PHAGOSOME: CHARACTERISTICS, FORMATION AND FUNCTIONS - BIOLOGY - 2025

A phagosome, also known as an endocytic vesicle, is a vesicle formed as an invagination of the plasma membrane to encompass a phagocytic particle or microorganism. Phagocytosis is the only feeding method for some protists and is also used for this purpose by some lower metazoans.

In most animals, however, the phagocytic function of some cells is maintained, but they cease to have a nutritional function to become a nonspecific protection mechanism against pathogens, as well as for the elimination of dead or senescent cells.

Basic illustration of phagocytosis. Taken and edited from: GrahamColm at English Wikipedia.

The phagosome, formed during phagocytosis, will then fuse to a lysosome, to give rise to a phagolysosome. In this the digestion of the ingested material takes place. In this way, the body can capture and kill bacteria. However, some of these are capable of surviving, and even thriving within phagosomes.

characteristics

For phagosomes to form, pathogens or opsonins must bind to a transmembrane receptor, which are randomly distributed on the surface of phagocyte cells.

Opsonins are molecules that serve as labels, such as antibodies, that will bind to pathogens and regulate the process of phagocytosis.

Because the phagosome is formed as an invagination of the plasma membrane, its membrane will have the same basic composition of the lipid bilayer.

Phagosomes have membrane-bound proteins to recruit and fuse with lysosomes to form mature phagolysosomes

Training

Phagocytosis is a sequential process that includes several stages such as: chemotaxis, adhesion, endocytosis, formation of the phagosome, formation of the phagolysosome, acidification of the phagolysosome, formation of reactive oxygen metabolites, activation of lysosomal hydrolases, release of the digested material, formation of the residual body and finally exocytosis.

Endocytosis and phagosome

Endocytosis is the mechanism by which particles or microorganisms go from outside to inside cells. This process can be optimized by the opsinization of the particles and generally occurs through receptors located in clathrin-coated regions of the membrane.

The process involves invagination of the plasma membrane, giving rise to the phagocytic vacuole. The adhesion of the particles or microorganisms to the membrane causes actin polymerization and also the formation of pseudopodia. These pseudopods will surround the material to be ingested and fuse behind it.

During this process, the participation of several proteins is important, such as kinase C, phosphoinositide 3-kinase and phospholipase C. When invagination ends up closing, a vesicle or phagosome is formed that separates from the membrane and drifts towards the inside the cell.

Phagolysosome

Shortly after phagosome formation, F-actin depolymerization occurs, which is initially associated with the phagosome. The membrane of this cell becomes accessible to early endosomes.

Then, the phagosome moves along the microtubules of the cytoskeleton, while it goes through a series of fusion and fission events, involving various proteins such as annexins and the rap7, rap5 and rap1 GTPases.

These events make the phagosome membrane and its contents mature and can fuse with late endosomes and later with lysosomes to form the phagolysosome.

The rate at which phagosome and lysosome fuse depends on the nature of the ingested particle, but generally takes 30 minutes to do so. That fusion does not necessarily require the membranes to come together completely for the phagolysosome to form.

In some cases, the junction between the phagosome and the lysosome is achieved through narrow aqueous bridges. These bridges only allow the limited exchange of the content of both structures.

Residual body

Once the hydrolysis of the particle or the microorganism has occurred, the resulting molecules will be released into the cytosol of the cell and the waste material will remain contained inside the vesicle, which becomes a residual body.

Later this waste material will be released outside the cell through a procedure called exocytosis.

Features

Elimination of pathogens

Phagosomes known as macrophages and neutrophils are called professional phagocytes and are the cells responsible for most of the capture and elimination of pathogens. These two types of cells have different methods of breaking down bacteria.

Neutrophils produce toxic oxygen, as well as chlorine derivatives to kill bacteria, as well as using proteases and antimicrobial peptides. Macrophages, for their part, depend more on the acidification of phagolysosomes, as well as the use of proteolytic and glycolytic enzymes to destroy pathogens.

Inflammation

The phagosome formation process is related to inflammation processes through common signaling molecules. PI-3 kinase and phospholipase C, for example, participate in the formation of phagosomes and are also important components of the innate immune response.

These proteins induce the production of pro-inflammatory cytokines in a strictly regulated process whose inflammatory response will depend on the type of particle that is encompassed within the phagosome.

Presentation of antigens

Immature dendritic cells are capable of phagocytosis of pathogenic elements. The phagosomes of these cells only partially degrade the phagocytosed pathogens.

As a result of this partial degradation, protein fragments of adequate size for specific bacterial recognition result. These fragments are targeted to the major histocompatibility complex and are used to activate T cells for immune response.

Nutrition

Many protists use phagocytosis as a feeding mechanism. In some cases, this is even their only mechanism for obtaining nutrients. In these cases, the time elapsed between engulfing the food particle and digesting it within the phagosome is much shorter than that used by professional phagocytes.

The stages of phagocytosis in Amoeba. Taken and edited from: Miklos.

Cell elimination

Phagosomes are responsible for the elimination of old and apoptotic cells as a mechanism to achieve tissue homeostasis. For example, red blood cells have one of the highest turnover rates in the body. Thus, senescent erythrocytes are phagocytosed by macrophages located in the liver and spleen.

References

1. G. Karp (2008). Cell and molecular biology. Concepts and experiments. 5th Edition. John Wiley & Sons, Inc.
2. SL Wolfe (1977). Cell biology. Ediciones Omega, SA
3. O. Rojas-Espinosa & P. ​​Arce-Paredes (2003). Phagocytosis: mechanisms and consequences. First part. Biochemistry.
4. O. Rojas-Espinosa & P. ​​Arce-Paredes (2004). Phagocytosis: mechanisms and consequences. Second part. Biochemistry.
5. O. Rojas-Espinosa & P. ​​Arce-Paredes (2004). Phagocytosis: mechanisms and consequences. Third part. Biochemistry.
6. Phagosome. On Wikipedia. Recovered from en.wilipedia.org
7. Phagosome: What is it? Formation, Structure, Function, Maturation Process and Bacterial Manipulation. Recovered from arribasalud.com