A family of membrane proteins associated with presenilin expression and gamma-secretase function

Expression Patterns and Prognostic Values of ORMDL1 in Different Cancers

The mammalian orosomucoid-like gene family (ORMDL), containing ORMDL1, ORMDL2, and ORMDL3, is the important regulator of sphingolipid metabolism, which is relevant to cell growth, proliferation, migration, and invasion. Since the role of ORMDL1 in cancers remained unclear, the main purpose of our study was to explore the expression patterns and prognostic values of ORMDL1 in different tumors, especially in cholangiocarcinoma (CHOL), lymphoid neoplasm diffuse large B cell lymphoma (DLBCL), acute myeloid leukemia (LAML), and thymoma (THYM). Bioinformatics tools including GEPIA, CCLE, LinkedOmics, cBioPortal, and TIMER databases were used. As a result, the expression levels of ORMDL1 in tumor tissues and normal tissues varied in different cancers, especially significantly upregulated in CHOL, DLBCL, LAML, and THYM. Moreover, ORMDL1 mRNA was also highly expressed in cell lines of DLBCL and LAML. Further studies showed that ORMDL1 overexpression was associated with poor prognosis in DLBCL, but not significant in CHOL, LAML, and THYM. Consistently, there were genetic alterations of ORMDL1 in DLBCL, and patients with genetic alterations indicated worse survival. Coexpressed genes and related biological events with ORMDL1 in DLBCL were found via LinkedOmics, Gene Ontology (GO), and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis. The relationship between ORMDL1 and cancer immune cells was investigated, and ORMDL1 expression was positively correlated with infiltrating levels of B cells. In conclusion, ORMDL1 is suggested to be a tumorigenic factor and considered as the potential therapeutic target and prognostic biomarker in DLBCL.

Transmembrane topology of mammalian ORMDL proteins in the endoplasmic reticulum as revealed by the substituted cysteine accessibility method (SCAM™)

Sphingolipids are diverse lipids with essential, and occasionally opposing, functions in the cell and therefore tight control over biosynthesis is vital. Mechanisms governing this regulation are not understood. Initial steps in sphingolipid biosynthesis take place on the cytosolic face of the endoplasmic reticulum (ER). Serine palmitoyltransferase (SPT) is an ER-resident enzyme catalyzing the first-committed step in sphingolipid biosynthesis. Not surprisingly, SPT activity is tightly regulated. ORMDLs are ER-resident proteins recently identified as regulators of SPT activity. ORMDL proteins interact directly with SPT but the nature of this interaction is unknown. ORMDL protein sequences contain hydrophobic regions, yet algorithm-based predictions of transmembrane segments are highly ambiguous, making topology of this key regulator unclear. Here we report use of substituted cysteine accessibility to analyze topology of mammalian ORMDLs. We constructed multiple mutant ORMDLs, each containing a single cysteine strategically placed along the protein length. Combined use of selective membrane permeabilization with an impermeant cysteine modification reagent allowed us to assign transmembrane and cytosolic segments of ORMDL. We confirmed that mammalian ORMDL proteins transit the membrane four times, with amino- and carboxy termini facing the cytosol along with a large cytosolic loop. This model will allow us to determine details of the ORMDL-SPT interaction and identify regions acting as the "lipid sensor" to detect changes in cellular sphingolipid levels. We also observe that SPT and ORMDL are substantially resistant to extraction from membranes with non-ionic detergent, indirectly suggesting that both proteins reside in a specialized subdomain of the ER.

Orm/ORMDL proteins: Gate guardians and master regulators

Sphingolipids comprise a diverse family of lipids that perform multiple functions in both structure of cellular membranes and intra- and inter-cellular signaling. The diversity of this family is generated by an array of enzymes that produce individual classes and molecular species of family members and enzymes which catabolize those lipids for recycling pathways. However, all of these lipids begin their lives with a single step, the condensation of an amino acid, almost always serine, and a fatty acyl-CoA, almost always the 16-carbon, saturated fatty acid, palmitate. The enzyme complex that accomplishes this condensation is serine palmitoyltransferase (SPT), a membrane-bound component of the endoplasmic reticulum. This places SPT in the unique position of regulating the production of the entire sphingolipid pool. Understanding how SPT activity is regulated is currently a central focus in the field of sphingolipid biology. In this review we examine the regulation of SPT activity by a set of small, membrane-bound proteins of the endoplasmic reticulum, the Orms (in yeast) and ORMDLs (in vertebrates). We discuss what is known about how these proteins act as homeostatic regulators by monitoring cellular levels of sphingolipid, but also how the Orms/ORMDLs regulate SPT in response to other stimuli. Finally, we discuss the intriguing connection between one of the mammalian ORMDL isoforms, ORMDL3, and the pervasive pulmonary disease, asthma, in humans.

The role of ORMDL proteins, guardians of cellular sphingolipids, in asthma

A family of widely expressed ORM-like (ORMDL) proteins has been recently linked to asthma in genomewide association studies in humans and extensively explored in in vivo studies in mice. ORMDL proteins are key regulators of serine palmitoyltransferase, an enzyme catalyzing the initial step of sphingolipid biosynthesis. Sphingolipids play prominent roles in cell signaling and response to stress, and they affect the mechanistic properties of cellular membranes. Deregulation of sphingolipid biosynthesis and their recycling has been proven to support and even cause several diseases including allergy, inflammation, and asthma. ORMDL3, the most extensively studied member of the ORMDL family, has been shown to be important for endoplasmic reticulum homeostasis by regulating the unfolded protein response and calcium response. In immune cells, ORMDL3 is involved in migration and in the production of proinflammatory cytokines. Furthermore, changes in the expression level of ORMDL3 are important in allergen-induced asthma pathologies. This review focuses on functional aspects of the ORMDL family proteins, which may serve as new therapeutic targets for the treatment of asthma and some other life-threatening diseases.

Unraveling the role of the Target of Rapamycin signaling in sphingolipid metabolism

Sphingolipids are important bioactive molecules that regulate basic aspects of cellular metabolism and physiology, including cell growth, adhesion, migration, senescence, apoptosis, endocytosis, and autophagy in yeast and higher eukaryotes. Since they have the ability to modulate the activation of several proteins and signaling pathways, variations in the relative levels of different sphingolipid species result in important changes in overall cellular functions and fate. Sphingolipid metabolism and their route of synthesis are highly conserved from yeast to mammalian cells. Studies using the budding yeast Saccharomyces cerevisiae have served in many ways to foster our understanding of sphingolipid dynamics and their role in the regulation of cellular processes. In the past decade, studies in S. cerevisiae have unraveled a functional association between the Target of Rapamycin (TOR) pathway and sphingolipids, showing that both TOR Complex 1 (TORC1) and TOR Complex 2 (TORC2) branches control temporal and spatial aspects of sphingolipid metabolism in response to physiological and environmental cues. In this review, we report recent findings in this emerging and exciting link between the TOR pathway and sphingolipids and implications in human health and disease.

Oxidative stress up-regulates presenilin 1 in lipid rafts in neuronal cells

Oxidative stress is associated with beta-amyloid peptide (A beta) accumulation in the brains of Alzheimer's disease patients. A beta is generated upon the sequential proteolytic cleavage of transmembrane amyloid precursor protein (APP) by two membrane-bound proteases, beta-secretase (BACE1) and the gamma-secretase complex comprising presenilin 1 (PS1), nicastrin, APH-1 and PEN-2. Recent evidence suggests that significant amounts of BACE1 and gamma-secretase components localize in the cholesterol-rich region of membranes known as lipid rafts, where A beta production occurs preferentially. In this study, we investigated the effects of oxidative stress on the BACE1 and gamma-secretase components in lipid rafts using human neuroblastoma SH-SY5Y cells exposed to ethacrynic acid (EA), a compound that induces cellular glutathione depletion. Following exposure of cells to EA, heme oxygenase-1, a marker protein of oxidative stress, was strongly induced. Moreover, treatment with EA resulted in a significant increase in PS1 protein levels, but not those of nicastrin, APH-1, PEN-2 or BACE1, in both cell lysates and the lipid raft fraction. This increase in PS1 protein expression was prevented by co-treatment with an antioxidant, N-acetylcysteine (NAC). EA additionally induced a significant increase in PS1 mRNA expression, which was inhibited by NAC. Finally, EA treatment was found to promote A beta secretion from cells expressing Swedish mutant APP. It appears that in our cell culture model, oxidative stress enhances PS1 protein levels in lipid rafts via up-regulation of PS1 transcription, which may constitute the mechanism underlying the oxidative stress-associated promotion of A beta production.

Orm1 and Orm2 are conserved endoplasmic reticulum membrane proteins regulating lipid homeostasis and protein quality control

Yeast members of the ORMDL family of endoplasmic reticulum (ER) membrane proteins play a central role in lipid homeostasis and protein quality control. In the absence of yeast Orm1 and Orm2, accumulation of long chain base, a sphingolipid precursor, suggests dysregulation of sphingolipid synthesis. Physical interaction between Orm1 and Orm2 and serine palmitoyltransferase, responsible for the first committed step in sphingolipid synthesis, further supports a role for the Orm proteins in regulating sphingolipid synthesis. Phospholipid homeostasis is also affected in orm1Delta orm2Delta cells: the cells are inositol auxotrophs with impaired transcriptional regulation of genes encoding phospholipid biosynthesis enzymes. Strikingly, impaired growth of orm1Delta orm2Delta cells is associated with constitutive unfolded protein response, sensitivity to stress, and slow ER-to-Golgi transport. Inhibition of sphingolipid synthesis suppresses orm1Delta orm2Delta phenotypes, including ER stress, suggesting that disrupted sphingolipid homeostasis accounts for pleiotropic phenotypes. Thus, the yeast Orm proteins control membrane biogenesis by coordinating lipid homeostasis with protein quality control.

The two-hydrophobic domain tertiary structure of reticulon proteins is critical for modulation of beta-secretase BACE1

beta-Site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1) is a membrane-bound protease that is essential for the production of beta-amyloid protein (Abeta). Given the crucial role of Abeta accumulation in Alzheimer's disease (AD), inhibition of BACE1 activity may represent a feasible therapeutic strategy in the treatment of AD. Recently, we and others identified reticulon 3 (RTN3) and reticulon 4-B/C (RTN4-B/C or Nogo-B/C) as membrane proteins that interact with BACE1 and inhibit its ability to produce Abeta. In this study, we employed various mutants of RTN3 and RTN4-C and C. elegans RTN to investigate the molecular mechanisms by which RTNs regulate BACE1. We found that RTN3 mutants lacking the N-terminal or C-terminal or loop domain as well as a RTN4-C mutant lacking the C-terminal domain bound to BACE1 comparably to wild-type RTN3 and RTN4-C. Furthermore, overexpression of wild-type RTN3, RTN4-C, and these RTN mutants similarly reduced Abeta40 and Abeta42 secretion by cells expressing Swedish mutant APP. C. elegans RTN, which has low homology to human RTNs, also interacted with BACE1 and inhibited Abeta secretion. In contrast, two RTN3 mutants containing deletions of the first or second potential transmembrane domains and an RTN3 swap mutant of the second transmembrane domain bound BACE1 but failed to inhibit Abeta secretion. Collectively, these results suggest that the two-transmembrane-domain tertiary structure of RTN proteins is critical for the ability of RTNs to modulate BACE1 activity, whereas N-terminal, C-terminal and loop regions are not essential for this function.

An E3 ubiquitin ligase, Synoviolin, is involved in the degradation of immature nicastrin, and regulates the production of amyloid beta-protein

The presenilin complex, consisting of presenilin, nicastrin, anterior pharynx defective-1 and presenilin enhancer-2, constitutes gamma-secretase, which is required for the generation of amyloid beta-protein. In this article, we show that Synoviolin (also called Hrd1), which is an E3 ubiquitin ligase implicated in endoplasmic reticulum-associated degradation, is involved in the degradation of endogenous immature nicastrin, and affects amyloid beta-protein generation. It was found that the level of immature nicastrin was dramatically increased in synoviolin-null cells as a result of the inhibition of degradation, but the accumulation of endogenous presenilin, anterior pharynx defective-1 and presenilin enhancer-2 was not changed. This was abolished by the transfection of exogenous Synoviolin. Moreover, nicastrin was co-immunoprecipitated with Synoviolin, strongly suggesting that nicastrin is the substrate of Synoviolin. Interestingly, amyloid beta-protein generation was increased by the overexpression of Synoviolin, although the nicastrin level was decreased. Thus, Synoviolin-mediated ubiquitination is involved in the degradation of immature nicastrin, and probably regulates amyloid beta-protein generation. Structured digital abstract: * MINT-7255352: Synoviolin (uniprotkb:Q9DBY1) physically interacts (MI:0915) with NCT (uniprotkb:P57716) by anti tag coimmunoprecipitation (MI:0007) * MINT-7255377: Ubiquitin (uniprotkb:P62991) physically interacts (MI:0915) with NCT (uniprotkb:P57716) by anti bait coimmunoprecipitation (MI:0006) * MINT-7255363: NCT (uniprotkb:P57716) physically interacts (MI:0915) with Synoviolin (uniprotkb:Q9DBY1) by anti bait coimmunoprecipitation (MI:0006).