Transcytosis It is the transport of material from one side of the extracellular space to the other. Although this phenomenon can occur in all types of cells – including osteoclasts and neurons – it is characteristic of epithelia and endothelia.
During transcytosis, molecules are transported through endocytosis, mediated by some molecular receptor. The membranous vesicle migrates through the microtubule fibers that make up the cytoskeleton and, on the opposite side of the epithelium, the contents of the vesicle are released by exocytosis.
In endothelial cells, transcytosis is an indispensable mechanism. The endothelium forms an impenetrable barrier for macromolecules such as proteins and nutrients.
Furthermore, these molecules are too large for transporters to pass through. Thanks to the process of transcytosis, the transport of the said particles is achieved.
The existence of transsilosis was postulated in the 1950s by Palde studying the permeability of capillaries, where he describes a population of vesicle enhancers. Subsequently, such transport was discovered in the blood vessels present in striated and cardiac muscle.
The term “transcytosis” was coined by Dr. N. Simionescu combined with his work group to describe the passage of molecules from the luminal side of endothelial cells to the interstitial space in membranous vesicles.
Features of the process
The movement of materials within the cell can follow various transcellular pathways: by membrane transporters, by channels or pores, or by cancer.
This phenomenon is a combination of the processes of endocytosis, the transport of vesicles through cells and exocytosis.
Endocytosis consists in the introduction of molecules into cells, which engulf them in an invasion coming from the cytoplasmic membrane. The formed vesicle is incorporated into the cytosol of the cell.
Exocytosis is the reverse process of endocytosis, where the cell excretes the products. During exocytosis, the membranes of the vesicles fuse with the plasma membrane and the contents are released into the extracellular medium. Both mechanisms are important in the transport of large molecules.
Transcytosis allows various molecules and particles to cross the cytoplasm of a cell and move from one extracellular region to another. For example, the passage of molecules through endothelial cells to circulate in the blood.
It is a process that requires energy – it is ATP dependent – and involves the structures of the cytoskeleton, where actin microfilaments have an engine role and microtubules indicate the direction of movement.
Transcytosis is a strategy used by multicellular organisms for the selective movement of materials between two environments, without altering their composition.
This mechanism of transport involves the following steps: The first molecule binds to a specific receptor that can be found on the apical or basal surface of cells. Then the process of endocytosis through covered vesicles takes place.
Third, intracellular transduction of the vesicle occurred on the opposite surface from where it was internalized. The process ends with the exocytosis of the transported molecule.
Some signals are capable of triggering transcytosis processes. It has been determined that a polymeric receptor of immunoglobulin called PIG-R ( polymer immunoglobin receptor ) experiences transcytosis in polarized epithelial cells.
When phosphorylation of the amino acid serine residue occurs at position 664 of the cytoplasmic domain of PIG-R, it is induced in the process of transcytosis.
In addition, there are transcytosis-associated proteins (TAPs, transcytosis-associated proteins ) found in the membrane of vesicles that participate in the process and interfere with the process of membrane fusion. There are markers for this process and they are proteins of about 180 kD.
types of transcytosis
Depending on the molecule involved in the process, there are two types of transcytosis. One is clathrin, a molecule of a protein nature that participates in the trafficking of vesicles inside cells and caveolin, which is a specific protein called caveolae.
The first type of transport, which involves clathrin, involves a highly specialized type of transport, as this protein has high affinity for certain receptors that bind ligands. The protein participates in the process of membrane stabilization that produces the membranous vesicle.
The second type of transport, mediated by the caveolin molecule, is essential in the transport of albumin, hormones, and fatty acids. These vesicles formed are less specific than in the previous group.
Transcytosis allows the cellular aggregation of large molecules, mainly in epithelial tissue, while retaining the structure of the traveling particle.
In addition, it is the means by which infants manage to absorb antibodies from breast milk and are excreted from the intestinal epithelium into the extracellular fluid.
Immunoglobulin G, abbreviated IgG, is a class of antibodies produced in the presence of microorganisms, whether fungi, bacteria or viruses.
It is often found in body fluids, such as blood and cerebrospinal fluid. Furthermore, it is the only type of immunoglobulin that is able to cross the placenta.
The most studied example of transcytosis is the transport of IgG from breast milk in rodents, which leaves behind the intestinal epithelium in the offspring.
IgG binds to Fc receptors located in the luminal part of brush cells, the ligand receptor complex is endocytosed in covered vesicular structures, is transported through the cell and released in the basal portion.
The lumen of the intestine has a pH of 6, so this pH level is optimal for the association of the complex. Similarly, the pH for dissociation is 7.4, corresponding to the intracellular fluid of the basal side.
This difference in pH between the two sides of the epithelial cells of the intestine makes it possible for immunoglobulins to reach the blood. In mammals, this same process makes it possible to transmit antibodies from the cells of the yolk sac to the embryo.