OSBPL8

OSBPL8 (ORP8) is an ER-anchored lipid transfer protein that operates at multiple membrane contact sites to exchange phosphatidylserine (PS) and phosphatidylinositol 4-phosphate (PI4P) between the ER and apposing organelle membranes, thereby governing lipid homeostasis, calcium signaling, phagosome maturation, and lipid droplet biogenesis. Its ORD domain, which adopts a β-barrel fold with a large lipid-binding cavity, mediates PI4P/PS countertransport at ER–plasma membrane junctions (where its PH domain binds PI(4,5)P2), at ER–mitochondria contacts (via interaction with PTPIP51), and at ER–phagosome contacts (via Sec22b), coupling PI4P consumption by ER-resident Sac1 phosphatase to PS enrichment on target membranes (PMID:26206935, PMID:27113756, PMID:37794132, PMID:37566053). Beyond lipid transfer, ORP8 functions as an AMPK-regulated lipophagy receptor that directly binds LC3/GABARAPs on lipid droplets to promote autophagic LD degradation, recruits GPX1 to the ER to reduce oxidized phosphatidic acid and suppress ferroptosis, and negatively regulates ABCA1 transcription and nuclear SREBP levels through interaction with the nucleoporin Nup62 (PMID:37707322, PMID:41720096, PMID:17991739, PMID:21698267). The PS/PI4P ratio set by ORP8 at ER–PM junctions controls STIM1-Orai1 store-operated calcium entry and NFAT signaling, linking its lipid transport activity to receptor-evoked calcium oscillation patterns (PMID:37607230).

1. 2007 High

   ORP8 was first characterized as an ER-localized oxysterol-binding protein that negatively regulates ABCA1 transcription and macrophage cholesterol efflux, establishing its role in cholesterol metabolism beyond simple lipid binding.

   Evidence siRNA knockdown in THP-1 macrophages with ABCA1 promoter-reporter assays, cholesterol efflux measurements, and 25-hydroxycholesterol binding assay

   PMID:17991739

   Open questions at the time

   • Mechanism by which ORP8 represses ABCA1 promoter activity not fully delineated
   • Physiological sterol ligand in vivo not determined
2. 2011 High

   Discovery that ORP8 interacts with nucleoporin Nup62 and suppresses nuclear SREBP levels in a Nup62-dependent manner revealed a non-canonical nuclear-envelope-associated function linking lipid sensing to transcriptional control of cholesterol biosynthesis.

   Evidence Yeast two-hybrid, BiFC, co-immunoprecipitation, epistasis (Nup62 knockdown reverses ORP8 effects on nSREBPs), adenoviral overexpression in mouse liver

   PMID:21698267

   Open questions at the time

   • Structural basis for ORP8–Nup62 interaction unknown
   • Whether ORP8 transits through the NPC or acts at the nuclear envelope surface unresolved
3. 2012 Medium

   ORP8 was shown to modulate cell migration by competing with Exo70 for Nup62 binding and reorganizing microtubule cytoskeleton, expanding its functional repertoire beyond lipid metabolism.

   Evidence Stable shRNA knockdown in RAW264.7 macrophages, co-immunoprecipitation, migration assays, epistasis with Nup62 knockdown

   PMID:22683860

   Open questions at the time

   • Single lab finding; independent replication in other cell types lacking
   • Mechanism linking Nup62–Exo70 balance to cytoskeletal reorganization not resolved
4. 2014 Medium

   Identification of SPAG5/Astrin as an ORP8 interactor and demonstration that ORP8 overexpression causes G2/M accumulation suggested an oxysterol-dependent cell-cycle regulatory role, though the mechanism remained indirect.

   Evidence Yeast two-hybrid, pulldown, co-immunoprecipitation, flow cytometry cell cycle analysis, epistasis with SPAG5 knockdown

   PMID:24424245

   Open questions at the time

   • Whether endogenous ORP8 levels regulate cell cycle under physiological conditions untested
   • Mechanism connecting ER-localized ORP8 to mitotic spindle component SPAG5 unclear
5. 2015 High

   The foundational discovery that ORP8 (with ORP5) mediates PI4P/PS countertransport at ER–PM contact sites, with its PH domain tethering to the PM and its ORD shuttling lipids, established ORP8 as a bona fide lipid transfer protein rather than merely a sensor.

   Evidence Reconstituted lipid transfer in vitro, gain/loss-of-function in cells, lipid binding assays

   PMID:26206935

   Open questions at the time

   • Relative contributions of ORP5 versus ORP8 not separated
   • Whether transport is diffusion-mediated or requires energy coupling unknown
6. 2016 High

   Extension of ORP8's contact-site activity to ER–mitochondria junctions, where it interacts with outer mitochondrial membrane protein PTPIP51 and supports mitochondrial morphology and respiration, demonstrated that ORP8 operates at multiple MCS types.

   Evidence Co-immunoprecipitation, confocal imaging, siRNA knockdown with mitochondrial morphology and respiration readouts

   PMID:27113756

   Open questions at the time

   • Specific lipid species transported at MAMs by ORP8 not identified
   • Whether ORP8 and ORP5 function redundantly at MAMs unresolved
7. 2017 High

   Clarification that the ORP8 PH domain binds PI(4,5)P2 rather than PI4P refined the model of ER–PM tethering, and demonstration that ORP8 depletion elevates PM PI(4,5)P2 revealed feedback between lipid transfer and phosphoinositide homeostasis.

   Evidence In vitro PH domain binding assays, lipid extraction/transport assays, siRNA knockdown with phosphoinositide measurements

   PMID:28970484

   Open questions at the time

   • Whether PH domain selectivity differs between cell types unexplored
   • Mechanism by which ORP8 loss increases PI(4,5)P2 (direct transport vs. indirect regulation) not fully resolved
8. 2022 High

   ORP5/ORP8 were found to control lipid droplet biogenesis at MAM subdomains by regulating seipin recruitment in a phosphatidic-acid-enriched environment, linking ER–mitochondria contacts to LD formation.

   Evidence siRNA knockdown, live-cell imaging, proximity ligation assay, LD biogenesis assays

   PMID:35969857

   Open questions at the time

   • Whether ORP8 directly transfers PA or acts indirectly through PI4P/PS exchange at MAM-LD contacts undetermined
   • Individual contribution of ORP8 versus ORP5 to seipin recruitment not separated
9. 2023 High

   The crystal structure of the ORP8 ORD revealed a β-barrel with a ~1860 ų cavity and a regulatory lid that slows lipid exchange, providing the first atomic-level framework for understanding its PS/PI4P countertransport mechanism.

   Evidence X-ray crystallography, in vitro fluorescence lipid transport assays, mutagenesis (lid deletion), coarse-grained MD simulations

   PMID:37566053

   Open questions at the time

   • No co-crystal with bound lipid substrate obtained
   • Whether the lid opens spontaneously at membranes or requires protein partners unknown
10. 2023 High

    Multiple studies in 2023 vastly expanded ORP8 biology: (1) the PS/PI4P ratio set by ORP8 at ER–PM junctions was shown to control STIM1-Orai1 calcium signaling and NFAT translocation; (2) ORP8 mediates PS/PI4P exchange at ER–phagosome contacts via Sec22b to regulate phagosome maturation and antigen degradation; (3) ORP8 was identified as an AMPK-phosphorylated lipophagy receptor that binds LC3/GABARAPs independently of lipid transfer; and (4) ORP8 promotes Stathmin1 proteasomal degradation to stabilize microtubules.

    Evidence Ca2+ imaging and FRET for STIM1 interactions; Co-IP plus mutant rescue for phagosome studies; Osbpl8 KO mouse, AMPK phosphorylation mapping, in vivo rescue in ob/ob mice for lipophagy; ubiquitination and microtubule polymerization assays for Stathmin1

    PMID:37054771 PMID:37607230 PMID:37707322 PMID:37794132

    Open questions at the time

    • How ORP8 switches between lipid-transfer and lipophagy-receptor modes unclear
    • Stathmin1 degradation finding from single lab and mechanism of ORP8-mediated ubiquitination indirect
    • Whether phagosomal role is macrophage-specific or general not tested
11. 2026 High

    ORP8 was discovered to recruit the antioxidant enzyme GPX1 to the ER to reduce oxidized phosphatidic acid, establishing a non-canonical ferroptosis-suppressive mechanism at the ER membrane.

    Evidence Co-immunoprecipitation, in vitro lipid peroxidation assay, ferroptosis assay, knockdown in cells and tumor xenograft model

    PMID:41720096

    Open questions at the time

    • Whether ORP8 directly presents oxidized PA to GPX1 or acts as a scaffold unknown
    • Relationship between ORP8's lipid transfer activity and its ferroptosis-suppressive role not tested

• Key unresolved questions include: how ORP8 coordinates its multiple MCS activities and lipophagy receptor function in a single cell; whether ORP8 and ORP5 are truly functionally redundant at specific contact sites; the structural basis for ORP8's interactions with non-lipid partners (Nup62, GPX1, PTPIP51); and how post-translational modifications beyond AMPK phosphorylation regulate ORP8 targeting and activity.
• No systematic separation of ORP5 versus ORP8 functions at each MCS type
• No full-length ORP8 structure available
• Regulation of ORP8 expression and turnover largely uncharacterized