Angiogenesis, the formation of new vessels from existing microvessels, is important in embryogenesis, wound healing, inflammation, ischemic heart- and peripheral vascular disease, myocardial infarction, diabetic retinopathy, and cancer. Platelet-derived growth factor (PDGF) has been implicated in most of these processes and can induce the generation of a vascularized connective tissue stroma. PDGF homo- and heterodimers, composed of a PDGF A- and/or B-chain, and PDGF regeptor subunits (α- and β) are widely expressed upon tissue injury and repair. Hypoxia and other angiogenesis-related stimuli can induce expression of the PDGF B-chain. PDGF may modulate angiogenesis by attracting inflammatory or connective tissue cells which in turn control angiogenesis. Additionally, PDGF may act directly on specific phenotypes of endothelial cells that are engaged in angiogenesis or that are of microvascular origin and that express PDGF receptor β-subunits. PDGF induces the formation of a vascularized connective tissue stroma upon exogenous administration or local overexpression. The presence of PDGF and its receptors in angiogenesis-related processes or diseases such as placenta formation, embryogenesis, wound healing, atherosclerosis, and cancer is consistent with a role for PDGF in angiogenesis. Thus, PDGF-BB can elicit the formation of a vascularized connective tissue stroma. New vessel formation in response to PDGF-BB may be partially due to its direct effects on endothelial cells that express PDGF receptor β-subunits.
Appicans are secreted and cell-associated chondroitin sulfate proteoglycans containing Alzheimer amyloid precursor protein (APP) as a core protein. Appicans are found in brain tissue and their expression in cell cultures depends on both cell type and growth conditions. In rat brain primary cell cultures appicans are mainly expressed by astroglial cells. Cellular appicans contain full-length APP and are found on the cell surface whereas secreted appicans contain proteolytically cleaved APP. Following the discovery of appicans it was shown that their core protein is the L-APP isoform which lacks exon 15 of the APP gene. Fusion of exon 14 to exon 16 creates an amino acid consensus sequence for the attachment of the glycosaminoglycan chain present in appican. Site directed mutagenesis was used to identify serine-619 of L-APP733 as the single chondroitin sulfate attachment site. C6 cells transfected with L-APP produced high levels of appican whereas C6 cells trans-fected with L-APPs/a, where serine-619 was mutated to alanine, produced no additional appican other than the endogenous one. Extracellular matrix prepared from C6 cells trans-fected with L-APP was a much better substrate for the attachment of neuro 2A neuroblastoma or pheochromocytoma PC12 cells than extracellular matrix prepared from C6 cells trans-fected with L-APPs/a suggesting that appicans function as cell adhesion molecules for neural cells. The recent cloning of new genes linked to familial Alzheimer disease offers new opportunities to investigate whether appican interacts with the familial Alzheimer disease genes and whether it plays any role in the development of this disease.
Over the last ten years since the discovery of GPI-an-choring as an alternative principle of membrane anchoring of proteins the research on these anchors has expanded dramatically. An enormous body of knowledge about their structure, biosynthesis and function has accumulated. Several systems have been studied, ranging from protozoa, yeast to mammalian cell lines, to elaborate this information. Analysis on the functional level revealed that GPIs exhibit a variety of functions beyond their mere function as membrane anchors e.g. in the maturation and transport of membrane proteins or their role in signal transduction processes and as pathogenicity factors. The recognition site for the transfer of the GPI-anchor to the native protein by the so called GPI-transamidase has been determined by the combination of biochemical and molecular genetic methods. Mutant cell lines and yeast strains with defined defects in the pathway of GPI-biosynthesis allowed the identification of the genes involved and have led to the successful cloning of several such genes. These key subjects will be dealt with in this article.
The blood group Sda antigen is a carbohydrate structure inherited as a dominant character and the β-linked N-acetylgalactosamine is the immunodominant sugar. This antigen is not confined to red cells but is mainly present in colon and kidney and is excreted in urine associated with the Tamm-Horsfall glycoprotein. A pentasaccharide fragment isolated from the Tamm-Horsfall glycoprotein with the GalNAcβ1, 4 (NeuAcα2, 3) Galβ1, 4GlcNAcβ1, 3Gal structure was found to have a very high Sda activity. The β1, 4-N-acetylgalactosaminyl-transferase involved in the biosynthesis of the Sda antigen (Sda-βGalNAc-transferase) strictly requires in acceptors a terminal galactose residue substituted at the O-3 position with N-acetylneuraminic acid. The tissue distribution of this enzyme correlates with the predominant localization of Sda antigen in kidney and colon. The Sda-βGalNAc-transferase is practically absent in neonatal guinea-pig kidney and in large intestine of suckling rat and is dramatically reduced in human colon carcinomas indicating that the expression is onco-developmentally regulated. In human colon carcinoma cells in culture only Caco-2 cells express the Sda-βGalNAc-transferase at a level that parallels the degree of enterocyte differentiation. A large amount of the enzyme is released in the soluble form by differentiated Caco-2 cells, preferentially from the basolateral face. One may postulate that a reduced susceptibility to infections caused by enterotoxigenic and pyelonephritogenic Escherichia coli strains which specifically bind to the NeuAcα2, 3Galβ-units has been the selective agent responsible for the dominant expression of the Sda-βGalNAc-transferase in distal kidney and colon.
Recently, the glycosciences have begun to have a presence on the Internet. We review this development and give some examples of the resources that can now be accessed via the World Wide Web (WWW) such as a directory of glycoscientists, structural analysis systems and a guide to cloned glycosyltransferases. In addition the electronic glycoscience conferences are a means of presenting results and discussing them in real time over the Internet. These tools are only the beginning of a process in which the glycosciences should see the pursuit of many other Internet-related developments.