Nuclear lipid droplets in Caco2 cells originate from nascent precursors and in situ at the nuclear envelope.

Intestinal epithelial cells convert excess fatty acids into triglyceride (TAG) for storage in cytoplasmic lipid droplets and secretion in chylomicrons. Nuclear lipid droplets (nLDs) are present in intestinal cells but their origin and relationship to cytoplasmic TAG synthesis and secretion is unknown. nLDs and related lipid-associated promyelocytic leukemia structures (LAPS) were abundant in oleate-treated Caco2 but less frequent in other human colorectal cancer cell lines and mouse intestinal organoids. nLDs and LAPS in undifferentiated oleate-treated Caco2 cells harbored the phosphatidate phosphatase Lipin1, its product diacylglycerol, and CTP:phosphocholine cytidylyltransferase (CCT)α. CCTα knockout Caco2 cells had fewer but larger nLDs, indicating a reliance on de novo PC synthesis for assembly. Differentiation of Caco2 cells caused large nLDs and LAPS to form regardless of oleate treatment or CCTα expression. nLDs and LAPS in Caco2 cells did not associate with apoCIII and apoAI and formed dependently of microsomal triglyceride transfer protein expression and activity, indicating they are not derived from endoplasmic reticulum luminal LDs precursors. Instead, undifferentiated Caco2 cells harbored a constitutive pool of nLDs and LAPS in proximity to the nuclear envelope that expanded in size and number with oleate treatment. Inhibition of TAG synthesis did affect the number of nascent nLDs and LAPS but prevented their association with promyelocytic leukemia protein, Lipin1α, and diacylglycerol, which instead accumulated on the nuclear membranes. Thus, nLD and LAPS biogenesis in Caco2 cells is not linked to lipoprotein secretion but involves biogenesis and/or expansion of nascent nLDs by de novo lipid synthesis.

Lipid- and phospho-regulation of CTP:Phosphocholine Cytidylyltransferase α association with nuclear lipid droplets.

Fatty acids stored in triacylglycerol-rich lipid droplets are assembled with a surface monolayer composed primarily of phosphatidylcholine (PC). Fatty acids stimulate PC synthesis by translocating CTP:phosphocholine cytidylyltransferase (CCT) α to the inner nuclear membrane, nuclear lipid droplets (nLD) and lipid associated promyelocytic leukemia (PML) structures (LAPS). Huh7 cells were used to identify how CCTα translocation onto these nuclear structures are regulated by fatty acids and phosphorylation of its serine-rich P-domain. Oleate treatment of Huh7 cells increased nLDs and LAPS that became progressively enriched in CCTα. In cells expressing the phosphatidic acid phosphatase Lipin1α or 1ß, the expanded pool of nLDs and LAPS had a proportional increase in associated CCTα. In contrast, palmitate induced few nLDs and LAPS and inhibited the oleate-dependent translocation of CCTα without affecting total nLDs. Phospho-memetic or phospho-null mutations in the P-domain revealed that a 70% phosphorylation threshold, rather than site-specific phosphorylation, regulated CCTα association with nLDs and LAPS. In vitro candidate kinase and inhibitor studies in Huh7 cells identified cyclin-dependent kinase (CDK) 1 and 2 as putative P-domain kinases. In conclusion, CCTα translocation onto nLDs and LAPS is dependent on available surface area and fatty acid composition, as well as threshold phosphorylation of the P-domain potentially involving CDKs.

Differential contributions of phosphotransferases CEPT1 and CHPT1 to phosphatidylcholine homeostasis and lipid droplet biogenesis.

The cytidine diphosphate-choline (Kennedy) pathway culminates with the synthesis of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) by choline/ethanolamine phosphotransferase 1 (CEPT1) in the endoplasmic reticulum (ER), and PC synthesis by choline phosphotransferase 1 (CHPT1) in the Golgi apparatus. Whether the PC and PE synthesized by CEPT1 and CHPT1 in the ER and Golgi apparatus has different cellular functions has not been formally addressed. Here, we used CRISPR editing to generate CEPT1-and CHPT1-KO U2OS cells to assess the differential contribution of the enzymes to feedback regulation of nuclear CTP:phosphocholine cytidylyltransferase (CCT)α, the rate-limiting enzyme in PC synthesis, and lipid droplet (LD) biogenesis. We found that CEPT1-KO cells had a 50 and 80% reduction in PC and PE synthesis, respectively, while PC synthesis in CHPT1-KO cells was also reduced by 50%. CEPT1 KO caused the posttranscriptional induction of CCTα protein expression as well as its dephosphorylation and constitutive localization on the inner nuclear membrane and nucleoplasmic reticulum. This activated CCTα phenotype was prevented by incubating CEPT1-KO cells with PC liposomes to restore end-product inhibition. Additionally, we determined that CEPT1 was in close proximity to cytoplasmic LDs and CEPT1 KO resulted in the accumulation of small cytoplasmic LDs, as well as increased nuclear LDs enriched in CCTα. In contrast, CHPT1 KO had no effect on CCTα regulation or LD biogenesis. Thus, CEPT1 and CHPT1 contribute equally to PC synthesis; however, only PC synthesized by CEPT1 in the ER regulates CCTα and the biogenesis of cytoplasmic and nuclear LDs.

Novel Role of CETP in Macrophages: Reduction of Mitochondrial Oxidants Production and Modulation of Cell Immune-Metabolic Profile.

Plasma cholesteryl ester transfer protein (CETP) activity diminishes HDL-cholesterol levels and thus may increase atherosclerosis risk. Experimental evidence suggests CETP may also exhibit anti-inflammatory properties, but local tissue-specific functions of CETP have not yet been clarified. Since oxidative stress and inflammation are major features of atherogenesis, we investigated whether CETP modulates macrophage oxidant production, inflammatory and metabolic profiles. Comparing macrophages from CETP-expressing transgenic mice and non-expressing littermates, we observed that CETP expression reduced mitochondrial superoxide anion production and H2O2 release, increased maximal mitochondrial respiration rates, and induced elongation of the mitochondrial network and expression of fusion-related genes (mitofusin-2 and OPA1). The expression of pro-inflammatory genes and phagocytic activity were diminished in CETP-expressing macrophages. In addition, CETP-expressing macrophages had less unesterified cholesterol under basal conditions and after exposure to oxidized LDL, as well as increased HDL-mediated cholesterol efflux. CETP knockdown in human THP1 cells increased unesterified cholesterol and abolished the effects on mitofusin-2 and TNFα. In summary, the expression of CETP in macrophages modulates mitochondrial structure and function to promote an intracellular antioxidant state and oxidative metabolism, attenuation of pro-inflammatory gene expression, reduced cholesterol accumulation, and phagocytosis. These localized functions of CETP may be relevant for the prevention of atherosclerosis and other inflammatory diseases.

Running 'LAPS' Around nLD: Nuclear Lipid Droplet Form and Function.

The nucleus harbours numerous protein subdomains and condensates that regulate chromatin organization, gene expression and genomic stress. A novel nuclear subdomain that is formed following exposure of cells to excess fatty acids is the nuclear lipid droplet (nLD), which is composed of a neutral lipid core surrounded by a phospholipid monolayer and associated regulatory and lipid biosynthetic enzymes. While structurally resembling cytoplasmic LDs, nLDs are formed by distinct but poorly understood mechanisms that involve the emergence of lipid droplets from the lumen of the nucleoplasmic reticulum and de novo lipid synthesis. Luminal lipid droplets that emerge into the nucleoplasm do so at regions of the inner nuclear membrane that become enriched in promyelocytic leukemia (PML) protein. The resulting nLDs that retain PML on their surface are termed lipid-associated PML structures (LAPS), and are distinct from canonical PML nuclear bodies (NB) as they lack key proteins and modifications associated with these NBs. PML is a key regulator of nuclear signaling events and PML NBs are sites of gene regulation and post-translational modification of transcription factors. Therefore, the subfraction of nLDs that form LAPS could regulate lipid stress responses through their recruitment and retention of the PML protein. Both nLDs and LAPS have lipid biosynthetic enzymes on their surface suggesting they are active sites for nuclear phospholipid and triacylglycerol synthesis as well as global lipid regulation. In this review we have summarized the current understanding of nLD and LAPS biogenesis in different cell types, their structure and composition relative to other PML-associated cellular structures, and their role in coordinating a nuclear response to cellular overload of fatty acids.

Global Proximity Interactome of the Human Macroautophagy Pathway.

ABBREVIATIONS: BioID: proximity-dependent biotin identification; GO: gene ontology; OSBPL: oxysterol binding protein like; VAPA: VAMP associated protein A; VAPB: VAMP associated protein B and C.

Mechanisms by Which Probiotic Bacteria Attenuate the Risk of Hepatocellular Carcinoma.

Hepatocellular carcinoma (HCC) is the most common primary liver cancer and the second leading cause of cancer-related deaths worldwide. Chronic infections with hepatitis B virus (HBV) and hepatitis C virus (HCV), alcoholic liver disease (ALD), and non-alcoholic fatty liver disease (NAFLD)/non-alcoholic steatohepatitis (NASH) are the major extrinsic risk factors of HCC development. Genetic background is pivotal in HCC pathogenesis, and both germline mutations and single nucleotide polymorphism (SNP) are intrinsic risk factors of HCC. These HCC risk factors predispose to hepatic injury and subsequent activation of fibrogenesis that progresses into cirrhosis and HCC. Probiotic bacteria can mitigate HCC risk by modulating host gut microbiota (GM) to promote growth of beneficial microbes and inhibit HCC-associated dysbiosis, thus preventing pathogen-associated molecular patterns (PAMPs)-mediated hepatic inflammation. Probiotics have antiviral activities against HBV and HCV infections, ameliorate obesity and risk of NAFLD/NASH, and their antioxidant, anti-proliferative, anti-angiogenic, and anti-metastatic effects can prevent the HCC pathogenesis. Probiotics also upregulate the expression of tumor suppressor genes and downregulate oncogene expression. Moreover, metabolites generated by probiotics through degradation of dietary phytochemicals may mitigate the risk of HCC development. These multiple anticancer mechanisms illustrate the potential of probiotics as an adjuvant strategy for HCC risk management and treatment.

Lipid-associated PML structures assemble nuclear lipid droplets containing CCTα and Lipin1.

Nuclear lipid droplets (nLDs) form on the inner nuclear membrane by a mechanism involving promyelocytic leukemia (PML), the protein scaffold of PML nuclear bodies. We report that PML structures on nLDs in oleate-treated U2OS cells, referred to as lipid-associated PML structures (LAPS), differ from canonical PML nuclear bodies by the relative absence of SUMO1, SP100, and DAXX. These nLDs were also enriched in CTP:phosphocholine cytidylyltransferase α (CCTα), the phosphatidic acid phosphatase Lipin1, and DAG. Translocation of CCTα onto nLDs was mediated by its α-helical M-domain but was not correlated with its activator DAG. High-resolution imaging revealed that CCTα and LAPS occupied distinct polarized regions on nLDs. PML knockout U2OS (PML KO) cells lacking LAPS had a 40-50% reduction in nLDs with associated CCTα, and residual nLDs were almost devoid of Lipin1 and DAG. As a result, phosphatidylcholine and triacylglycerol synthesis was inhibited in PML KO cells. We conclude that in response to excess exogenous fatty acids, LAPS are required to assemble nLDs that are competent to recruit CCTα and Lipin1.

Differential dephosphorylation of CTP:phosphocholine cytidylyltransferase upon translocation to nuclear membranes and lipid droplets.

CTP:phosphocholine cytidylyltransferase-alpha (CCTα) and CCTß catalyze the rate-limiting step in phosphatidylcholine (PC) biosynthesis. CCTα is activated by association of its α-helical M-domain with nuclear membranes, which is negatively regulated by phosphorylation of the adjacent P-domain. To understand how phosphorylation regulates CCT activity, we developed phosphosite-specific antibodies for pS319 and pY359+pS362 at the N- and C-termini of the P-domain, respectively. Oleate treatment of cultured cells triggered CCTα translocation to the nuclear envelope (NE) and nuclear lipid droplets (nLDs) and rapid dephosphorylation of pS319. Removal of oleate led to dissociation of CCTα from the NE and increased phosphorylation of S319. Choline depletion of cells also caused CCTα translocation to the NE and S319 dephosphorylation. In contrast, Y359 and S362 were constitutively phosphorylated during oleate addition and removal, and CCTα-pY359+pS362 translocated to the NE and nLDs of oleate-treated cells. Mutagenesis revealed that phosphorylation of S319 is regulated independently of Y359+S362, and that CCTα-S315D+S319D was defective in localization to the NE. We conclude that the P-domain undergoes negative charge polarization due to dephosphorylation of S319 and possibly other proline-directed sites and retention of Y359 and S362 phosphorylation, and that dephosphorylation of S319 and S315 is involved in CCTα recruitment to nuclear membranes.

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.

Substrate channeling in the glycerol-3-phosphate pathway regulates the synthesis, storage and secretion of glycerolipids.

The successive acylation of glycerol-3-phosphate (G3P) by glycerol-3-phosphate acyltransferases and acylglycerol-3-phosphate acyltransferases produces phosphatidic acid (PA), a precursor for CDP-diacylglycerol-dependent phospholipid synthesis. PA is further dephosphorylated by LIPINs to produce diacylglycerol (DG), a substrate for the synthesis of triglyceride (TG) by DG acyltransferases and a precursor for phospholipid synthesis via the CDP-choline and CDP-ethanolamine (Kennedy) pathways. The channeling of fatty acids into TG for storage in lipid droplets and secretion in lipoproteins or phospholipids for membrane biogenesis is dependent on isoform expression, activity and localization of G3P pathway enzymes, as well as dietary and hormonal and tissue-specific factors. Here, we review the mechanisms that control partitioning of substrates into lipid products of the G3P pathway.

Phosphorylation of a serine/proline-rich motif in oxysterol binding protein-related protein 4L (ORP4L) regulates cholesterol and vimentin binding.

The family of oxysterol binding protein (OSBP) and OSBP-related proteins (ORPs) mediate sterol and phospholipid transfer and signaling at membrane contact sites (MCS). The activity of OSBP at MCS is regulated by phosphorylation, but whether this applies to ORPs is unknown. Here we report the functional characterization of a unique proline/serine-rich phosphorylation motif (S762SPSSPSS769) in the lipid binding OSBP-related domain of full-length ORP4L and a truncated variant ORP4S. Phosphorylation was confirmed by mass spectrometry and [32P]PO4 incorporation, and in silico and in vitro assays using purified ORP4L identified putative proline-directed kinases that phosphorylate the site. The functional significance of the phospho-site was assessed by mutating serine 762, S763, S766 and S768 to aspartate or alanine to produce phosphomimetic (S4D) and phosphorylation-deficient (S4A) mutants, respectively. Solution binding of 25-hydroxycholesterol and cholesterol by recombinant ORP4L-S4D and -S4A was similar to wild-type but ORP4L-S4D more effectively extracted cholesterol from liposomes. ORP4L homo-dimerization was unaffected by phosphorylation but gel filtration of ORP4L-S4D indicated that the native conformation was affected. Confocal microscopy revealed that ORP4L-S4D also strongly associated with bundled vimentin filaments, a feature shared with ORP4S which lacks the PH and dimerization domains. We conclude that phosphorylation of a unique serine/proline motif in the ORD induces a conformation change in ORP4L that enhances interaction with vimentin and cholesterol extraction from membranes.

Phosphatidylcholine synthesis regulates triglyceride storage and chylomicron secretion by Caco2 cells.

Intracellular lipid droplets (LDs) supply fatty acids for energy, membrane biogenesis, and lipoprotein secretion. The surface monolayer of LDs is composed of phospholipids, primarily phosphatidylcholine (PC), that stabilize the neutral lipid core of triglyceride (TG). To determine the relationship between PC synthesis and TG storage and secretion in chylomicrons, we used a model of intestinal-derived human epithelial colorectal adenocarcinoma (Caco2) cells with knockout of PCYT1A, which encodes the rate-limiting enzyme CTP:phosphocholine cytidylyltransferase (CCT)α in the CDP-choline pathway, that were treated with the fatty acid oleate. CRISPR/Cas9 knockout of CCTα in Caco2 cells (Caco2-KO cells) reduced PC synthesis by 50%. Compared with Caco2 cells, Caco2-KO cells exposed to oleate had fewer and larger LDs and greater TG accumulation as a result. The addition of exogenous lysophosphatidylcholine to Caco2-KO cells reversed the LD morphology defect. Caco2-KO cells, differentiated into epithelial monolayers, accumulated intracellular TG and had deficient TG and chylomicron-associated apoB48 secretion; apoB100 secretion was unaffected by CCTα knockout or oleate. Metabolic-labeling and LD imaging of Caco2-KO cells indicated preferential shuttling of de novo synthesized TG into larger LDs rather than into chylomicrons. Thus, reduced de novo PC synthesis in Caco2 cells enhances TG storage in large LDs and inhibits apoB48 chylomicron secretion.

Protein kinase D1 and oxysterol-binding protein form a regulatory complex independent of phosphorylation.

Protein kinase D (PKD) controls secretion from the trans-Golgi network (TGN) by phosphorylating phosphatidylinositol 4-kinase IIIß and proteins that bind and/or transfer phosphatidylinositol 4-phosphate (PtdIns-4P), such as oxysterol-binding protein (OSBP) and ceramide transfer protein. Here, we investigated the consequences of PKD phosphorylation of OSBP at endoplasmic reticulum (ER)-Golgi membrane contact sites (MCS). Results with OSBP phospho-mutants revealed that PKD phosphorylation did not affect sterol and PtdIns-4P binding, activation of sphingomyelin (SM) synthesis at Golgi-ER MCS or other OSBP phospho-sites. Instead, an interaction was identified between the N-terminal region of OSBP and PKD1 that was independent of kinase activity and OSBP phosphorylation status. S916 autophosphorylation of PKD1 was inhibited by OSBP expression suggesting the interaction negatively regulates PKD1 activity. Stimulation of PKD1 activity by phorbol ester promoted the Golgi-localization of wild-type and phospho-mutants of OSBP but did not affect OSBP-dependent SM synthesis. Only when wild-type or kinase-dead PKD1 was overexpressed was 25-hydroxycholesterol-activated SM synthesis inhibited. We conclude that OSBP and PKD1 form a complex that inhibits both the oxysterol-dependent activity of OSBP at the ER-Golgi and activation of PKD1. Formation of the complex was independent of PKD1 activity and phosphorylation of OSBP.

Golgi localization of oxysterol binding protein-related protein 4L (ORP4L) is regulated by ligand binding.

Oxysterol binding protein (OSBP)-related protein 4L (ORP4L, also known as OSBPL2), a closely related paralogue and interacting partner of OSBP, binds sterols and phosphatidylinositol 4-phosphate [PI(4)P] and regulates cell proliferative signalling at the plasma membrane (PM). Here, we report that ORP4L also interacts with the trans-Golgi network (TGN) in an OSBP-, sterol- and PI(4)P-dependent manner. Characterization of ORP4L lipid and VAP binding mutants indicated an indirect mechanism for translocation to ER-Golgi contact sites in response to 25-hydroxycholesterol that was dependent on OSBP and PI(4)P. shRNA silencing revealed that ORP4L was required to maintain the organization and PI(4)P content of the Golgi and TGN. In contrast, the interaction of ORP4L with the PM was not dependent on its sterol, PI(4)P or VAP binding activities. At the PM, ORP4L partially localized with a genetically encoded probe for PI(4)P but not with a probe for phosphatidylinositol 4,5-bisphosphate. We conclude that ORP4L is differentially localized to the PM and ER-Golgi contacts sites. OSBP-, lipid- and VAP-regulated interactions of ORP4L with ER-Golgi contact sites are involved in the maintenance of Golgi and TGN structure.

Lipid and membrane recognition by the oxysterol binding protein and its phosphomimetic mutant using dual polarization interferometry.

OSBP binds, extracts and transfers sterols and phosphatidylinositol-4-phosphate (PI(4)P between liposomes, but the sequence of steps at the membrane surface leading to ligand removal is poorly characterized. In this study, we used dual polarization interferometry (DPI), a label-free surface analytical technique, to characterize the interaction of recombinant, purified OSBP as it flows over immobilized dioleoyl-phosphatidylcholine (DOPC) bilayers containing PI(4)P, cholesterol or 25-hydroxycholesterol. Kinetics of membrane interaction were analyzed for PI(4)P-binding and phosphorylation mutants of OSBP. Wild-type OSBP demonstrated a distinctive association with immobilized DOPC bilayers containing 1-8 mol% PI(4)P that was characterized by initial saturable binding followed by desorption, indicative of PI(4)P extraction. In support of this conclusion, an OSBP mutant with impaired binding and extraction of PI(4)P was stably absorbed to PI(4)P-containing membranes, while a pleckstrin homology domain mutant did not associate with PI(4)P-containing membranes. The inclusion of >2 mol% cholesterol, but not 25-hydroxycholesterol, in membranes, enhanced the absorption of the wild-type OSBP. A phosphomimetic of OSBP with enhanced in vitro sterol binding activity displayed membrane interaction properties similar to wild-type. These real-time flow studies allow us to dissect the association of OSBP with PI(4)P into discrete components; initial recruitment to PI(4)P membranes by the PH domain, detection and extraction of PI(4)P, and desorption due to ligand depletion.

How CCTα puts a leash on phospholipid synthesis.

The proportion of phosphatidylcholine (PC) in the membrane is controlled by CTP:phosphocholine cytidylyltransferase α (CCTα), which is known to be regulated by a dual auto-inhibitory and membrane-binding domain. However, the detailed mechanism by which this domain regulates CCTα activity is not clear. Ramezanpour et al. use a combined computational and biochemical approach to define new details of this mechanism, providing an elegant illustration of how the lipid-sensing domain of a phospholipid biosynthetic enzyme controls membrane homeostasis.

Cholesterol transfer at endosomal-organelle membrane contact sites.

PURPOSE OF REVIEW: Cholesterol is delivered to the limiting membrane of late endosomes by Niemann-Pick Type C1 and C2 proteins. This review summarizes recent evidence that cholesterol transfer from endosomes to the endoplasmic reticulum and other organelles is mediated by lipid-binding proteins that localize to membrane contact sites (MCS). RECENT FINDINGS: LDL-cholesterol in the late endosomal/lysosomes is exported to the plasma membrane, where most cholesterol resides, and the endoplasmic reticulum, which harbors the regulatory complexes and enzymes that control the synthesis and esterification of cholesterol. A major advance in dissecting these cholesterol transport pathways was identification of frequent and dynamic MCS between endosomes and the endoplasmic reticulum, peroxisomes and plasma membrane. Positioned at these MCS are members of the oxysterol-binding protein (OSBP) and steroidogenic acute regulatory protein-related lipid-transfer family of lipid transfer proteins that bridge the opposing membranes and directly or indirectly mediate cholesterol transfer. OSBP-related protein 1L (ORP1L), ORP5 and ORP6 mediate cholesterol transfer to the endoplasmic reticulum that regulates cholesterol homeostasis. ORP1L and STARD3 also move cholesterol from the endoplasmic reticulum-to-late endosomal/lysosomes under low-cholesterol conditions to facilitate intraluminal vesicle formation. Cholesterol transport also occurs at MCS with peroxisomes and possibly the plasma membrane. SUMMARY: Frequent contacts between organelles and the endo-lysosomal vesicles are sites for bidirectional transfer of cholesterol.

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.

19q13.12 microdeletion syndrome fibroblasts display abnormal storage of cholesterol and sphingolipids in the endo-lysosomal system.

Microdeletions in 19q12q13.12 cause a rare and complex haploinsufficiency syndrome characterized by intellectual deficiency, developmental delays, and neurological movement disorders. Variability in the size and interval of the deletions makes it difficult to attribute the complex clinical phenotype of this syndrome to an underlying gene(s). As an alternate approach, we examined the biochemical and metabolic features of fibroblasts from an affected individual to derive clues as to the molecular basis for the syndrome. Immunofluorescence and electron microscopy of affected fibroblasts revealed an abnormal endo-lysosomal compartment that was characterized by rapid accumulation of lysosomotropic dyes, elevated LAMP1 and LAMP2 expression and vacuoles containing membrane whorls, common features of lysosomal lipid storage disorders. The late endosomes-lysosomes (LE/LY) of affected fibroblasts accumulated low-density lipoprotein cholesterol, and displayed reduced cholesterol esterification and increased de novo cholesterol synthesis, indicative of defective cholesterol transport to the endoplasmic reticulum. Affected fibroblasts also had increased ceramide and sphingolipid mass, altered glycosphingolipid species and accumulation of a fluorescent lactosylceramide probe in LE/LY. Autophagosomes also accumulated in affected fibroblasts because of decreased fusion with autolysosomes, a defect associated with other lysosomal storage diseases. Attempts to correct the cholesterol/sphingolipid storage defect in fibroblasts with cyclodextrin, sphingolipid synthesis inhibitors or by altering ion transport were unsuccessful. Our data show that 19q13.12 deletion fibroblasts have abnormal accumulation of cholesterol and sphingolipids in the endo-lysosomal system that compromises organelle function and could be an underlying cause of the clinical features of the syndrome.