Correlation between oxysterol binding to a cytosolic binding protein and potency in the repression of hydroxymethylglutaryl coenzyme A reductase

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.

Lipid Sorting and Organelle Identity

The sorting and trafficking of lipids between organelles gives rise to a dichotomy of bulk membrane properties between organelles of the secretory and endolysosome networks, giving rise to two "membrane territories" based on differences in lipid-packing density, net membrane charge, and bilayer leaflet asymmetries. The cellular organelle membrane dichotomy emerges from ER-to-PM anterograde membrane trafficking and the synthesis of sphingolipids and cholesterol flux at the trans-Golgi network, which constitutes the interface between the two membrane territories. Organelle homeostasis is maintained by vesicle-mediated retrieval of bulk membrane from the distal organelles of each territory to the endoplasmic reticulum or plasma membrane and by soluble lipid transfer proteins that traffic particular lipids. The concept of cellular membrane territories emphasizes the contrasting features of organelle membranes of the secretory and endolysosome networks and the essential roles of lipid-sorting pathways that maintain organelle function.

Structure-Activity Relationships of Ligand Binding to Oxysterol-Binding Protein (OSBP) and OSBP-Related Protein 4

Oxysterol-binding protein (OSBP) and OSBP-related protein 4 (ORP4) have emerged as potentially druggable targets in antiviral and precision cancer drug development. Multiple structurally diverse small molecules function through targeting the OSBP/ORP family of proteins, including the antiviral steroidal compounds OSW-1 and T-00127-HEV2. Here, the structure-activity relationships of oxysterols and related compound binding to human OSBP and ORP4 are characterized. Oxysterols with hydroxylation at various side chain positions (i.e., C-20, C-24, C-25, and C-27)─but not C-22─confer high affinity interactions with OSBP and ORP4. A library of 20(S)-hydroxycholesterol analogues with varying sterol side chains reveal that side chain length modifications are not well tolerated for OSBP and ORP4 interactions. This side chain requirement is contradicted by the high affinity binding of T-00127-HEV2, a steroidal compound lacking the side chain. The binding results, in combination with docking studies using homology models of OSBP and ORP4, suggest multiple modes of steroidal ligand binding to OSBP and ORP4.

Fungal oxysterol-binding protein-related proteins promote pathogen virulence and activate plant immunity

Oxysterol-binding protein-related proteins (ORPs) are a conserved class of lipid transfer proteins that are closely involved in multiple cellular processes in eukaryotes, but their roles in plant-pathogen interactions are mostly unknown. We show that transient expression of ORPs of Magnaporthe oryzae (MoORPs) in Nicotiana benthamina plants triggered oxidative bursts and cell death; treatment of tobacco Bright Yellow-2 suspension cells with recombinant MoORPs elicited the production of reactive oxygen species. Despite ORPs being normally described as intracellular proteins, we detected MoORPs in fungal culture filtrates and intercellular fluids from barley plants infected with the fungus. More importantly, infiltration of Arabidopsis plants with recombinant Arabidopsis or fungal ORPs activated oxidative bursts, callose deposition, and PR1 gene expression, and enhanced plant disease resistance, implying that ORPs may function as endogenous and exogenous danger signals triggering plant innate immunity. Extracellular application of fungal ORPs exerted an opposite impact on salicylic acid and jasmonic acid/ethylene signaling pathways. Brassinosteroid Insensitive 1-associated Kinase 1 was dispensable for the ORP-activated defense. Besides, simultaneous knockout of MoORP1 and MoORP3 abolished fungal colony radial growth and conidiation, whereas double knockout of MoORP1 and MoORP2 compromised fungal virulence on barley and rice plants. These observations collectively highlight the multifaceted role of MoORPs in the modulation of plant innate immunity and promotion of fungal development and virulence in M. oryzae.

Coordination of inter-organelle communication and lipid fluxes by OSBP-related proteins

Oxysterol-binding protein (OSBP) and OSBP-related proteins (ORPs) constitute one of the largest families of lipid-binding/transfer proteins (LTPs) in eukaryotes. The current view is that many of them mediate inter-organelle lipid transfer over membrane contact sites (MCS). The transfer occurs in several cases in a 'counter-current' fashion: A lipid such as cholesterol or phosphatidylserine (PS) is transferred against its concentration gradient driven by transport of a phosphoinositide in the opposite direction. In this way ORPs are envisioned to maintain the distinct organelle lipid compositions, with impacts on multiple organelle functions. However, the functions of ORPs extend beyond lipid homeostasis to regulation of processes such as cell survival, proliferation and migration. Important expanding areas of mammalian ORP research include their roles in viral and bacterial infections, cancers, and neuronal function. The yeast OSBP homologue (Osh) proteins execute multifaceted functions in sterol and glycerophospholipid homeostasis, post-Golgi vesicle transport, phosphatidylinositol-4-phosphate, sphingolipid and target of rapamycin (TOR) signalling, and cell cycle control. These observations identify ORPs as lipid transporters and coordinators of signals with an unforeseen variety of cellular processes. Understanding their activities not only enlightens the biology of the living cell but also allows their employment as targets of new therapeutic approaches for disease.

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.

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.

The crystal structure of ORP3 reveals the conservative PI4P binding pattern

Oxysterol-binding protein (OSBP) and its related protein (ORP) constitute a conserved family of lipid transfer proteins (LTPs). ORPs have been implicated as intracellular lipid exchanger and sensor in recent years, which regulate the lipid homeostasis and signal pathway. OSBP-related protein 3 plays key role in controlling cell adhesion and migration and could be developed as the drug target for cancer therapy. Here, we report the crystal structures of human ORP3 ORD to 2.1 Å and ORD-PI4P complex to 3.2 Å. The binding assay in vitro confirms the ORP3 has the capability of PI4P binding. This study further verifies that the PI4P is the common ligand of all ORPs and ORPs should be the lipid exchanger in membrane contact sites(MCS).

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.

Oxysterol-binding protein-related protein 1 variants have opposing cholesterol transport activities from the endolysosomes

OSBPL1 encodes the full-length oxysterol-binding protein-related protein ORP1L, which transports LDL-derived cholesterol at membrane contacts between the late endosomes/lysosomes (LEL) and the endoplasmic reticulum (ER). OSBPL1 also encodes the truncated variant ORP1S that contains only the C-terminal lipid binding domain. HeLa cells in which both variants were knocked out (ORP1-null) were used to determine the functional relationship between ORP1L and ORP1S with respect to cellular cholesterol localization and regulation. ORP1-null cells accumulated cholesterol in LEL and had reduced plasma membrane (PM) cholesterol. PM cholesterol was restored by expression of wild-type ORP1S or a phosphatidylinositol phosphate-binding mutant but not by a sterol-binding mutant. Expression of ORP2, another truncated variant, also restored PM cholesterol in ORP1-null cells. Consistent with a LEL-to-PM cholesterol transport activity, a small fraction of ORP1S was detected on the PM. As a consequence of reduced delivery of cholesterol to the PM in ORP1-null cells, cholesterol was diverted to the ER resulting in normalization of de novo cholesterol synthesis. The deficiency in PM cholesterol also reduced ABCA1-dependent cholesterol efflux and LDL receptor activity in ORP1-null cells. We conclude that ORP1S, which lacks discrete membrane-targeting motifs, transports cholesterol from LEL to the PM.

The role of mitochondria-associated membranes in cellular homeostasis and diseases

Mitochondria and endoplasmic reticulum (ER) are fundamental in the control of cell physiology regulating several signal transduction pathways. They continuously communicate exchanging messages in their contact sites called MAMs (mitochondria-associated membranes). MAMs are specific microdomains acting as a platform for the sorting of vital and dangerous signals. In recent years increasing evidence reported that multiple scaffold proteins and regulatory factors localize to this subcellular fraction suggesting MAMs as hotspot signaling domains. In this review we describe the current knowledge about MAMs' dynamics and processes, which provided new correlations between MAMs' dysfunctions and human diseases. In fact, MAMs machinery is strictly connected with several pathologies, like neurodegeneration, diabetes and mainly cancer. These pathological events are characterized by alterations in the normal communication between ER and mitochondria, leading to deep metabolic defects that contribute to the progression of the diseases.

ORP4L couples IP3 to ITPR1 in control of endoplasmic reticulum calcium release

Oxysterol-binding protein-related protein (ORP) 4L acts as a scaffold protein assembling CD3-ε, G-α_q/11, and PLC-β3 into a complex at the plasma membrane that mediates inositol (1,4,5)-trisphosphate (IP₃)-induced endoplasmic reticulum (ER) Ca²⁺ release and oxidative phosphorylation in T-cell acute lymphoblastic leukemia cells. Here, we offer new evidence that ORP4L interacts with the carboxyl terminus of the IP₃ receptor type 1 (ITPR1) in Jurkat T cells. ORP4L enables IP₃ binding to ITPR1; a truncated construct that lacks the ITPR1-binding region retains the ability to increase IP₃ production but fails to mediate IP₃ and ITPR1 binding. In association with this ability of ORP4L, it enhances Ca²⁺ release from the ER and subsequent cytosolic and mitochondrial parallel Ca²⁺ spike oscillations that stimulate mitochondrial energetics and thus maintains cell survival. These data support a novel model in which ORP4L is a cofactor of ITPR1, which increases ITPR1 sensitivity to IP₃ and enables ER Ca²⁺ release.-Cao, X., Chen, J., Li, D., Xie, P., Xu, M., Lin, W., Li, S., Pan, G., Tang, Y., Xu, J., Olkkonen, V. M., Yan, D., Zhong, W. ORP4L couples IP₃ to ITPR1 in control of endoplasmic reticulum calcium release.

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 4L promotes phospholipase Cβ3 translocation from the nucleus to the plasma membrane in Jurkat T-cells

Phosphoinositide phospholipases C (PLCs) are a family of eukaryotic intracellular enzymes with important roles in signal transduction. In addition to their location at the plasma membrane, PLCs also exist within the cell nucleus where they are stored. We previously demonstrated that OSBP-related protein 4L (ORP4L) anchors cluster of differentiation 3ϵ (CD3ϵ) to the heterotrimeric G protein subunit (Gα_q/11) to control PLCβ3 relocation and activation. However, the underlying mechanism by which ORP4L facilitates PLCβ3 translocation remains unknown. Here, using confocal immunofluorescence microscopy and coimmunoprecipitation assays, we report that ORP4L stimulates PLCβ3 translocation from the nucleus to the plasma membrane in Jurkat T-cells in two steps. First, we found that ORP4L is required for the activation of Ras-related nuclear protein (RAN), a GTP-binding nuclear protein that binds to exportin 1 and eventually promotes the nuclear export of PLCβ3. Second, we also observed that ORP4L interacts with vesicle-associated membrane protein-associated protein A (VAPA) through its two phenylalanines in an acidic tract (FFAT) motif. This complex enabled PLCβ3 movement to the plasma membrane, indicating that PLCβ3 translocation occurs in a VAPA-dependent manner. This study reveals detailed mechanistic insight into the role of ORP4L in PLCβ3 redistribution from storage within the nucleus to the plasma membrane via RAN activation and interaction with VAPA in Jurkat T-cells.

Intracellular and Plasma Membrane Events in Cholesterol Transport and Homeostasis

Cholesterol transport between intracellular compartments proceeds by both energy- and non-energy-dependent processes. Energy-dependent vesicular traffic partly contributes to cholesterol flux between endoplasmic reticulum, plasma membrane, and endocytic vesicles. Membrane contact sites and lipid transfer proteins are involved in nonvesicular lipid traffic. Only “active" cholesterol molecules outside of cholesterol-rich regions and partially exposed in water phase are able to fast transfer. The dissociation of partially exposed cholesterol molecules in water determines the rate of passive aqueous diffusion of cholesterol out of plasma membrane. ATP hydrolysis with concomitant conformational transition is required to cholesterol efflux by ABCA1 and ABCG1 transporters. Besides, scavenger receptor SR-B1 is involved also in cholesterol efflux by facilitated diffusion via hydrophobic tunnel within the molecule. Direct interaction of ABCA1 with apolipoprotein A-I (apoA-I) or apoA-I binding to high capacity binding sites in plasma membrane is important in cholesterol escape to free apoA-I. ABCG1-mediated efflux to fully lipidated apoA-I within high density lipoprotein particle proceeds more likely through the increase of “active” cholesterol level. Putative cholesterol-binding linear motifs within the structure of all three proteins ABCA1, ABCG1, and SR-B1 are suggested to contribute to the binding and transfer of cholesterol molecules from cytoplasmic to outer leaflets of lipid bilayer. Together, plasma membrane events and intracellular cholesterol metabolism and traffic determine the capacity of the cell for cholesterol efflux.

The Oxysterol-Binding Protein Cycle: Burning Off PI(4)P to Transport Cholesterol

To maintain an asymmetric distribution of ions across membranes, protein pumps displace ions against their concentration gradient by using chemical energy. Here, we describe a functionally analogous but topologically opposite process that applies to the lipid transfer protein (LTP) oxysterol-binding protein (OSBP). This multidomain protein exchanges cholesterol for the phosphoinositide phosphatidylinositol 4-phosphate [PI(4)P] between two apposed membranes. Because of the subsequent hydrolysis of PI(4)P, this counterexchange is irreversible and contributes to the establishment of a cholesterol gradient along organelles of the secretory pathway. The facts that some natural anti-cancer molecules block OSBP and that many viruses hijack the OSBP cycle for the formation of intracellular replication organelles highlight the importance and potency of OSBP-mediated lipid exchange. The architecture of some LTPs is similar to that of OSBP, suggesting that the principles of the OSBP cycle-burning PI(4)P for the vectorial transfer of another lipid-might be general.

A Facile Synthesis of 3β,25-dihydroxy-5α-Cholest-7-ene, the Precursor to HMG-CoA Reductase Inhibitor

An efficient four-step procedure is described for the synthesis of 3β,25-dihydroxy-5α-cholest-7-ene from 3β-hydroxy-5α-cholesta-7,24-diene in an overall yield of 34%. This product can serve as a precursor for the synthesis of an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase. The 3β-toluene- p-sulfonate ester of 5α-cholesta-7,24-diene was obtained from natural lanolin followed by treatment with toluene- p-sulfonyl chloride. The key step in the synthesis is a very mild method for the hydroxybromination of this ester with N-bromosuccinimide in water, followed by reduction with LiAlH4to obtain the 3β-toluene- p-sulfonate ester of 25-hydroxy-5α-cholesta-7-ene. The final product was obtained after desulfonation of 3β-toluene- p-sulfonyoxy-25-hydroxy-5α-cholesta-7-ene to afford 3β,25-dihydroxy-5α-cholest-7-ene in excellent yield. The reagents are all relatively cheap, non-toxic and stable.

The role of oxysterol-binding protein and its related proteins in cancer

Oxysterol-binding protein (OSBP) and its related proteins (ORPs) constitute a large, evolutionarily conserved family of lipid-binding proteins that are associated with a wide range of cellular activities. The core function of OSBP/ORPs appears to be moving lipids between cellular membranes in a non-vesicular manner. Recent studies have unveiled a novel, counter-transport mechanism of cellular lipid transfer mediated by OSBP/ORPs at the membrane contact sites that involves phosphatidylinositol 4-phosphate. Importantly, the OSBP/ORPs family has also been implicated in cell signalling pathways and cancer development. Here, we summarize recent progress in understanding the role of OSBP/ORPs in cancer development, and discuss how the lipid transfer function of OSBP/ORPs may underpin their role in tumorigenesis.

Over Six Decades of Discovery and Characterization of the Architecture at Mitochondria-Associated Membranes (MAMs)

The discovery of proteins regulating ER-mitochondria tethering including phosphofurin acidic cluster sorting protein 2 (PACS-2) and mitofusin-2 has pushed contact sites between the endoplasmic reticulum (ER) and mitochondria into the spotlight of cell biology. While the field is developing rapidly and controversies have come and gone multiple times during its history, it is sometimes overlooked that significant research has been done decades ago with the original discovery of these structures in the 1950s and the first characterization of their function (and coining of the term mitochondria-associated membrane, MAM) in 1990. Today, an ever-increasing array of proteins localize to the MAM fraction of the endoplasmic reticulum (ER) to regulate the interaction of this organelle with mitochondria. These mitochondria-ER contacts, sometimes referred to as MERCs, regulate a multitude of biological functions, including lipid metabolism, Ca²⁺ signaling, bioenergetics, inflammation, autophagy, mitochondrial structure, and apoptosis.

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.

Oxysterol binding protein-related protein 8 mediates the cytotoxicity of 25-hydroxycholesterol

Oxysterols are 27-carbon oxidized derivatives of cholesterol or by-products of cholesterol biosynthesis that can induce cell apoptosis in addition to a number of other bioactions. However, the mechanisms underlying this cytotoxicity are not completely understood. ORP8 is a member of the oxysterol binding protein-related protein (ORP) family, implicated in cellular lipid homeostasis, migration, and organization of the microtubule cytoskeleton. Here, we report that 25-hydroxycholesterol (OHC) induced apoptosis of the hepatoma cell lines, HepG2 and Huh7, via the endoplasmic reticulum (ER) stress response pathway, and ORP8 overexpression resulted in a similar cell response as 25-OHC, indicating a putative functional relationship between oxysterol cytotoxicity and ORP8. Further experiments demonstrated that ORP8 overexpression significantly enhanced the 25-OHC effect on ER stress and apoptosis in HepG2 cells. A truncated ORP8 construct lacking the ligand-binding domain or a closely related protein, ORP5, was devoid of this activity, evidencing for specificity of the observed effects. Importantly, ORP8 knockdown markedly dampened such responses to 25-OHC. Taken together, the present study suggests that ORP8 may mediate the cytotoxicity of 25-OHC.

Oxysterols: An emerging class of broad spectrum antiviral effectors

Oxysterols are a family of cholesterol oxidation derivatives that contain an additional hydroxyl, epoxide or ketone group in the sterol nucleus and/or a hydroxyl group in the side chain. The majority of oxysterols in the blood are of endogenous origin, derived from cholesterol via either enzymatic or non-enzymatic mechanisms. A large number of reports demonstrate multiple physiological roles of specific oxysterols. One such role is the inhibition of viral replication. This biochemical/biological property was first characterised against a number of viruses endowed with an external lipid membrane (enveloped viruses), although antiviral activity has since been observed in relation to several non-enveloped viruses. In the present paper, we review the recent findings about the broad antiviral activity of oxysterols against enveloped and non-enveloped human viral pathogens, and provide an overview of their putative antiviral mechnism(s).

OSBP-Related Protein Family: Mediators of Lipid Transport and Signaling at Membrane Contact Sites

Oxysterol-binding protein (OSBP) and its related protein homologs, ORPs, constitute a conserved family of lipid-binding/transfer proteins (LTPs) expressed ubiquitously in eukaryotes. The ligand-binding domain of ORPs accommodates cholesterol and oxysterols, but also glycerophospholipids, particularly phosphatidylinositol-4-phosphate (PI4P). ORPs have been implicated as intracellular lipid sensors or transporters. Most ORPs carry targeting determinants for the endoplasmic reticulum (ER) and non-ER organelle membrane. ORPs are located and function at membrane contact sites (MCSs), at which ER is closely apposed with other organelle limiting membranes. Such sites have roles in lipid transport and metabolism, control of Ca(2+) fluxes, and signaling events. ORPs are postulated either to transport lipids over MCSs to maintain the distinct lipid compositions of organelle membranes, or to control the activity of enzymes/protein complexes with functions in signaling and lipid metabolism. ORPs may transfer PI4P and another lipid class bidirectionally. Transport of PI4P followed by its hydrolysis would in this model provide the energy for transfer of the other lipid against its concentration gradient. Control of organelle lipid compositions by OSBP/ORPs is important for the life cycles of several pathogenic viruses. Targeting ORPs with small-molecular antagonists is proposed as a new strategy to combat viral infections. Several ORPs are reported to modulate vesicle transport along the secretory or endocytic pathways. Moreover, antagonists of certain ORPs inhibit cancer cell proliferation. Thus, ORPs are LTPs, which mediate interorganelle lipid transport and coordinate lipid signals with a variety of cellular regimes.

OSBP-Related Protein Family in Lipid Transport Over Membrane Contact Sites

Increasing evidence suggests that oxysterol-binding protein-related proteins (ORPs) localize at membrane contact sites, which are high-capacity platforms for inter-organelle exchange of small molecules and information. ORPs can simultaneously associate with the two apposed membranes and transfer lipids across the interbilayer gap. Oxysterol-binding protein moves cholesterol from the endoplasmic reticulum to trans-Golgi, driven by the retrograde transport of phosphatidylinositol-4-phosphate (PI4P). Analogously, yeast Osh6p mediates the transport of phosphatidylserine from the endoplasmic reticulum to the plasma membrane in exchange for PI4P, and ORP5 and -8 are suggested to execute similar functions in mammalian cells. ORPs may share the capacity to bind PI4P within their ligand-binding domain, prompting the hypothesis that bidirectional transport of a phosphoinositide and another lipid may be a common theme among the protein family. This model, however, needs more experimental support and does not exclude a function of ORPs in lipid signaling.

Sterol liganding of OSBP-related proteins (ORPs) regulates the subcellular distribution of ORP-VAPA complexes and their impacts on organelle structure

Oxysterol-binding protein (OSBP) and its homologues (ORPs) are lipid-binding/transfer proteins with affinity for oxysterols, cholesterol and glycerophospholipids. In addition to a ligand-binding domain, a majority of the ORPs carry a pleckstrin homology domain that targets organelle membranes via phosphoinositides, and a motif targeting the endoplasmic reticulum (ER) via VAMP-associated proteins (VAPs). We employed here Bimolecular Fluorescence Complementation (BiFC) to systematically assess the effects of sterol manipulation of HuH7 cells on complexes of established sterol-binding ORPs with their ER receptor, VAMP-associated protein A (VAPA). Depletion of cellular cholesterol with lipoprotein-deficient medium and Mevastatin caused concentration of OSBP-VAPA complexes and Golgi complex markers at a juxtanuclear position, an effect reversed by low-density lipoprotein treatment. A similar redistribution of OSBP-VAPA but not of sterol-binding deficient mutant OSBP(ΔELSK)-VAPA, occurred upon treatment with the high-affinity ligand, 25-hydroxycholesterol (25OHC), which reduced total and free cholesterol. ORP2-VAPA complexes, which localize in untreated cells at blob-like ER structures with associated lipid droplets, were redistributed upon treatment with the ORP2 ligand 22(R)OHC to a diffuse cytoplasmic/ER pattern and the plasma membrane. Analogously, distribution of ORP4L-VAPA complexes between the plasma membrane and vimentin intermediate filament associated compartments was modified by statin or 25OHC treatment. The treatments resulted in loss of vimentin co-localization, and sterol-binding deficient ORP4L(ΔELSR)-VAPA localized predominantly to the plasma membrane. In conclusion, treatment with statin or oxysterol ligands modify the subcellular targeting of ORP-VAPA complexes, consistent with the notion that this machinery controls lipid homeostasis and signaling at organelle interfaces.

Ligand-dependent localization and function of ORP-VAP complexes at membrane contact sites

Oxysterol-binding protein/OSBP-related proteins (ORPs) constitute a conserved family of sterol/phospholipid-binding proteins with lipid transporter or sensor functions. We investigated the spatial occurrence and regulation of the interactions of human OSBP/ORPs or the S. cerevisiae orthologs, the Osh (OSBP homolog) proteins, with their endoplasmic reticulum (ER) anchors, the VAMP-associated proteins (VAPs), by employing bimolecular fluorescence complementation and pull-down set-ups. The ORP-VAP interactions localize frequently at distinct subcellular sites, shown in several cases to represent membrane contact sites (MCSs). Using established ORP ligand-binding domain mutants and pull-down assays with recombinant proteins, we show that ORP liganding regulates the ORP-VAP association, alters the subcellular targeting of ORP-VAP complexes, or modifies organelle morphology. There is distinct protein specificity in the effects of the mutants on subcellular targeting of ORP-VAP complexes. We provide evidence that complexes of human ORP2 and VAPs at ER-lipid droplet interfaces regulate the hydrolysis of triglycerides and lipid droplet turnover. The data suggest evolutionarily conserved, complex ligand-dependent functions of ORP-VAP complexes at MCSs, with implications for cellular lipid homeostasis and signaling.

N-Hydroxyphthalimide catalyzed allylic oxidation of steroids with t-butyl hydroperoxide

A new and optimized procedure for the allylic oxidation of Δ(5)-steroids with t-butyl hydroperoxide in the presence of catalytic amounts of N-hydroxyphthalimide (NHPI) under mild conditions was developed, showing excellent regioselectivity and chemoselectivity (functional group compatibility). It was found that Co(OAc)2 could enhance the catalytic ability of NHPI resulting in better yields and shorter reaction times. The reaction mechanism and the scope of the reaction with a variety of Δ(5)-steroidal substrates were also investigated.

Genetic basis of haloperidol resistance in Saccharomyces cerevisiae is complex and dose dependent

The genetic basis of most heritable traits is complex. Inhibitory compounds and their effects in model organisms have been used in many studies to gain insights into the genetic architecture underlying quantitative traits. However, the differential effect of compound concentration has not been studied in detail. In this study, we used a large segregant panel from a cross between two genetically divergent yeast strains, BY4724 (a laboratory strain) and RM11_1a (a vineyard strain), to study the genetic basis of variation in response to different doses of a drug. Linkage analysis revealed that the genetic architecture of resistance to the small-molecule therapeutic drug haloperidol is highly dose-dependent. Some of the loci identified had effects only at low doses of haloperidol, while other loci had effects primarily at higher concentrations of the drug. We show that a major QTL affecting resistance across all concentrations of haloperidol is caused by polymorphisms in SWH1, a homologue of human oxysterol binding protein. We identify a complex set of interactions among the alleles of the genes SWH1, MKT1, and IRA2 that are most pronounced at a haloperidol dose of 200 µM and are only observed when the remainder of the genome is of the RM background. Our results provide further insight into the genetic basis of drug resistance.

Variation in response to a drug can be determined by many factors. In the model organism baker's yeast, many studies of chemical resistance traits have uncovered a complex genetic basis of such resistance. However, an in-depth study of how drug dose alters the effects of underlying genetic factors is lacking. Here, we employed linkage analysis to map the specific genetic loci underlying response to haloperidol, a small molecule therapeutic drug, using a large panel of segregants from a cross between two genetically divergent yeast strains BY (a laboratory strain) and RM (a vineyard strain). We found that loci associated with haloperidol resistance are dose-dependent. We also showed that variants in the oxysterol-binding-protein-like domain of the gene SWH1 underlie the major locus detected at all doses of haloperidol. Genetic interactions among genes SWH1, MKT1, and IRA2 in the RM background contribute to the differential response at high concentrations of haloperidol.

Binding domain-driven intracellular trafficking of sterols for synthesis of steroid hormones, bile acids and oxysterols

Steroid hormones, bioactive oxysterols and bile acids are all derived from the biological metabolism of lipid cholesterol. The enzymatic pathways generating these compounds have been an area of intense research for almost a century, as cholesterol and its metabolites have substantial impacts on human health. Owing to its high degree of hydrophobicity and the chemical properties that it confers to biological membranes, the distribution of cholesterol in cells is tightly controlled, with subcellular organelles exhibiting highly divergent levels of cholesterol. The manners in which cells maintain such sterol distributions are of great interest in the study of steroid and bile acid synthesis, as limiting cholesterol substrate to the enzymatic pathways is the principal mechanism by which production of steroids and bile acids is regulated. The mechanisms by which cholesterol moves within cells, however, remain poorly understood. In this review, we examine the subcellular machinery involved in cholesterol metabolism to steroid hormones and bile acid, relating it to both lipid- and protein-based mechanisms facilitating intracellular and intraorganellar cholesterol movement and delivery to these pathways. In particular, we examine evidence for the involvement of specific protein domains involved in cholesterol binding, which impact cholesterol movement and metabolism in steroidogenesis and bile acid synthesis. A better understanding of the physical mechanisms by which these protein- and lipid-based systems function is of fundamental importance to understanding physiological homeostasis and its perturbation.

Oligo-astheno-teratozoospermia in mice lacking ORP4, a sterol-binding protein in the OSBP-related protein family

Oligo-astheno-teratozoospermia (OAT), a condition that includes low sperm number, low sperm motility and abnormal sperm morphology, is the commonest cause of male infertility. Because genetic analysis is frequently impeded by the infertility phenotype, the genetic basis of many of OAT conditions has been hard to verify. Here, we show that deficiency of ORP4, a sterol-binding protein in the oxysterol-binding protein (OSBP)-related protein family, causes male infertility due to severe OAT in mice. In ORP4-deficient mice, spermatogonia proliferation and subsequent meiosis occurred normally, but the morphology of elongating and elongated spermatids was severely distorted, with round-shaped head, curled back head or symplast. Spermatozoa derived from ORP4-deficient mice had little or no motility and no fertilizing ability in vitro. In ORP4-deficient testis, postmeiotic spermatids underwent extensive apoptosis, leading to a severely reduced number of spermatozoa. At the ultrastructural level, nascent acrosomes appeared to normally develop in round spermatids, but acrosomes were detached from the nucleus in elongating spermatids. These results suggest that ORP4 is essential for the postmeiotic differentiation of germ cells.

OSBP-related proteins: liganding by glycerophospholipids opens new insight into their function

Oxysterol-binding protein (OSBP) and its homologs designated OSBP-related (ORP) or OSBP-like (OSBPL) proteins constitute a conserved family of lipid binding/transfer proteins (LTP) in eukaryotes. The mechanisms of ORP function have remained incompletely understood, but they have been implicated as intracellular sterol sensors or transporters. A number of studies have provided evidence for the roles of ORPs at membrane contact sites (MCS), where endoplasmic reticulum is closely apposed with other organelle limiting membranes. ORPs are postulated to either transport sterols over MCSs or control the activity of enzymatic effectors or assembly of protein complexes with functions in signaling and lipid metabolism. Studies of yeast Saccharomyces cerevisiae ORPs Osh4p, Osh3p, Osh6p and Osh7p have revealed that ORPs do not exclusively bind sterols within their OSBP-related ligand-binding domain (ORD): The Osh4p ORD accommodates either sterols or phosphatidylinositol-4-phosphate (PI4P), and the Osh3p ORD was shown to specifically bind PI4P, the binding cavity being too narrow for a sterol to fit in. Most recently, Osh6p and Osh7p were demonstrated to show specific affinity for phosphatidylserine (PS), and to play a role in the intracellular transport of this glycerophospholipid; Additionally, two mammalian ORPs were shown to bind PS. Thus, the term frequently used for ORPs/OSBPLs, oxysterol-binding proteins, is a misnomer. While a number of ORPs bind oxysterols or cholesterol, other family members appear to interact with phospholipid ligands to regulate lipid fluxes, organelle lipid compositions and cell signaling. As a conclusion, ORPs are LTPs with a wide ligand spectrum and marked functional heterogeneity.

Oxysterol-binding protein (OSBP) is required for the perinuclear localization of intra-Golgi v-SNAREs

OSBP regulates the Golgi cholesterol level. This study demonstrates that OSBP and cholesterol are essential for localization of Golgi v-SNAREs. Knockdown of ArfGAP1 restores v-SNARE localization in OSBP-depleted cells, suggesting that OSBP-regulated cholesterol ensures proper COP-I vesicle transport.

Oxysterol-binding protein (OSBP) and OSBP-related proteins (ORPs) have been implicated in the distribution of sterols among intracellular organelles. OSBP regulates the Golgi cholesterol level, but how it relates to Golgi function is elusive. Here we report that OSBP is essential for the localization of intra-Golgi soluble vesicle N-ethylmaleimide-sensitive fusion attachment protein receptors (v-SNAREs). Depletion of OSBP by small interfering RNA causes mislocalization of intra-Golgi v-SNAREs GS28 and GS15 throughout the cytoplasm without affecting the perinuclear localization of Golgi target-SNARE syntaxin5 and reduces the abundance of a Golgi enzyme, mannosidase II (Man II). GS28 mislocalization and Man II reduction are also induced by cellular cholesterol depletion. Three domains of OSBP—an endoplasmic reticulum–targeting domain, a Golgi-targeting domain, and a sterol-binding domain—are all required for Golgi localization of GS28. Finally, GS28 mislocalization and Man II reduction in OSBP-depleted cells are largely restored by depletion of ArfGAP1, a regulator of the budding of coat protein complex (COP)-I vesicles. From these results, we postulate that Golgi cholesterol level, which is controlled by OSBP, is essential for Golgi localization of intra-Golgi v-SNAREs by ensuring proper COP-I vesicle transport.

Synthesis and structure of 16,22-diketocholesterol bound to oxysterol-binding protein Osh4

We have synthesized 16,22-diketocholesterol, a novel ligand for oxysterol-binding protein Osh4, and determined X-ray structure of the diketocholesterol in complex with Osh4. The X-ray structure shows that α7 helix of Osh4 assumes open conformation while the rest of Osh4, closed conformation, implying this diketocholesterol-bound Osh4 structure may represent a structural intermediate between the two conformations.

Structure of Osh3 reveals a conserved mode of phosphoinositide binding in oxysterol-binding proteins

The oxysterol-binding protein (OSBP)-related proteins (ORPs) are conserved from yeast to humans, and implicated in the regulation of lipid homeostasis and in signaling pathways. Saccharomyces cerevisiae has seven ORPs (Osh1-Osh7) that share one unknown essential function. Here, we report the 1.5-2.3 Å structures of the PH domain and ORD (OSBP-related domain) of yeast Osh3 in apo-form or in complex with phosphatidylinositol 4-phosphate (PI[4]P). Osh3 recognizes PI(4)P by the highly conserved residues in the tunnel of ORD whereas it lacks sterol binding due to the narrow hydrophobic tunnel. Yeast complementation tests suggest that PI(4)P binding to PH and ORD is essential for function. This study suggests that the unifying feature in all ORP homologs is the binding of PI(4)P to ORD and sterol binding is additional to certain homologs. Structural modeling of full-length Osh3 is consistent with the concept that Osh3 is a lipid transfer protein or regulator in membrane contact sites.

Osbpl8 deficiency in mouse causes an elevation of high-density lipoproteins and gender-specific alterations of lipid metabolism

OSBP-related protein 8 (ORP8) encoded by Osbpl8 is an endoplasmic reticulum sterol sensor implicated in cellular lipid metabolism. We generated an Osbpl8^−/− (KO) C57Bl/6 mouse strain. Wild-type and Osbpl8KO animals at the age of 13-weeks were fed for 5 weeks either chow or high-fat diet, and their plasma lipids/lipoproteins and hepatic lipids were analyzed. The chow-fed Osbpl8KO male mice showed a marked elevation of high-density lipoprotein (HDL) cholesterol (+79%) and phospholipids (+35%), while only minor increase of apolipoprotein A-I (apoA-I) was detected. In chow-fed female KO mice a less prominent increase of HDL cholesterol (+27%) was observed, while on western diet the HDL increment was prominent in both genders. The HDL increase was accompanied by an elevated level of HDL-associated apolipoprotein E in male, but not female KO animals. No differences between genotypes were observed in lecithin:cholesterol acyltransferase (LCAT) or hepatic lipase (HL) activity, or in the fractional catabolic rate of fluorescently labeled mouse HDL injected in chow-diet fed animals. The Osbpl8KO mice of both genders displayed reduced phospholipid transfer protein (PLTP) activity, but only on chow diet. These findings are consistent with a model in which Osbpl8 deficiency results in altered biosynthesis of HDL. Consistent with this hypothesis, ORP8 depleted mouse hepatocytes secreted an increased amount of nascent HDL into the culture medium. In addition to the HDL phenotype, distinct gender-specific alterations in lipid metabolism were detected: Female KO animals on chow diet showed reduced lipoprotein lipase (LPL) activity and increased plasma triglycerides, while the male KO mice displayed elevated plasma cholesterol biosynthetic markers cholestenol, desmosterol, and lathosterol. Moreover, modest gender-specific alterations in the hepatic expression of lipid homeostatic genes were observed. In conclusion, we report the first viable OsbplKO mouse model, demonstrating a HDL elevating effect of Osbpl8 knock-out and additional gender- and/or diet-dependent impacts on lipid metabolism.

Silencing of OSBP-related protein 8 (ORP8) modifies the macrophage transcriptome, nucleoporin p62 distribution, and migration capacity

ORP8 is an oxysterol/cholesterol binding protein anchored to the endoplasmic reticulum and the nuclear envelope, and is abundantly expressed in the macrophage. We created and characterized mouse RAW264.7 macrophages with ORP8 stably silenced using shRNA lentiviruses. A microarray transcriptome and gene ontology pathway analysis revealed significant alterations in several nuclear pathways and ones associated with centrosome and microtubule organization. ORP8 knockdown resulted in increased expression and altered subcellular distribution of an interaction partner of ORP8, nucleoporin NUP62, with an intranuclear localization aspect and association with cytoplasmic vesicular structures and lamellipodial edges of the cells. Moreover, ORP8 silenced cells displayed enhanced migration, and a more pronounced microtubule cytoskeleton than controls expressing a non-targeting shRNA. ORP8 was shown to compete with Exo70 for interaction with NUP62, and NUP62 knockdown abolished the migration enhancement of ORP8-silenced cells, suggesting that the endogenous ORP8 suppresses migration via binding to NUP62. As a conclusion, the present study reveals new, unexpected aspects of ORP8 function in macrophages not directly involving lipid metabolism, but rather associated with nuclear functions, microtubule organization, and migration capacity.

Characterization of the oxysterol-binding protein gene family in the yellow fever mosquito, Aedes aegypti

The oxysterol-binding protein (OSBP) and OSBP-related proteins (ORPs) are sterol-binding proteins that may be involved in cellular sterol transportation, sterol metabolism and signal transduction pathways. Four ORP genes were cloned from Aedes aegypti. Based on amino acid sequence homology to human proteins, they are AeOSBP, AeORP1, AeORP8 and AeORP9. Splicing variants of AeOSBP and AeORP8 were identified. The temporal and spatial transcription patterns of members of the AeOSBP gene family through developmental stages and the gonotrophic cycle were profiled. AeORP1 transcription seemed to be head tissue-specific, whereas AeOSBP and AeORP9 expression was induced by a bloodmeal. Furthermore, over-expression of AeORPs facilitated [(3)H]-cholesterol uptake in Ae. aegypti cultured Aag -2 cells.

OSBP-related protein 8 (ORP8) regulates plasma and liver tissue lipid levels and interacts with the nucleoporin Nup62

We earlier identified OSBP-related protein 8 (ORP8) as an endoplasmic reticulum oxysterol-binding protein implicated in cellular lipid homeostasis. We now investigated its action in hepatic cells in vivo and in vitro. Adenoviral overexpression of ORP8 in mouse liver induced a decrease of cholesterol, phospholipids, and triglycerides in serum (−34%, −26%, −37%, respectively) and liver tissue (−40%, −12%, −24%), coinciding with reduction of nuclear (n)SREBP-1 and -2 and mRNA levels of their target genes. Consistently, excess ORP8 reduced nSREBPs in HuH7 cells, and ORP8 overexpression or silencing by RNA interference moderately suppressed or induced the expression of SREBP-1 and SREBP-2 target genes, respectively. In accordance, cholesterol biosynthesis was reduced by ORP8 overexpression and enhanced by ORP8 silencing in [³H]acetate pulse-labeling experiments. ORP8, previously shown to bind 25-hydroxycholesterol, was now shown to bind also cholesterol in vitro. Yeast two-hybrid, bimolecular fluorescence complementation (BiFC), and co-immunoprecipitation analyses revealed the nuclear pore component Nup62 as an interaction partner of ORP8. Co-localization of ORP8 and Nup62 at the nuclear envelope was demonstrated by BiFC and confocal immunofluorescence microscopy. Furthermore, the impact of overexpressed ORP8 on nSREBPs and their target mRNAs was inhibited in cells depleted of Nup62. Our results reveal that ORP8 has the capacity to modulate lipid homeostasis and SREBP activity, probably through an indirect mechanism, and provide clues of an entirely new mode of ORP action.

OSBP-related protein 7 interacts with GATE-16 and negatively regulates GS28 protein stability

ORP7 is a member of oxysterol-binding protein (OSBP) family, the function of which has remained obscure. In this study, we identified by yeast two-hybrid screening an interaction partner of ORP7, GATE-16, which (i) regulates Golgi SNARE of 28kDa (GS28) function and stability, and (ii) plays a role in autophagosome biogenesis. The interaction was confirmed by bimolecular fluorescence complementation (BiFC) assay in living cells. The interacting regions were delineated within aa 1-142 of ORP7 and aa 30-117 of GATE-16. ORP7 knock-down in 293A cells resulted in a 40% increase of GS28 protein while ORP7 overexpression had the opposite effect (25% decrease of GS28). We show evidence that the regulation of GS28 by ORP7 does not occur at the level of transcription, but involves degradation of GS28 on proteasomes. Truncated ORP7 that lacks the GATE-16 binding region failed to affect GS28 stability, evidencing for specificity of the observed effect. Similar to ORP7 overexpression, treatment of cells with 25-hydroxycholesterol (25-OH) resulted in GS28 destabilization, which was potentiated by excess ORP7 and inhibited by ORP7 silencing. Overexpression of ORP7 led in most cells to formation of vacuolar structures positive for RFP-LC3, thus representing autophagic elements. Also GATE-16 was found in the vacuolar ORP7-positive elements, suggesting that excess ORP7 increases entrapment of GATE-16 in autophagosomes. Taken together, our results suggest that ORP7 negatively regulates GS28 protein stability via sequestration of GATE-16, and may mediate the effect of 25-OH on GS28 and Golgi function.

Conversion of 7-ketocholesterol to oxysterol metabolites by recombinant CYP27A1 and retinal pigment epithelial cells

Of the different oxygenated cholesterol metabolites, 7-ketocholesterol (7KCh) is considered a noxious oxy-sterol implicated in the development of certain pathologies, including those found in the eye. Here we elucidated whether sterol 27-hydroxylase cytochrome P450 27A1 (CYP27A1) is involved in elimination of 7KCh from the posterior part of the eye: the neural retina and underlying retinal pigment epithelium (RPE). We first established that the affinities of purified recombinant CYP27A1 for 7KCh and its endogenous substrate cholesterol are similar, yet 7KCh is metabolized at a 4-fold higher rate than cholesterol in the reconstituted system in vitro. Lipid extracts from bovine neural retina and RPE were then analyzed by isotope dilution GC-MS for the presence of the 7KCh-derived oxysterols. Two metabolites, 3β,27-dihydroxy-5-cholesten-7-one (7KCh-27OH) and 3β-hydroxy-5-cholesten-7-one-26-oic acid (7KCh-27COOH), were detected in the RPE but not in the neural retina. 7KCh-27OH was also formed when RPE homogenates were supplemented with NADPH and the mitochondrial redox system. Quantifications in human RPE showed that CYP27A1 is indeed expressed in the RPE at 2-4-fold higher levels than in the neural retina. The data obtained represent evidence for the role of CYP27A1 in retinal metabolism of 7KCh and suggest that, in addition to cholesterol removal, the functions of this enzyme could also include elimination of toxic endogenous compounds.

Sterol binding by OSBP-related protein 1L regulates late endosome motility and function

ORP1L is an oxysterol binding homologue that regulates late endosome (LE) positioning. We show that ORP1L binds several oxysterols and cholesterol, and characterize a mutant, ORP1L Δ560-563, defective in oxysterol binding. While wild-type ORP1L clusters LE, ORP1L Δ560-563 induces LE scattering, which is reversed by disruption of the endoplasmic reticulum (ER) targeting FFAT motif, suggesting that it is due to enhanced LE-ER interactions. Endosome motility is reduced upon overexpression of ORP1L. Both wild-type ORP1L and the Δ560-563 mutant induce the recruitment of both dynactin and kinesin-2 on LE. Most of the LE decorated by overexpressed ORP1L fail to accept endocytosed dextran or EGF, and the transfected cells display defective degradation of internalized EGF. ORP1L silencing in macrophage foam cells enhances endosome motility and results in inhibition of [(3)H]cholesterol efflux to apolipoprotein A-I. These data demonstrate that LE motility and functions in both protein and lipid transport are regulated by ORP1L.