OSBP-related protein 2 is a sterol receptor on lipid droplets that regulates the metabolism of neutral lipids

Lipid droplets, autophagy, and ageing: A cell-specific tale

Lipid droplets are the essential organelle for storing lipids in a cell. Within the variety of the human body, different cells store, utilize and release lipids in different ways, depending on their intrinsic function. However, these differences are not well characterized and, especially in the context of ageing, represent a key factor for cardiometabolic diseases. Whole body lipid homeostasis is a central interest in the field of cardiometabolic diseases. In this review we characterize lipid droplets and their utilization via autophagy and describe their diverse fate in three cells types central in cardiometabolic dysfunctions: adipocytes, hepatocytes, and macrophages.

Insight Into the Dynamics of the Ixodes ricinus Nymphal Midgut Proteome

Ticks are ectoparasites that feed on blood and have an impressive ability to consume and process enormous amounts of host blood, allowing extremely long periods of starvation between blood meals. The central role in the parasitic lifestyle of ticks is played by the midgut. This organ efficiently stores and digests ingested blood and serves as the primary interface for the transmission of tick-borne pathogens. In this study, we used a label-free quantitative approach to perform a novel dynamic proteomic analysis of the midgut of Ixodesricinus nymphs, covering their development from unfed to pre-molt stages. We identified 1534 I. ricinus-specific proteins with a relatively low proportion of host proteins. This proteome dataset, which was carefully examined by manual scrutiny, allowed precise annotation of proteins important for blood meal processing and their dynamic changes during nymphal ontogeny. We focused on midgut molecules related to lipid hydrolysis, storage, and transport, opening a yet unexplored avenue for studying lipid metabolism in ticks. Further dynamic profiling of the tick's multi-enzyme digestive network, protease inhibitors, enzymes involved in redox homeostasis and detoxification, antimicrobial peptides, and proteins responsible for midgut colonization by Borrelia spirochetes promises to uncover new targets for targeting tick nymphs, the most critical life stage for transmission the pathogens that cause tick-borne diseases.

Comparative proximity biotinylation implicates the small GTPase RAB18 in sterol mobilization and biosynthesis

Loss of functional RAB18 causes the autosomal recessive condition Warburg Micro syndrome. To better understand this disease, we used proximity biotinylation to generate an inventory of potential RAB18 effectors. A restricted set of 28 RAB18 interactions were dependent on the binary RAB3GAP1-RAB3GAP2 RAB18-guanine nucleotide exchange factor complex. Twelve of these 28 interactions are supported by prior reports, and we have directly validated novel interactions with SEC22A, TMCO4, and INPP5B. Consistent with a role for RAB18 in regulating membrane contact sites, interactors included groups of microtubule/membrane-remodeling proteins, membrane-tethering and docking proteins, and lipid-modifying/transporting proteins. Two of the putative interactors, EBP and OSBPL2/ORP2, have sterol substrates. EBP is a Δ8-Δ7 sterol isomerase, and ORP2 is a lipid transport protein. This prompted us to investigate a role for RAB18 in cholesterol biosynthesis. We found that the cholesterol precursor and EBP-product lathosterol accumulates in both RAB18-null HeLa cells and RAB3GAP1-null fibroblasts derived from an affected individual. Furthermore, de novo cholesterol biosynthesis is impaired in cells in which RAB18 is absent or dysregulated or in which ORP2 expression is disrupted. Our data demonstrate that guanine nucleotide exchange factor-dependent Rab interactions are highly amenable to interrogation by proximity biotinylation and may suggest that Micro syndrome is a cholesterol biosynthesis disorder.

The Janus-Faced Role of Lipid Droplets in Aging: Insights from the Cellular Perspective

It is widely accepted that nine hallmarks-including mitochondrial dysfunction, epigenetic alterations, and loss of proteostasis-exist that describe the cellular aging process. Adding to this, a well-described cell organelle in the metabolic context, namely, lipid droplets, also accumulates with increasing age, which can be regarded as a further aging-associated process. Independently of their essential role as fat stores, lipid droplets are also able to control cell integrity by mitigating lipotoxic and proteotoxic insults. As we will show in this review, numerous longevity interventions (such as mTOR inhibition) also lead to strong accumulation of lipid droplets in Saccharomyces cerevisiae, Caenorhabditis elegans, Drosophila melanogaster, and mammalian cells, just to name a few examples. In mammals, due to the variety of different cell types and tissues, the role of lipid droplets during the aging process is much more complex. Using selected diseases associated with aging, such as Alzheimer's disease, Parkinson's disease, type II diabetes, and cardiovascular disease, we show that lipid droplets are "Janus"-faced. In an early phase of the disease, lipid droplets mitigate the toxicity of lipid peroxidation and protein aggregates, but in a later phase of the disease, a strong accumulation of lipid droplets can cause problems for cells and tissues.

Pathogen vacuole membrane contact sites - close encounters of the fifth kind

Vesicular trafficking and membrane fusion are well-characterized, versatile, and sophisticated means of 'long range' intracellular protein and lipid delivery. Membrane contact sites (MCS) have been studied in far less detail, but are crucial for 'short range' (10-30 nm) communication between organelles, as well as between pathogen vacuoles and organelles. MCS are specialized in the non-vesicular trafficking of small molecules such as calcium and lipids. Pivotal MCS components important for lipid transfer are the VAP receptor/tether protein, oxysterol binding proteins (OSBPs), the ceramide transport protein CERT, the phosphoinositide phosphatase Sac1, and the lipid phosphatidylinositol 4-phosphate (PtdIns(4)P). In this review, we discuss how these MCS components are subverted by bacterial pathogens and their secreted effector proteins to promote intracellular survival and replication.

Evolutionary History of Oxysterol-Binding Proteins Reveals Complex History of Duplication and Loss in Animals and Fungi

Cells maintain the specific lipid composition of distinct organelles by vesicular transport as well as non-vesicular lipid trafficking via lipid transport proteins. Oxysterol-binding proteins (OSBPs) are a family of lipid transport proteins that transfer lipids at various membrane contact sites (MCSs). OSBPs have been extensively investigated in human and yeast cells where 12 have been identified in Homo sapiens and 7 in Saccharomyces cerevisiae. The evolutionary relationship between these well-characterized OSBPs is still unclear. By reconstructing phylogenies of eukaryote OSBPs, we show that the ancestral Saccharomycotina had four OSBPs, the ancestral fungus had five OSBPs, and the ancestral animal had six OSBPs, whereas the shared ancestor of animals and fungi as well as the ancestral eukaryote had only three OSBPs. Our analyses identified three undescribed ancient OSBP orthologues, one fungal OSBP (Osh8) lost in the lineage leading to yeast, one animal OSBP (ORP12) lost in the lineage leading to vertebrates, and one eukaryotic OSBP (OshEu) lost in both the animal and fungal lineages.

Roles of Phosphatidylinositol 4-Phosphorylation in Non-vesicular Cholesterol Trafficking

Cholesterol (Chol) is an essential component of all eukaryotic cell membranes that affects the function of numerous peripheral as well as integral membrane proteins. Chol is synthesized in the ER, but it is selectively enriched within the plasma membrane (PM) and other endomembranes, which requires Chol to cross the aqueous phase of the cytoplasm. In addition to the classical vesicular trafficking pathways that are known to facilitate the bulk transport of membrane intermediates, Chol is also transported via non-vesicular lipid transfer proteins that work primarily within specialized membrane contact sites. Some of these transport pathways work against established concentration gradients and hence require energy. Recent studies highlight the unique role of phosphoinositides (PPIns), and phosphatidylinositol 4-phosphate (PI4P) in particular, for the control of non-vesicular Chol transport. In this chapter, we will review the emerging connection between Chol, PPIns, and lipid transfer proteins that include the important family of oxysterol-binding protein related proteins, or ORPs.

The lipid transporter ORP2 regulates synaptic neurotransmitter release via two distinct mechanisms

Cholesterol is crucial for neuronal synaptic transmission, assisting in the molecular and structural organization of lipid rafts, ion channels, and exocytic proteins. Although cholesterol absence was shown to result in impaired neurotransmission, how cholesterol locally traffics and its route of action are still under debate. Here, we characterized the lipid transfer protein ORP2 in murine hippocampal neurons. We show that ORP2 preferentially localizes to the presynapse. Loss of ORP2 reduces presynaptic cholesterol levels by 50%, coinciding with a profoundly reduced release probability, enhanced facilitation, and impaired presynaptic calcium influx. In addition, ORP2 plays a cholesterol-transport-independent role in regulating vesicle priming and spontaneous release, likely by competing with Munc18-1 in syntaxin1A binding. To conclude, we identified a dual function of ORP2 as a physiological modulator of the synaptic cholesterol content and a regulator of neuronal exocytosis.

OSBPL2 mutations impair autophagy and lead to hearing loss, potentially remedied by rapamycin

Intracellular accumulation of mutant proteins causes proteinopathies, which lack targeted therapies. Autosomal dominant hearing loss (DFNA67) is caused by frameshift mutations in OSBPL2. Here, we show that DFNA67 is a toxic proteinopathy. Mutant OSBPL2 accumulated intracellularly and bound to macroautophagy/autophagy proteins. Consequently, its accumulation led to defective endolysosomal homeostasis and impaired autophagy. Transgenic mice expressing mutant OSBPL2 exhibited hearing loss, but osbpl2 knockout mice or transgenic mice expressing wild-type OSBPL2 did not. Rapamycin decreased the accumulation of mutant OSBPL2 and partially rescued hearing loss in mice. Rapamycin also partially improved hearing loss and tinnitus in individuals with DFNA67. Our findings indicate that dysfunctional autophagy is caused by mutant proteins in DFNA67; hence, we recommend rapamycin for DFNA67 treatment.Abbreviations: ABR: auditory brainstem response; ACTB: actin beta; CTSD: cathepsin D; dB: decibel; DFNA67: deafness non-syndromic autosomal dominant 67; DPOAE: distortion product otoacoustic emission; fs: frameshift; GFP: green fluorescent protein; HsQ53R-TG: human p.Q53Rfs*100-transgenic: HEK 293: human embryonic kidney 293; HFD: high-fat diet; KO: knockout; LAMP1: lysosomal associated membrane protein 1; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MTOR: mechanistic target of rapamycin kinase; NSHL: non-syndromic hearing loss; OHC: outer hair cells; OSBPL2: oxysterol binding protein-like 2; SEM: scanning electron microscopy; SGN: spiral ganglion neuron; SQSTM1/p62: sequestosome 1; TEM: transmission electron microscopy; TG: transgenic; WES: whole-exome sequencing; YUHL: Yonsei University Hearing Loss; WT: wild-type.

Structure, function and small molecule modulation of intracellular sterol transport proteins

Intracellular sterol transport proteins (STPs) are crucial for maintaining cellular lipid homeostasis by regulating local sterol pools. Despite structural similarities in their sterol binding domains, STPs have different substrate specificities, intracellular localisation and biological functions. In this review, we highlight recent advances in the determination of STP structures and how this regulates their lipid specificities. Furthermore, we cover the important discoveries relating to the intracellular localisation of STPs, and the organelles between which lipid transport is carried out, giving rise to specific functions in health and disease. Finally, serendipitous and targeted efforts to identify small molecule modulators of STPs, as well as their ability to act as tool compounds and potential therapeutics, will be discussed.

ORP5 and ORP8 orchestrate lipid droplet biogenesis and maintenance at ER-mitochondria contact sites

Lipid droplets (LDs) are the primary organelles of lipid storage, buffering energy fluctuations of the cell. They store neutral lipids in their core that is surrounded by a protein-decorated phospholipid monolayer. LDs arise from the endoplasmic reticulum (ER). The ER protein seipin, localizing at ER-LD junctions, controls LD nucleation and growth. However, how LD biogenesis is spatially and temporally coordinated remains elusive. Here, we show that the lipid transfer proteins ORP5 and ORP8 control LD biogenesis at mitochondria-associated ER membrane (MAM) subdomains, enriched in phosphatidic acid. We found that ORP5/8 regulates seipin recruitment to these MAM-LD contacts, and their loss impairs LD biogenesis. Importantly, the integrity of ER-mitochondria contact sites is crucial for ORP5/8 function in regulating seipin-mediated LD biogenesis. Our study uncovers an unprecedented ORP5/8 role in orchestrating LD biogenesis and maturation at MAMs and brings novel insights into the metabolic crosstalk between mitochondria, ER, and LDs at the membrane contact sites.

Cholesterol transport in the late endocytic pathway: Roles of ORP family proteins

Oxysterol-binding protein (OSBP) homologues, designated ORP or OSBPL proteins, constitute one of the largest families of intracellular lipid-binding/transfer proteins (LTP). This review summarizes the mounting evidence that several members of this family participate in the machinery facilitating cholesterol trafficking in the late endocytic pathway. There are indications that OSBP, besides acting as a cholesterol/phosphatidylinositol 4-phosphate (PI4P) exchanger at the endoplasmic reticulum (ER)-trans-Golgi network (TGN) membrane contact sites (MCS), also exchanges these lipids at ER-lysosome (Lys) contacts, increasing Lys cholesterol content. The long isoform of ORP1 (ORP1L), which also targets ER-late endosome (LE)/Lys MCS, has the capacity to mediate cholesterol transport either from ER to LE or in the opposite direction. Moreover, it regulates the motility, positioning and fusion of LE as well as autophagic flux. ORP2, the closest relative of ORP1, is mainly cytosolic, but also targets PI(4,5)P₂-rich endosomal compartments. Our latest data suggest that ORP2 transfers cholesterol from LE to recycling endosomes (RE) in exchange for PI(4,5)P₂, thus stimulating the recruitment of focal adhesion kinase (FAK) on the RE and cell adhesion. FAK activates phosphoinositide kinase on the RE to enhance PI(4,5)P₂ synthesis. ORP2 in turn transfers PI(4,5)P₂ from RE to LE, thus regulating LE tubule formation and transport activity.

Protein Quality Control and Lipid Droplet Metabolism

Lipid droplets (LDs) are endoplasmic reticulum-derived organelles that consist of a core of neutral lipids encircled by a phospholipid monolayer decorated with proteins. As hubs of cellular lipid and energy metabolism, LDs are inherently involved in the etiology of prevalent metabolic diseases such as obesity and nonalcoholic fatty liver disease. The functions of LDs are regulated by a unique set of associated proteins, the LD proteome, which includes integral membrane and peripheral proteins. These proteins control key activities of LDs such as triacylglycerol synthesis and breakdown, nutrient sensing and signal integration, and interactions with other organelles. Here we review the mechanisms that regulate the composition of the LD proteome, such as pathways that mediate selective and bulk LD protein degradation and potential connections between LDs and cellular protein quality control.

Functions of Oxysterol-Binding Proteins at Membrane Contact Sites and Their Control by Phosphoinositide Metabolism

Lipids must be correctly transported within the cell to the right place at the right time in order to be fully functional. Non-vesicular lipid transport is mediated by so-called lipid transfer proteins (LTPs), which contain a hydrophobic cavity that sequesters lipid molecules. Oxysterol-binding protein (OSBP)-related proteins (ORPs) are a family of LTPs known to harbor lipid ligands, such as cholesterol and phospholipids. ORPs act as a sensor or transporter of those lipid ligands at membrane contact sites (MCSs) where two different cellular membranes are closely apposed. In particular, a characteristic functional property of ORPs is their role as a lipid exchanger. ORPs mediate counter-directional transport of two different lipid ligands at MCSs. Several, but not all, ORPs transport their lipid ligand from the endoplasmic reticulum (ER) in exchange for phosphatidylinositol 4-phosphate (PI4P), the other ligand, on apposed membranes. This ORP-mediated lipid “countertransport” is driven by the concentration gradient of PI4P between membranes, which is generated by its kinases and phosphatases. In this review, we will discuss how ORP function is tightly coupled to metabolism of phosphoinositides such as PI4P. Recent progress on the role of ORP-mediated lipid transport/countertransport at multiple MCSs in cellular functions will be also discussed.

Role of Lipid Transfer Proteins (LTPs) in the Viral Life Cycle

Viruses are obligate parasites that depend on the host cell machinery for their replication and dissemination. Cellular lipids play a central role in multiple stages of the viral life cycle such as entry, replication, morphogenesis, and egress. Most viruses reorganize the host cell membranes for the establishment of viral replication complex. These specialized structures allow the segregation of replicating viral RNA from ribosomes and protect it from host nucleases. They also facilitate localized enrichment of cellular components required for viral replication and assembly. The specific composition of the lipid membrane governs its ability to form negative or positive curvature and possess a rigid or flexible form, which is crucial for membrane rearrangement and establishment of viral replication complexes. In this review, we highlight how different viruses manipulate host lipid transfer proteins and harness their functions to enrich different membrane compartments with specific lipids in order to facilitate multiple aspects of the viral life cycle.

Oxysterol-binding protein-like 2 contributes to the developmental progression of preadipocytes by binding to β-catenin

Oxysterol-binding protein-like 2 (OSBPL2), also known as oxysterol-binding protein-related protein (ORP) 2, is a member of lipid transfer protein well-known for its role in regulating cholesterol homeostasis. A recent study reported that OSBPL2/ORP2 localizes to lipid droplets (LDs) and is associated with energy metabolism and obesity. However, the function of OSBPL2/ORP2 in adipocyte differentiation is poorly understood. Here, we report that OSBPL2/ORP2 contributes to the developmental progression of preadipocytes. We found that OSBPL2/ORP2 binds to β-catenin, a key effector in the Wnt signaling pathway that inhibits adipogenesis. This complex plays a role in regulating the protein level of β-catenin only in preadipocytes, not in mature adipocytes. Our data further indicated that OSBPL2/ORP2 mediates the transport of β-catenin into the nucleus and thus regulates target genes related to adipocyte differentiation. Deletion of OSBPL2/ORP2 markedly reduces β-catenin both in the cytoplasm and in the nucleus, promotes preadipocytes maturation, and ultimately leads to obesity-related characteristics. Altogether, we provide novel insight into the function of OSBPL2/ORP2 in the developmental progression of preadipocytes and suggest OSBPL2/ORP2 may be a potential therapeutic target for the treatment of obesity-related diseases.

Interactions of Lipid Droplets with the Intracellular Transport Machinery

Historically, studies of intracellular membrane trafficking have focused on the secretory and endocytic pathways and their major organelles. However, these pathways are also directly implicated in the biogenesis and function of other important intracellular organelles, the best studied of which are peroxisomes and lipid droplets. There is a large recent body of work on these organelles, which have resulted in the introduction of new paradigms regarding the roles of membrane trafficking organelles. In this review, we discuss the roles of membrane trafficking in the life cycle of lipid droplets. This includes the complementary roles of lipid phase separation and proteins in the biogenesis of lipid droplets from endoplasmic reticulum (ER) membranes, and the attachment of mature lipid droplets to membranes by lipidic bridges and by more conventional protein tethers. We also discuss the catabolism of neutral lipids, which in part results from the interaction of lipid droplets with cytosolic molecules, but with important roles for both macroautophagy and microautophagy. Finally, we address their eventual demise, which involves interactions with the autophagocytotic machinery. We pay particular attention to the roles of small GTPases, particularly Rab18, in these processes.

Structural and Functional Specialization of OSBP-Related Proteins

Lipids are precisely distributed in the eukaryotic cell where they help to define organelle identity and function, in addition to their structural role. Once synthesized, many lipids must be delivered to other compartments by non-vesicular routes, a process that is undertaken by proteins called Lipid Transfer Proteins (LTPs). OSBP and the closely-related ORP and Osh proteins constitute a major, evolutionarily conserved family of LTPs in eukaryotes. Most of these target one or more subcellular regions, and membrane contact sites in particular, where two organelle membranes are in close proximity. It was initially thought that such proteins were strictly dedicated to sterol sensing or transport. However, over the last decade, numerous studies have revealed that these proteins have many more functions, and we have expanded our understanding of their mechanisms. In particular, many of them are lipid exchangers that exploit PI(4)P or possibly other phosphoinositide gradients to directionally transfer sterol or PS between two compartments. Importantly, these transfer activities are tightly coupled to processes such as lipid metabolism, cellular signalling and vesicular trafficking. This review describes the molecular architecture of OSBP/ORP/Osh proteins, showing how their specific structural features and internal configurations impart unique cellular functions.

ORP/Osh mediate cross-talk between ER-plasma membrane contact site components and plasma membrane SNAREs

OSBP-homologous proteins (ORPs, Oshp) are lipid binding/transfer proteins. Several ORP/Oshp localize to membrane contacts between the endoplasmic reticulum (ER) and the plasma membrane, where they mediate lipid transfer or regulate lipid-modifying enzymes. A common way in which they target contacts is by binding to the ER proteins, VAP/Scs2p, while the second membrane is targeted by other interactions with lipids or proteins.We have studied the cross-talk of secretory SNARE proteins and their regulators with ORP/Oshp and VAPA/Scs2p at ER-plasma membrane contact sites in yeast and murine primary neurons. We show that Oshp-Scs2p interactions depend on intact secretory SNARE proteins, especially Sec9p. SNAP-25/Sec9p directly interact with ORP/Osh proteins and their disruption destabilized the ORP/Osh proteins, associated with dysfunction of VAPA/Scs2p. Deleting OSH1-3 in yeast or knocking down ORP2 in primary neurons reduced the oligomerization of VAPA/Scs2p and affected their multiple interactions with SNAREs. These observations reveal a novel cross-talk between the machineries of ER-plasma membrane contact sites and those driving exocytosis.

ORP5 localizes to ER-lipid droplet contacts and regulates the level of PI(4)P on lipid droplets

Lipid droplets (LDs) are important organelles for cell metabolism. Here, Du et al. show that phosphatidylinositol-4-phosphate produced by PI4K2A can exist on LDs and is used/consumed by ORP5, which localizes to ER–LD contacts during the growth of LDs.

Lipid droplets (LDs) are evolutionarily conserved organelles that play important roles in cellular metabolism. Each LD is enclosed by a monolayer of phospholipids, distinct from bilayer membranes. During LD biogenesis and growth, this monolayer of lipids expands by acquiring phospholipids from the endoplasmic reticulum (ER) through nonvesicular mechanisms. Here, in a mini-screen, we find that ORP5, an integral membrane protein of the ER, can localize to ER–LD contact sites upon oleate loading. ORP5 interacts with LDs through its ligand-binding domain, and ORP5 deficiency enhances neutral lipid synthesis and increases the size of LDs. Importantly, there is significantly more phosphatidylinositol-4-phosphate (PI(4)P) and less phosphatidylserine (PS) on LDs in ORP5-deficient cells than in normal cells. The increased presence of PI(4)P on LDs in ORP5-deficient cells requires phosphatidylinositol 4-kinase 2-α. Our results thus demonstrate the existence of PI(4)P on LDs and suggest that LD-associated PI(4)P may be primarily used by ORP5 to deliver PS to LDs.

Lipid Exchangers: Cellular Functions and Mechanistic Links With Phosphoinositide Metabolism

Lipids are amphiphilic molecules that self-assemble to form biological membranes. Thousands of lipid species coexist in the cell and, once combined, define organelle identity. Due to recent progress in lipidomic analysis, we now know how lipid composition is finely tuned in different subcellular regions. Along with lipid synthesis, remodeling and flip-flop, lipid transfer is one of the active processes that regulates this intracellular lipid distribution. It is mediated by Lipid Transfer Proteins (LTPs) that precisely move certain lipid species across the cytosol and between the organelles. A particular subset of LTPs from three families (Sec14, PITP, OSBP/ORP/Osh) act as lipid exchangers. A striking feature of these exchangers is that they use phosphatidylinositol or phosphoinositides (PIPs) as a lipid ligand and thereby have specific links with PIP metabolism and are thus able to both control the lipid composition of cellular membranes and their signaling capacity. As a result, they play pivotal roles in cellular processes such as vesicular trafficking and signal transduction at the plasma membrane. Recent data have shown that some PIPs are used as energy by lipid exchangers to generate lipid gradients between organelles. Here we describe the importance of lipid counter-exchange in the cell, its structural basis, and presumed links with pathologies.

OSBPL2 Is Required for the Binding of COPB1 to ATGL and the Regulation of Lipid Droplet Lipolysis

The accumulation of giant lipid droplets (LDs) increases the risk of metabolic disorders including obesity and insulin resistance. The lipolysis process involves the activation and transfer of lipase, but the molecular mechanism is not completely understood. The translocation of ATGL, a critical lipolysis lipase, from the ER to the LD surface is mediated by an energy catabolism complex. Oxysterol-binding protein-like 2 (OSBPL2/ORP2) is one of the lipid transfer proteins that regulates intracellular cholesterol homeostasis. A recent study has proven that Osbpl2^−/− pigs exhibit hypercholesterolemia and obesity phenotypes with an increase in adipocytes. In this study, we identified that OSBPL2 links the endoplasmic reticulum (ER) with LDs, binds to COPB1, and mediates ATGL transport. We provide important insights into the function of OSBPL2, indicating that it is required for the regulation of lipid droplet lipolysis.

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LD lipolysis is impaired in OSBPL2/osbpl2b-mutant HepG2 cells and zebrafish

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OSBPL2 interacts with COPB1, a subunit of the COPI complex located on LDs

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Altered COPI complexes on LDs may perturb the trafficking of lipolysis lipase ATGL

Enhanced acyl-CoA:cholesterol acyltransferase activity increases cholesterol levels on the lipid droplet surface and impairs adipocyte function

Cholesterol plays essential structural and signaling roles in mammalian cells, but too much cholesterol can cause cytotoxicity. Acyl-CoA:cholesterol acyltransferases 1 and 2 (ACAT1/2) convert cholesterol into its storage form, cholesteryl esters, regulating a key step in cellular cholesterol homeostasis. Adipose tissue can store >50% of whole-body cholesterol. Interestingly, however, almost no ACAT activity is present in adipose tissue, and most adipose cholesterol is stored in its free form. We therefore hypothesized that increased cholesterol esterification may have detrimental effects on adipose tissue function. Here, using several approaches, including protein overexpression, quantitative RT-PCR, immunofluorescence, and various biochemical assays, we found that ACAT1 expression is significantly increased in the adipose tissue of the ob/ob mice. We further demonstrated that ACAT1/2 overexpression partially inhibited the differentiation of 3T3-L1 preadipocytes. In mature adipocytes, increased ACAT activity reduced the size of lipid droplets (LDs) and inhibited lipolysis and insulin signaling. Paradoxically, the amount of free cholesterol increased on the surface of LDs in ACAT1/2-overexpressing adipocytes, accompanied by increased LD localization of caveolin-1. Moreover, cholesterol depletion in adipocytes by treating the cells with cholesterol-deficient media or β-cyclodextrins induced changes in cholesterol distribution that were similar to those caused by ACAT1/2 overexpression. Our results suggest that ACAT1/2 overexpression increases the level of free cholesterol on the LD surface, thereby impeding adipocyte function. These findings provide detailed insights into the role of free cholesterol in LD and adipocyte function and suggest that ACAT inhibitors have potential utility for managing disorders associated with extreme obesity.

Come a little bit closer! Lipid droplet-ER contact sites are getting crowded

Not so long ago, contact sites between the endoplasmic reticulum (ER) and lipid droplets (LDs) were largely unexplored on a molecular level. In recent years however, numerous proteins have been identified that are enriched or exclusively located at the interfaces between LDs and the ER. These comprise members of protein classes typically found in diverse types of contacts, such as organelle tethers and lipid transfer proteins, but also proteins that have no similarities to known contact site machineries. This structurally heterogeneous group of contact site residents might be required to fulfill unique aspects of LD-ER contact biology, such as de novo LD biogenesis, and maintenance of lipidic connections between LDs and ER. Here, we summarize the current knowledge on the molecular components of this special organelle contact site, and discuss their features and functions.

OSBPL2 deficiency upregulate SQLE expression increasing intracellular cholesterol and cholesteryl ester by AMPK/SP1 and SREBF2 signalling pathway

Previous studies have shown that oxysterol binding protein like 2 (OSBPL2) knockdown is closely related to cholesterol metabolism. However, whether there is a direct relation between OSBPL2 and cholesterol synthesis is unknown. This study explored the mechanism of OSBPL2 deficiency in the upregulation of squalene epoxidase (SQLE) and the subsequent accumulation of intracellular cholesterol and cholesteryl ester. Here, we constructed an OSBPL2-deleted HeLa cell line using CRISPR/Cas9 technology, screened differentially expressed genes and examined the transcriptional regulation of SQLE using a dual-luciferase reporter gene. RNA-seq analysis showed that SQLE was upregulated significantly and the dual luciferase reporter gene assay revealed that two new functional transcription factor binding sites of Sp1 transcription factor (SP1) and sterol regulatory element-binding transcription factor 2 (SREBF2) in the SQLE promoter participated in the SQLE transcription and expression. In addition, we also observed that OSBPL2 deletion inhibited the AMPK signalling pathway and that the inhibition of AMPK signalling promoted SP1 and SREBF2 entry into the nuclear to upregulate SQLE expression. Therefore, these data support that OSBPL2 deficiency upregulates SQLE expression and increases the accumulation of cholesterol and cholesteryl ester by suppressing AMPK signalling, which provides new evidence of the connection between OSBPL2 and cholesterol synthesis.

Localisation of oxysterols at the sub-cellular level and in biological fluids

Oxysterols are oxidized derivatives of cholesterol that are formed enzymatically or via reactive oxygen species or both. Cholesterol or oxysterols ingested as food are absorbed and packed into lipoproteins that are taken up by hepatic cells. Within hepatic cells, excess cholesterol is metabolised to form bile acids. The endoplasmic reticulum acts as the main organelle in the bile acid synthesis pathway. Metabolised sterols originating from this pathway are distributed within other organelles and in the cell membrane. The alterations to membrane oxysterol:sterol ratio affects the integrity of the cell membrane. The presence of oxysterols changes membrane fluidity and receptor orientation. It is well documented that hydroxylase enzymes located in mitochondria facilitate oxysterol production via an acidic pathway. More recently, the presence of oxysterols was also reported in lysosomes. Peroxisomal deficiencies favour intracellular oxysterols accumulation. Despite the low abundance of oxysterols compared to cholesterol, the biological actions of oxysterols are numerous and important. Oxysterol levels are implicated in the pathogenesis of multiple diseases ranging from chronic inflammatory diseases (atherosclerosis, Alzheimer's disease and bowel disease), cancer and numerous neurodegenerative diseases. In this article, we review the distribution of oxysterols in sub-cellular organelles and in biological fluids.

OSBP-related protein 2 (ORP2): Unraveling its functions in cellular lipid/carbohydrate metabolism, signaling and F-actin regulation

Oxysterol-binding protein (OSBP)-related proteins (ORPs) constitute a family of intracellular lipid-binding/transport proteins (LTPs) in eukaryotes. They typically have a modular structure comprising a lipid-binding domain and membrane targeting determinants, being thus suited for function at membrane contact sites. Among the mammalian ORPs, ORP2/OSBPL2 is the only member that only exists as a 'short' variant lacking a membrane-targeting pleckstrin homology domain. ORP2 is expressed ubiquitously and has been assigned a multitude of functions. Its OSBP-related domain binds cholesterol, oxysterols, and phosphoinositides, and its overexpression enhances cellular cholesterol efflux. Consistently, the latest observations suggest a function of ORP2 in cholesterol transport to the plasma membrane (PM) in exchange for phosphatidylinositol 4,5-bisphosphate (PI4,5P₂), with significant impacts on the concentrations of PM cholesterol and PI4,5P₂. On the other hand, ORP2 localizes at the surface of cytoplasmic lipid droplets (LDs) and at endoplasmic-reticulum-LD contact sites, and its depletion modifies cellular triglyceride (TG) metabolism. Study in an adrenocortical cell line further suggested a function of ORP2 in the synthesis of steroid hormones. Our recent knock-out of ORP2 in human hepatoma cells revealed its function in hepatocellular PI3K/Akt signaling, glucose and triglyceride metabolism, as well as in actin cytoskeletal regulation, cell adhesion, migration and proliferation. ORP2 was shown to interact physically with F-actin regulators such as DIAPH1, ARHGAP12, SEPT9 and MLC12, as well as with IQGAP1 and the Cdc37-Hsp90 chaperone complex controlling the activity of Akt. Interestingly, mutations in OSBPL2 encoding ORP2 are associated with autosomal dominant non-syndromic hearing loss, and the protein was found to localize in cochlear hair cell stereocilia. The functions assigned to ORP2 suggest that this protein, in concert with other LTPs, controls the subcellular distribution of cholesterol in various cell types and steroid hormone synthesis in adrenocortical cells. However, it also impacts cellular TG and carbohydrate metabolism and F-actin-dependent functions, revealing a bewildering spectrum of activities.

Deletion of OSBPL2 in auditory cells increases cholesterol biosynthesis and drives reactive oxygen species production by inhibiting AMPK activity

Oxysterol-binding protein like 2 (OSBPL2) was identified as a novel causal gene for autosomal dominant nonsyndromic hearing loss. However, the pathogenesis of OSBPL2 deficits in ADNSHL was still unclear. The function of OSBPL2 as a lipid-sensing regulator in multiple cellular processes suggested that OSBPL2 might play an important role in the regulation of cholesterol-homeostasis, which was essential for inner ear. In this study the potential roles of OSBPL2 in cholesterol biosynthesis and ROS production were investigated in Osbpl2-KO OC1 cells and osbpl2b-KO zebrafish. RNA-seq-based analysis suggested that OSBPL2 was implicated in cholesterol biosynthesis and AMPK signaling pathway. Furthermore, Osbpl2/osbpl2b-KO resulted in a reduction of AMPK activity and up-regulation of Srebp2/srebp2, Hmgcr/hmgcr and Hmgcs1/hmgcs1, key genes in the sterol biosynthetic pathway and associated with AMPK signaling. In addition, OSBPL2 was also found to interact with ATIC, key activator of AMPK. The levels of total cholesterol and ROS in OC1 cells or zebrafish inner ear were both increased in Osbpl2/osbpl2b-KO mutants and the mitochondrial damage was detected in Osbpl2-KO OC1 cells. This study uncovered the regulatory roles of OSBPL2 in cellular cholesterol biosynthesis and ROS production. These founds might contribute to the deep understanding of the pathogenesis of OSBPL2 mutation in ADNSHL.

OSBPL2-disrupted pigs recapitulate dual features of human hearing loss and hypercholesterolaemia

Oxysterol binding protein like 2 (OSBPL2), an important regulator in cellular lipid metabolism and transport, was identified as a novel deafness-causal gene in our previous work. To resemble the phenotypic features of OSBPL2 mutation in animal models and elucidate the potential genotype-phenotype associations, the OSBPL2-disrupted Bama miniature (BM) pig model was constructed using CRISPR/Cas9-mediated gene editing, somatic cell nuclear transfer (SCNT) and embryo transplantation approaches, and then subjected to phenotypic characterization of auditory function and serum lipid profiles. The OSBPL2-disrupted pigs displayed progressive hearing loss (HL) with degeneration/apoptosis of cochlea hair cells (HCs) and morphological abnormalities in HC stereocilia, as well as hypercholesterolaemia. High-fat diet (HFD) feeding aggravated the development of HL and led to more severe hypercholesterolaemia. The dual phenotypes of progressive HL and hypercholesterolaemia resembled in OSBPL2-disrupted pigs confirmed the implication of OSBPL2 mutation in nonsydromic hearing loss (NSHL) and contributed to the potential linkage between auditory dysfunction and dyslipidaemia/hypercholesterolaemia.

A novel pathogenic variant in OSBPL2 linked to hereditary late-onset deafness in a Mongolian family

BACKGROUND

To investigate the clinical features and the underlying causal gene of a family with hereditary late-onset deafness in Inner Mongolia of China, and to provide evidence for the early genetic screening and diagnosis of this disease.

METHODS

Family data were collected to draw a pedigree. Audiological testing and physical examination of the family members were conducted following questionnaire. Genomic DNA was extracted from peripheral blood of 5 family members (3 patients and 2 normal control) and subjected to whole genome sequencing for identifying deafness casual genes. The pathogenic variant in the deafness gene was further confirmed by Sanger sequencing.

RESULTS

The family is composed of a total of 6 generations, with 53 traceable individuals. In this family,19 of them were diagnosed with post lingual deafness with the age of onset between 10 and 40 years, displaying delayed and progressive hearing loss. Patients with hearing loss showed bilateral symmetry and mild to severe sensorineural deafness. The pattern of deafness inheritance in this family is autosomal dominant. Whole genome sequencing identified a novel pathogenic frameshift mutation, c.158_159delAA (p.Gln53Arg fs*100) in the gene OSBPL2 (Oxysterol-binding protein-related protein 2, NM_144498.2), which is absent from genomic data of 201 unrelated normal subjects. This pathogenic variant was further validated by Sanger sequencing, and was found to co-segregate in this family.

CONCLUSIONS

Whole genome sequencing identified a two-nucleotide deletion in OSBPL2 (c.158_159delAA) as the pathogenic variant for deafness in the family. Our finding expands the mutational spectrum of OSBPL2 and contributes to the pathogenic variant list in genetic counseling for deafness screening.

25-hydroxycholesterol down-regulates oxysterol binding protein like 2 (OSBPL2) via the p53/SREBF2/NFYA signaling pathway

Oxysterol Binding Protein Like 2 (OSBPL2) is a lipid-binding protein implicated in various cellular processes. Previous studies have shown that depression of OSBPL2 significantly increases the level of cellular 25-hydroxycholesterol (25-OHC) which regulates the expression of lipid-metabolism-related genes. However, whether 25-OHC can regulate the expression of OSBPL2 remains unanswered. This study aimed to explore the molecular mechanism of 25-OHC regulating the expression of OSBPL2. Using dual-luciferase reporter assay, we found a decrease of nuclear transcription factor Y subunit alpha (NFYA) bound with OSBPL2 promoter when HeLa cells were treated with 25-OHC. Furthermore, transcriptome sequencing and RNA interference results revealed that the p53/sterol regulatory element binding transcription factor 2 (SREBF2) signaling pathway was involved in the NFYA-dependent transcription of OSBPL2 induced by 25-OHC. Based on these results, we concluded that pleomorphic adenoma gene 1 (PLAG1) and NFYA participated in the basal transcription of OSBPL2 and that 25-OHC decreased the transcription of OSBPL2 via the p53/SREBF2/NFYA signaling pathway. 25-OHC will accumulate over time in OSBPL2 knockdown cells. These results may provide a new insight into the deafness caused by OSBPL2 mutation.

ORP2 interacts with phosphoinositides and controls the subcellular distribution of cholesterol

ORP2 is a sterol-binding protein with documented functions in lipid and glucose metabolism, Akt signaling, steroidogenesis, cell adhesion, migration and proliferation. Here we investigate the interactions of ORP2 with phosphoinositides (PIPs) by surface plasmon resonance (SPR), its affinity for cholesterol with a pull-down assay, and its capacity to transfer sterol in vitro. Moreover, we determine the effects of wild-type (wt) ORP2 and a mutant with attenuated PIP binding, ORP2(mHHK), on the subcellular distribution of cholesterol, and analyze the interaction of ORP2 with the related cholesterol transporter ORP1L. ORP2 showed specific affinity for PI(4,5)P₂, PI(3,4,5)P₃ and PI(4)P, with suggestive K_d values in the μM range. Also binding of cholesterol by ORP2 was detectable, but a K_d could not be determined. Wt ORP2 was in HeLa cells mainly detected in the cytosol, ER, late endosomes, and occasionally on lipid droplets (LDs), while ORP2(mHHK) displayed an enhanced LD localization. Overexpression of wt ORP2 shifted the D4H cholesterol probe away from endosomes, while ORP2(mHHK) caused endosomal accumulation of the probe. Although ORP2 failed to transfer dehydroergosterol in an in vitro assay where OSBP is active, its knock-down resulted in the accumulation of cholesterol in late endocytic compartments, as detected by both D4H and filipin probes. Interestingly, ORP2 was shown to interact and partially co-localize on late endosomes with ORP1L, a cholesterol transporter/sensor at ER-late endosome junctions. Our data demonstrates that ORP2 binds several phosphoinositides, both PI(4)P and multiply phosphorylated species. ORP2 regulates the subcellular distribution of cholesterol dependent on its PIP-binding capacity. The interaction of ORP2 with ORP1L suggests a concerted action of the two ORPs.

OSBP-related protein-2 (ORP2): a novel Akt effector that controls cellular energy metabolism

ORP2 is a ubiquitously expressed OSBP-related protein previously implicated in endoplasmic reticulum (ER)-lipid droplet (LD) contacts, triacylglycerol (TG) metabolism, cholesterol transport, adrenocortical steroidogenesis, and actin-dependent cell dynamics. Here, we characterize the role of ORP2 in carbohydrate and lipid metabolism by employing ORP2-knockout (KO) hepatoma cells (HuH7) generated by CRISPR-Cas9 gene editing. The ORP2-KO and control HuH7 cells were subjected to RNA sequencing, analyses of Akt signaling, carbohydrate and TG metabolism, the extracellular acidification rate, and the lipidome, as well as to transmission electron microscopy. The loss of ORP2 resulted in a marked reduction of active phosphorylated Akt(Ser473) and its target Glycogen synthase kinase 3β(Ser9), consistent with defective Akt signaling. ORP2 was found to form a physical complex with the key controllers of Akt activity, Cdc37, and Hsp90, and to co-localize with Cdc37 and active Akt(Ser473) at lamellipodial plasma membrane regions, in addition to the previously reported ER-LD localization. ORP2-KO reduced glucose uptake, glycogen synthesis, glycolysis, mRNA-encoding glycolytic enzymes, and SREBP-1 target gene expression, and led to defective TG synthesis and storage. ORP2-KO did not reduce but rather increased ER-LD contacts under basal culture conditions and interfered with their expansion upon fatty acid loading. Together with our recently published work (Kentala et al. in FASEB J 32:1281-1295, 2018), this study identifies ORP2 as a new regulatory nexus of Akt signaling, cellular energy metabolism, actin cytoskeletal function, cell migration, and proliferation.

Bridging the molecular and biological functions of the oxysterol-binding protein family

Oxysterol-binding protein (OSBP) and OSBP-related proteins (ORPs) constitute a large eukaryotic gene family that transports and regulates the metabolism of sterols and phospholipids. The original classification of the family based on oxysterol-binding activity belies the complex dual lipid-binding specificity of the conserved OSBP homology domain (OHD). Additional protein- and membrane-interacting modules mediate the targeting of select OSBP/ORPs to membrane contact sites between organelles, thus positioning the OHD between opposing membranes for lipid transfer and metabolic regulation. This unique subcellular location, coupled with diverse ligand preferences and tissue distribution, has identified OSBP/ORPs as key arbiters of membrane composition and function. Here, we will review how molecular models of OSBP/ORP-mediated intracellular lipid transport and regulation at membrane contact sites relate to their emerging roles in cellular and organismal functions.

Posaconazole inhibits dengue virus replication by targeting oxysterol-binding protein

Dengue virus (DENV) is associated with an estimated 390 million infections per year, occurring across approximately 100 countries in tropical and sub-tropical regions. To date, there are no antiviral drugs or specific therapies to treat DENV infection. Posaconazole and itraconazole are potent antifungal drugs that inhibit ergosterol biosynthesis in fungal cells, but also target a number of human proteins. Here, we show that itraconazole and posaconazole have antiviral activity against DENV. Posaconazole inhibited replication of multiple serotypes of DENV and the related flavivirus Zika virus, and reduced viral RNA replication, but not translation of the viral genome. We used a combination of knockdown and drug sensitization assays to define the molecular target of posaconazole that mediates its antiviral activity. We found that knockdown of oxysterol-binding protein (OSBP) inhibited DENV replication. Moreover, knockdown of OSBP, but not other known targets of posaconazole, enhanced the inhibitory effect of posaconazole. Our findings imply OSBP as a potential target for the development of antiviral compounds against DENV.

Analysis of ORP2-knockout hepatocytes uncovers a novel function in actin cytoskeletal regulation

ORP2 is implicated in cholesterol transport, triglyceride metabolism, and adrenocortical steroid hormone production. We addressed ORP2 function in hepatocytes by generating ORP2-knockout (KO) HuH7 cells by CRISPR-Cas9 gene editing, followed by analyses of transcriptome, F-actin morphology, migration, adhesion, and proliferation. RNA sequencing of ORP2-KO cells revealed >2-fold changes in 579 mRNAs. The Ingenuity Pathway Analysis (IPA) uncovered alterations in the following functional categories: cellular movement, cell-cell signaling and interaction, cellular development, cellular function and maintenance, cellular growth and proliferation, and cell morphology. Many pathways in these categories involved actin cytoskeleton, cell migration, adhesion, or proliferation. Analysis of the ORP2 interactome uncovered 109 putative new partners. Their IPA analysis revealed Ras homolog A (RhoA) signaling as the most significant pathway. Interactions of ORP2 with SEPT9, MLC12, and ARHGAP12 were validated by independent assays. ORP2-KO resulted in abnormal F-actin morphology characterized by impaired capacity to form lamellipodia, migration defect, and impaired adhesion and proliferation. Rescue of the migration phenotype and generation of typical cell surface morphology required an intact ORP2 phosphoinositide binding site, suggesting that ORP2 function involves phosphoinositide binding and transport. The results point at a novel function of ORP2 as a lipid-sensing regulator of the actin cytoskeleton, with impacts on hepatocellular migration, adhesion, and proliferation.-Kentala, H., Koponen, A., Kivelä, A. M., Andrews, R., Li, C., Zhou, Y., Olkkonen, V. M. Analysis of ORP2-knockout hepatocytes uncovers a novel function in actin cytoskeletal regulation.

Modulation of Lipid Droplet Metabolism-A Potential Target for Therapeutic Intervention in Flaviviridae Infections

Lipid droplets (LDs) are endoplasmic reticulum (ER)-related dynamic organelles that store and regulate fatty acids and neutral lipids. They play a central role in cellular energy storage, lipid metabolism and cellular homeostasis. It has become evident that viruses have co-evolved in order to exploit host lipid metabolic pathways. This is especially characteristic of the Flaviviridae family, including hepatitis C virus (HCV) and several flaviviruses. Devoid of an appropriate lipid biosynthetic machinery of their own, these single-strand positive-sense RNA viruses can induce dramatic changes in host metabolic pathways to establish a favorable environment for viral multiplication and acquire essential components to facilitate their assembly and traffic. Here we have reviewed the current knowledge on the intracellular life cycle of those from the Flaviviridae family, with particular emphasis on HCV and dengue virus (DENV), and their association with the biosynthesis and metabolism of LDs, with the aim to identify potential antiviral targets for development of novel therapeutic interventions.

Blood hsa-miR-122-5p and hsa-miR-885-5p levels associate with fatty liver and related lipoprotein metabolism-The Young Finns Study

MicroRNAs are involved in disease development and may be utilized as biomarkers. We investigated the association of blood miRNA levels and a) fatty liver (FL), b) lipoprotein and lipid pathways involved in liver lipid accumulation and c) levels of predicted mRNA targets in general population based cohort. Blood microRNA profiling (TaqMan OpenArray), genome-wide gene expression arrays and nuclear magnetic resonance metabolomics were performed for Young Finns Study participants aged 34–49 years (n = 871). Liver fat status was assessed ultrasonographically. Levels of hsa-miR-122-5p and -885-5p were up-regulated in individuals with FL (fold change (FC) = 1.55, p = 1.36 * 10⁻¹⁴ and FC = 1.25, p = 4.86 * 10⁻⁴, respectively). In regression model adjusted with age, sex and BMI, hsa-miR-122-5p and -885-5p predicted FL (OR = 2.07, p = 1.29 * 10⁻⁸ and OR = 1.41, p = 0.002, respectively). Together hsa-miR-122-5p and -885-5p slightly improved the detection of FL beyond established risk factors. These miRNAs may be associated with FL formation through the regulation of lipoprotein metabolism as hsa-miR-122-5p levels associated with small VLDL, IDL, and large LDL lipoprotein subclass components, while hsa-miR-885-5p levels associated inversely with XL HDL cholesterol levels. Hsa-miR-885-5p levels correlated inversely with oxysterol-binding protein 2 (OSBPL2) expression (r = −0.143, p = 1.00 * 10⁻⁴) and suppressing the expression of this lipid receptor and sterol transporter could link hsa-miR-885-5p with HDL cholesterol levels.

Oxysterols: Old Tale, New Twists

Oxysterols have long been known for their important role in cholesterol homeostasis, where they are involved in both transcriptional and posttranscriptional mechanisms for controlling cholesterol levels. However, they are increasingly associated with a wide variety of other, sometimes surprising cell functions. They are activators of the Hedgehog pathway (important in embryogenesis), and they act as ligands for a growing list of receptors, including some that are of importance to the immune system. Oxysterols have also been implicated in several diseases such as neurodegenerative diseases and atherosclerosis. Here, we explore the latest research into the roles oxy-sterols play in different areas, and we evaluate the current evidence for these roles. In addition, we outline critical concepts to consider when investigating the roles of oxysterols in various situations, which includes ensuring that the concentration and form of the oxysterol are relevant in that context--a caveat with which many studies have struggled.

Oxysterols: From cholesterol metabolites to key mediators

Oxysterols are cholesterol metabolites that can be produced through enzymatic or radical processes. They constitute a large family of lipids (i.e. the oxysterome) involved in a plethora of physiological processes. They can act through GPCR (e.g. EBI2, SMO, CXCR2), nuclear receptors (LXR, ROR, ERα) and through transporters or regulatory proteins. Their physiological effects encompass cholesterol, lipid and glucose homeostasis. Additionally, they were shown to be involved in other processes such as immune regulatory functions and brain homeostasis. First studied as precursors of bile acids, they quickly emerged as interesting lipid mediators. Their levels are greatly altered in several pathologies and some oxysterols (e.g. 4β-hydroxycholesterol or 7α-hydroxycholestenone) are used as biomarkers of specific pathologies. In this review, we discuss the complex metabolism and molecular targets (including binding properties) of these bioactive lipids in human and mice. We also discuss the genetic mouse models currently available to interrogate their effects in pathophysiological settings. We also summarize the levels of oxysterols reported in two key organs in oxysterol metabolism (liver and brain), plasma and cerebrospinal fluid. Finally, we consider future opportunities and directions in the oxysterol field in order to gain a better insight and understanding of the complex oxysterol system.

Intracellular cholesterol transport proteins: roles in health and disease

Effective cholesterol homoeostasis is essential in maintaining cellular function, and this is achieved by a network of lipid-responsive nuclear transcription factors, and enzymes, receptors and transporters subject to post-transcriptional and post-translational regulation, whereas loss of these elegant, tightly regulated homoeostatic responses is integral to disease pathologies. Recent data suggest that sterol-binding sensors, exchangers and transporters contribute to regulation of cellular cholesterol homoeostasis and that genetic overexpression or deletion, or mutations, in a number of these proteins are linked with diseases, including atherosclerosis, dyslipidaemia, diabetes, congenital lipoid adrenal hyperplasia, cancer, autosomal dominant hearing loss and male infertility. This review focuses on current evidence exploring the function of members of the ‘START’ (steroidogenic acute regulatory protein-related lipid transfer) and ‘ORP’ (oxysterol-binding protein-related proteins) families of sterol-binding proteins in sterol homoeostasis in eukaryotic cells, and the evidence that they represent valid therapeutic targets to alleviate human disease.

Oxysterols in Metabolic Syndrome: From Bystander Molecules to Bioactive Lipids

Oxysterols are cholesterol metabolites now considered bona fide bioactive lipids. Recent studies have identified new receptors for oxysterols involved in immune and inflammatory processes, hence reviving their appeal. Through multiple receptors, oxysterols are involved in numerous metabolic and inflammatory processes, thus emerging as key mediators in metabolic syndrome. This syndrome is characterized by complex interactions between inflammation and a dysregulated metabolism. Presently, the use of synthetic ligands and genetic models has facilitated a better understanding of the roles of oxysterols in metabolism, but also raised interesting questions. We discuss recent findings on the absolute levels of oxysterols in tissues, their newly identified targets, and the mechanistic studies emphasizing their importance in metabolic disease, as there is a pressing need to further comprehend these intriguing bioactive lipids.

Oxysterol-related-binding-protein related Protein-2 (ORP2) regulates cortisol biosynthesis and cholesterol homeostasis

Oxysterol binding protein-related protein 2 (ORP2) is a lipid binding protein that has been implicated in various cellular processes, including lipid sensing, cholesterol efflux, and endocytosis. We recently identified ORP2 as a member of a protein complex that regulates glucocorticoid biosynthesis. Herein, we examine the effect of silencing ORP2 on adrenocortical function and show that the ORP2 knockdown cells exhibit reduced amounts of multiple steroid metabolites, including progesterone, 11-deoxycortisol, and cortisol, but have increased concentrations of androgens, and estrogens. Moreover, silencing ORP2 suppresses the expression of most proteins required for cortisol production and reduces the expression of steroidogenic factor 1 (SF1). ORP2 silencing also increases cellular cholesterol, concomitant with decreased amounts of 22-hydroxycholesterol and 7-ketocholesterol, two molecules that have been shown to bind to ORP2. Further, we show that ORP2 binds to liver X receptor (LXR) and is required for nuclear LXR expression. LXR and ORP2 are recruited to the CYP11B1 promoter in response to cAMP signaling. Additionally, ORP2 is required for the expression of other LXR target genes, including ABCA1 and the LDL receptor (LDLR). In summary, we establish a novel role for ORP2 in regulating steroidogenic capacity and cholesterol homeostasis in the adrenal cortex.

Spatial and temporal expression patterns of Osbpl2a and Osbpl2b during zebrafish embryonic development

OBJECTIVE

The mutated OSBPL2 (OMIM: 606731), encoding oxysterol binding protein-like protein 2, was recently identified as a novel causative gene for autosomal dominant nonsyndromic hearing loss (ADNSHL). We reported the expression patterns of Osbpl2 in zebrafish, in order to further understand the role of OSBPL2 in hearing formation and development.

METHODS

Zebrafish was used as an animal model, and the expression of Osbpl2 was investigated by whole mount in situ hybridization.

RESULTS

Bioinformatics analysis indicates that zebrafish has two homologues of Osbpl2 gene (Osbpl2a and Osbpl2b) and Osbpl2b is the orthologous gene of human OSBPL2. No expression of Osbpl2a and Osbpl2b mRNA is detected at 75% epiboly. The zygotical expression of the two genes has not been started at 11-somite stage. At 24h post-fertilization (hpf), both Osbpl2a and Osbpl2b are found at ventricle zone of brain, however, the expression level of Osbpl2a is higher than that of Osbpl2b. When embryos are 48hpf, the expression level of Osbpl2a and Osbpl2b becomes higher at the ventricle zone. At 72hpf, Osbpl2b is only found at liver primordium, while Osbpl2a is not detected anywhere obviously. At 96hpf, Osbpl2b is found at pharyngeal arches, liver, digestive tract and otic vesicle, while Osbpl2a remains undetected.

CONCLUSION

Osbpl2b was demonstrated to be the orthologous gene of human OSBPL2, which has strong maternal expression, while Osbpl2a was detected without obvious maternal expression. This work would contribute to the further study of the molecular mechanism and function of OSBPL2 implicated with ADNSHL.

miRNA Targeting of Oxysterol-Binding Protein-Like 6 Regulates Cholesterol Trafficking and Efflux

OBJECTIVE

Cholesterol homeostasis is fundamental to human health and is, thus, tightly regulated. MicroRNAs exert potent effects on biological pathways, including cholesterol metabolism, by repressing genes with related functions. We reasoned that this mode of pathway regulation could be exploited to identify novel genes involved in cholesterol homeostasis.

APPROACH AND RESULTS

Here, we identify oxysterol-binding protein-like 6 (OSBPL6) as a novel target of 2 miRNA hubs regulating cholesterol homeostasis: miR-33 and miR-27b. Characterization of OSBPL6 revealed that it is transcriptionally regulated in macrophages and hepatocytes by liver X receptor and in response to cholesterol loading and in mice and nonhuman primates by Western diet feeding. OSBPL6 encodes the OSBPL-related protein 6 (ORP6), which contains dual membrane- and endoplasmic reticulum-targeting motifs. Subcellular localization studies showed that ORP6 is associated with the endolysosomal network and endoplasmic reticulum, suggesting a role for ORP6 in cholesterol trafficking between these compartments. Accordingly, knockdown of OSBPL6 results in aberrant clustering of endosomes and promotes the accumulation of free cholesterol in these structures, resulting in reduced cholesterol esterification at the endoplasmic reticulum. Conversely, ORP6 overexpression enhances cholesterol trafficking and efflux in macrophages and hepatocytes. Moreover, we show that hepatic expression of OSBPL6 is positively correlated with plasma levels of high-density lipoprotein cholesterol in a cohort of 200 healthy individuals, whereas its expression is reduced in human atherosclerotic plaques.

CONCLUSIONS

These studies identify ORP6 as a novel regulator of cholesterol trafficking that is part of the miR-33 and miR-27b target gene networks that contribute to the maintenance of cholesterol homeostasis.