Glicocálix functions it fulfills and components

The glycocalyx or glucocalyx It is a cell covering composed mainly of carbohydrates that protects and covers the plasma membrane of certain protozoa, some endothelial cells and many species of bacteria.

This outer layer, very prone to hydration, is essentially made up of the polysaccharides that make up the carbohydrate portions of the integral membrane glycoproteins, glycolipids and proteoglycans associated with the outer layer of the plasma membrane and / or cell wall..

The exact composition of the glucocalyx, as well as its structure, depends on the specific cell type considered, as well as the physicochemical and mechanical conditions to which said cell is subjected at the time it is analyzed..

Glycocalyx fulfills various functions at the cellular level, including fixation to different surfaces, protection against harmful agents and prevention against desiccation (in bacteria), regulation of vascular permeability, and transmission of physical forces. to the cytoskeleton (in eukaryotes).

Article index

• 1 Where is the glycocalyx found and what functions does?
  • 1.1 Glycocalyx in prokaryotes
  • 1.2 Glycocalyx in eukaryotes
• 2 Components of glycocalyx
  • 2.1 Proteoglycans
  • 2.2 Glycoproteins
  • 2.3 Some soluble components
• 3 References

Where is it found and what functions does the glycocalyx?

Many cells in nature present glycocalyx, but among them prokaryotes such as bacteria and eukaryotes such as vascular endothelial cells of animals with a circulatory system stand out..

The most relevant examples among living beings that are known are presented below:

Glycocalyx in prokaryotes

Prokaryotes are represented by bacteria and archaea. Both types of unicellular organisms usually present complex envelopes, which perform very important functions in terms of preserving their integrity..

The glycocalyx of bacteria has been, perhaps, the most studied of the prokaryotes, so it is known that depending on the conditions of growth and nutrition, these cells can modify not only the composition, but also the appearance and / or texture of his glycocalyx.

There are many species of archaea and bacteria that present glycocalyx, among whose varied functions are:

- Environmental protection barrier

- Cell stability

- Mobility

- Adherence to biotic or abiotic surfaces

- Biofilm formation or biofilms

- Communication with the surrounding environment and with other cells around

- Establishment of infections

- Evasion of the immune system from the organisms they infect

- Among other

• What is a biofilm?

For certain species of bacteria it is common to observe the establishment of large communities in which the glycocalyx secreted by individual cells allow the formation of well-defined “layers” or “films”, that is, a kind of “continuum” of bacteria..

These films allow bacterial communities to adhere to solid surfaces, while protecting the cells contained therein against numerous external agents..

In the biofilms cells in a community can more easily communicate with each other through a process called quorum sensing, which involves the production and release of signaling molecules to the extracellular medium that, upon reaching a certain concentration, can induce changes in the genetic expression of many cells at the same time.

This intercellular communication capacity, in addition to the ability to exchange genetic material, allows the development of antibiotic resistance, so the establishment of these films can be a great advantage for pathogenic microbes..

Glycocalyx in eukaryotes

A large number of eukaryotic cells secrete a glycocalyx around them and, for many multicellular organisms, the presence of this is essential for intercellular communication and adhesion.

In humans and other mammals, for example, glycocalyx plays important roles in the digestive and vascular systems.

• In the vascular system

Endothelial cells, that is, those that line the internal portion of the "pipes" that make up the vascular system, constantly experience different forces and types of stress, which they endure thanks to the production of glycocalyx, which dampens the different forces and pressures.

Due to the glycocalyx, which, like that of bacteria, forms a thick gelatinous layer around the plasma membrane of endothelial cells, these cells are able to bind to others that are transported in the blood, such as leukocytes. and thrombocytes, very important for coagulation.

• In the digestive system

The microvilli that line the internal portion of the small intestine, those responsible for the absorption of nutrients during digestion, secrete a glycocalyx around them that allows them to protect themselves from the stress to which they are constantly subjected in the intestinal environment, especially in relation to the presence of substances with extremely low pH (acids).

At the same time, it has been determined that some of the enzymes necessary for the decomposition and absorption of nutrients from food are present in glycocalyx, hence its importance..

Many other eukaryotic cells secrete a glycocalyx around them, which forms, as in bacteria, an amorphous gel-like layer. Some additional functions that this layer can perform include:

- Cell signaling (by recognition of glycosylation patterns on the cell surface)

- Elicitation of growth factor release

- Cellular protection against exogenous substances or physical pressures

- Facilitation of cell movement and displacement

- Cell adhesion

- Transmission of mechanical forces exerted on a cell to the internal cytoskeleton

Glycocalyx components

Glycocalyx, as already mentioned, consists of a fibrous mesh or network made up of “threads” of sugars and proteins that bind together, resulting in a thick and sticky layer, capable of hydrating in aqueous environments..

Therefore, the more or less generic components of this extracellular coat are mainly glycoproteins, glycolipids and proteoglycans, their composition in terms of sugars varies significantly between different cells..

So much so that cell recognition in many animals depends on the identification of specific glycosylation patterns on the surface of cells, not only their own, but also foreign and potentially dangerous.

In endothelial cells, for example, the composition of endothelial cells varies constantly, as well as their thickness, since it is in dynamic equilibrium with the components that flow in the blood..

Proteoglycans

Proteoglycans are an important part of the glycocalyx, many authors point to them as the main "skeleton" of this layer.

These molecules consist of a protein nucleus of variable size to which variable numbers of chains of glycosaminoglycans are attached, composed, in turn, by different types of sugars..

The protein nucleus allows the connection between the molecule and the cell membrane, either by hydrophobic transmembrane segments or by the presence of a glycosylphosphatidylinositol anchor (GPI, in eukaryotes).

Among the glycosaminoglycan chains that may be present in proteoglycans are heparan sulfate, chondroitin sulfate, dermatan sulfate, keratan sulfate, and hyaluronic acid; all of these containing a uronic acid and a hexosamine.

Glycoproteins

Glycoproteins are also very abundant molecules in glycocalyx. They also consist of proteins "decorated" with simple or branched chains of sugars of varying lengths. Some of these proteins have cytoplasmic tails, while others have only transmembrane segments..

Some soluble components

Depending on the type of organism, the glycocalyx of some cells may be enriched with soluble factors that are also present in the cellular environment. In the vascular endothelium, for example, glycocalyx may contain albumin, mucoids, and other soluble proteins, as well as ions and other small molecules..

References

1. Alberts, B., Bray, D., Hopkin, K., Johnson, A. D., Lewis, J., Raff, M.,… & Walter, P. (2013). Essential cell biology. Garland science.
2. Cooper, G. M., & Hausman, R. E. (2004). The cell: Molecular approach. Medicinska naklada.
3. Costerton, J. W., Irvin, R. T., & Cheng, K. J. (1981). The bacterial glycocalyx in nature and disease. Annual Reviews in Microbiology, 35 (1), 299-324.
4. Devuyst, O. (2014). Glycocalyx: the fuzzy coat now regulates cell signaling. Peritoneal Dialysis International, 34 (6), 574-575.
5. Egberts, H. J. A., Koninkx, J. F. J. G., Van Dijk, J. E., & Mouwen, J. M. V. M. (1984). Biological and pathobiological aspects of the glycocalyx of the small intestinal epithelium. A review. Veterinary Quarterly, 6 (4), 186-199.
6. Harriott, M. M. (2019). Biofilms and Antibiotics.
7. Reitsma, S., Slaaf, D. W., Vink, H., Van Zandvoort, M. A., & oude Egbrink, M. G. (2007). The endothelial glycocalyx: composition, functions, and visualization. Pflügers Archiv-European Journal of Physiology, 454 (3), 345-359.
8. Robert, P., Limozin, L., Benoliel, A. M., Pierres, A., & Bongrand, P. (2006). Glycocalyx regulation of cell adhesion. In Principles of Cellular Engineering (pp. 143-169). Academic Press.