Glycosphingolipid Sulfates

1. Occurrence and Structure

Glycosphingolipid sulfates (sometimes termed "sulfatides" or "sulfoglycosphingolipids") are glycosphingolipids carrying a sulfate ester group attached to the carbohydrate moiety. They were first identified in brain tissue by the pioneering lipid chemist Thudichum in 1884, although it was much later (1962) before the structure of the main component was fully characterized. Although sulfoglycosphingolipids tend to be minor components of tissues, 3'-sulfo-galactosylceramide (or galactosylceramide-I3-sulfate or 'cerebroside sulfate') illustrated is one of the more abundant glycolipid constituents of brain myelin, and it is also present in many other organs, especially the kidney, but also the gastointestinal tract, islet of Langerhans, trachea and many cancer cell lines. In contrast, 3’-sulfo-glucosylceramide is only encountered at trace levels in tissues.

Formula of 3'-sulfo-galactosylceramide

Many other sulfoglycosphingolipids have now been characterized but from animal tissues only (sea urchins to vertebrates). For example, sulfo-lactosylceramide or lactosylceramide-II3-sulfate and other sulfate esters derived from oligoglycosylceramides of the globo- and ganglio-series have been isolated from human kidney, where they show some structural kinship with the "brain-type" gangliosides. Such lipids with one to four hexose units, and usually one but occasionally two sulfate groups have been isolated from the kidneys of rats and mice. As of 2009, 24 such lipids with variations in the carbohydrate chain had been characterized in vertebrates alone.

Two oligoglycosphingolipids with terminal glucuronic acid residues having sulfate ester moieties in the 3' position occur in the peripheral nervous system. Further oligoglycosphingolipids, including gangliosides, with sulfate groups have been isolated from human, mouse and monkey kidney cells. For example, with kidney cells from the African green monkey, nine distinct sulfated glycolipids were characterized. In most if not all of these, the sulfate ester moiety is attached to the C3 hydroxyl group and has an equatorial conformation.

Many parallels can be drawn between the biosynthesis, metabolism and function of the sulfoglycoglycerolipid, seminolipid, and sphingolipid sulfates, as described elsewhere on this web site. Also, there are separate web pages dealing with the plant sulfonolipid, sulfoquinovosyldiacylglycerol, and the microbial sulfo and sulfonolipids.

The fatty acid components of sulfolipids in animals vary with the nature of the lipid and the tissue. In myelin from the central nervous system, 24:0, 24:1 and 2-hydroxy saturated fatty acids predominate in the sulfo-galactosylceramide. Hydroxy fatty acids predominate in grey matter but not in the white. The corresponding lipid from peripheral tissues such as the pancreas often contains a high proportion of 16:0 and 18:0 fatty acids, and this can also be the case in some of the sulfo-oligoglycolipids of brain, especially the cortical grey matter. On the other hand, 22:0 together with 23:0 and 24:0 are the main fatty acids of kidney sulfolipids.

Sponges have been found to contain many novel lipid compounds, and the freshwater sponge Ephydatia syriaca contains a strange sulfated ceramide glycoside, termed ‘syriacin’, in which a fucose residue is linked to ceramide via a sulfate bridge. The fatty acid linked to the ceramide is also novel, i.e. (all Z)-34S-methylhexatriaconta-5,9,12,15,18,21-hexaenoic acid.

 

2. Biochemistry and Function

The biosynthetic precursor of sulfatide galactosylceramide is synthesised in the endoplasmic reticulum and is transported to the Golgi. Sulfation of galactosylceramide is then catalysed by the enzyme galactosylceramide sulfotransferase in the lumen of the Golgi apparatus, with 3'-phosphoadenosine-5'-phosphosulfate as the activated sulfate donor. Oligoglycosphingolipids are presumably sulfated by similar mechanisms.

Galactosylceramide-I3-sulfate is located exclusively on the extracellular leaflet of the cell membrane of the myelin sheath. The most active period for sulfatide synthesis in brain coincides with myelin formation during fetal development in animal models, and there is considerable evidence pointing to a specific role in this process. During development, there is a rapid increase in the relative concentration of molecular species with C24 as opposed to C18 fatty acids. Experiments with genetically modified animals have confirmed that sphingolipid sulfates are essential for myelin development, maintenance and function. It is believed that sulfatide is a key regulator of the differentiation of oligodendrocytes. The physical properties of sulfatides may be relevant here also, as it is believed that sulfatide interacts with galactosylceramide in myelin via the carbohydrate moieties to maintain a compact myelin structure.

Like the gangliosides, sulfoglycosphingolipids are acidic and relatively soluble in aqueous systems, properties that must have a bearing on their functions in tissues, especially in ion transport. Indeed stable layers of water up to 44 Angstroms thick can form around the polar head group. The free hydroxyl groups in the fatty acid and sphingoid base constituents greatly strengthen hydrogen bonding effects in surface membrane, where sulfoglycosphingolipids may be essential components as amphiphilic donors of negative charges. For example, sulfatides have been shown to be important in the transport of sodium and potassium ions in salt glands of ducks and in organs associated with osmoregulation in fish. There are strong indications that they may have a similar role in kidney. A high content of sulfatides in the gastric and duodenal mucosa, where membranes can be attacked by acid, pepsin and bile salts, may be closely related to a function in mucosal protection.

In addition, sulfatides affect the behaviour of a number of cell types and are involved in platelet aggregation via a strong anticoagulant activity. They bind specifically to certain proteins, including the N-terminal domain of sphingosine kinase, and they can act as antigens in the immune system.

In the pancreas, sulfatide is located in the same cellular compartment as insulin, and it is involved in insulin processing and secretion through activation of ion channels. It is believed to promote folding of proinsulin and may serve as a molecular chaperone for insulin. Antibodies to sulfatide are often present in serum before the onset of diabetes, and sulfatide may influence the progression of the disease.

Catabolism: The principles of lysosomal degradation of sphingolipids are outlined in our web page dealing with glycosylceramides. Catabolism of cerebroside sulfate within the lysosome involves hydrolysis of the sulfate bond by an arylsulfatase and is aided by a nonenzymic protein, known as ‘saposin B’ (or as the ‘cerebroside sulfate activator’ or 'sphingolipid activator protein-1'), which is one of a group of four cysteine-rich proteins with a common ability to interact with membranes, amongst other functions. It is believed that saposin B acts by binding to the lipid, extracting it from the membrane and presenting it to the hydrolase in a form that facilitates reaction. Structural studies have revealed that the molecule is a dimer with a large hydrophobic cavity into which the lipid fits and is presumably orientated in an appropriate manner to enable attack by the hydrolase.

 

3. Sulfatides and Disease

Aberrant sulfoglycosphingolipid metabolism has been associated with various pathogenic conditions, including cancer, autoimmune diseases and sphingolipid storage disorders. For example, sulfoglycosphingolipids accumulate as a consequence of elevated galactosylceramide sulfotransferase activity in a number of human cancers. In contrast, reduced levels of brain sulfatides have been found at the earliest stages of Alzheimer’s disease, possibly as a consequence of an impaired sulfatide transport mechanism mediated by apolipoprotein E. Substantial changes in brain sulfatide levels have been noted in Parkinson’s disease. Reduced formation of sulfatides has also been noted in polycystic kidney disease.

Sulfatide has been implicated in infection by viruses, including the human immunodeficiency virus (HIV-1) and the hepatitis C and influenza A viruses, by facilitating entry into cells. Similarly, many types of pathogenic bacteria bind to sulfatide at the mucosal surface with effects on the development of disease.

Lysosulfatide (sulfogalactosylsphingosine), i.e. without the fatty acid constituent, in addition to the acylated form accumulates in tissues from patients with the metabolic disease - metachromatic leukodystrophy. The enzyme arylsulfatase A (or saposin B) is lacking, leading to fatal demyelination of both central and peripheral nervous systems (large amounts of sulfatides may be present in the urine). Lysosulfatide also appears to have signalling functions akin to those of sphingosine 1-phosphate and sphingosine phosphorylcholine. All three lysolipids are transported in plasma in the high-density lipoprotein fraction.

 

4. Analysis

Sulfoglycolipids tend not to be quite as water-soluble as the gangliosides, but they resemble them in some of their physical properties, and comparable methods are used for analysis. One advantageous strategy is to remove the sulfate ester moiety to reduce the polarity, so that the methodology devised for neutral glycosphingolipids can be employed. Modern mass spectrometric methods are now being used increasingly for characterization of intact sulfatides.

 

Recommended Reading

  • Ahn, V.E., Faull, K.F., Whitelegge, J.P., Fluharty, A.L. and Prive, G.G. Crystal structure of saposin B reveals a dimeric shell for lipid binding. Proc. Nat. Acad. Sci., USA, 100, 38-43 (2003) (DOI: 10.1073/pnas.0136947100).
  • Christie, W.W. and Han, X. Lipid Analysis - Isolation, Separation, Identification and Lipidomic Analysis (4th edition), 446 pages (Oily Press, Bridgwater, U.K.) (2010) - www.pjbarnes.co.uk/op/la4.htm.
  • Honke, K., Zhang, Y., Cheng, X., Kotani, N. and Taniguchi, N. Biological roles of sulfoglycolipids and pathophysiology of their deficiency. Glycoconjugate J., 21, 59-62 (2004) (DOI: 10.1023/B:GLYC.0000043749.06556.3d).
  • Ishizuka, I. Chemistry and functional distribution of sulfoglycolipids. Prog. Lipid Res., 36, 245-319 (1997) (DOI: 10.1016/S0163-7827(97)00011-8).
  • Merrill, A.H. and Sandhoff, K. Sphingolipids: metabolism and cell signalling. In: Biochemistry of Lipids, Lipoproteins and Membranes (4th Edition). pp. 373-407 (Vance, D.E. and Vance, J. (editors), Elsevier, Amsterdam) (2002).
  • Takahashi, T. and Suzuki, T. Role of sulfatide in normal and pathological cells and tissues. J. Lipid Res., 53, 1437-1450 (2012) (DOI: 10.1194/jlr.R026682).
  • Vukelic, E., Zamfir, A.D., Bindila, L., Froesch, M., Peter-Katalinic, J., Usuki, S. and Yu, R.K. Screening and sequencing of complex sialylated and sulfated glycosphingolipid mixtures by negative ion electrospray Fourier transform ion cyclotron resonance mass spectrometry. J. Am. Soc. Mass Spectrom., 16, 571-580 (2005) (DOI: 10.1016/j.jasms.2005.01.013).
  • Yu, R.K., Yanagisawa, M. and Ariga, T. Glycosphingolipid structures. In: Comprehensive Glycoscience. pp. 73-122 (ed. J.P. Kamerling, Elsevier, Oxford, UK) (2007).

 

Updated July 16, 2012

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