The INDIVIDUALIST

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Glycomics

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Description

A glycocalyx is a network of polysaccharides that project from cellular surfaces, e.g. those of bacteria. It allows the bacterium to attach itself to inert surfaces (like teeth or rocks), prokaryotes (e.g. streptococcus pneumoniae attaches itself to lung cells), or other bacteria (their glycocalyxes can fuse to envelop the colony).

Its presence on inert materials (such as metal hardware implanted for fracture fixation or total joint replacement) make it difficult to eradicate deep infections as the bacteria will 'cling' on to the material via the glycocalyx. It is therefore often necessary to completely remove the hardware device in order to fully eradicate a wound infection.

The glycocalyx can be found just outside the cell wall of a bacterium. When its components are organized, it is called a capsule and when it is not organized, it can be called a slime layer. The bacteria can use this glycocalyx to its advantage by using it to aid in the protection from phagocytosis. It also helps in the formation of biofilms such as a coating on inert surfaces such as your teeth or rocks.

Discussion

The glycocalyx is chemically unique in everyone but identical twins, and acts like an identification tag that enables the body to distinguish its own healthy cells from transplanted tissues, invading organisms and diseased cells. Human blood types and transfusion compatibility are determined by glycoproteins.

The glycocalyx, the "sugar-rich" covering that coats plasma membranes, may be an extraneous coat and/or an integral part of the plasma membrane.(1) It is considered to form a hydrophilic polyanionic gel coat on the enterocyte surface and is thought to maintain cell surface charge, protect against physical trauma, regulate ionic and macromolecular access, and form a cationic store. (2)

Thus absence of the glycocalyx would have profound consequences on normal cell function and may result in an inability to maintain cellular homeostasis. This could explain why the vast majority of cases of microvillus atrophy do not exhibit hydramnios,1 as the amniotic fluid, which is swallowed by the fetus and bathes the luminal surface of the gut, is comparable in osmolality to serum,(3,4) resulting in no osmotic pressure across the gut in utero. This changes dramatically following birth and could explain the rapid onset of symptoms.

In the intestine, there are many glycosylated proteins associated with the apical epithelial membrane and it is uncertain which ones provide the physical characteristics of the glycocalyx listed above, or whether a particular component is responsible. Components include brush border enzymes, mucins, and a filamentous matrix (termed the filamentous brush border glycocalyx (FBBG)), the major component of which is an approximately 400 kDa molecular weight transmembrane mucin-type glycoprotein (5) containing O-acetylated sialic acid. (6) The 400 kDa FBBG in the rabbit has the same morphological appearance as the fine filamentous material on the microvilli of humans and various other species, including bats, rodents, and amphibians. (7)

As stated above, brush border enzymes are normally inserted into the apical membrane in microvillous atrophy,(8) and we could not detect differences in alcian blue staining of goblet cells and mucus between controls and microvillous atrophy cases. Thus the FBBG could be responsible for the abnormal histochemical staining pattern.

Functions of the glycocalyx

Protection Cushions the plasma membrane and protects it from physical and chemical injury.
Immunity To Infection Enables the immune system to recognize and selectively attack foreign organisms.
Defense Against Cancer Changes in the glycocalyx of cancerous cells enable the immune system to recognize and destroy them.
Transplant Compatibility Forms the basis for compatibility of blood transfusions, tissue grafts, and organ transplants.
Cell Adhesion Binds cells together so that tissues do not fall apart.
Fertilization Enables sperm to recognize and bind to eggs.
Embryonic Development Guides embryonic cells to their destinations in the body.

Links

References


1. Ito S . Structure and function of the glycocalyx. Fedn Proc 1969;28:12–25.

2. Gupta BL. The relationship of mucoid substances and ion and water transport, with new data on intestinal goblet cells and a model for gastric secretion. Symp Soc Exp Biol 1989;43:81–110

3. Gillibrand PN. Changes in the electrolytes, urea and osmolality of the amniotic fluid with advancing pregnancy. J Obstet Gynaec Brit Cwlth 1969;76:898–905.

4. Sinha RS, Carlton M. The volume and composition of amniotic fluid in early pregnancy. J Obstet Gynaec Brit Cwlth 1970;77:211–14.

5. Maury J , Nicoletti C, Guzzo-Chambraud L, et al. The filamentous brush border glycocalyx, a mucin-like marker of enterocyte hyper-polarisation. Eur J Biochem 1995;228:323–31.

6. Maury J , Bernadac A, Rigal A, et al. Expression and glycosylation of the filamentous brush border glycocalyx (FBBG) during rabbit enterocyte differentiation along the crypt-villus axis. J Cell Sci 1995;108:2705–13.

7. Ito S . The enteric surface coat on cat intestinal microvilli. J Cell Biol 1965;27:475–91.

8. Phillips AD, Fransen JAM, Hauri HP, et al. The constitutive exocytotic pathway in microvillous atrophy. J Pediatr Gastroenterol Nutr 1993;17:239–46.

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