors were assessed by qRT-PCR. For in vivo studies, mouse models of OVX- and HFD-induced obesity were orally administered with loganin, body weight was measured, and hepatic steatosis and development of excessive fat were evaluated by histological analysis. Loganin treatment reduced adipocyte differentiation by accumulating lipid droplets through the downregulation of adipogenesis-related factors, including peroxisome proliferator-activated receptor γ (Pparg), CCAAT/enhancer-binding protein α (Cebpa), perilipin 2 (Plin2), fatty acid synthase (Fasn), and sterol regulatory element binding transcription protein 1 (Srebp1). Loganin administration prevented weight gain in mouse models of obesity induced by OVX and HFD. Further, loganin inhibited metabolic abnormalities, such as hepatic steatosis and adipocyte enlargement, and increased the serum levels of leptin and insulin in both OVX- and HFD-induced obesity models. These results suggest that loganin is a potential candidate for preventing and treating obesity.

Effects of carnosic acid on arsenic-induced liver injury in mice: A comparative transcriptomics analysis

BACKGROUND

Long-term chronic exposure to arsenic can cause different degrees of liver injury. Till date, its molecular mechanism has not meant fully elucidated. Evidence indicates that Carnosic acid (CA) has a protective role in arsenic-induced liver injury. This study aimed to reveal the potential targets and evaluate the potential effect of CA intervention at transcriptional level, and provide reference for the intervention of arsenic-induced liver injury.

METHODS

Arsenic-induced liver injury and CA intervention models were established in C57BL/6 mice. RNA sequencing technique was carried out to obtain the differentially expressed gene (DEG) profiles. The common covariant DEGs between arsenic induction and CA intervention was screened by comparative transcriptomic analysis methods. QRT-PCR was used to verify the covariant DEGs.

RESULTS

Transcriptome results showed that 220 DEGs were identified after arsenic induction. 267 DEGs were identified after CA intervention (|fold change| > 2.0 and adjusted P < 0.05). 42 covariant DEGs were discovered between the comparison of "AS vs Control" and "AS & CA vs AS". In addition, hub gene analysis revealed a total of 8 covariant DEGs (Ehhadh, Fgf21, Cyp2b10, Plin2, Aacs, Cyp7a1, Per2 and Mylip). The mRNA expressions of Fgf21 and Plin2 were significantly increased (P < 0.05) and the mRNA expressions of Cyp2b10, Cyp7a1, Per2 and Mylip were significantly decreased (P < 0.05) after arsenic induction. On the contrary, the changes of these DEGs were reversed after CA intervention.

CONCLUSION

The present study would be helpful to understand the potential health effects of arsenic-induced liver injury and identify new potential targets, and provide a reference for the intervention of CA.

Transcriptomics Reveals Discordant Lipid Metabolism Effects between In Vitro Models Exposed to Elafibranor and Liver Samples of NAFLD Patients after Bariatric Surgery

BACKGROUND AND AIMS

Non-alcoholic steatohepatitis (NASH) is a life-threatening stage of non-alcoholic fatty liver disease (NAFLD) for which no drugs have been approved. We have previously shown that human-derived hepatic in vitro models can be used to mimic key cellular mechanisms involved in the progression of NASH. In the present study, we first characterize the transcriptome of multiple in vitro NASH models. Subsequently, we investigate how elafibranor, which is a peroxisome proliferator-activated receptor (PPAR)-α/δ agonist that has recently failed a phase 3 clinical trial as a potential anti-NASH compound, modulates the transcriptome of these models. Finally, we compare the elafibranor-induced gene expression modulation to transcriptome data of patients with improved/resolved NAFLD/NASH upon bariatric surgery, which is the only proven clinical NASH therapy.

METHODS

Human whole genome microarrays were used for the transcriptomics evaluation of hepatic in vitro models. Comparison to publicly available clinical datasets was conducted using multiple bioinformatic application tools.

RESULTS

Primary human hepatocytes (PHH), HepaRG, and human skin stem cell-derived hepatic progenitors (hSKP-HPC) exposed to NASH-inducing triggers exhibit up to 35% overlap with datasets of liver samples from NASH patients. Exposure of the in vitro NASH models to elafibranor partially reversed the transcriptional modulations, predicting an inhibition of toll-like receptor (TLR)-2/4/9-mediated inflammatory responses, NFκB-signaling, hepatic fibrosis, and leukocyte migration. These transcriptomic changes were also observed in the datasets of liver samples of patients with resolved NASH. Peroxisome Proliferator Activated Receptor Alpha (PPARA), PPARG Coactivator 1 Alpha (PPARGC1A), and Sirtuin 1 (SIRT1) were identified as the major common upstream regulators upon exposure to elafibranor. Analysis of the downstream mechanistic networks further revealed that angiopoietin Like 4 (ANGPTL4), pyruvate dehydrogenase kinase 4 (PDK4), and perilipin 2 (PLIN2), which are involved in the promotion of hepatic lipid accumulation, were also commonly upregulated by elafibranor in all in vitro NASH models. Contrarily, these genes were not upregulated in liver samples of patients with resolved NASH.

CONCLUSION

Transcriptomics comparison between in vitro NASH models exposed to elafibranor and clinical datasets of NAFLD patients after bariatric surgery reveals commonly modulated anti-inflammatory responses, but discordant modulations of key factors in lipid metabolism. This discordant adverse effect of elafibranor deserves further investigation when assessing PPAR-α/δ agonism as a potential anti-NASH therapy.

Perilipins at a glance

Lipid droplets (LDs) are ubiquitous organelles that store and supply lipids for energy metabolism, membrane synthesis and production of lipid-derived signaling molecules. While compositional differences in the phospholipid monolayer or neutral lipid core of LDs impact their metabolism and function, the proteome of LDs has emerged as a major influencer in all aspects of LD biology. The perilipins (PLINs) are the most studied and abundant proteins residing on the LD surface. This Cell Science at a Glance and the accompanying poster summarize our current knowledge of the common and unique features of the mammalian PLIN family of proteins, the mechanisms through which they affect cell metabolism and signaling, and their links to disease.

Validating an artificial organelle: Studies of lipid droplet-specific proteins on adiposome platform

New strategies are urgently needed to characterize the functions of the lipid droplet (LD). Here, adiposome, an artificial LD mimetic platform, was validated by comparative in vitro bioassays. Scatchard analysis found that the binding of perilipin 2 (PLIN2) to the adiposome surface was saturable. Phosphatidylinositol (PtdIns) was found to inhibit PLIN2 binding while it did not impede perilipin 3 (PLIN3). Structural analysis combined with mutagenesis revealed that the 73^rd glutamic acid of PLIN2 is significant for the effect of PtdIns on the PLIN2 binding. Furthermore, adiposome was also found to be an ideal platform for in situ enzymatic activity measurement of adipose triglyceride lipase (ATGL). The significant serine mutants of ATGL were found to cause the loss of lipase activity. Our study demonstrates the adiposome as a powerful, manipulatable model system that mimics the function of LD for binding and enzymatic activity studies of LD proteins in vitro.

Choline kinase alpha 2 acts as a protein kinase to promote lipolysis of lipid droplets

Lipid droplets are important for cancer cell growth and survival. However, the mechanism underlying the initiation of lipid droplet lipolysis is not well understood. We demonstrate here that glucose deprivation induces the binding of choline kinase (CHK) α2 to lipid droplets, which is sequentially mediated by AMPK-dependent CHKα2 S279 phosphorylation and KAT5-dependent CHKα2 K247 acetylation. Importantly, CHKα2 with altered catalytic domain conformation functions as a protein kinase and phosphorylates PLIN2 at Y232 and PLIN3 at Y251. The phosphorylated PLIN2/3 dissociate from lipid droplets and are degraded by Hsc70-mediated autophagy, thereby promoting lipid droplet lipolysis, fatty acid oxidation, and brain tumor growth. In addition, levels of CHKα2 S279 phosphorylation, CHKα2 K247 acetylation, and PLIN2/3 phosphorylation are positively correlated with one another in human glioblastoma specimens and are associated with poor prognosis in glioblastoma patients. These findings underscore the role of CHKα2 as a protein kinase in lipolysis and glioblastoma development.

Rare Variants in Autophagy and Non-Autophagy Genes in Late-Onset Pompe Disease: Suggestions of Their Disease-Modifying Role in Two Italian Families

Pompe disease is an autosomal recessive disorder caused by a deficiency in the enzyme acid alpha-glucosidase. The late-onset form of Pompe disease (LOPD) is characterized by a slowly progressing proximal muscle weakness, often involving respiratory muscles. In LOPD, the levels of GAA enzyme activity and the severity of the clinical pictures may be highly variable among individuals, even in those who harbour the same combination of GAA mutations. The result is an unpredictable genotype–phenotype correlation. The purpose of this study was to identify the genetic factors responsible for the progression, severity and drug response in LOPD. We report here on a detailed clinical, morphological and genetic study, including a whole exome sequencing (WES) analysis of 11 adult LOPD siblings belonging to two Italian families carrying compound heterozygous GAA mutations. We disclosed a heterogeneous pattern of myopathic impairment, associated, among others, with cardiac defects, intracranial vessels abnormality, osteoporosis, vitamin D deficiency, obesity and adverse response to enzyme replacement therapy (ERT). We identified deleterious variants in the genes involved in autophagy, immunity and bone metabolism, which contributed to the severity of the clinical symptoms observed in the LOPD patients. This study emphasizes the multisystem nature of LOPD and highlights the polygenic nature of the complex phenotype disclosed in these patients.

Differentially expressed proteins obtained by label-free quantitative proteomic analysis reveal affected biological processes and functions in Western diet-induced steatohepatitis

Nonalcoholic steatohepatitis (NASH) is a pathological manifestation with a progressive incidence in response to the epidemic of hepatic steatosis caused primarily by excessive energy intake. The present study unravels affected biological processes and functions by the presence of NASH in rats using a label-free quantitative proteomic strategy. NASH was induced by a Western high-sugar and high-fat diet for 20 weeks. The liver tissue was collected for histology and for a mass spectrometry-based proteomic protocol. The NASH group showed severe lipidosis, hepatocyte ballooning, and the presence of collagen deposition. Among upregulated proteins in NASH perilipin-2 (Plin-2; F6QBA3; difference [diff]: 2.29), ferritin heavy (Fth1; Q66HI5; diff: 2.19) and light (Ftl1; P02793; diff: 1.75) chains, macrophage migration inhibitory factor 1 (Mif; P30904; diff: 1.69), and fibronectin (Fn1; F1LST1; diff: 0.35) were observed, whereas among downregulated proteins, plectin (Q6S399; diff: -3.34), some Cyp2 family proteins of the cytochrome P450 complex, glutathione S-transferases, flavin-containing monooxygenase 1 (Fmo1; P36365; diff: -2.08), acetyl-CoA acetyltransferase 2 (Acat2; Q5XI22; diff: -2.25), acyl-CoA oxidase 2 (Acox2; F1LNW3; diff: -1.59), and acyl-CoA oxidase 3 (Acox3; F1M9A7; diff: -2.41) were observed. Also, biological processes and functions such as LPS/IL-1 inhibition of RXR, fatty acid metabolism, Nrf2-mediated oxidative stress response, xenobiotic metabolism, and PXR/RXR and CAR/RXR activations were predicted to be affected. In conclusion, the liver of rats with NASH induced by Western diet shows a decreased capacity of metabolizing lipids, fatty acids, and xenobiotic compounds that predispose fibrosis development.

Rab18 binds PLIN2 and ACSL3 to mediate lipid droplet dynamics

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

The major cap-binding protein eIF4E regulates lipid homeostasis and diet-induced obesity

Obesity is a global epidemic leading to increased mortality and susceptibility to comorbidities, with few viable therapeutic interventions. A hallmark of disease progression is the ectopic deposition of lipids in the form of lipid droplets in vital organs such as the liver. However, the mechanisms underlying the dynamic storage and processing of lipids in peripheral organs remain an outstanding question. Here, we show an unexpected function for the major cap-binding protein, eIF4E, in high-fat-diet-induced obesity. In response to lipid overload, select networks of proteins involved in fat deposition are altered in eIF4E-deficient mice. Specifically, distinct messenger RNAs involved in lipid metabolic processing and storage pathways are enhanced at the translation level by eIF4E. Failure to translationally upregulate these mRNAs results in increased fatty acid oxidation, which enhances energy expenditure. We further show that inhibition of eIF4E phosphorylation genetically-and by a potent clinical compound-restrains weight gain following intake of a high-fat diet. Together, our study uncovers translational control of lipid processing as a driver of high-fat-diet-induced weight gain and provides a pharmacological target to treat obesity.

Hepatic lipid droplets: A balancing act between energy storage and metabolic dysfunction in NAFLD

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) is defined by the abundance of lipid droplets (LDs) in hepatocytes. While historically considered simply depots for energy storage, LDs are increasingly recognized to impact a wide range of biological processes that influence cellular metabolism, signaling, and function. While progress has been made toward understanding the factors leading to LD accumulation (i.e. steatosis) and its progression to advanced stages of NAFLD and/or systemic metabolic dysfunction, much remains to be resolved.

SCOPE OF REVIEW

This review covers many facets of LD biology. We provide a brief overview of the major pathways of lipid accretion and degradation that contribute to steatosis and how they are altered in NAFLD. The major focus is on the relationship between LDs and cell function and the detailed mechanisms that couple or uncouple steatosis from the severity and progression of NAFLD and systemic comorbidities. The importance of specific lipids and proteins within or on LDs as key components that determine whether LD accumulation is linked to cellular and metabolic dysfunction is presented. We discuss emerging areas of LD biology and future research directions that are needed to advance our understanding of the role of LDs in NAFLD etiology.

MAJOR CONCLUSIONS

Impairments in LD breakdown appear to contribute to disease progression, but inefficient incorporation of fatty acids (FAs) into LD-containing triacylglycerol (TAG) and the consequential changes in FA partitioning also affect NAFLD etiology. Increased LD abundance in hepatocytes does not necessarily equate to cellular dysfunction. While LD accumulation is the prerequisite step for most NAFLD cases, the protein and lipid composition of LDs are critical factors in determining the progression from simple steatosis. Further defining the detailed molecular mechanisms linking LDs to metabolic dysfunction is important for designing effective therapeutic approaches targeting NAFLD and its comorbidities.

Cell type-specific lipid storage changes in Parkinson's disease patient brains are recapitulated by experimental glycolipid disturbance

Neurons are dependent on proper trafficking of lipids to neighboring glia for lipid exchange and disposal of potentially lipotoxic metabolites, producing distinct lipid distribution profiles among various cell types of the central nervous system. Little is known of the cellular distribution of neutral lipids in the substantia nigra (SN) of Parkinson's disease (PD) patients and its relationship to inflammatory signaling. This study aimed to determine human PD SN neutral lipid content and distribution in dopaminergic neurons, astrocytes, and microglia relative to age-matched healthy subject controls. The results show that while total neutral lipid content was unchanged relative to age-matched controls, the levels of whole SN triglycerides were correlated with inflammation-attenuating glycoprotein non-metastatic melanoma protein B (GPNMB) signaling in human PD SN. Histological localization of neutral lipids using a fluorescent probe (BODIPY) revealed that dopaminergic neurons and midbrain microglia significantly accumulated intracellular lipids in PD SN, while adjacent astrocytes had a reduced lipid load overall. This pattern was recapitulated by experimental in vivo inhibition of glucocerebrosidase activity in mice. Agents or therapies that restore lipid homeostasis among neurons, astrocytes, and microglia could potentially correct PD pathogenesis and disease progression.

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

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

Lipid Droplet-Derived Monounsaturated Fatty Acids Traffic via PLIN5 to Allosterically Activate SIRT1

Lipid droplets (LDs) provide a reservoir for triacylglycerol storage and are a central hub for fatty acid trafficking and signaling in cells. Lipolysis promotes mitochondrial biogenesis and oxidative metabolism via a SIRT1/PGC-1α/PPARα-dependent pathway through an unknown mechanism. Herein, we identify that monounsaturated fatty acids (MUFAs) allosterically activate SIRT1 toward select peptide-substrates such as PGC-1α. MUFAs enhance PGC-1α/PPARα signaling and promote oxidative metabolism in cells and animal models in a SIRT1-dependent manner. Moreover, we characterize the LD protein perilipin 5 (PLIN5), which is known to enhance mitochondrial biogenesis and function, to be a fatty-acid-binding protein that preferentially binds LD-derived monounsaturated fatty acids and traffics them to the nucleus following cAMP/PKA-mediated lipolytic stimulation. Thus, these studies identify the first-known endogenous allosteric modulators of SIRT1 and characterize a LD-nuclear signaling axis that underlies the known metabolic benefits of MUFAs and PLIN5.

Manduca sexta Perilipin 1B: A new PLIN1 isoform linked to fat storage prior to pupation

Perilipins (PLINs) are proteins that associate with lipid droplets (LDs) and play roles in the control of triglycerides (TG) metabolism. Two types of PLINs - 1 and 2- occur in insects. Following previous work on MsPLIN1A (a 42 kDa protein formerly called MsLsd1), here we report a new PLIN1 isoform, MsPLIN1B. MsPLIN1B cDNA was cloned and the 1835bp cDNA contains an ORF encoding a 47.9 kDa protein whose expression was confirmed by mass spectrometry. Alternative transcripts A and B, which differ in the alternative use of exon 1, were the most abundant PLIN1 transcripts in the fat body. These transcripts encode nearly identical proteins except that the B isoform contains 59 additional residues in its amino terminus. No conserved domain was identified in the extra region of MsPLIN1B. The novel PLIN1 isoform is found in lepidopteran species. In Manduca, PLIN1B was expressed only in the 5th instar larva and its levels correlated with fat storage. Furthermore, PLIN1B levels increased with the fat content of the diet in insects of the same age confirming a direct relationship between PLIN1B and TG storage irrespective of development. The nutritional status impacted PLIN1B levels, which decreased in starvation and increased with subsequent re-feeding. Altogether data support a link between PLIN1B and TG storage in caterpillars prior to pupation. The combined findings suggest distinct roles for PLIN1A, PLIN1B and PLIN2. MsPLIN1A abundance correlates with mobilization of TG stores, MsPLIN2 with the synthesis of new LDs and MsPLIN1B abundance correlates with high levels of TG storage and large LD sizes at the end of the last feeding period.

Evidence for a Novel Regulatory Interaction Involving Cyclin D1, Lipid Droplets, Lipolysis, and Cell Cycle Progression in Hepatocytes

During normal proliferation, hepatocytes accumulate triglycerides (TGs) in lipid droplets (LDs), but the underlying mechanisms and functional significance of this steatosis are unknown. In the current study, we examined the coordinated regulation of cell cycle progression and LD accumulation. As previously shown, hepatocytes develop increased LD content after mitogen stimulation. Cyclin D1, in addition to regulating proliferation, was both necessary and sufficient to promote LD accumulation in response to mitogens. Interestingly, cyclin D1 promotes LD accumulation by inhibiting the breakdown of TGs by lipolysis through a mechanism involving decreased lipophagy, the autophagic degradation of LDs. To examine whether inhibition of lipolysis is important for cell cycle progression, we overexpressed adipose TG lipase (ATGL), a key enzyme involved in TG breakdown. As expected, ATGL reduced LD content but also markedly inhibited hepatocyte proliferation, suggesting that lipolysis regulates a previously uncharacterized cell cycle checkpoint. Consistent with this, in mitogen-stimulated cells with small interfering RNA-mediated depletion of cyclin D1 (which inhibits proliferation and stimulates lipolysis), concurrent ATGL knockdown restored progression into S phase. Following partial hepatectomy, a model of robust hepatocyte proliferation in vivo, ATGL overexpression led to decreased LD content, cell cycle inhibition, and marked liver injury, further indicating that down-regulation of lipolysis is important for normal hepatocyte proliferation. Conclusion: We suggest a new relationship between steatosis and proliferation in hepatocytes: cyclin D1 inhibits lipolysis, resulting in LD accumulation, and suppression of lipolysis is necessary for cell cycle progression.

Visualization of lipid directed dynamics of perilipin 1 in human primary adipocytes

Perilipin 1 is a lipid droplet coating protein known to regulate lipid metabolism in adipocytes by serving as a physical barrier as well as a recruitment site for lipases to the lipid droplet. Phosphorylation of perilipin 1 by protein kinase A rapidly initiates lipolysis, but the detailed mechanism on how perilipin 1 controls lipolysis is unknown. Here, we identify specific lipid binding properties of perilipin 1 that regulate the dynamics of lipolysis in human primary adipocytes. Cellular imaging combined with biochemical and biophysical analyses demonstrate that perilipin 1 specifically binds to cholesteryl esters, and that their dynamic properties direct segregation of perilipin 1 into topologically distinct micro domains on the lipid droplet. Together, our data points to a simple unifying mechanism that lipid assembly and segregation control lipolysis in human primary adipocytes.

The Perilipins: Major Cytosolic Lipid Droplet-Associated Proteins and Their Roles in Cellular Lipid Storage, Mobilization, and Systemic Homeostasis

The discovery by Dr. Constantine Londos of perilipin 1, the major scaffold protein at the surface of cytosolic lipid droplets in adipocytes, marked a fundamental conceptual change in the understanding of lipolytic regulation. Focus then shifted from the enzymatic activation of lipases to substrate accessibility, mediated by perilipin-dependent protein sequestration and recruitment. Consequently, the lipid droplet became recognized as a unique, metabolically active cellular organelle and its surface as the active site for novel protein-protein interactions. A new area of investigation emerged, centered on lipid droplets' biology and their role in energy homeostasis. The perilipin family is of ancient origin and has expanded to include five mammalian genes and a growing list of evolutionarily conserved members. Universally, the perilipins modulate cellular lipid storage. This review provides a summary that connects the perilipins to both cellular and whole-body homeostasis.

Whole exome sequence-based association analyses of plasma amyloid-β in African and European Americans; the Atherosclerosis Risk in Communities-Neurocognitive Study

Objective

We performed single-variant and gene-based association analyses of plasma amyloid-β (aβ) concentrations using whole exome sequence from 1,414 African and European Americans. Our goal was to identify genes that influence plasma aβ₄₂ concentrations and aβ₄₂:aβ₄₀ ratios in late middle age (mean = 59 years), old age (mean = 77 years), or change over time (mean = 18 years).

Methods

Plasma aβ measures were linearly regressed onto age, gender, APOE ε4 carrier status, and time elapsed between visits (fold-changes only) separately by race. Following inverse normal transformation of the residuals, seqMeta was used to conduct race-specific single-variant and gene-based association tests while adjusting for population structure. Linear regression models were fit on autosomal variants with minor allele frequencies (MAF)≥1%. T5 burden and Sequence Kernel Association (SKAT) gene-based tests assessed functional variants with MAF≤5%. Cross-race fixed effects meta-analyses were Bonferroni-corrected for the number of variants or genes tested.

Results

Seven genes were associated with aβ in late middle age or change over time; no associations were identified in old age. Single variants in KLKB1 (rs3733402; p = 4.33x10^-10) and F12 (rs1801020; p = 3.89x10^-8) were significantly associated with midlife aβ₄₂ levels through cross-race meta-analysis; the KLKB1 variant replicated internally using 1,014 additional participants with exome chip. ITPRIP, PLIN2, and TSPAN18 were associated with the midlife aβ₄₂:aβ₄₀ ratio via the T5 test; TSPAN18 was significant via the cross-race meta-analysis, whereas ITPRIP and PLIN2 were European American-specific. NCOA1 and NT5C3B were associated with the midlife aβ₄₂:aβ₄₀ ratio and the fold-change in aβ₄₂, respectively, via SKAT in African Americans. No associations replicated externally (N = 725).

Conclusion

We discovered age-dependent genetic effects, established associations between vascular-related genes (KLKB1, F12, PLIN2) and midlife plasma aβ levels, and identified a plausible Alzheimer’s Disease candidate gene (ITPRIP) influencing cell death. Plasma aβ concentrations may have dynamic biological determinants across the lifespan; plasma aβ study designs or analyses must consider age.

Analysis of the lipid body proteome of the oleaginous alga Lobosphaera incisa

BACKGROUND

Lobosphaera incisa (L. incisa) is an oleaginous microalga that stores triacylglycerol (TAG) rich in arachidonic acid in lipid bodies (LBs). This organelle is gaining attention in algal research, since evidence is accumulating that proteins attached to its surface fulfill important functions in TAG storage and metabolism.

RESULTS

Here, the composition of the LB proteome in L incisa was investigated by comparing different cell fractions in a semiquantitative proteomics approach. After applying stringent filters to the proteomics data in order to remove contaminating proteins from the list of possible LB proteins (LBPs), heterologous expression of candidate proteins in tobacco pollen tubes, allowed us to confirm 3 true LBPs: A member of the algal Major Lipid Droplet Protein family, a small protein of unknown function and a putative lipase. In addition, a TAG lipase that belongs to the SUGAR DEPENDENT 1 family of TAG lipases known from oilseed plants was identified. Its activity was verified by functional complementation of an Arabidopsis thaliana mutant lacking the major seed TAG lipases.

CONCLUSIONS

Here we describe 3 LBPs as well as a TAG lipase from the oleaginous microalga L. incisa and discuss their possible involvement in LB metabolism. This study highlights the importance of filtering LB proteome datasets and verifying the subcellular localization one by one, so that contaminating proteins can be recognized as such. Our dataset can serve as a valuable resource in the identification of additional LBPs, shedding more light on the intriguing roles of LBs in microalgae.

Perilipins: a diversity of intracellular lipid droplet proteins

Intracellular lipid droplets (LDs) are found in a wide variety of cell types and have been recognized as organelles with unique spherical structures. Although LDs are not stable lipid-depots, they are active sites of neutral lipid metabolism, and comprise neutral lipid or cholesterol cores surrounded by phospholipid monolayers containing specialized proteins. However, sizes and protein compositions vary between cell and tissue types. Proteins of the perilipin family have been associated with surfaces of LDs and all carry a conserved 11-mer repeat motif. Accumulating evidence indicates that all perilipins are involved in LD formation and that all play roles in LD function under differing conditions. In this brief review, we summarize current knowledge of the roles of perilipins and lipid metabolizing enzymes in a variety of mammalian cell types.

Perilipin 2 and Age-Related Metabolic Diseases: A New Perspective

Perilipin 2 (Plin2), a protein associated with the metabolism of intracellular lipid droplets (LDs), has long been considered only for its role in lipid storage. However, the manipulation of its expression affects the severity of a variety of metabolic and age-related diseases, such as fatty liver, insulin resistance and type 2 diabetes (T2D), cardiovascular disease, atherosclerosis, sarcopenia, and cancer, suggesting that this protein may play a role in these pathological conditions. In particular, its downregulation in mice prevents or mitigates some of the above mentioned diseases. Conversely, in humans high levels of Plin2 are present in sarcopenia, hepatic steatosis, atherosclerosis, and some types of cancer. We propose that inhibition of Plin2 might be a strategy to counteract several metabolic and age-related diseases.

The perilipin 2 (PLIN2) gene Ser251Pro missense mutation is associated with reduced insulin secretion and increased insulin sensitivity in Italian obese subjects

BACKGROUND

Perilipin 2 (PLIN2), a member of the family of perilipin lipid droplets coating proteins, is very widely expressed. The Ser251Pro (rs35568725) missense mutation in exon 6 of PLIN2 gene was previously associated with increased lipid accumulation, decreased lipolysis and increased number of small lipid droplets per cell. Furthermore, the Pro251 mutation was associated with decreased plasma triglyceride and very low density lipoprotein concentrations in population studies. The aim of this study was to evaluate the effect of the Ser251Pro mutation of PLIN2 gene in a cohort with a higher predisposition to obesity-associated metabolic alterations, such as insulin resistance, decreased insulin-secretion, hyperglycaemia, and dyslipidaemia.

METHODS

A large cohort (N = 1692) of Italian obese subjects (mean body mass index = 41 kg/m(2) ) was genotyped for the Ser251Pro mutation. All participants underwent oral glucose tolerance tests (OGTT), with measurement of glucose and insulin levels. Indices of insulin resistance and of insulin secretion were also calculated. Clinical and biochemical parameters were collected for all participants.

RESULTS

We observed that insulin concentration was significantly reduced at 120 min after the administration of glucose in Pro251 allele carriers, whereas glucose levels were similar in Pro251 allele carriers and non-carriers throughout the OGTT. Furthermore, the CIR120 index of insulin secretion was significantly lower (P < 0.035) and the ISI index of insulin-sensitivity was significantly higher (P < 0.031) in carriers of the Pro251 allele. When we analysed men and women separately to test for gender-specific associations, we observed that in women insulin levels were significantly lower in Pro251 allele carriers compared with wild-type subjects throughout the whole OGTT. In men, we confirmed a significant reduction in insulin concentration only at 120 min after the OGTT. No significant differences between genotype groups regarding triglyceride levels and anyother clinical and metabolic parameters were observed.

CONCLUSION

We observed a strong significant association between the PLIN2 Pro251 mutation and lower insulin secretion associated with an increased insulin sensitivity. Copyright © 2015 John Wiley & Sons, Ltd.

Liver-specific loss of Perilipin 2 alleviates diet-induced hepatic steatosis, inflammation, and fibrosis

Hepatic inflammation and fibrosis are key elements in the pathogenesis of nonalcoholic steatohepatitis (NASH), a progressive liver disease initiated by excess hepatic lipid accumulation. Lipid droplet protein Perilipin 2 (Plin2) alleviates dietary-induced hepatic steatosis when globally ablated; however, its role in the progression of NASH remains unknown. To investigate this further, we challenged Plin2 liver-specific knockout mice (designated L-KO) and their respective wild-type (WT) controls with a methionine-choline-deficient (MCD) diet for 15 days to induce a NASH phenotype of increased hepatic triglyceride levels through impaired phosphatidylcholine (PC) synthesis and very-low-density lipoprotein (VLDL) secretion. Results on liver weights, body weights, fat tissue mass, and histology in WT and L-KO mice fed the MCD diet revealed signs of hepatic steatosis, fibrosis, and inflammation; however, these effects were blunted in L-KO mice. In addition, levels of PC and VLDL were unchanged, and hepatic steatosis was reduced in L-KO mice fed the MCD diet, due in part to an increase in remodeling of PE to PC via the enzyme phosphatidylethanolamine N-methyltransferase (PEMT). These mice also exhibited decreased hepatic expression of proinflammatory markers cyclooxygenase 2, IL-6, TNF-α, IL-1β, and reduced expression of endoplasmic reticulum (ER) stress proteins C/EBP homologous protein and cleaved caspase-1. Taken together, these results suggest that Plin2 liver-specific ablation alleviates diet-induced hepatic steatosis and inflammation via a PEMT-mediated mechanism that involves compensatory changes in proteins involved in phospholipid remodeling, inflammation, and ER stress that work to alleviate diet-induced NASH. Overall, these findings support a role for Plin2 as a target for NASH therapy.

Conserved Amphipathic Helices Mediate Lipid Droplet Targeting of Perilipins 1-3

Perilipins (PLINs) play a key role in energy storage by orchestrating the activity of lipases on the surface of lipid droplets. Failure of this activity results in severe metabolic disease in humans. Unlike all other lipid droplet-associated proteins, PLINs localize almost exclusively to the phospholipid monolayer surrounding the droplet. To understand how they sense and associate with the unique topology of the droplet surface, we studied the localization of human PLINs inSaccharomyces cerevisiae,demonstrating that the targeting mechanism is highly conserved and that 11-mer repeat regions are sufficient for droplet targeting. Mutations designed to disrupt folding of this region into amphipathic helices (AHs) significantly decreased lipid droplet targetingin vivoandin vitro Finally, we demonstrated a substantial increase in the helicity of this region in the presence of detergent micelles, which was prevented by an AH-disrupting missense mutation. We conclude that highly conserved 11-mer repeat regions of PLINs target lipid droplets by folding into AHs on the droplet surface, thus enabling PLINs to regulate the interface between the hydrophobic lipid core and its surrounding hydrophilic environment.

Targeting Fat: Mechanisms of Protein Localization to Lipid Droplets

How proteins specifically localize to the phospholipid monolayer surface of lipid droplets (LDs) is being unraveled. We review here the major known pathways of protein targeting to LDs and suggest a classification framework based on the localization origin for the protein. Class I proteins often have a membrane-embedded, hydrophobic 'hairpin' motif, and access LDs from the endoplasmic reticulum (ER) either during LD formation or after formation via ER-LD membrane bridges. Class II proteins access the LD surface from the cytosol and bind through amphipathic helices or other hydrophobic domains. Other proteins require lipid modifications or protein-protein interactions to bind to LDs. We summarize knowledge for targeting and removal of the different classes, and highlight areas needing investigation.