Current Organic Chemistry - Volume 12, Issue 11, 2008

This issue of Current Organic Chemistry on Complex Carbohydrates shall illustrate several medical contexts where carbohydrates and their recognition are central to the disease process, or to the development of therapeutic strategies. On a more theoretical ground, the problem of how to analyse multifunctionality of proteins regulated either genetically or by posttranslational modifications shall be dealt with in the last review. The review by Stoykova and Scanlin deals with the “glycosylation phenotype” of cystic fibrosis and the infection by P. aeruginosa to which it contributes in 80% of the patients with cystic fibrosis. Diskin and Panjwani focus on glaucoma, an eye disease in which lectin-carbohydrate interactions perturb the outflow of the aqueous humor through the trabecular meshwork, and result in elevated intraocular pressure. In their discussion of the therapeutic potential of mistletoe lectin I, Hoessli and Ahmad provide information as to how this plant lectin could be utilized therapeutically to eliminate cancer cells and increase the patient's immune defenses against the tumor. Mendonca-Previato et al. review the vast field of carbohydrates elaborated by protozoan parasites, which consist mainly of complex sugars linked to lipids and membrane-bound. Interfering with the biosynthesis of such sugars makes it possible to modify the host-parasite interface and propose new therapies. In their review on galactose, Walker-Nasir et al. discuss how carbohydrates bearing terminal galactose such as blood group B determinants, are involved in determining carbohydrate recognition and how carbohydrate databases could further our understanding of such biological contexts. Finally, Nasir-ud-Din et al. approach the complex subject of post-translational modifications including structure-function relationship, and discuss how the in silico approach will help us understanding the functional switches that occur in proteins following post-translational modifications.

Cystic Fibrosis (CF) is a common, ultimately fatal, inherited disorder of exocrine glands. The dominant clinical feature is a chronic progressive lung disease mediated by airways infection and inflammation. The most important pathogen in the lungs is Pseudomonas aeruginosa (P. aeruginosa). Eventually more than 80 percent of all CF individuals are chronically infected with this organism. The responsible gene, named the CF Transmembrane Regulator (CFTR), was identified in 1989. Although our understanding of the disease has increased, treatment is symptomatic and the details of pathogenesis at the molecular level remain to be elucidated. The decreased conductance of chloride at the surface of airway epithelial cells is the most characteristic pathophysiologic abnormality and is the target of much of the research to develop new therapies. Shortly after the identification of the sweat chloride defect in CF, biochemical analysis of mucins defined an altered carbohydrate composition of CF mucins when compared to controls. The altered terminal glycosylation of CF glycoproteins has been described by many investigators, although there is not a consensus. Since the recent determination of the crystal structure of the major binding protein of P. aeruginosa - (PA-IIL) and the demonstration of its high affinity binding to the same fucosylated oligosaccharides (Lewis x and Lewis a) that are characteristic of the CF glycophenotype, there has been a renewed interest in the role of altered terminal glycosylation in the pathogenesis of CF. This review will provide an update of studies on the altered terminal glycosylation in CF and will discuss the possible role of glycosylation in the pathogenesis of the pulmonary infection in CF and its link to CFTR function.

Primary Open Angle Glaucoma (POAG) is a blindness causing disease. The main risk factor for POAG is elevated intraocular pressure (IOP) due to insufficient outflow of aqueous humor from the eye into the vasculature. Carbohydrate- based recognition systems are increasingly recognized for their roles in physiological as well as pathogenic processes. These systems are comprised of a glycan message carried by a glycoprotein or glycolipid and a message decoder, a carbohydrate binding protein (lectin). The trabecular meshwork (TM), a prominent participant in the regulation of aqueous humor outflow facility shows expression of various lectins, including members of the classic families I-type lectins, C-type lectins and S-type lectins (Galectins). Some of these lectins, namely ICAM-1, E- and P-selectins have altered expression levels in TM derived from eyes with POAG. Another, CD44 shows elevated levels in the POAG aqueous humor. Many carbohydrate recognizing proteins, upon binding to their respective countereceptors can affect cellular processes shown to be paramount for the maintenance of outflow facility by TM. This review will survey lectins known to be expressed in TM, their known and hypothesized functions in the tissue and highlight some interesting venues for study of putative roles for lectins in the disease-promoting mechanisms taking place in POAG TM.

Mistletoe lectin I (ML-I) is a heterodimeric ribosome-inactivating protein composed of a sialic acid-specific Bchain that binds to cell surfaces, and an A-chain with the capacity to depurinate a critical adenosine in the 28S ribosomal RNA. ML-1, in purified or recombinant form, exerts an immunomodulatory effect on neutrophils and macrophages/ monocytes in the low-dose range, while at high doses, it induces apoptosis in both normal and tumoral cells. While mistletoe extracts are widely used as cancer adjuvant therapy, recombinant ML-I (rAviscumin) is a candidate antineoplastic agent that has successfully passed Phase I clinical trials. In immunodeficient mouse models, the efficacy of recombinant ML-I was demonstrated for ovarian carcinoma, melanoma and various hematological malignant cell lines. The clinical potential of recombinant ML-I as a non-mutagenic and non-genotoxic molecule is high and could be used to potentiate classical anti-neoplastic drugs. Its capacity to induce apoptosis in cancer cell lines lacking p53 allows considering its use against genetically unstable and highly metastatic cancers. The mechanisms of apoptosis induced by ML-I probably involves intracellular pathways akin to those described as the “ribotoxic stress response” that directly target the mitochondrion.

This review highlights the chemical structures of complex glycoconjugates from protozoan and helminth parasites, etiologic agents of major world-wide infections. Several studies on parasitic diseases indicate that glycan portions linked to proteins or lipids, expressed on the cell surface or secreted by protozoa Trypanosoma, Leishmania, and by trematodes and nematodes are virulence determinants responsible for host-parasite interaction and immunomodulation in infected animals and humans. Also, the unique chemical structures of the carbohydrate moieties of these glycoconjugates indicate a specific correlation between such compounds and parasite pathogenicity, suggesting that parasite glycan biosynthesis could be an important target to novel drugs, since vaccines are nonexistent and drugs for treatment are inadequate.

Glycoproteins and glycolipids carrying diverse oligosaccharide structures are involved in countless molecular interactions in physiologic and pathologic situations. Defining the specific carbohydrate moieties expressed in a particular set of molecules is a challenging task that could eventually explain how glycoproteins and glycolipids contribute to the physiology of normal cells and how their alterations could lead to pathologic states. A simple example is the ABO blood group system: in individuals with blood group B, the marker is defined by its terminal linked galactose, and substitution of its hydroxyl group at C2 by an N-acetyl group results in the formation of N-acetylgalactosamine, the blood group A marker. This review focuses on the importance of terminal linked galactose and its derivatives in different normal and pathological conditions. The involvement of various sugars residues sub-terminal to galactose and its derivatives was also evaluated on the basis of the galactosylation data taken from different publicly available carbohydrate databases. We conclude that those sugars penultimate to galactose, with their different types of linkages and anomery, contribute to the structure and functions of carbohydrate moieties with a terminal galactose.

Understanding the biological functions of proteins has been facilitated by the availability of powerful computational tools that greatly help analyzing the complex nature of protein structure-function relationships. The most important challenge faced by functional genomics and proteomics is to elucidate how the three-dimensional structure of a protein may change in vivo, and directly produce a new functional state. The key to understanding functional switches in proteins is to define how post-translational modifications contribute to the structural plasticity of those proteins and how new structures display new functions. Deciphering the protein functional switches regulated by transitory structural and conformational changes will require collaboration between computational scientists and experimentalists. The purpose of this review is to critically discuss the tools presently available in computational biology to approach these questions.