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General chemical structure of sphingolipids. Different substituents (R) give:
H -> ceramide
phosphocholine -> sphingomyelin
sugar(s) -> glycosphingolipid(s).

Sphingolipids are a class of lipids derived from the aliphatic amino alcohol sphingosine. These compounds play important roles in signal transmission and cell recognition. Sphingolipidoses, or disorders of sphingolipid metabolism, have particular impact on neural tissue.

Contents

Structure

The long-chain bases, sometimes simply known as sphingoid bases, are the first non-transient products of de novo sphingolipid synthesis in both yeast and mammals. These compounds, specifically known as phytosphingosine and dihydrosphingosine (also known as sphinganine [1], although this term is less common), are mainly C18 compounds, with somewhat lower levels of C20 bases.[2] Ceramides and glycosphingolipids are N-acyl derivatives of these compounds.[3]

The sphingosine backbone is O-linked to a (usually) charged head group such as ethanolamine, serine, or choline.

The backbone is also amide-linked to an acyl group, such as a fatty acid.

Types

  • Ceramide is the fundamental structural unit common to all sphingolipids. They consist simply of a fatty acid chain attached through an amide linkage to sphingosine.

There are three main types of sphingolipids, differing in their head groups:

Mammalian sphingolipid metabolism

De novo sphingolipid synthesis begins with formation of 3-keto-dihydrosphingosine by serine palmitoyltransferase[4]. The preferred substrates for this reaction are palmitoyl-CoA and serine. However, studies have demonstrated that serine palmitoyltransferase has some activity toward other species of fatty acyl-CoA[5] and alternative amino acids[6], and the diversity of sphingoid bases has recently been reviewed.[7] Next, 3-keto-dihydrosphingosine is reduced to form dihydrosphingosine. Dihydrosphingosine is acylated by a (dihydro)-ceramide synthase, such as Lass1p or Lass2p (also termed as CerS), to form dihydroceramide[8]. This is desaturated to form ceramide.[9]

Ceramide may subsequently have several fates. It may be phosphorylated by ceramide kinase to form ceramide-1-phosphate. Alternatively, it may be glycosylated by glucosylceramide synthase or galactosylceramide synthase. Additionally, it can be converted to sphingomyelin by the addition of a phosphorylcholine headgroup by sphingomyelin synthase. Diacylglycerol is generated by this process. Finally, ceramide may be broken down by a ceramidase to form sphingosine. Sphingosine may be phosphorylated to form sphingosine-1-phosphate. This may be dephosphorylated to reform sphingosine.[10]

Breakdown pathways allow the reversion of these metabolites to ceramide. The complex glycosphingolipids are hydrolyzed to glucosylceramide and galactosylceramide. These lipids are then hydrolyzed by beta-glucosidases and beta-galactosidases to regenerate ceramide. Similarly, sphingomyelin may be broken down by sphingomyelinase to form ceramide.

The only route by which sphingolipids are converted to non-sphingolipids is through sphingosine-1-phosphate lyase. This forms ethanolamine phosphate and hexadecenal [11].

Functions of mammalian sphingolipids

Sphingolipids are commonly believed to protect the cell surface against harmful environmental factors by forming a mechanically stable and chemically resistant outer leaflet of the plasma membrane lipid bilayer. Certain complex glycosphingolipids were found to be involved in specific functions, such as cell recognition and signaling. The first feature depends mainly on the physical properties of the sphingolipids, whereas signaling involves specific interactions of the glycan structures of glycosphingolipids with similar lipids present on neighboring cells or with proteins.

Recently, relatively simple sphingolipid metabolites, such as ceramide and sphingosine-1-phosphate, have been shown to be important mediators in the signaling cascades involved in apoptosis, proliferation, and stress responses.[12][13] Ceramide-based lipids self-aggregate in cell membranes and form separate phases less fluid than the bulk phospholipids. These sphingolipid-based microdomains, or "lipid rafts" were originally proposed to sort membrane proteins along the cellular pathways of membrane transport. At present, most research focuses on the organizing function during signal transduction.[14]

Sphingolipids are synthesized in a pathway that begins in the ER and is completed in the Golgi apparatus, but these lipids are enriched in the plasma membrane and in endosomes, where they perform many of their functions.[15] Transport occurs via vesicles and monomeric transport in the cytosol. Sphingolipids are virtually absent from mitochondria and the ER, but constitute a 20-35 molar fraction of plasma membrane lipids.[16]

Yeast sphingolipids

Because of the incredible complexity of mammalian systems, yeast are sometimes used as a model organism for working out new pathways. These single-celled organisms are often more genetically tractable than mammalian cells, and strain libraries are available to supply strains harboring almost any non-lethal open reading frame single deletion. The two most commonly used yeasts are Saccharomyces cerevisiae and Schizosaccharomyces pombe, although research is also done in the pathological yeast Candida albicans.

In addition to the important structural functions of complex sphingolipids (inositol phosphorylceramide and its mannosylated derivatives), the sphingoid bases phytosphingosine and dihydrosphingosine (sphinganine) play vital signaling roles in S. cerevisiae. These effects include regulation of endocytosis, ubiquitin-dependent proteolysis (and, thus, regulation of nutrient uptake [17]), cytoskeletal dynamics, the cell cycle, translation, posttranslational protein modification, and the heat stress response.[18] Additionally, modulation of sphingolipid metabolism by phosphatidylinositol (4,5)-bisphosphate signaling via Slm1p and Slm2p and calcineurin has recently been described[19]. Additionally, a substrate-level interaction has been shown between complex sphingolipid synthesis and cycling of phosphatidylinositol 4-phosphate by the phosphatidylinositol kinase Stt4p and the lipid phosphatase Sac1p.[20]

Plant sphingolipids

Higher plants contain a wider variety of sphingolipids than animals and fungi.

Disorders

There are several disorders of sphingolipid metabolism, known as sphingolipidoses. The most common is Gaucher's disease.

Also of note is Fabry's disease, an X-linked recessive condition wherein a buildup of glycosphingolipids in lysosomes of various tissues is due to alpha-galactosidase deficiency. These patients tend to present with peripheral neuropathies and develop chronic renal conditions.

Additional images

References

  1. ^ Product page at Sigma Aldrich[1]
  2. ^ Reviewed in Dickson, R.C. (1998)Annual Review of Biochemistry. 67, 27-48.
  3. ^ A brief, very comprehensible review is given in Gunstone, F. (1996) Fatty Acid and Lipid Chemistry, pp 43-44. Blackie Academic and Professional. ISBN 0 7514 0253 2
  4. ^ Merrill. "Characterization of serine palmitoyltransferase activity in Chinese hamster ovary cells." Biochim Biophys Acta (1983) 754(3):284-91.
  5. ^ Merrill and Williams. "Utilization of different fatty acyl-CoA thioesters by serine palmitoyltransferase from rat brain". Journal of Lipid Research (1984) 25 (2): 185-188.
  6. ^ Zitomer NC, Mitchell T, Voss KA, Bondy GS, Pruett ST, Garnier-Amblard EC, Liebeskind LS, Park H, Wang E, Sullards MC, Merrill AH Jr, Riley RT. "Ceramide Synthase Inhibition by Fumonisin B1 Causes Accumulation of 1-Deoxysphinganine: A Novel Category of Bioactive 1-Deoxysphingoid Bases And 1-Deoxydihydroceramides Biosynthesized By Mammalian Cell Lines And Animals". Journal of Biological Chemistry (2009) 284 (8): 4786-4795.
  7. ^ Pruett et al. "Biodiversity of sphingoid bases ("sphingosines") and related amino alcohols". Journal of Lipid Research. (2008) 49:1621-1639.
  8. ^ Pewzner-Jung et al. "When do Lasses (longevity assurrance genes) become CerS (ceramide synthases)?: insights into the regulation of ceramide synthesis". Journal of Biological Chemistry. (2006) 281, 25001-25005.
  9. ^ Causeret et al. "Further characterization of rat dihydroceramide desaturase: tissue distribution, subcellular localization, and substrate specificity". Lipids. (2005) 35:1117-1125.
  10. ^ Reviewed in Hannun and Obeid. "Principles of bioactive lipid signalling: lessons from sphingolipids". Nature Reviews Molecular Cell Biology. (2008) 9, 139-150.
  11. ^ Bandhuvulua & Saba. "Sphingosine-1-phosphate lyase in immunity and cancer: silencing the siren". Trends in Molecular Medicine. (2007) 13:210-217.
  12. ^ Hannun YA, Obeid LM (July 2002). "The Ceramide-centric universe of lipid-mediated cell regulation: stress encounters of the lipid kind". J. Biol. Chem. 277 (29): 25847–50. doi:10.1074/jbc.R200008200. PMID 12011103. http://www.jbc.org/cgi/content/full/277/29/25847.  
  13. ^ Spiegel S, Milstien S (July 2002). "Sphingosine 1-phosphate, a key cell signaling molecule". J. Biol. Chem. 277 (29): 25851–4. doi:10.1074/jbc.R200007200. PMID 12011102. http://www.jbc.org/cgi/content/full/277/29/25851.  
  14. ^ Brown DA, London E (June 2000). "Structure and function of sphingolipid- and cholesterol-rich membrane rafts". J. Biol. Chem. 275 (23): 17221–4. doi:10.1074/jbc.R000005200. PMID 10770957. http://www.jbc.org/cgi/content/full/275/23/17221.  
  15. ^ Futerman AH (December 2006). "Intracellular trafficking of sphingolipids: relationship to biosynthesis". Biochim. Biophys. Acta 1758 (12): 1885–92. doi:10.1016/j.bbamem.2006.08.004. PMID 16996025.  
  16. ^ van Meer G, Lisman Q (July 2002). "Sphingolipid transport: rafts and translocators". J. Biol. Chem. 277 (29): 25855–8. doi:10.1074/jbc.R200010200. PMID 12011105. http://www.jbc.org/cgi/content/full/277/29/25855.  
  17. ^ Chung N. (2001) "Phytosphingosine as a specific inhibitor of growth and nutrient import in Saccharomyces cerevisiae." J Biol Chem. Sep 21; 276(38):35614-21.
  18. ^ Cowart and Obeid. (2007) "Yeast sphingolipids: recent developments in understanding biosynthesis, regulation, and function." Biochim Biophys Acta. Mar;1771(3):421-31.
  19. ^ Dickson, RC. (2008) J Lipid Res. May;49(5):909-21.
  20. ^ Brice and Cowart. (2009) J Biol Chem. Jan 12. Epub ahead of print

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