**2.8 OSBPL6 (ORP6)**

**Structure, Tissue distribution and Intracellular localization:** ORP6 belongs to the subfamily III of OSBP/ORP homologues. ORP6 has an ORD, a PH domain and an FFAT motif. ORP6 shows the highest expression in brain and skeletal muscle (Lehto, Tienari et al. 2004). Endogenous ORP6 associated predominantly with the nuclear envelope. When expressed from the cDNA in cultured cells, ORP6 was distributed between the cytosol and ER membranes, with a minor portion found at the PM.

**Molecular functions related to dyslipidemia:** The direct evidence that ORP6 functions to regulate Dyslipidemia is yet to be reported.

**Epidemiological study:** Using the Framingham Heart Study data set, a quantitative trait locus in the chromosome 2q was found to be significantly involved in variations of HDL cholesterol levels. ORP6 is identified as one of the candidate genes that are possibly involved in the regulation of HDL cholesterol levels in this region (North, Martin et al. 2003).

#### **2.9 OSBPL7 (ORP7)**

**Structure, Tissue distribution and Intracellular localization:** ORP7 belongs to the subfamily III of OSBP/ORP homologues. ORP7 has an ORD, a PH domain and a an FFAT motif. ORP7 shows the highest expression in the gastrointestinal tract (Lehto, Tienari et al. 2004). When expressed from the cDNA in cultured cells, ORP7 was distributed between the cytosol and ER membranes, with a minor portion found at the PM. The N-terminal portion of the proteins, containing a PH domain, has markedly strong PM targeting specificity, while the C-terminal half remains largely cytosolic. The dual targeting of the proteins indicates a putative role in communication between the ER and the PM.

**Molecular functions related to dyslipidemia:** Recently, Zhong et al. identified by yeast twohybrid screening an interaction partner of ORP7, GATE-16, which (i) regulates the function and stability of Golgi SNARE of 28kDa (GS28), and (ii) plays a role in autophagosome biogenesis (Zhong, Zhou et al. 2011).

Functions of OSBP/ORP Family Proteins and Their Relation to Dyslipidemia 135

**Epidemiological study:** Ma et al. performed a genome-wide association analysis of total cholesterol and HDL cholesterol levels using the Framingham heart study data. In that study, single-locus effects and pairwise epistasis effects of 432,096 SNP markers were tested for their significance on log-transformed total cholesterol and HDL cholesterol levels. As a result, the OSBPL8-ZDHHC17 region (chr12) was detected for HDL cholesterol

**Structure, Tissue distribution and Intracellular localization:** ORP9 belongs to the subfamily V of OSBP/ORP homologues. ORP9 has an ORD, a PH domain and an FFAT motif. VAP binding FFAT motif and PH domains target ORP9 to the ER and a Golgi-COPII

**Molecular functions related to dyslipidemia:** Ngo et al. demonstrate that ORP9L partitioning between the trans-Golgi/trans-Golgi network (TGN), and the ER is mediated by a phosphatidylinositol 4-phosphate (PI-4P)-specific PH domain and VAP, respectively (Ngo and Ridgway 2009). In vitro, ORP9L mediates PI-4P-dependent cholesterol transport between liposomes, suggesting that its primary function in vivo is sterol transfer between the Golgi and ER. Depletion of ORP9L by RNAi caused Golgi fragmentation, inhibition of vesicular somatitus virus glycoprotein transport from the ER and accumulation of cholesterol in endosomes/lysosomes. These findings indicate that ORP9 maintains the integrity of the early secretory pathway by mediating transport of sterols between the ER

It is also reported that ORP9, in interaction with ORP11, may act as an intracellular lipid

**Structure, Tissue distribution and Intracellular localization:** ORP10 belongs to the subfamily VI of OSBP/ORP homologues. ORP10 has an ORD and a PH domain but does not

ORP10 was shown to associate dynamically with microtubules, being consistent with its involvement in intracellular transport or organelle positioning (Perttila, Merikanto et al.

Immunofluorescence localization in transiently transfected bovine aorta endothelial cells showed that EGFP-ORP10 co-localized with alpha-tubulin (Fig. 2 c, g) and not with actin (Fig. 2 a, e) or vimentin (Fig. 2 b, f). The microtubules co-localize with EGFP-ORP10 show the aberrant bundled structures. These structures were disrupted by treatment with

**Molecular functions related to dyslipidemia:** Silencing of ORP10 increased the incorporation of [(3)H]acetate into cholesterol and both [(3)H]acetate and [(3)H]oleate into triglycerides and enhanced the accumulation of secreted apolipoprotein B100 in growth medium, suggesting that ORP10 suppresses hepatic lipogenesis and very-low-density

**Epidemiological study:** We examined the association between polymorphisms in the ORP10 gene and risk factors for the metabolic syndrome in the Tanno and Sobetsu Study in

identified by one new SNP with genome-wide significance (Ma, Yang et al. 2010).

compartment, respectively (Wyles and Ridgway 2004).

sensor or transporter (Zhou, Li et al. 2010). (see also ORP11.)

have an FFAT motif or a transmembrane domain.

**Epidemiological study:** Epidemiological study of ORP9 is not reported yet.

**2.11 OSBPL9 (ORP9)** 

and trans-Golgi/TGN.

**2.12 OSBPL10 (ORP10)** 

nocodazol (Fig. 2 d, h).

lipoprotein production.

Japan (Koriyama, Nakagami et al. 2010).

2009).

GATE-16 is a ubiquitin-like low molecular weight peripheral membrane protein which was initially reported to localize at the Golgi complex and to regulate docking/fusing reactions in intra-Golgi traffic and Golgi assembly from mitotic fragments via interactions with NSF and the Golgi v-SNARE GS28 (Sagiv, Legesse-Miller et al. 2000). GS28 was identified as a SNARE protein, the majority of which is associated with the cis-Golgi, and is implicated in both ER-Golgi and intra-Golgi transport (Subramaniam, Peter et al. 1996). In the presence of NSF, SNAP and ATP, GATE-16 interacts with GS28, apparently maintaining GS28 in a transport competent form and protecting it from proteolysis.

Zhong et al. revealed that ORP7 knockdown in 293A cells resulted in a 40% increase of GS28 protein while ORP7 overexpression had the opposite 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. Their 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.

**Epidemiological study:** It is reported that SNPs near ORP7 gene show genome-wide significant association with LDL cholesterol (Teslovich, Musunuru et al. 2010).

#### **2.10 OSBPL8 (ORP8)**

**Structure, Tissue distribution and Intracellular localization:** ORP8 belongs to the subfamily IV of OSBP/ORP homologues. ORP8 has an ORD, a PH domain and a transmembrane domain. ORP8 is expressed at the highest levels in macrophages, liver, spleen, kidney, and brain (Yan, Mayranpaa et al. 2008). ORP8 is localized in the ER via its Cterminal transmembrane domain.

**Molecular functions related to dyslipidemia:** It is reported that silencing of ORP8 by RNA interference in THP-1 macrophages increased the expression of ABCA1 and concomitantly cholesterol efflux to lipid-free apolipoprotein A-I. Experiments employing an ABCA1 promoter-luciferase reporter confirmed that ORP8 silencing enhances ABCA1 transcription. These data identify ORP8 as a negative regulator of ABCA1 expression and macrophage cholesterol efflux. But the precise mechanism to regulate the expression of ABCA1 has not been revealed.

Recently, Zhou et al. investigated the action of ORP8 in hepatic cells in vivo and in vitro. They found that 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 [(3)H]acetate pulse-labeling experiments.

They also performed yeast two-hybrid, bimolecular fluorescence complementation (BiFC), and co-immunoprecipitation analyses, and revealed the nuclear pore component Nup62 as an interaction partner of ORP8. They showed that the impact of overexpressed ORP8 on nSREBPs and their target mRNAs was inhibited in cells depleted of Nup62.

These results reveal that ORP8 has the capacity to modulate lipid homeostasis and SREBP activity, probably through an indirect mechanism required Nup62.

**Epidemiological study:** Ma et al. performed a genome-wide association analysis of total cholesterol and HDL cholesterol levels using the Framingham heart study data. In that study, single-locus effects and pairwise epistasis effects of 432,096 SNP markers were tested for their significance on log-transformed total cholesterol and HDL cholesterol levels.

As a result, the OSBPL8-ZDHHC17 region (chr12) was detected for HDL cholesterol identified by one new SNP with genome-wide significance (Ma, Yang et al. 2010).
