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Hot roles for glycosylation: signal transduction, control of cell development and differentiation, and innate immunity
567
Carbohydrates and glycoconjugates Hot roles for glycosylation: signal transduction, control of cell development and differentiation, and innate immunity Editorial overview Mark R Wormald and Nathan Sharon Current Opinion in Structural Biology 2002, 12:567–568 0959-440X/02/$ — see front matter © 2002 Elsevier Science Ltd. All rights reserved.
Mark R Wormald Oxford Glycobiology Institute, Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, UK; e-mail: [email protected]
Mark’s research is focused on studying the conformational and dynamic properties of oligosaccharides, glycopeptides and glycoproteins, mostly using NMR spectroscopy combined with molecular modelling. Nathan Sharon Department of Biological Chemistry, The Weizmann Institute of Science, Rehovot 76100, Israel; e-mail: [email protected]
Nathan Sharon has been interested in carbohydrates and lectins for nearly half a century, and was instrumental in formulating and widely disseminating the concept that these substances function in cell recognition. His work has led to the development of a lectin-purging method of bone marrow used clinically in the treatment of ‘bubble children’
Glycobiology, sometimes referred to as the last unconquered frontier of molecular and cell biology [1], is starting to succumb to the combined assault of sophisticated biochemical techniques and genomics. In particular, this is highlighted by the elucidation of the key roles played by cell surface glycans in signal transduction, control of cell differentiation and innate immunity, some of which are surveyed in the reviews in this section. In the first review, Wilson (pp 569–577) surveys the current state of knowledge on the biosynthesis of glycoprotein glycans in plants and invertebrates. Although much work has been done to elucidate these biosynthetic pathways in mammals, plants and invertebrates have always been the ‘poor relations’. Based on structural studies, they were thought to produce a very limited range of N- and O-linked glycans in comparison to mammalian systems. The availability of (nearly complete) genomes for several species of plants and invertebrates has provided a very considerable boost to this area, allowing many putative glycosyltransferase genes to be identified and strongly suggesting that plant and invertebrate glycosylation is more complicated than first thought. However, much work remains to be done to translate the existence of these putative genes into knowledge of the glycans that are actually synthesised by these organisms. Prion protein, an endogenous glycoprotein with the ability to act as an infectious agent, is the subject of the second review. Rudd et al. (pp 578–586) summarise recent results relating to the effects of N-glycosylation on the structural and functional properties of this protein. These range from the effects on glycosylation of the way that the protein is anchored to the membrane to the way that the glycans modulate the neuronal localisation of the protein and its conversion from the normal to the pathogenic form. The review demonstrates very well that the effects of prion protein glycosylation are wide ranging, frequently subtle and not yet fully understood. Although, in the past, the area of glycolipids has received somewhat less attention than that of glycoproteins, this attitude is changing. A major reason for this is the realisation that they are involved in cell signalling. As discussed by Allende and Proia (pp 587–592), recent evidence shows that one of the most prominent families of glycolipids, the gangliosides, can serve two different roles in cell signalling. They act directly as ligands for specific receptors (such as the siglecs discussed in Crocker’s review) and, by participating in the formation of specialised protein/lipid domains (rafts) on the plasma membrane, they can modulate receptor functions (e.g. of tyrosine kinases). It is suggested that the signalling function of gangliosides may be a potential therapeutic target in cancer, diabetes and nerve regeneration. An additional important source of information on the biological function of gangliosides in vivo comes from knockout mice in which genes involved in glycolipid
568
Carbohydrates and glycoconjugates
biosynthesis have been disrupted. For example, disruption of the gene in mice that is responsible for the first step in the formation of almost all glycosphingolipids is embryonically lethal. Mice that lack enzymes involved in later steps of glycolipid biosynthesis are viable, but show a range of abnormalities, including infertility and neurological defects. Knowledge of glycolipid biosynthesis has led to novel approaches for treating diseases due to defects in the degradation of these substances. One such approach has recently reached fruition in the form of an Oxford GlycoSciences imino sugar that has been approved for the treatment of Gaucher disease. In his article, Haltiwanger (pp 593–598) reviews the role of O-linked fucose residues in regulating certain cell–cell signalling pathways involved in development in Drosophila, and in mice and zebrafish. One example in Drosophila is the Fringe protein, a fucose-specific N-acetylglucosaminyltransferase, which carries out a variety of essential development functions by modulating the activity of Notch, another protein in the development pathway. This modulation is achieved by the Fringe-catalysed addition of N-acetylglucosamine residues to fucose residues present on Notch. An example in mice and zebrafish is the Nodal protein, which plays an important role in the development of polarity in the developing embryo; mutations of this protein in these animals result in severe defects. Signal transduction by Nodal requires O-fucosylation of another group of proteins, designated as the EGF-CFC family. Although there is no single function for oligosaccharides (just as there is no single function for polypeptides), a clearly important function of glycans is as ligands for recognition by lectins (discussed to some degree in all the preceding reviews). The remaining three reviews concentrate on such recognition events. In his article, Kannagi (pp 599–608) reviews work on characterising the ligands for the different selectins (E-, L-and P-) and discusses their roles in lymphocyte homing. The 6-sulfated sialyl Lewis X epitope is involved in the routine trafficking of lymphocytes, whereas the sialyl Lewis X epitope is involved in leukocyte trafficking to sites of inflammation. The regulation of these epitopes is different, as might be expected from their very different functions. Sialyl Lewis X epitopes are considerably upregulated during inflammation by increased transcription of a fucosyltransferase gene. Expression of sialyl 6-sulfo Lewis X is regulated by a recently discovered unique modification of the sialic acid, its de-N-acetylation and conversion of the product into an internal cyclic form not encountered before. Kannagi also discusses the role of
sialyl 6-sulfo Lewis X present on P-selectin glycoprotein ligand-1 (PSLG-1), which is expressed on leukocytes, and suggests that it may serve as ligand for all three selectins, although with different affinity. One of the earlier roles proposed for the terminal sialic acid residues present on glycoprotein glycans was the modulation of the half-life of soluble glycoproteins by masking the underlying galactose residues that are required for the clearance of certain asialoglycoproteins from the circulation. However, they now appear to be much more important as recognition epitopes themselves. Sialic acids are ubiquitous on all mammalian cell surfaces, making them useful for mediating cell–cell contacts, but are absent from many lower organisms, such as some pathogens, thereby providing a means of distinguishing self from nonself within the immune system. It is now clear that an extensive family of closely related proteins, the siglecs, carries out these different functions. In his review, Crocker (pp 609–615) provides a wide-ranging discussion of this lectin family, from the chromosomal localisation of their genes and its implications for diversity to their functional role in modulating the innate immune response. The review emphasises the importance of competition between interaction of the siglecs with sialic acid residues on the surface of the same cell as opposed to their interaction with residues on the membranes of opposing cells, as a method of regulating siglec function. He emphasises the need to understand how glycosylation changes modulate these interactions. The interaction between a lectin and its glycan ligand is usually relatively weak. High-affinity binding can be achieved by a multivalent lectin interacting with either a multivalent glycan or multiply presented glycans (as in the recognition of multiply presented oligomannose glycans by the mannose-binding lectin, a key component of innate immunity in higher animals). In the last review, Brewer, Miceli and Baum (pp 616–623) discuss how the interactions between multivalent lectins and multivalent ligands can lead to the clustering of both into large assemblies. Such clustering can occur between selected components on the same membrane, leading to local organisation of the membrane and hence modulation of function, or it can occur during cell signalling events, the clustering providing a mechanism for signal transduction. Such clustering may account for the biological activities of several members of the galectin family, including cell proliferation, adhesion and apoptosis.
Reference 1.
Sharon N: The conquest of the last frontier of molecular and cell biology. Biochimie 2001, 83:555.
Carbohydrates and glycoconjugates Hot roles for glycosylation: signal transduction, control of cell development and differentiation, and innate immunity Editorial overview Mark R Wormald and Nathan Sharon Current Opinion in Structural Biology 2002, 12:567–568 0959-440X/02/$ — see front matter © 2002 Elsevier Science Ltd. All rights reserved.
Mark R Wormald Oxford Glycobiology Institute, Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, UK; e-mail: [email protected]
Mark’s research is focused on studying the conformational and dynamic properties of oligosaccharides, glycopeptides and glycoproteins, mostly using NMR spectroscopy combined with molecular modelling. Nathan Sharon Department of Biological Chemistry, The Weizmann Institute of Science, Rehovot 76100, Israel; e-mail: [email protected]
Nathan Sharon has been interested in carbohydrates and lectins for nearly half a century, and was instrumental in formulating and widely disseminating the concept that these substances function in cell recognition. His work has led to the development of a lectin-purging method of bone marrow used clinically in the treatment of ‘bubble children’
Glycobiology, sometimes referred to as the last unconquered frontier of molecular and cell biology [1], is starting to succumb to the combined assault of sophisticated biochemical techniques and genomics. In particular, this is highlighted by the elucidation of the key roles played by cell surface glycans in signal transduction, control of cell differentiation and innate immunity, some of which are surveyed in the reviews in this section. In the first review, Wilson (pp 569–577) surveys the current state of knowledge on the biosynthesis of glycoprotein glycans in plants and invertebrates. Although much work has been done to elucidate these biosynthetic pathways in mammals, plants and invertebrates have always been the ‘poor relations’. Based on structural studies, they were thought to produce a very limited range of N- and O-linked glycans in comparison to mammalian systems. The availability of (nearly complete) genomes for several species of plants and invertebrates has provided a very considerable boost to this area, allowing many putative glycosyltransferase genes to be identified and strongly suggesting that plant and invertebrate glycosylation is more complicated than first thought. However, much work remains to be done to translate the existence of these putative genes into knowledge of the glycans that are actually synthesised by these organisms. Prion protein, an endogenous glycoprotein with the ability to act as an infectious agent, is the subject of the second review. Rudd et al. (pp 578–586) summarise recent results relating to the effects of N-glycosylation on the structural and functional properties of this protein. These range from the effects on glycosylation of the way that the protein is anchored to the membrane to the way that the glycans modulate the neuronal localisation of the protein and its conversion from the normal to the pathogenic form. The review demonstrates very well that the effects of prion protein glycosylation are wide ranging, frequently subtle and not yet fully understood. Although, in the past, the area of glycolipids has received somewhat less attention than that of glycoproteins, this attitude is changing. A major reason for this is the realisation that they are involved in cell signalling. As discussed by Allende and Proia (pp 587–592), recent evidence shows that one of the most prominent families of glycolipids, the gangliosides, can serve two different roles in cell signalling. They act directly as ligands for specific receptors (such as the siglecs discussed in Crocker’s review) and, by participating in the formation of specialised protein/lipid domains (rafts) on the plasma membrane, they can modulate receptor functions (e.g. of tyrosine kinases). It is suggested that the signalling function of gangliosides may be a potential therapeutic target in cancer, diabetes and nerve regeneration. An additional important source of information on the biological function of gangliosides in vivo comes from knockout mice in which genes involved in glycolipid
568
Carbohydrates and glycoconjugates
biosynthesis have been disrupted. For example, disruption of the gene in mice that is responsible for the first step in the formation of almost all glycosphingolipids is embryonically lethal. Mice that lack enzymes involved in later steps of glycolipid biosynthesis are viable, but show a range of abnormalities, including infertility and neurological defects. Knowledge of glycolipid biosynthesis has led to novel approaches for treating diseases due to defects in the degradation of these substances. One such approach has recently reached fruition in the form of an Oxford GlycoSciences imino sugar that has been approved for the treatment of Gaucher disease. In his article, Haltiwanger (pp 593–598) reviews the role of O-linked fucose residues in regulating certain cell–cell signalling pathways involved in development in Drosophila, and in mice and zebrafish. One example in Drosophila is the Fringe protein, a fucose-specific N-acetylglucosaminyltransferase, which carries out a variety of essential development functions by modulating the activity of Notch, another protein in the development pathway. This modulation is achieved by the Fringe-catalysed addition of N-acetylglucosamine residues to fucose residues present on Notch. An example in mice and zebrafish is the Nodal protein, which plays an important role in the development of polarity in the developing embryo; mutations of this protein in these animals result in severe defects. Signal transduction by Nodal requires O-fucosylation of another group of proteins, designated as the EGF-CFC family. Although there is no single function for oligosaccharides (just as there is no single function for polypeptides), a clearly important function of glycans is as ligands for recognition by lectins (discussed to some degree in all the preceding reviews). The remaining three reviews concentrate on such recognition events. In his article, Kannagi (pp 599–608) reviews work on characterising the ligands for the different selectins (E-, L-and P-) and discusses their roles in lymphocyte homing. The 6-sulfated sialyl Lewis X epitope is involved in the routine trafficking of lymphocytes, whereas the sialyl Lewis X epitope is involved in leukocyte trafficking to sites of inflammation. The regulation of these epitopes is different, as might be expected from their very different functions. Sialyl Lewis X epitopes are considerably upregulated during inflammation by increased transcription of a fucosyltransferase gene. Expression of sialyl 6-sulfo Lewis X is regulated by a recently discovered unique modification of the sialic acid, its de-N-acetylation and conversion of the product into an internal cyclic form not encountered before. Kannagi also discusses the role of
sialyl 6-sulfo Lewis X present on P-selectin glycoprotein ligand-1 (PSLG-1), which is expressed on leukocytes, and suggests that it may serve as ligand for all three selectins, although with different affinity. One of the earlier roles proposed for the terminal sialic acid residues present on glycoprotein glycans was the modulation of the half-life of soluble glycoproteins by masking the underlying galactose residues that are required for the clearance of certain asialoglycoproteins from the circulation. However, they now appear to be much more important as recognition epitopes themselves. Sialic acids are ubiquitous on all mammalian cell surfaces, making them useful for mediating cell–cell contacts, but are absent from many lower organisms, such as some pathogens, thereby providing a means of distinguishing self from nonself within the immune system. It is now clear that an extensive family of closely related proteins, the siglecs, carries out these different functions. In his review, Crocker (pp 609–615) provides a wide-ranging discussion of this lectin family, from the chromosomal localisation of their genes and its implications for diversity to their functional role in modulating the innate immune response. The review emphasises the importance of competition between interaction of the siglecs with sialic acid residues on the surface of the same cell as opposed to their interaction with residues on the membranes of opposing cells, as a method of regulating siglec function. He emphasises the need to understand how glycosylation changes modulate these interactions. The interaction between a lectin and its glycan ligand is usually relatively weak. High-affinity binding can be achieved by a multivalent lectin interacting with either a multivalent glycan or multiply presented glycans (as in the recognition of multiply presented oligomannose glycans by the mannose-binding lectin, a key component of innate immunity in higher animals). In the last review, Brewer, Miceli and Baum (pp 616–623) discuss how the interactions between multivalent lectins and multivalent ligands can lead to the clustering of both into large assemblies. Such clustering can occur between selected components on the same membrane, leading to local organisation of the membrane and hence modulation of function, or it can occur during cell signalling events, the clustering providing a mechanism for signal transduction. Such clustering may account for the biological activities of several members of the galectin family, including cell proliferation, adhesion and apoptosis.
Reference 1.
Sharon N: The conquest of the last frontier of molecular and cell biology. Biochimie 2001, 83:555.