Cell-Surface Glycoproteins

FIGURE 25 11 Diagram of a cell surface glycoprotein showing the disaccharide unit that IS recognized by an invading influenza virus... 

The following protocols are generalized for the labeling of cell-surface glycoproteins or glycoproteins in solution. Some optimization may be necessary to achieve the best level of fluorescent modification for each particular application. 

The methods used for in vivo incorporation of azido-monomers and performing a labeling reaction with live cells are relatively simple. The following protocol is based on the methods of Saxon and Bertozzi (2000), which uses acetylated azidoacetylmannosamine as the azido-monomer source and a biotin-PEG-phosphine compound to biotinylate cell surface glycoproteins at the specific azide-sialic acid incorporation sites (Figure 17.19). 

It has been estimated that 1-2 per cent of the US population suffer from autoimmune conditions, including rheumatoid arthritis, MS and some forms of diabetes. In many instances, an autoimmune response results from the inappropriate activation of a specific subset of B- and/or T-lymphocytes. The most common immunotherapeutic approach to potentially treat such diseases is to induce depletion of the individual s T- and B-cell populations. This could be achieved by administration of an antibody raised against a surface antigen present on such cells. Initial trials, for example, have shown that injection of an (unconjugated) anti-CD4 antibody (cell surface glycoprotein present on many T-lymphocytes) over 7 days significantly reduced the clinical symptoms of rheumatoid arthritis for several months. 

Many transmembrane proteins that mediate intracellular signaling form complexes with both intra- and extracellular proteins. For example, neural cell adhesion molecules (NCAMs) are cell-surface glycoproteins (Ch. 7). The extracellular domains of NCAMs can activate fibroblast growth factor receptors when clustered by reaction with NCAM antibodies [4] or by homotypic binding to domains of adjacent cells (see Fig. 7-2). Activation was found to sequester a complex of NCAM, (31 spectrin and PKC(32 into rafts, as defined by the operational criteria discussed on p. 28. 

On the basis of this anti-proteolytic effect of sialic acids, a hypothetical model435 for the role of sialidase in clostridial infections is shown in Scheme 4. It is considered that the bacterial enzyme releases sialic acids from cell-surface glycoproteins of the infected tissue, which thereafter can be readily attacked by proteases. This cooperation between sialidase and protease may support the spreading of the bacteria. Acylneuraminate pyruvate-lyase, also shown in this model, degrades sialic acids for energy supply, and growth, of the bacteria. 

Many aspects of the social behavior of cells are determined by the composition, arrangement, and interaction of cell-surface molecules. Therefore, changes in the composition and structure of plasma membranes appear to contribute to differences in such characteristics as cell adhesion, contact inhibition, and tumorogenicity of cells. Cell-surface glycoproteins, in particular, participate in a number of membrane-modulated phenomena, including responsiveness to hormones, agglutination by lectins, recognition by antibodies, or uptake of nutri-... 

A prominent finding in transformed cells has been the absence, or marked lessening of the content, of a high-molecular-weight, cell-surface glycoprotein known as LETS protein, or fibronectin, that is involved in the attachment of the cells to the substratum (for a review, see Hynes and coworkers538). 

The blastocyst implants into the maternal endometrial wall on about the 7th day of embryonic life. Trophoblastic cells attach to the uterine mucosa by apposition and adhesion. Under the influence of progesterone and estrogen, the uterine lumen closes, which brings the blastocyst into close contact with the endometrium. Adhesion of the trophoblastto the uterine epithelium occurs with increasing apposition and involves cell surface glycoproteins. The uterine epithelium is penetrated by syncytial growths on the trophoblast into the adjacent uterine epithelial cells. Subsequently, the trophoblastic membranes... 

It has been argued that the glycoprotein composition of the cell surface is a determinant in cell behavior,146,149 and it is clear that many properties of cells may be affected by the action of glycosidases on cell-surface glycoproteins. In this respect, changes in their electrophoretic mobility, in the regulation of the cell cycle, and in cell-cell interactions, and the effects on lymphocyte stimulation of cells, their immunogenicity, their interaction with hormones, and their onco-... 

Cell surface glycoproteins/ oligosaccharides Developmental signals Extracellular matrix components Growth factors Hormones... 

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