Recognition and adhesion

The surface carbohydrates on a cell serve as points of attachment for other cells, infectious bacteria and viruses, toxins, hormones, and many other molecules. This ability depends on the presence of these sugars, their accessibility and their mode of presentation. The carbohydrates are recognized by the cell adhesion molecules which are glycoproteins expressed on the cell surface.

Several viruses, bacteria and parasites have exploited the presence of cell surface oligosaccharides using them to enter into the cells.

For example :

• Rhinoviruses attach to ICAM-1 (intercellular adhesion molecule-1)
• Parvoviruses attach to the erythrocyte- specific cell surface P-antigen
• Influenza virus and Plasmodium falciparum (protozoan causing malaria) use glycophorin (the MN blood group system) to enter the erythrocytes
• Herpesvirus type I enter the cells via FGFR (fibroblast growth factor receptor)

Bacteria adhere to tissues selectively due to their fimbriae ( containing lectins) which bind exclusively to certain surfece carbohydrates. These interactions determine which tissues are susceptible for the invasion (for example E.coli causes urinary tract infections while Streptococci of A group colonize upper respiratory tract and skin ).

• type 1 fimbriae of E.coli bind to glycoproteins containing mannose residue and P-fimbriae interact with P blood group glycolipid containing the disaccharide galabiose.
  Lectins (sugar binding proteins) recognize sugars at the end of carbohydrates and those localized within the structure. The way of the sugar presentation is very important for recognition ( determination of host specificity).
  For example there are different E.coli strains with lectins recognizing either exclusively human or exclusively canine galabiose from urinary tract glycoproteins. It appeared that the galabiose is presented in different ways in those two organisms.
• The other example of recognition between organisms can be the interactions between the legumes (plants) and the bacteria Rhizobium which infects the root hairs of legumes. Plant lectins recognize the bacterial carbohydrates and stimulate infection of the roots.

Carbohydrates can be strong antigens which may trigger harmful immune reactions.

• For example the ABO blood group antigens are the carbohydrate moieties attached to the lipids or proteins on the surface of erythrocytes and many others cell types. A, B and O antigens are structurally related. All people have enzymes which synthesize the O antigen (H substance). In people with B antigen there is an enzyme which adds a galactose residue to the H substance and people with A antigen have an enzyme adding GalNac residue.
  
  
• The other example can be Trypanosoma brucei (it causes dangerous disease within some animals) which contains highly variable surface glycoproteins (VSGs). They are strong antigens. Trypanosoma frequently changes one VSG to another thus enabling the protozoan to elude host responses that would inactivate it.

Adhesion molecules are important for reproduction. Binding of the sperm cell to the egg is mediated by glycoproteins serving as receptors on the surface of each two membranes. Glycoproteins surrounding the egg (forming zona pellucida) prevent the polyspermy. Adhesion carbohydrates are also essential for embryonic development. As the fertilized egg divides, the carbohydrates on resulting embryonic cells change in characteristic ways. For example a trisaccharide LeX appears at the 8 to 16 cell stage when the embryo compacts from a group of loose cells into a smooth ball. LeX trisaccharide plays an important role in compaction.

Cell - cell contacts mediated by glycoproteins play a role in growth control. For example contactinhibin (N-glycosylated protein) is responsible for density dependent inhibition of fibroblasts growth. Cells stop proliferation when contacts between them are close ; they form a monolayer. Contactinhibin is synthesized in highly sialylated, inactive form. After contacts between the cells are established, the glycoprotein is converted to low sialylated, biologically active form. It is placed in the plasma membrane and binds to a specific cell-surface receptor, CiR.

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Adhesion molecules

Other functions of glycoproteins