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 illustrated is one of the more abundant glycolipid constituents of myelin, and it is synthesised in oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. The terminology galactosylceramide-I3-sulfate or 'cerebroside sulfate' or the short-hand 'SM4' (by analogy with the GM nomenclature used with gangliosides) can also be employed.
Sulfatides are of particular importance in brain, where galactosylceramide and its sulfatide comprise 23% and 4%, respectively, of the total lipid content of the myelin sheath. In mouse brain, they are reportedly concentrated in the substantia nigra region. However, they are vital constituents of many other organs, especially the kidney, but also the gastrointestinal tract, islet of Langerhans, trachea and many cancer cell lines. 3'-Sulfo-galactosylceramide is present at the surface of blood cells, such as erythrocytes, neutrophils and platelets, and it is a component of the serum lipoproteins.
Many other sulfoglycosphingolipids have now been characterized but primarily from animal tissues (sea urchins to vertebrates). For example, sulfo-lactosylceramide or lactosylceramide-II3-sulfate (or 'SM3') is frequently found in tissues, 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, while analogous lipids containing from one up to five hexose units and one sulfate were detected in a human renal carcinoma. There are substantial regional differences in the content and composition of the sulfatides in kidney, with particularly high concentrations in the distal nephron segments and the renal medulla, reflecting specific functional requirements. Sulfated forms of galactosylceramide, lactosylceramide and globopentaosylceramide are present on the surface membrane of undifferentiated human pluripotent stem cells. The bis-sulfo-gangliotetraosylceramide (SB1a) or gangliotetraosylceramide-II3,IV3-bis-sulfate from mouse kidney is illustrated.
As of 2009, 24 such lipids with variations in the carbohydrate chain had been characterized in vertebrates alone. Important non-animal exceptions are the parasitic protozoa Plasmodium falciparum, and the trypanosomatid parasite, Trypanosoma cruzi, from which a sulfated dihexosylceramide has been isolated. As further examples, two oligoglycosphingolipids with terminal glucuronic acid residues having sulfate ester moieties in the 3'-position occur in the peripheral nervous system. Other complex oligoglycosphingolipids, including gangliosides, with sulfate groups have been isolated from human, mouse and monkey kidney cells. 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. 3’‑Sulfo-glucosylceramide is only encountered at trace levels in tissues, although glucosylceramide-I6 sulfate has been isolated from the Ascidian Ciona intestinalis.
The fatty acid components of sulfolipids in animals vary with the nature of each 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 occur in high concentrations in gray matter but not in the white. During myelination and development of the brain in rats, short-chain sulfatides with C16 non-hydroxylated fatty acids and C18 non-hydroxy and hydroxy fatty acids are synthesised first in restricted regions of the early embryonic spinal cord, while C22 hydroxy fatty acids accumulate later in oligodendrocyte development; C24 fatty acids are most abundant in adulthood. 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, although there are appreciable variations in the long-chain base compositions within structures in this organ that are presumably related in some manner to function. Thus, the renal papillae contain sulfatides with C20 bases, whereas conventional C18-sphingosine-containing compounds predominate in the medulla, and sulfatides with C18-phytosphingosine are restricted to special cortical structures.
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. Similarly, ceramides linked directly to sulfate have been isolated from marine Zoanthids ('palyosulfonoceramides').
Other acidic glycosphingolipids: Gangliosides and ceramide inositol phosphates are of such biological importance and have such distinctive properties that they have their own web pages on this site. In addition, oligoglycosphingolipids containing glucuronic acid as one of the carbohydrate units are present in the insect genus Arthropoda and in several species of marine invertebrates.
Sulfoglycoglycerolipids: Many parallels can be drawn between the biosynthesis, metabolism and function of seminolipid and those of sphingolipid sulfates. Also, there are separate web pages dealing with the plant sulfonolipid, sulfoquinovosyldiacylglycerol, and the microbial sulfo and sulfonolipids.
2. Biochemistry and Function
The biosynthetic precursor of sulfatide galactosylceramide is synthesised in the endoplasmic reticulum and is transported to the Golgi. Then, sulfation of galactosylceramide is catalysed by the enzyme glycosylceramide sulfotransferase in the lumen of the Golgi apparatus with 3'-phosphoadenosine-5'-phosphosulfate as the activated sulfate donor. Lactosylceramide and oligoglycosphingolipids are sulfated by the same enzyme, which prefers a β-glycoside and especially a β-galactoside at the non-reducing termini of sugar chains attached to ceramide; it was first isolated and then cloned from a cell line derived from human renal carcinoma, where it is especially active. The same enzyme and sulfate donor is utilized for the biosynthesis of the glycoglycerolipid seminolipid. A second sulfotransferase from rat brain catalyses the transfer of sulfate to glucuronylglycolipids. Within the cell, sulfatides are redistributed by means of a glycolipid transfer protein (GLTP), which transfers glycolipids from the cytosolic leaflet of the endoplasmic reticulum or plasma membrane and acts as a sensor of glycolipid levels.
Galactosylceramide-I3-sulfate is located exclusively on the outer leaflet of the myelin sheath, which is a lipid-rich membrane produced as an extension of the plasma membrane and forms multilamellar and spirally wrapped sheaths around neuronal axons; it is also an essential component of the axo-glial junction. Simplistically, the lipid coating acts as an insulator although the various lipid constituent also have specific functions. 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 acid constituents. Experiments with genetically modified animals, for example with targeted enzyme deletion, have confirmed that sphingolipid sulfates are essential for myelin development and function, and especially for its maintenance. Mice in which the galactosylceramide sulfotransferase in the brain was eliminated in this manner appeared normal at birth, but soon developed neurological disorders. In addition, there is evidence that sulfatide is a key regulator of the terminal differentiation of oligodendrocytes. The physical properties of sulfatides may be relevant here, as it is believed that sulfatide interacts with galactosylceramide in myelin via the carbohydrate moieties to maintain a compact myelin structure. It is believed also that sulfatide has a role in the lateral organization of myelin membranes with effects upon the sorting, lateral assembly and membrane dynamics, as well as upon the function of myelin proteins in different substructures of the myelin sheath.
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, as targeted gene deletion has demonstrated that sulfoglycosphingolipids are critical for renal ammonium handling, urinary acidification and acid-base homeostasis, i.e. to maintain a stable blood pH. 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.
Sulfatides participate in many different cellular processes throughout the body including trafficking of proteins, cell adhesion and aggregation, immune responses and signal transmission. Many of these effects are a consequence of binding to specific proteins, which usually have a hydrophobic cavity that interacts with the ceramide component, with the hydrophilic moiety often exposed for further intermolecular associations. For example, sulfatides are involved in platelet aggregation by binding to selectins to form stable aggregates, and they can act as antigens in the immune system by binding to cluster of differentiation 1 (CD1) molecules to form complexes recognized by T cells. They are ligands for Toll-like receptor 4 (TLR4), an innate immune receptor that initiates inflammation when activated by bacterial lipopolysaccharide (lipid A). The endogenous sulfatides can thus cause autoinflammation and autoimmunity. Whether they are concentrated in the microdomains known as rafts, which function as signalling platforms in the plasma membrane, has still to be determined.
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, where molecular species containing palmitic acid are important. 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 an initial hydrolysis of the sulfate bond by an arylsulfatase to form galactosylceramide and is aided by a non-enzymic 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. Finally, the galactose unit is removed by a β-galactosidase.
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, including renal cell carcinoma, adenocarcinoma of colon and lung, and ovarian cancer. They are also known to play a critical role in the development of cardiovascular disease. 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, and reduced formation of sulfatides has also been found 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. With the influenza A virus, for example, sulfatide is recognized by the glycoprotein hemagglutinin of the viral envelope and binds to it on the surface membrane of infected cells. This induces apoptosis and enables the virus to interact with the nucleus to facilitate virus replication. Similarly, many types of pathogenic bacteria or bacterial protein toxins bind to sulfatide at the mucosal surface with effects on the development of disease. In contrast, sulfoglycolipids have been associated with protection against Chagas disease and the vaccinia virus.
Lyso-sulfatide (sulfogalactosylsphingosine), i.e. without the fatty acid constituent, in addition to the acylated form accumulates in the brain of patients with the metabolic disease - metachromatic leukodystrophy. The enzyme arylsulfatase A (or saposin B) is lacking, leading to fatal de-myelination of both central and peripheral nervous systems (large amounts of sulfatides may be present in the urine). Lysosulfatide is cytotoxic in vitro but occurs naturally at low levels in tissues such as brain, where it appears to have signalling functions that may oppose those of sphingosine 1-phosphate and sphingosine phosphocholine. It inhibits the activities of protein kinase C and cytochrome c oxidase, and also inhibits and perturbs the migration of neuronal precursor cells.
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.
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|Credits/disclaimer||Updated: August 25th, 2021||Author: William W. Christie|