Transcytosis is a type of mass transfer in which a specific substance is taken up into the cell via endocytosis and released back into the extracellular space by exocytosis. Transcytosis is receptor-driven and occurs primarily in the epithelium of the intestine, at the blood–brain barrier, and in the placenta. The consequences of disruption of transcytosis depend on the localization.
What is transcytosis?
Transcytosis is a type of mass transfer in which a specific substance is taken up into the cell via endocytosis and released back into the extracellular space via exocytosis. The area behind a biomembrane is a largely controlled area that shields the inside from the outside and allows the cell, for example, to build and later maintain its cellular milieu. This specific milieu is vital for cells as it enables their essential functional processes. The double layer of a biological membrane consists of phospholipids and can therefore only be passed through by gases and tiny, uncharged molecules. Ions and other substances with biological activity cannot easily pass through this layer. Because of their hydrophilic nature, they are stopped by the lipid bilayer of the biomembrane as if by a barrier. For this reason, transport mechanisms are necessary to introduce specific ions into specific cells. Such mechanisms correspond to the mechanisms of membrane transport, which transports substances across a biological membrane. Membrane transport can correspond to transmembrane transport in the sense of diffusion, active or passive transport. In addition to transmembrane transport, so-called membrane-displacing transport of substances takes place in the human body. Of these membrane-displacing transports, three types exist. In addition to endocytosis and exocytosis, they also include transcytosis. In medicine, transcytosis is the transport of substances by receptor mediation. Substances are passed through cells with the help of receptors.
Function and task
Transcytosis is also known as cytopempsis. It is a transport of substances with the help of receptors. The receptors of the human body are mostly cellular receptors, which mostly correspond to proteins. Some of them are located inform of membrane receptors within cell membranes, such as opioid receptors. Nuclear receptors lie within the cytosol or nucleus of a cell, as do steroid receptors, for example. All receptors in the human body have a specific fit for particular molecules. The fit may be to ligands or larger molecule parts. The binding of substances to receptors works according to the fit-in principle: only certain substances fit certain receptors. The transport process of transcytosis takes advantage of the structure and specific function of receptors. Material outside a specific biomembrane or cell can be passed through the respective cell with the help of receptor-dependent transport. Thus, the principles of endocytosis and exocytosis meet in transcytosis. In endocytosis, materials foreign to the cell are inverted into the cell and strangled off as certain parts of the cell membrane turn inside out. Exocytosis, in turn, expels materials from the cell. Both principles are relevant to transcytosis in that in this type of mass transfer, the substance must first enter the cell to exit on the other side. As in endocytosis, vesicles are formed during the uptake of substances in transcytosis. Similar to the processes of exocytosis, the vesicles with the substance they contain are released back to the outside during transcytosis. In transcytosis, this outward transport corresponds to a transfer of the vesicles to the neighboring cell or to a transport into the extracellular space. Nothing changes in the content and composition of the transported substances. Transcytosis is mainly carried out by the epithelial cells of the vessels and the cells of the intestinal epithelium. Other transport of substances is not possible in these cases because of the tight junctions in the intercellular spaces. Transcytotic receptors are, for example, the membrane-bound Fc receptors located in the placenta. Such receptors are also present at the apical fetal intestinal epithelium, where they transport maternal IgG to the fetus via transcytosis. In addition, receptor-mediated transcytosis occurs at the blood–brain barrier.During transcytosis, the receptor recognizes the respective substance and takes it up into the cell via endocytosis. The transduction through the cell takes place in a vesicle, which is discharged on the other side of the cell by exocytosis.
Diseases and disorders
If the processes of transcytosis are impaired, this can have serious health consequences, because in this way numerous substances no longer reach their site of action. For example, transcytosis disruption during pregnancy is particularly fatal. The passage of maternal antibodies into the embryo is associated with nest protection. This is a natural protection of newborns against infectious diseases in the context of passive immunization. The mother’s IgG antibodies pass through the placenta by transcytosis in the last weeks of pregnancy and reach the child. Thus, after delivery, the newborn has basic protection against many pathogens. In the first weeks after birth, this protection is the only one available, since the child does not yet produce its own antibodies. After about three months, the child begins to produce its own transferred antibodies. If no antibodies are transferred from the mother to the child as part of a disturbed transcytosis, there is no nest protection after birth. The newborn is conspicuously susceptible to infectious diseases and may even require inpatient care. Disorders of transcytosis at the blood–brain barrier are also fatal. The brain lacks important substances in such disorders. Since the brain is the control center of all bodily processes, the consequences can be correspondingly severe.
