Current Organic Chemistry, Volume 8, No. 5, 2004
Contents
Complex
Carbohydrates
Guest
Editors: Nasir-ud-Din and Daniel C. Hoessli
Nucleocytoplasmic Glycosylation, O-linked b-N-Acetylglucosamine Pp.369-383
N.E.
Zachara, W.D. Cheung and G.W. Hart
The Glycobiology of Cystic Fibrosis Pp.385-394
A.D.
Rhim, L.I. Stoykova, P.J. Park, T.F. Scanlin and M.C. Glick
Metabolism and Structure-Function Relationships of
Connective Tissue Glycosaminoglycans and
Proteoglycans Pp.395-411
Jeremiah
E. Silbert
The Diversity of Epithelial Secreted Mucins Pp.413-437
Philippe
Roussel and Philippe Delmotte
Glycosphingolipid Clusters as Organizers of Plasma
Membrane Rafts and Caveolae
Pp.439-452
Daniel
C. Hoessli, Isabelle Semac, Asma Iqbal, Nasir-ud-Din and Bettina Borisch
Combinatorial Metabolism in Plasmodium
falciparum-Infected Erythrocyte and Interplay of Glycosylation &
Phosphorylation Pp.453-461
Nasir-ud-Din, Ishtiaq Ahmad, Asma Iqbal and Daniel C. Hoessli
Abstracts
[Back to top] Nucleocytoplasmic Glycosylation, O-linked b-N-Acetylglucosamine
N.E. Zachara, W.D. Cheung and G.W. Hart
O-linked b-N-acetylglucosmaine (O-GlcNAc) is an essential, ubiquitous, dynamic modification of metazoan nucleocytoplasmic proteins. Unlike prototypical glycosylation, O-GlcNAc is not elongated into more complex structures and it is localized almost exclusively to nuclear and cytoplasmic proteins. O-GlcNAc modifies Ser/Thr residues in peptide motifs either identical or similar to those used by kinases. In some instances, O-GlcNAc and phosphorylation occur at the same site, suggesting that a complex interplay exists between these post-translational modifications. Deletion of the gene that adds O-GlcNAc to the protein backbone, the UDP-GlcNAc: polypeptide O-b-N-acetylglucosaminyltransferase, is lethal at the single cell level underlying the importance of O-GlcNAc. O-GlcNAc is rapidly emerging as a key nutrient sensor regulating signaling, transcription and cellular responses to stress.
[Back to top] The Glycobiology of Cystic Fibrosis
A.D.
Rhim, L.I. Stoykova, P.J. Park, T.F. Scanlin and M.C. Glick
Cystic fibrosis (CF), the most common lethal genetic disease of Caucasians, is characterized by pathology to the exocrine organs, especially the lungs which are the site of most of the morbidity and mortality of the disease. The gene causing CF was defined more than a decade ago and a deletion of F508 has been shown to be the most common mutation. The gene product the cystic fibrosis transmembrane conductance regulator (CFTR) is a Cl- channel on the apical surface of epithelial cells and has been extensively studied. It is a large glycoprotein but the glycosylation has not yet been defined. On the other hand, examination of glycosylation on the surface of CF airway epithelial cells has shown a glycophenotype, which is usually expressed as increased fucosyl residues and/or decreased sialic acid. Studies have shown a direct relationship of CFTR to the CF glycophenotype in airway epithelial cells. The terminal glycosyltransferases which are responsible for the CF glycophenotype have been studied. A difference in these enzymes and the mRNA expression was not sufficient to account for the altered surface glycoproteins of CF and non CF cells. It may be possible that a compartmental difference in the Golgi between CF and non CF glycosyltransferases may be responsible for the CF glycophenotype.
[Back to top] Metabolism and Structure-Function Relationships of
Connective Tissue Glycosaminoglycans and
Proteoglycans
Jeremiah
E. Silbert
Connective tissue matrix contains the linear polysaccharide (glycosaminoglycan) hyaluronan, and the glycosaminoglycans chondroitin/dermatan sulfate, and keratan sulfate covalently linked to protein (proteoglycans). Hyaluronan is a linear polymer consisting of alternating N-acetylglucosamine and glucuronic acid residues; chondroitin/dermatan has alternating N-acetylgalactosamine and glucuronic or iduronic acid residues; keratan has galactose and N-acetylglucosamine residues. There are two classes of matrix proteoglycans named “hyalectin” and “SLRP (small leucine-rich protein).” Hyalectins have similar large core proteins and large numbers of glycosaminoglycan chains, while SLRPs have similar small core proteins with few glycosaminoglycan chains. Synthesis proceeds by modification of glucose to form specific sugar nucleotides that act as the direct precusors for forming the glycosaminoglycans and linkage oligosaccharides on specific amino acids of specific proteins. The chondroitin/dermatan and keratan are modified by variable sulfation and in the case of dermatan, by epimerization of some of the glucuronic to iduronic. These modifications provide for multiple structural variations that in turn provide a high degree of specificity in function. In general, the functions of matrix hyaluronan and the matrix proteoglycans relate to interactions with other matrix substances such as collagen to provide scaffolding and shape. Their variable structures provide specificity of function for particular tissues, and specific aspects of interaction with other matrix molecules.
[Back to top] The Diversity of Epithelial Secreted Mucins
Philippe
Roussel and Philippe Delmotte
Secreted mucins are a family of polydisperse, high molecular mass and highly glycosylated glycoproteins synthesized by mucosae, or skin of amphibians and fish. Most secreted mucins are synthesized in goblet cells of epithelial surfaces or mucous cells of exocrine glands. They are probably the most complicated biological molecules ever described. Except for a low molecular mass species, the peptide part of mucin, or apomucin, is very long and can contain from # 5,000 to more than 13,000 amino acids. In man, the apomucins synthesized in goblet or mucous cells are encoded by four mucin genes (MUC), organized in a cluster. Apomucins are covered by hundreds of carbohydrate chains. Their biosynthesis implicates different glycosylation processes, mostly O-glycosylation, which are responsible for about 80 % of the mucin molecular mass. During their biosynthesis, secreted mucins may dimerize and multimerize via disulfide bridges. The carbohydrate chains resulting from the O-glycosylation process are extremely diverse and contribute to the possible formation of different mucin glycoforms from a single apomucin peptide. In man, the glycosylation differs from one individual to another according to histo-blood bood groups (ABO, secretor, Lewis...). Moreover, there are large differences in the O-glycosylation processes between different animal species. Preliminary data suggest that different factors endogeneous (cytokines and/or hormones), and exogeneous (microbial), are involved in the regulation of mucin biosynthesis. Owing to their physical properties, secreted mucins have an essential role in the protection of underlying epithelia. Mice genetically deficient in one of the mucin genes (MUC2) frequently develop tumors. Finally, the wide diversity of carbohydrate chains, specific for a given species, is very important in specific recognition phenomena (interactions with microbes, fertilization...).
[Back to top] Glycosphingolipid
Clusters as Organizers of Plasma Membrane Rafts and Caveolae
Daniel C. Hoessli, Isabelle Semac, Asma Iqbal, Nasir-ud-Din and Bettina Borisch
Glycosphingolipids aggregate with sphingomyelin and cholesterol in the outer leaflet of the plasma membranes to become included in transmembrane domains comprising saturated phospholipids in the inner leaflet, select transmembrane proteins and acylated proteins anchored to the cytofacial aspect. Such domains constitute liquid-ordered, planar and short-lived rafts, floating in the liquid-disordered glycerophospholipid medium of the membrane outer leaflet. When scaffolded by the protein caveolin, such rafts assume the shape of microscopically detectable small invaginations or caveolae.
Rafts and caveolae are plasma membrane microdomains involved in membrane trafficking, endocytosis, transcytosis, signal transduction and adhesion, and may be opportunistically utilized by various microbial pathogens to enter many different types of host cells. By virtue of the greater cohesion of their sphingolipid and cholesterol components, rafts and caveolae constitute stable membrane platforms that associate receptors, kinases, phosphatases and adaptor proteins, and promote their interactions in a membrane environment conducive to optimal signaling. Such membrane platforms are involved in a variety of cellular interactions with extracellular matrices, cells, microbes and soluble ligands. The capacity of protein receptors to associate with or dissociate from rafts is in part due to the affinity of their “lipid shells” for the sphingolipid aggregates, as well as to their extramembranous properties and characteristics. In particular, carbohydrate side-chains of N-glycosylated receptors may interact with gangliosides in rafts and then change the conformation, affinity and signaling properties of the glycoprotein receptors.
Rafts and caveolae are targeted in several therapeutic approaches aiming at modifying their contents and function. Such approaches have been applied successfully to the treatment of cancer cells and of cells unresponsive to physiological insulin triggering.
[Back to top] Combinatorial
Metabolism in Plasmodium falciparum-Infected Erythrocyte and Interplay of
Glycosylation & Phosphorylation
Nasir-ud-Din, Ishtiaq Ahmad, Asma Iqbal and Daniel C. Hoessli
The nature of protein modifications, particularly surface glycosylation, in Plasmodium falciparum has long been observed, but in recent times controversial reports appeared. It is considered that the surface modifications, O- and N-glycosylation, have been of particular importance to the malarial parasite itself as well as to the erythrocyte. The modifications in the erythrocytic membranes and changes in the parasitic structure have been the subject of intense investigations in recent years. Comprehensive elaborations resulted in the development of means to describe the extensively modified structural and antigenic properties of the host cell and parasitic molecules that are of vital significance in the understanding of the functions of biologically important molecules such as the merozoite surface protein-1 (MSP-1). A combinatorial behavior of the parasitic and host cell molecules becomes the basis for molecular modifications of the parasite proteins and the events leading to invasion and survival of the parasite. The MSP-1 protein is modified on the surface by a sugar moiety O-glycosidically linked in b-configuration. This b-O-GlcNAc modication of MSP-1 has been predicted by computer assistance and experimentally verified. The Plasmodium falciparum proteins are known to be phosphorylated, and phosphorylation of MSP-1 has been predicted; moreover, there is sufficient experimental evidence to suggest phosphorylation. Computer assisted studies suggest a strong possibility of interplay between glycosylation and phosphorylation in proteins with b-O-GlcNAc modification.