Sphingolipid Transport: Rafts and Translocators (2002)

endoplasmic reticulum galactosylceramide glucosylceramide lactosylceramide sphingomyelin trans-Golgi network glycosylphosphatidylinositol phosphatidylinositol Neu5Acα3Galβ4GlcCer Galβ3GalNAcβ4(Neu5Acα3)Galβ4GlcCer Neu5Acα8Neu5Acα3Galβ4GlcCer Until some 15 years ago, sphingolipids were generally believed to protect the cell surface against harmful factors in the environment by forming a mechanically stable and chemically resistant outer leaflet of the plasma membrane lipid bilayer. Furthermore, complex glycosphingolipids were found to be involved in specific functions like recognition and signaling (1Hakomori S. J. Biol. Chem. 1990; 265: 18713-18716Abstract Full Text PDF PubMed Google Scholar). Whereas the first feature would depend on physical properties of the sphingolipids, the signaling functions involve specific interactions of the complex glycan structures on the glycosphingolipids with similar lipids on neighboring cells or with proteins. Since then, two findings have revolutionized the field. (i) Simple sphingolipid metabolites, like ceramide and sphingosine 1-phosphate, have been found to be important mediators in signaling cascades of apoptosis, proliferation, and stress responses (reviews by Hannun and Obeid (66Hannun Y.A. Obeid L.M. J. Biol. Chem. 2002; 277: 25847-25850Abstract Full Text Full Text PDF PubMed Scopus (756) Google Scholar) and Spiegel and Milstien (67Spiegel S. Milstien S. J. Biol. Chem. 2002; 277: 25851-25854Abstract Full Text Full Text PDF PubMed Scopus (512) Google Scholar)). (ii) It has been realized that ceramide-based lipids self-aggregate in cellular membranes to form a separate phase that is less fluid (liquid-ordered) than the bulk liquid-disordered phospholipids based on diacylglycerol. Sphingolipid-based microdomains or “rafts” were originally proposed to sort membrane proteins along the cellular pathways of membrane transport (2Simons K. van Meer G. Biochemistry. 1988; 27: 6197-6202Crossref PubMed Scopus (1091) Google Scholar). Presently, most excitement focuses on their organizing functions in signal transduction (3Brown D.A. London E. J. Biol. Chem. 2000; 275: 17221-17224Abstract Full Text Full Text PDF PubMed Scopus (2065) Google Scholar). Sphingolipids are synthesized in the ER1 and the Golgi but are enriched in plasma membrane and endosomes where they perform many of their functions. Thus, sphingolipids travel between organelles. Transport occurs via transport vesicles and via monomeric transport through the cytosol. Furthermore, some sphingolipids efficiently translocate across cellular membranes. That transport is not random is clear from the heterogeneous distribution of sphingolipids over the cell; sphingolipids are virtually absent from mitochondria and the ER but constitute 20–35 mol % of the plasma membrane lipids (TableI). Furthermore, signaling pools of sphingolipids do not freely mix with pools of biosynthesis and degradation (reviews by Hannun and Obeid (66Hannun Y.A. Obeid L.M. J. Biol. Chem. 2002; 277: 25847-25850Abstract Full Text Full Text PDF PubMed Scopus (756) Google Scholar), Merrill (68Merrill A.H., Jr. J. Biol. Chem. 2002; 277: 25843-25846Abstract Full Text Full Text PDF PubMed Scopus (498) Google Scholar), and Spiegel and Milstien (67Spiegel S. Milstien S. J. Biol. Chem. 2002; 277: 25851-25854Abstract Full Text Full Text PDF PubMed Scopus (512) Google Scholar)). The specificity in sphingolipid transport is the topic of the present review.Table ISphingolipid content of plasma membranesSphingolipidsGlycerophospholipidsSterol mol/molIntestinal epithelium Apical1-aIn this membrane, 50% of the sphingolipids were GlcCer, the rest consisted of the complex glycosphingolipid globoside and SM. The major glycerophospholipid, phosphatidylcholine, makes up 8 mol % of the lipids in the apicalversus 29 mol % of the lipids in the basolateral membrane (62).382933 Basolateral1-aIn this membrane, 50% of the sphingolipids were GlcCer, the rest consisted of the complex glycosphingolipid globoside and SM. The major glycerophospholipid, phosphatidylcholine, makes up 8 mol % of the lipids in the apicalversus 29 mol % of the lipids in the basolateral membrane (62).195625Myelin1-bThe major sphingolipid in myelin is GalCer followed by sulfated GalCer and SM (63).282844Yeast plasma membrane1-cSphingolipids inSaccharomyces cerevisiae consist of nearly equal fractions of inositolphosphoceramide, mannosylinositolphosphoceramide, and mannosyldiinositolphosphoceramide (64). In addition, other yeasts generally contain GlcCer-based glycosphingolipids.163648Plasma membranes are manyfold enriched in sphingolipids and cholesterol as compared to the ER and the mitochondria (34Keenan T.W. Morré D.J. Biochemistry. 1970; 9: 19-25Crossref PubMed Scopus (207) Google Scholar, 61Lange Y. Swaisgood M.H. Ramos B.V. Steck T.L. J. Biol. Chem. 1989; 264: 3786-3793Abstract Full Text PDF PubMed Google Scholar). Indeed, electron microscopy after immunolabeling demonstrated that the concentration of the complex glycosphingolipid Forssman antigen was 10-fold higher in the plasma membrane than in the ER, with no label over mitochondria and peroxisomes (53van Genderen I.L. van Meer G. Slot J.W. Geuze H.J. Voorhout W.F. J. Cell Biol. 1991; 115: 1009-1019Crossref PubMed Scopus (124) Google Scholar).1-a In this membrane, 50% of the sphingolipids were GlcCer, the rest consisted of the complex glycosphingolipid globoside and SM. The major glycerophospholipid, phosphatidylcholine, makes up 8 mol % of the lipids in the apicalversus 29 mol % of the lipids in the basolateral membrane (62Kawai K. Fujita M. Nakao M. Biochim. Biophys. Acta. 1974; 369: 222-233Crossref PubMed Scopus (135) Google Scholar).1-b The major sphingolipid in myelin is GalCer followed by sulfated GalCer and SM (63Morell P. Quarles R.H. Norton W.T. Siegel G.J. Agranoff B.W. Albers R.W. Molinoff P.B. Basic Neurochemistry. Molecular, Cellular, and Medical Aspects. 4th Ed. Raven Press, New York1994: 117-143Google Scholar).1-c Sphingolipids inSaccharomyces cerevisiae consist of nearly equal fractions of inositolphosphoceramide, mannosylinositolphosphoceramide, and mannosyldiinositolphosphoceramide (64Patton J.L. Lester R.L. J. Bacteriol. 1991; 173: 3101-3108Crossref PubMed Google Scholar). In addition, other yeasts generally contain GlcCer-based glycosphingolipids. Open table in a new tab Plasma membranes are manyfold enriched in sphingolipids and cholesterol as compared to the ER and the mitochondria (34Keenan T.W. Morré D.J. Biochemistry. 1970; 9: 19-25Crossref PubMed Scopus (207) Google Scholar, 61Lange Y. Swaisgood M.H. Ramos B.V. Steck T.L. J. Biol. Chem. 1989; 264: 3786-3793Abstract Full Text PDF PubMed Google Scholar). Indeed, electron microscopy after immunolabeling demonstrated that the concentration of the complex glycosphingolipid Forssman antigen was 10-fold higher in the plasma membrane than in the ER, with no label over mitochondria and peroxisomes (53van Genderen I.L. van Meer G. Slot J.W. Geuze H.J. Voorhout W.F. J. Cell Biol. 1991; 115: 1009-1019Crossref PubMed Scopus (124) Google Scholar). The first steps in sphingolipid synthesis are the condensation of l-serine and palmitoyl-CoA to ketosphinganine and its reduction to sphinganine in the ER membrane. In yeast, these lipids do not feed into signaling pools (4Skrzypek M.S. Nagiec M.M. Lester R.L. Dickson R.C. J. Bacteriol. 1999; 181: 1134-1140Crossref PubMed Google Scholar), and exogenous sphingoid bases need to go through a cycle of phosphorylation and dephosphorylation before they can be utilized for ceramide synthesis (5Zanolari B. Friant S. Funato K. Sutterlin C. Stevenson B.J. Riezman H. EMBO J. 2000; 19: 2824-2833Crossref PubMed Scopus (210) Google Scholar). This suggests that sphingoid bases synthesized de novoare channeled through the pathway into ceramide without being able to escape. In yeast, ceramide is then converted to inositolphosphoceramide and the mannosyl derivatives mannosylinositolphosphoceramide and mannosyldiinositolphosphoceramide on the lumenal surface of the Golgi (6Levine T.P. Wiggins C.A. Munro S. Mol. Biol. Cell. 2000; 11: 2267-2281Crossref PubMed Scopus (134) Google Scholar). In mammals, ceramide is utilized for the synthesis of glucosylceramide (GlcCer) on the cytosolic side of the Golgi, sphingomyelin (SM) on the lumenal surface of the Golgi, and in specialized cells, e.g. many epithelial cells, of galactosylceramide (GalCer) in the lumen of the ER (Fig.1) (7van Meer G. Holthuis J.C.M. Biochim. Biophys. Acta. 2000; 1486: 145-170Crossref PubMed Scopus (135) Google Scholar). Because ceramide synthesis occurs on the cytosolic side of the ER, the rate of ceramide translocation toward the lumena of ER and Golgi affects the relative synthesis of the various products. If the t 12 of spontaneous ceramide translocation would be tens of minutes (8Bai J. Pagano R.E. Biochemistry. 1997; 36: 8840-8848Crossref PubMed Scopus (190) Google Scholar), this is slow compared with the vesicular transport between ER and Golgi (minutes). However, translocation may be faster in the unsaturated lipid environment of the ER. In addition, ER and Golgi may possess proteins that stimulate ceramide translocation. Ceramide transport to the site of SM synthesis can be inhibited under conditions where transport to the site of GlcCer synthesis and ER-Golgi vesicle transport are normal (9Yasuda S. Kitagawa H. Ueno M. Ishitani H. Fukasawa M. Nishijima M. Kobayashi S. Hanada K. J. Biol. Chem. 2001; 276: 43994-44002Abstract Full Text Full Text PDF PubMed Scopus (115) Google Scholar), and besides the vesicular pathway, a non-vesicular mechanism delivers ceramide to the Golgi in mammalian cells and yeast (10Kok J.W. Babia T. Klappe K. Egea G. Hoekstra D. Biochem. J. 1998; 333: 779-786Crossref PubMed Scopus (46) Google Scholar, 11Funato K. Riezman H. J. Cell Biol. 2001; 155: 949-959Crossref PubMed Scopus (152) Google Scholar). In yeast, this alternative pathway depends on ER-Golgi membrane contact and on a cytosolic factor and is energy-independent (11Funato K. Riezman H. J. Cell Biol. 2001; 155: 949-959Crossref PubMed Scopus (152) Google Scholar). Interestingly, close apposition of the ER to cisternae of the trans-Golgi has been observed in mammalian cells (12Marsh B.J. Mastronarde D.N. Buttle K.F. Howell K.E. McIntosh J.R. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 2399-2406Crossref PubMed Scopus (308) Google Scholar). In the model of Fig. 1, GlcCer synthase in the cis-Golgi receives ceramide via the vesicular pathway whereas GlcCer synthase and SM synthase in the trans-Golgi (13Sadeghlar F. Sandhoff K. van Echten-Deckert G. FEBS Lett. 2000; 478: 9-12Crossref PubMed Scopus (25) Google Scholar) receive ceramide from the ER via membrane contacts. Similar contacts have often been observed between ER and mitochondria (12Marsh B.J. Mastronarde D.N. Buttle K.F. Howell K.E. McIntosh J.R. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 2399-2406Crossref PubMed Scopus (308) Google Scholar). They may be responsible for the transfer of signaling ceramide to mitochondria. A mitochondrial ceramidase has been identified (Hannun and Obeid (66Hannun Y.A. Obeid L.M. J. Biol. Chem. 2002; 277: 25847-25850Abstract Full Text Full Text PDF PubMed Scopus (756) Google Scholar)). GlcCer synthesized on the cytosolic surface of the Golgi is partially converted to complex glycosphingolipids in the Golgi lumen (review by Kolter et al. (69Kolter T. Proia R.L. Sandhoff K. J. Biol. Chem. 2002; 277: 25859-25862Abstract Full Text Full Text PDF PubMed Scopus (259) Google Scholar)). Experiments with brefeldin A, which fuses the cis-medial Golgi with the ER, have suggested that the enzymes synthesizing lactosylceramide (LacCer; Fig.1) and the first complex glycosphingolipids are in the early Golgi. However, the bulk of these events is thought to occur in the trans-Golgi or trans-Golgi network (TGN) in vivo (14Lannert H. Gorgas K. Meissner I. Wieland F.T. Jeckel D. J. Biol. Chem. 1998; 273: 2939-2946Abstract Full Text Full Text PDF PubMed Scopus (106) Google Scholar, 15Allende M.L., Li, J. Darling D.S. Worth C.A. Young W.W., Jr. Glycobiology. 2000; 10: 1025-1032Crossref PubMed Scopus (20) Google Scholar). 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The in the of signal transduction and have to a in the of sphingolipid the of sphingolipids in membrane It is being realized that in most cellular membranes. be to and the of to their in and to the of the various sphingolipids in their important be to the of lipid be most important to the interactions between sphingolipids and proteins. are proteins involved in signaling but proteins involved in vesicular for sphingolipids functions in their Because sphingolipid functions are by the enzymes that and transport the sphingolipids, major be to these enzymes and their is in the