Glucosyl- and Galactosylceramides (Cerebrosides)
There are two natural monoglycosylceramides of special importance in animals, plants and fungi, i.e. glucosylceramide and galactosylceramide. Both have biological functions in their own right, for example as structural components of membranes, especially in the brain, but glucosylceramide is also a vital component of human skin and the key intermediate in the biosynthesis of lactosylceramide and thence of complex oligoglycosphingolipids, including gangliosides, in animal tissues. Some bacteria of the order Sphingomonadales of α-proteobacteria produce other monoglycosylceramides.
1. Structure and Occurrence
Galactosylceramide (Galβ1-1'Cer) is the principal glycosphingolipid in brain tissue, hence the trivial name "cerebroside", which was first conferred on it in 1874, although it was much later before it was properly characterized. In fact, β-D-galactosylceramides are found in all nervous tissues and indeed at low levels in all organs, but in brain they can amount to 2% of the dry weight of grey matter and 12% of white matter or 23% of myelin lipids, where they insulate the axons of neuronal cells and constitute a substantial component of the extended plasma membrane of oligodendrocytes. It is also present in some fungal species.
Glucosylceramide (Glcβ1-1'Cer), trivial name "glucocerebroside", is present in animal tissues such as spleen and erythrocytes as well as in nervous tissues, especially in the neurons if at low levels, and it is also found in plants. In addition, β-D-glucosylceramide is a major constituent of skin lipids, where it is essential for the maintenance of the water permeability barrier of the skin. Similarly, higher than normal concentrations of this glycosphingolipid have been reported for the apical plasma membrane domain of epithelial cells from the intestines (especially the absorptive villous cells) and urinary bladder. The d18:1/16:0 molecular species of the two lipids are illustrated.
However, of equal or greater importance to the natural occurrence of glucosylceramide per se is its role as the biosynthetic precursor of lactosylceramide in animals, and thence of most of the complex neutral oligoglycolipids and gangliosides. In contrast, glucosylceramide is the end-product of the biosynthetic pathway in plants and fungi. While galactosylceramide can be sulfated to form a sulfatide or sialylated to form ganglioside GM4, only a small proportion is subjected to further galactosylation to form Gal2Cer as the precursor for the limited gala-series of oligoglycosphingolipids.
Interestingly, the proportion of galactosylceramides relative to glucosylceramides in myelin glycolipids increases greatly in the ascending phylogenic tree, and the ratio of hydroxy- to nonhydroxy-fatty acids in cerebrosides increases with the complexity of the central nervous system. There is also an intriguing sex difference in kidney, where it has been shown that galactosylceramide rather than glucosylceramide occurs in male mice only (or androgen-treated adult females). Only glucosylceramide is present in the nerves of the most primitive animals (protostomes).
In brain, the galactosylceramides are enriched in very-long-chain fatty acids (C22–C26). The fatty acid and long-chain base compositions of cerebrosides from intestines of the Japanese quail are listed in Table 1 for illustrative purposes. The fatty acid components resemble those of other sphingolipids, although the percentage of 2-hydroxy acids is higher than that in sphingomyelin, for example. They are exclusively saturated in this instance, though a small proportion of monoenoic components may also be found in other tissues. The proportion of trihydroxy bases is perhaps higher than in other many other tissues or species studied, probably reflecting the diet. Usually, sphingosine is the main long-chain base in cerebrosides of animal tissues.
Table 1. Composition of fatty acids and long-chain bases (wt % of the total) in cerebrosides of intestines from the Japanese quail.*
|Long‑chain bases||Fatty acids||Non-hydroxy
|* The cerebrosides comprised 81% galactosylceramide and 19% glucosylceramide.
From Hirabayashi, Y. et al., Lipids, 21, 710-714 (1986).
Small amounts of glucosyl- and galactosylceramide that are O-acylated with a fatty acid in various positions of the carbohydrate moiety, especially position 6, have been found in brain tissue of some animal species. Novel galactosylceramides acetylated at position 3 of the sphingosine moiety were first located in myelin from rat brain, and molecular species with further galactose O-acetyl modifications are now known to be present in this tissue. In addition, a galactosylceramide with a long-chain cyclic acetal at the sugar moiety, plasmalo-galactosylceramide, has been isolated from equine brain, i.e. the 4',6'-O-acetal derivative with a clearly defined stereochemistry.
Plants: Glucosylceramide is the only glycosphingolipid common to plants, fungi and animals. It has often been described incorrectly as the main sphingolipid in plants, but this has been because the more polar complex glycosylinositol phosphoceramides are not easily extracted and until relatively recently were missed in conventional analyses. Nonetheless, glucosylceramide is abundant in photosynthetic tissues and constitutes approximately a third of the total sphingolipids, where the main long-chain bases are C18 4,8-diunsaturated (Z/Z and E/Z) (not sphingosine as illustrated above); it is a major component of the outer layer of the plasma membrane and is also enriched in the late endosomes and plant tonoplast. Small amounts of monoglycosylceramides containing a β‑D‑mannopyranosyl unit may be present in non-photosynthetic tissues, but galactosylceramides have not been found in plants. Glucosylceramide is a common component of the lipids of yeast and other fungi, including most fungal pathogens. However, it does not occur in the yeast Saccharomyces cerevisiae, which is widely used as an experimental model, although trace levels of galactosylceramide have been detected.
The fatty acid and long-chain base compositions of glucosylceramides from two plant sources are listed in Table 2 (data for the sphingoid bases in glucosylceramide in Arabidopsis are listed elsewhere). Perhaps surprisingly, the fatty acid components are not very different in nature from those in animal tissues, comprising mainly longer-chain saturated and monoenoic acids, with a high proportion being saturated and having a hydroxyl group in position 2. In the examples selected for the table here, both di- and tri-hydroxy long-chain bases were found, mainly diunsaturated (Z/Z and E/Z) and almost entirely C18 in chain-length. Much higher concentrations of glucosylceramides are found in pollen than in leaves, with substantial compositional differences. For example, the long-chain bases in Arabidopsis leaves consist mainly of t18:1, with relatively little d18:1, t18:0 and d18:0 (with 16:0, 24:0 and 24:1 hydroxy fatty acids mainly), but no d18:2 base although this is 50% of those in pollen. While saturated 2-hydroxy acids predominate in most plants, some cereal glucosylceramides contain high proportions of mono-unsaturated very-long-chain fatty acids of the n-9 family. Glucosylceramides from algae tend to resemble those from higher plants, although some novel structures have been reported from microalgae.
Table 2. Composition of fatty acids and long-chain bases (wt % of the total) in glucosylceramides of seeds from scarlet runner beans and kidney beans.
|Fatty acidsa||Long-chain basesb|
|Runner beans||Kidney beans||Runner beans||Kidney beans|
|From Kojima, M. et al., J. Agric. Food. Chem., 39,
a including 2-hydroxy acids; b di- and tri-hydroxy bases with cis or trans double bonds in the positions indicated
In fungi, ceramide monohexosides are highly conserved molecules, with the ceramide moiety containing the distinctive sphingoid base, (4E,8E)‑9‑methyl-4,8-sphingadienine (or rarely phytosphingosine), linked to 2-hydroxy-octadecanoic or hexadecanoic acids (occasionally these with a trans-double bond in position 3), and with the carbohydrate portion consisting of one residue of either glucose or less often galactose (in contrast to plants). However, the nature of these can vary dramatically during different stages of growth (yeast versus mycelial forms).
Other glycosyl ceramides: Other monoglycosylceramides found in nature include fucosylceramide, which has been isolated from a colon carcinoma, a xylose-containing cerebroside from an avian salt gland, glycosylceramides containing mannose from certain microorganisms, and neuraminic acid linked to ceramide in the phototrophic green sulfur bacterium, Chlorobium limicola. The genus Sphingomonas is unique among gram-negative bacteria in that it lacks lipopolysaccharides in its outer membrane, and instead has two sphingolipids, a tetraglycosylceramide and α-glucuronosylceramide, i.e. with a galacturonic acid moiety with an α- rather than a β-linkage to the ceramide unit. For example, α-glucuronosylceramide from Zymomonas mobilis, a Gram-negative bacterium, contains 2-hydroxy-myristic acid as the main fatty acid with sphinganine as the long-chain base. Related species produce α-galactosylceramide also (see below).
Cerebrosides linked to α-D- rather than β-D-galactose occur in a marine sponge (Agelas mauritianus). A few other bacterial species contain a similar lipid, and it may be of significance that α-galactosylceramides are produced by Bacteroides fragilis, an important component of the human intestinal microflora (see the note on the function of this stereoisomer below). In the latter, the fatty acid and long-chain bases are saturated and contain iso-methyl-branches. It is now recognized that trace amounts of α-glycosylceramides are present in mammalian cells and function as part of the immune response (0.02% of the galactosylceramides in RBL-CD1d cells, for example).
The biosynthesis of monoglycosylceramides in animal tissues resembles that discussed elsewhere on this website for glycosyldiacylglycerols, i.e. there is a direct transfer of the carbohydrate moiety from a sugar-nucleotide, e.g. uridine 5-diphosphate(UDP)-galactose, UDP-glucose, etc, to the ceramide unit. During the transfer, which is catalysed by specific glycosyl-transferases, inversion of the glycosidic bond occurs (from alpha to beta). Synthesis of β-D-galactosylceramide takes place on the lumenal surface of the endoplasmic reticulum, although it has free access to the cytosolic surface by an energy-independent flip-flop process. Expression of the UDP-galactose:ceramide galactosyl transferase is restricted to oligodendrocytes, Schwann cells, kidneys and testes.
In contrast, glucosylceramide is produced on cytosolic side of the early Golgi membranes, with the possible exception of neuronal tissues, by means of a glucosylceramide synthase present in the membrane. If it is to be converted to more complex oligoglycosylceramides, this must be translocated to the luminal leaflet of the trans-Golgi membranes, a process that occurs mainly by non-vesicular transport and is mediated by a conserved clade of integral membrane proteins, i.e. phospholipid flippases (P4-ATPases) designated ATP10A and ATP10D in humans with related enzymes in fungi, which utilize the energy from ATP catalysis to translocate lipids across cellular membranes. The human enzymes are entirely specific for glucosylceramide and not galactosylceramide. For their functions in protein interactions and signalling, both galactosyl- and glucosylceramide must be transported to and then across the plasma membrane. Some glucosylceramide is carried by lipoproteins (VLDL, LDL and HDL) in the circulation and presumably requires active transport for absorption and distribution across the membranes of target tissues.
In certain animal cells, studied in vitro, ceramides with 2-hydroxy acids are converted to galactosylceramide, whereas those with normal fatty acids are used for glucosylceramides, but this is not a universal rule. It is apparent that both ceramides synthesised de novo and those produced by catabolism of sphingomyelin are used for synthesis of glucosylceramide.
In plants, glucosylceramides are also formed by an evolutionarily conserved glucosylceramide synthase involving UDP-glucose in the endoplasmic reticulum, although an alternative mechanism has been described that utilizes sterol glucoside as the immediate glucose donor to ceramide. There is also evidence for a requirement for ceramides containing Δ4 trans-double bonds for synthesis of glucosylceramides but not other sphingolipids in some plant and fungal tissues. However, there is a distinct ceramide synthase in the yeast Pichia pastoris, which produces ceramides of defined composition exclusively for the production of glucosylceramides (see our web page on long-chain bases). A separate ceramide synthase with different specificities produces the ceramide precursors for ceramide phosphorylinositol, which contains only phytosphingosine as the long-chain base. In fungi, glucosylceramide synthases have been characterized, but a galactosylceramide synthase has yet to be identified. Enzymes responsible for the biosynthesis of glucuronosylceramide and α-galactosylceramide in some bacterial species have been characterized.
Galactosylceramides: A remarkable property of cerebrosides is that their 'melting point' is well above physiological body temperature, so that glycolipids have a para-crystalline structure at this temperature. Each cerebroside molecule may form up to eight inter- or intramolecular hydrogen bonds by lateral interaction between the polar hydrogens of the sugar and the hydroxy and amide groups of the sphingosine base of the ceramide moiety, and this dense network of hydrogen bonds is believed to contribute to the high transition temperature and the compact alignment of cerebrosides in membranes. As with sphingomyelin, monoglycosylceramides tend to be concentrated in the outer leaflet of the plasma membrane together with cholesterol and thence in myelin in the specific membrane domains termed 'rafts'. Indeed, the latter appear to facilitate segregation to a greater extent than sphingomyelin via the combination of hydrogen bonds and hydrophobic interactions, and these forces are also of great importance for binding to the wide range of proteins, including enzymes and receptors, which are found in raft domains.
Galactosylceramide is essential to myelin structure and function and it is involved in oligodendrocytes differentiation. While molecular species with 2’-hydroxy fatty acid constituents are not essential for myelin formation, they are critical for the long-term stability of myelin, presumably because increased hydrogen bonding with neighbouring lipids in membranes stabilizes the phase structure. Galactosylceramide is important as a precursor of 3’-sulfo-galactosylceramide, which is also essential to brain development in addition to numerous functions in other tissues. By interacting with sulfatide located in the membrane of opposing layers in the myelin sheath to form what is known as a glycosynapse, galactosylceramide provides a further contribution to the long-term stability of myelin.
Glucosylceramides: As mentioned briefly above glucosylceramide is the primary precursor for most of the more complex oligoglycosphingolipids in animal tissues. This is evident especially in brain, where its synthesis is vital for the production of most neuronal oligoglycosphingolipids, and glucosylceramide per se is essential for axonal growth. It is a major constituent of skin lipids, where it is essential for lamellar body formation in the stratum corneum and to maintain the water permeability barrier of the skin. In addition, the epidermal glucosylceramides (together with sphingomyelin) are the source of the unusual complex ceramides that are found in the stratum corneum including those with terminal hydroxyl groups and estolide-linked fatty acids. Some of the glucosylceramide in skin is linked covalently to proteins via terminal hydroxyl groups, presumably to strengthen the epidermal barrier.
Much of the evidence for the function of glycosylceramides in animals has been derived from cell lines in which synthesis of the lipid has been suppressed by various means in vitro. It appears that glucosylceramide is not essential for the viability of certain cell lines in culture, but disruption of the global synthase gene results in the death of embryos. It is essential for the survival of cancer cells, and deletion from other cell types can lead to abnormalities. In addition to being an intermediate in the biosynthesis of more complex glycosphingolipids and its role in the permeability barrier of the skin (discussed above), glucosylceramide is believed to be required for intracellular membrane transport, cell proliferation and survival, and for various functions in the immune system. In contrast, there are indications that it may have adverse implications for various disease states. For example, over-expression of glucosylceramide synthase in cancer cells has been linked to tumour progression with a reduction in ceramide concentration, resulting in an increased resistance to chemotherapy. The lipid has also been associated with drug resistance in a wider context.
In Arabidopsis, glucosylceramides are critical for cell differentiation and organogenesis, but not necessarily for the viability of cells. It has been proposed that glycosphingolipids could impose positive curvature to membranes, thereby facilitating vesicle fusion. There is evidence that glycosylceramides (but not glycosyldiacylglycerols) together with sterols are located in 'rafts' in plant membranes in an analogous manner to sphingolipids in animal tissues, and that they are associated with specific proteins. Correlative studies suggest that glucosylceramides help the plasma membrane in plants to withstand stresses brought about by cold and drought. For example, glycosylceramides containing 2-hydroxy monounsaturated very-long-chain fatty acids and long-chain bases with 4-cis double bonds appear to be present in higher concentrations in plants that are more tolerant of chilling and freezing. While fungal glucosylceramides with a 9-methyl group within the sphingosine backbone elicit defence responses in rice, cerebrosides with double bonds in positions 4 and/or 8 of the long-chain base appear to be involved in the defence of some plant species against fungal attack.
Less is known of the function of glucosylceramides in fungi, although they are certainly major constituents of the plasma membrane and cell wall. They are believed to be involved in such processes as cell wall assembly, cell division and differentiation, and signalling. The presence of the methyl branch in the long-chain base is essential for cell division and alkali-tolerance. In the case of fungal pathogens, glucosylceramides are recognized by the host immune system and regulate virulence, often after export into the external environment as extracellular vesicles. In contrast to animals, ceramide monohexosides are not precursors for oligoglycosylceramides in fungi. Some molecular species of this lipid from plants (a Δ8 double bond in the long-chain base is essential) show fruiting-inducing activity in the fungus Schizophyllum commune.
α-D-Galactosylceramides: Cerebrosides linked to an α-D- rather than a β-D-galactosyl unit such as that found in the marine sponge Agelas mauritianus, in human gut microflora and even cow's milk are potent stimulators of mammalian immune systems by binding to the protein CD1d on the surface of antigen-presenting cells and activating invariant natural killer T cells. Indeed this was one of the first pieces of evidence to show that glycolipids, like glycoproteins, could invoke an immune response. Subsequently, it was demonstrated that α-galactosylceramide with a 24:1 fatty acid, though present in very small amounts, is loaded onto the CD1d or CD40 protein and is presented as the natural endogenous ligand for NKT cells in the thymus and the periphery. Once activated, NKT cells secrete a range of pro-and anti-inflammatory cytokines to modulate innate and adaptive immune responses. The α-glucosyl and α-psychosine analogues show similar activity.
It is not certain whether α-galactosylceramide is synthesised in animal tissues, and it is likely that is derived primarily from members of the gut microbiome such as Bacteroides fragilis and related species (although in general few bacterial species produce sphingolipids). In mouse gut, the main molecular form consisted of a 2‑(R)‑hydroxylated hexadecanoyl chain linked to C18-sphinganine, while that in B. fragilis contained longer-chain components. Ceramide-galactosyltransferases responsible for synthesis of this lipid in two species of bacteria from the intestinal microbiome have been identified. A decrease in production of this lipid was observed in mice exposed to stress conditions that alter the composition of the gut microbiota, including Western type diet, colitis, and influenza A virus infection with potential consequences upon the systemic immune responses. Its concentration within animal tissues is controlled by catabolic enzymes in a two-step mechanism: removal of the acyl chain by an acid ceramidase followed by hydrolysis of the sugar residue by an α-glycosidase. Initial studies with animal models suggest that treatment with α‑D‑galactosylceramides may be effective against lung and colorectal cancers, melanomas and leukemia, and clinical trials are planned for this lipid and synthetic analogues as anti-tumor immunotherapeutic agents and vaccine adjuvants. Indeed, a phase I trial with high-risk melanoma patients has given promising preliminary results.
In animal tissues, the main sites for the degradation of all glycosphingolipids, including the monoglycosylceramides, oligoglycosphingolipids and gangliosides, are the lysosomes. These are membrane-bound organelles that comprise a limiting external membrane and internal lysosomal vesicles, which contain soluble digestive enzymes that are active at the acidic pH of this organelle. All membrane components are actively transported to the lysosomes to be broken down into their various primary components. In the case of glycosphingolipids, this means to fatty acids, sphingoid bases and monosaccharides, which can be recovered for re-use or further degraded. Thus, sections of the plasma membrane enter the cell by a process of endocytosis, and they are then transported through the endosomal compartment to the lysosomes. The compositional and physical arrangement of the lysosomal membranes is such that they are themselves resistant to digestion with bis(monoacylglycero)phosphate (lysobisphosphatidic acid) as a characteristic component of the inner membrane. A glycocalyx of highly N-glycosylated integral membrane proteins protects the perimeter membrane with the aid of the ganglioside GM3, which is resistant to degradation. This glycocalix forms an efficient hydrophilic barrier at the luminal surface of the lysosomal perimeter membrane to protect it from degradation by proteases and hydrolases, and to prevent lipids and their hydrolysis products from escaping from the lumen of the lysosome.
Degradation of oligoglycosylceramides and gangliosides occurs by sequential removal of monosaccharide units via the action of specific exohydrolases from the non-reducing end until a monoglycosylceramide unit is reached, when glucosylceramide β-glucosidase or an analogous β-galactosidase removes the final carbohydrate moiety. Several glucosylceramidases are known; GBA1 is a lysosomal hydrolase, GBA2 is a ubiquitous non-lysosomal enzyme and GBA3 is a cytosolic β-glucosidase, found in the kidney, liver, spleen and a few other tissues of mammals, the function of which is not clear.
As glycolipids with fewer than four carbohydrate residues are embedded in intralysosomal membranes, while the degradative enzymes are soluble, the process requires the presence of negatively charged lipids and specific activator proteins, which are water-soluble glycoproteins of low molecular weight. These are not themselves active catalytically but are required as cofactors either by directing the enzyme to the substrate or by activating the enzyme by binding to it in some manner. Five such proteins are known, the GM2-activator protein (specific for gangliosides) and Sphingolipid Activator Proteins or saposins A, B, C and D, which perturb the membranes sufficiently to enable the degradative enzymes to reach the glycolipid substrates. The four saposins are derived by proteolytic processing from a single precursor protein, prosaposin, which is synthesised in the endoplasmic reticulum, transported to the Golgi for glycosylation and then to the lysosomes. Of these, saposin C is essential for the degradation of galactosyl- and glucosylceramide, while saposin B is required for hydrolysis of sulfatide, globotriaosylceramide and digalactosylceramide. The products of the hydrolysis reaction with monoglycosylceramides are ceramides and monosaccharides with net retention of the stereochemistry of the latter in the process.
The reactions are aided by the presence of anionic lipids such as bis(monoacylglycero)phosphate. In particular, this increases the ability of the GM2-activator to solubilize lipids and stimulates the hydrolysis of membrane-bound GM1, GM2 and some of the kidney sulfatides. Saposin D stimulates degradation of lysosomal ceramide by acid ceramidase, and it is also involved in the solubilization of negatively charged lipids at an appropriate pH. Eventually, the ceramides can in turn be hydrolysed by an acid ceramidase to fatty acids and sphingoid bases.
β-Glucosylceramidase and saposin C are also required for the generation of the structural ceramides from glucosylceramide in the outer region of the skin, a process essential for optimal skin barrier function and survival. Some glucosylceramide is hydrolysed by the enzyme GBA2 at the plasma membrane, where the ceramide formed is rapidly converted to sphingomyelin by the sphingomyelinase 2, which may be co-located with the glucosidase. In addition, it has been established that cellular β-glucosidases are able to transfer the glucose moiety from glucosylceramide to and from other lipids as in the formation of cholesterol glucoside.
Small but significant amounts of glucosyl- and galactosylceramides are ingested as part of the human diet. They are not hydrolysed by pancreatic enzymes but are degraded in the brush border of the intestines by the enzyme lactase-phlorizin hydrolase (which also hydrolyses the lactose in milk) to ceramides and thence to sphingosine (see also our web page on sphingomyelin.
5. Genetic disorders
Harmful quantities of glucosylceramide accumulate in the spleen, liver, lungs, bone marrow, and, in rare cases, the brain of patients with Gaucher disease, the most common of the inherited metabolic disorders (autosomal recessive) involving storage of excessive amounts of complex sphingolipids. Three clinical forms (phenotypes) of the disease are commonly recognized of which by far the most dangerous are those affecting the brain (Types 2 and 3). All of the patients exhibit a deficiency in the activity of the lysosomal glucosylceramide-β-glucosidase (GBA1), which catalyses the first step in the catabolism of glucosylceramide. The enzyme may be present, but a mutation prevents it assuming its correct conformation, although other factors may be involved as patients with a defective saposin C, the lysosomal activator protein, develop similar symptoms.
In the brain, glucosylceramide accumulates when complex lipids turn over during brain development and during the formation of the myelin sheath of nerves. Other than in the brain, the excess glucosylceramide arises mainly from the biodegradation of old red and white blood cells. The result is that the glucosylceramide remains stored within the lysosomes of macrophages, i.e. the specialized cells that remove worn-out cells by degrading them to simple molecules for recycling, thus preventing them from functioning normally and often leading to chronic inflammation. The enlarged macrophages containing undigested glucosylceramide are termed Gaucher cells. They over express and secrete certain proteins into the circulation, and some of these are used as biomarkers. In addition, glucosylceramide is converted more rapidly to gangliosides in these cells, leading to an increase in ganglioside GM3 in plasma and spleen of patients with Gaucher disease. Fortunately, there are now effective enzyme replacement therapies for patients with the milder (non-neurological or Type 1) form of Gaucher disease that successfully reverse most manifestations of the disorder, including decreasing liver and spleen size and reducing skeletal abnormalities. Two oral drugs that inhibit glucosylceramide synthesis have also been approved.
Defective GBA1 enzyme activity in humans has been implicated in an increased risk of multiple myeloma and other cancers. Similarly, a deficiency in glucocerebrosidase activity may predispose individuals to more common disorders such as Parkinson's disease and Lewy body dementia. Excess glucosylceramide production and thence of more complex glycosphingolipids is a factor in polycystic kidney disease. It appears to be a general rule that the mere process of lysosomal substrate accumulation in all lysosomal storage disorders impairs lysosome integrity and results in more general disruptions to lipid metabolism and membrane structure and function. On the other hand, inhibition of glucosylceramidases may be of benefit in cystic fibrosis. Krabbe disease is discussed in the next section.
Psychosine is the trivial name for galactosylsphingosine, the non-acylated or lyso form of galactosylceramide, sometimes termed lyso-galactosylceramide. It is present in bovine but not normal human brain at very low concentrations, and it is a minor intermediate in the catabolism of monoglycosylceramides. However, it may have some specific function in animal cells, for example in pathophysiology or in signalling, since specific receptors have been found. It is unusual in being a basic (cationic) lipid, so it may have binding properties that differ from those of other lipids.
Psychosine is synthesised, together with galactosylceramide, by the action of UDP-galactose:ceramide galactosyltransferase on sphingosine in the oligodendrocytes (as described above), but under normal conditions the levels of the former are kept low by the action of the acid hydrolase β-galactosylceramidase (galactosylceramide β-galactosidase). A deficiency of this enzyme can lead to accumulation of psychosine in tissues. In particular, psychosine accumulates in the brain in the genetic lysosomal storage disorder Krabbe disease (globoid cell leukodystrophy), leading to widespread degeneration of oligodendrocytes and then to demyelination. Psychosine is believed to inhibit cytokinesis, i.e. the last stage in the process by which a single cell divides to produce two daughter cells, with production of multinucleate cells instead. While it can inhibit protein kinase C activity, it is believed that psychosine exerts its effects primarily through perturbation of membranes, especially of raft domains.
Similarly, glucosylceramide can react with β-galactosylceramidase to produce the related lipid glucosylsphingosine, which is cytotoxic and may exert pathological effects by stimulating the release of Ca2+ from the endoplasmic reticulum in the brain. It inhibits glucosylceramide-β-glucosidase and accumulates in severe forms of Gaucher disease (discussed above). Some babies with this genetic defect have no functional water barrier in the epidermis and die shortly after birth. This lipid can accumulate also in Niemann Pick type C disease, caused by deficient intracellular cholesterol transporter proteins, and it is present in elevated concentrations in glaucoma.
O-Acyl and plasmalogen forms of psychosine with hexadecanal or octadecanal linked to the carbohydrate moiety through 4,6- or 3,4-cyclic acetal bonds, termed 'plasmalopsychosines', have been detected in brain tissues of certain species, including humans. They are not cytotoxic, and 4,6‑plasmalopsychosine in particular displays distinctive neurological effects. Although two stereoisomers can exist in theory, only the endo form appears to occur naturally. An analogous glycero-plasmalopsychosine has also been characterized from brain tissue.
Glycosphingolipids in which the carbohydrate moiety is phosphorylated have been described, i.e. where the ceramide is linked directly to carbohydrate moieties not via phosphate. These can be compared with similar diacylglycerol-linked phosphoglycolipids. As an example, cholinephosphoryl–6-Gal-(β1–1)-Cer and cholinephosphoryl–6-Gal-(β1–6)-Gal-(β1–1)-Cer have been isolated and characterized from the earthworm, Pheretima hilgendorfi.
In this instance, the main fatty acids are 22:0 and 24:0, and the sphingoid bases are octadeca- and nonadeca-4-sphingenine. Subsequently, related triglycosylsphingophospholipids with either a terminal mannose or galactose unit linked to phosphorylcholine were found in the same species, while a similar lipid to that illustrated was found in a clam worm, Marphysa sanguinea. Analogous phosphonolipids, i.e. with a carbon-phosphorus bond, have been found in some marine invertebrates.
The model nematode Caenorhabditis elegans contains a novel glucosylceramide with phosphoethanolamine or its monomethylated form attached to carbon 6 of the glucose moiety. The ceramide moiety contained an iso-branched C17 sphingoid base of the phytosphinganine type (i.e. with a 4-hydroxyl group) and amide-linked 2-hydroxy long-chain fatty acids with variable chain lengths (C22, C23 and C24). This lipid is essential for the development of C. elegans through its regulation of sterol mobilization (the organism requires an exogenous source of cholesterol). It is able to rescue larval arrest that has been induced by sterol starvation.
Methods involving high-resolution thin-layer chromatography and high-performance liquid chromatography (HPLC) are well established for the separation and analysis of monoglycosylceramides. Although separation of α-D-galactosylceramides from the β-form is more of an analytical challenge, it has been accomplished by HPLC. HPLC in the reversed-phase mode was for many years the standard method for separation of molecular species, often after benzoylation for sensitive UV detection, but modern mass spectrometric methods are now being used increasingly for characterization purposes.
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