Membrane flux is the term used to summarize all processes of intercellular mass transport across the endomembrane system. This primarily includes endo-, exo-, and transcytosis, which allow cells to take up and release substances by displacing the membrane. Disruption of membrane flux can cause cell death (apoptosis).
What is membrane flux?
Membrane flux is the term used to summarize all processes of intercellular mass transport across the endomembrane system. The transport of substances between the individual cells of the body corresponds to membrane transport. The two main processes of intercellular mass transport are endocytosis and exocytosis. In exocytosis, substances are discharged from a cell. In endocytosis, on the other hand, materials foreign to the cell are taken up into a cell. With this goal in mind, the cell engorges itself, thus constricting the substance into its cell membrane. Vesicles and vacuoles are formed during this process. When a biomembrane partially self-dislocates during mass transfer, it is referred to as membrane flux. Membrane flux occurs in the endomembrane system. This system corresponds to the totality of all eukaryotic organelles between which there is a connection for vesicular transport. Besides the endoplasmic reticulum, the endosomes, the lysosomes and the plasma membrane (cell membrane), the Golgi apparatus also belongs to the endomembrane system. Excluded from the system, however, are peroxisomes, mitochondria, and the nucleus. S
ow endocytosis, as well as exocytosis, can be mentioned as transport processes in the context of membrane flux. A third transport pathway of membrane-displaced mass transfer is transcytosis, which is the receptor-mediated transport of a substance across a cell membrane.
Function and task
Endocytosis of membrane flux (or membrane-displacing mass transport) corresponds to an invagination of the biomembrane. Through invagination, the cell incorporates, for example, a drop of fluid, certain macromolecules, or large pieces of food. Sometimes it even invaginates smaller cells into its membrane. After incorporating the substance, the cell laces off an endosome into the cytoplasm, which henceforth belongs to the endomembrane system. The surrounding medium thus partially migrates into the cell interior. Endocytosis is either phagocytosis and thus targets solid particles or it corresponds to pinocytosis and takes up dissolved molecules. Also relevant in the context of membrane flux is receptor-mediated endocytosis, in which asialoglycoprotein receptors provide particle recognition and thus trigger uptake into the cell interior. This type of membrane flux is relevant, for example, to cholesterol uptake. Exocytosis must be distinguished from endocytosis of membrane-displacing mass transport. In this process, substances are transported out of the interior of the cell and released into the cell’s environment. The substances transported in this way are, for example, substances produced by the cell itself. But indigestible remains can also leave the cell in this way. A so-called exome or transport vesicle fuses with the membrane of the cell during the transport process. A lipid bilayer coats the exome from the outside. Normally, an exocytosis is associated with an endocytosis and is then called exocytosis-coupled endocytosis. The combination of endocytosis and exocytosis prevents unhindered cell enlargement. Exocytosis-coupled endocytosis also spares the cell the synthesis of vesicles and membrane proteins for transport purposes. For this reason, this is often referred to as vesicle recycling. The third transport pathway of membrane flux is transcytosis, also known as cytopempsis. This is a receptor-dependent transport, which transports extracellular material through a cell and thus corresponds to a combination of exocytosis and endocytosis. The vesicle formed in this process is released by the cell to a neighboring cell or transported into the extracellular space. The content of the vesicle remains unchanged. This type of transport mainly affects epithelial cells in vessels and in the intestine. Receptors for transcytosis are mainly known to be Fc receptors of a particular group, which are found in the placenta and in the apical infantile intestinal epithelium. In the placenta, they mainly take over the transport of maternal IgG into the child.
Diseases and ailments
When the transport of substances by membrane-modifying transport processes is impaired, this is often due to a mutation of the transport proteins, transport enzymes, or receptors involved. Quite a few diseases are associated with defective membrane transport. Tumors, for example, are associated with impaired endocytosis. The same applies to infections and neurogenerative diseases such as neuropathies with impaired walking ability and reduced nerve conduction velocity or sensory disturbances. For example, endocytosis is impaired in Huntington’s disease. In this disease, dying nerve cells trigger symptoms such as movement disorders and changes in character. A mutation of the protein huntingtin is the cause of the disease. Disturbed exocytosis can have equally serious consequences. Exocytosis of neurotransmitters, for example, may be impeded by toxins. Bacterial toxins can cause convulsions or paralysis by obstructing membrane flow. Exocytosis is also affected by disorders in metabolic diseases such as cystic fibrosis. In this disease, in addition to bronchial secretions, pancreatic and biliary secretions become viscous, resulting in organ dysfunction. Disturbed pinocytosis is now associated with diseases such as Alzheimer’s, with metabolic disorders, elevated cholesterol levels and with changes in character. Finally, impaired membrane flux can also result in severe disorders in the gastrointestinal tract and is thus sometimes associated with gastrointestinal disease. The effects of restrictions in membrane flux are correspondingly diverse and, in the worst case, result in cell death. In the context of membrane flow disorders, the cells are often unable to absorb any or only a few vital substances and can hardly or not at all excrete superfluous or even toxic substances.
