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.

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 3ß(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.

OSBP-related protein 3 (ORP3) coupling with VAMP-associated protein A regulates R-Ras activity.

ORP3 is an R-Ras interacting oxysterol-binding protein homolog that regulates cell adhesion and is overexpressed in several cancers. We investigated here a novel function of ORP3 dependent on its targeting to both the endoplasmic reticulum (ER) and the plasma membrane (PM). Using biochemical and cell imaging techniques we demonstrate the mechanistic requirements for the subcellular targeting and function of ORP3 in control of R-Ras activity. We show that hyperphosphorylated ORP3 (ORP3-P) selectively interacts with the ER membrane protein VAPA, and ORP3-VAPA complexes are targeted to PM sites via the ORP3 pleckstrin homology (PH) domain. A novel FFAT (two phenylalanines in an acidic tract)-like motif was identified in ORP3; only disruption of both the FFAT-like and canonical FFAT motif abolished the phorbol-12-myristate-13-acetate (PMA) stimulated interaction of ORP3-P with VAPA. Co-expression of ORP3 and VAPA induced R-Ras activation, dependent on the interactions of ORP3 with VAPA and the PM. Consistently, downstream AktS473 phosphorylation and ß1-integrin activity were enhanced by ORP3-VAPA. To conclude, phosphorylation of ORP3 controls its association with VAPA. Furthermore, we present evidence that ORP3-VAPA complexes stimulate R-Ras signaling.

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.

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 Kd values in the µM range. Also binding of cholesterol by ORP2 was detectable, but a Kd 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 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.

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.