Endocytosis

As Endocytosis refers to the uptake of liquid or solid substances by a cell. With the help of phagocytosis, the cell takes up solid particles, while with pinocytosis, dissolved molecules are internalized.

What is endocytosis?

Eukaryotic cells have a semipermeable cell membrane that only a few particles can pass through. The membrane must be penetrated so that macromolecules can also get into the cell. Through endocytosis, particles can be taken up from the extracellular space, whereby there are two different forms of endocytosis - pinocytosis and phagocytosis.

With the help of endocytosis, mainly fluids and macromolecules are absorbed, pathogens are eliminated and the metabolism is maintained. Endocytosis also plays an important role in the transmission of extracellular signals.

Function & task

With the help of endocytosis, larger particles, macromolecules and molecules are absorbed into the cell, which happens via transport vesicles. After the signal molecules have been bound to the surface of the cell, the cell membrane is inverted and the absorbed charge is enclosed. A vesicle called an endosome forms inside the cell. Thousands of these vesicles then carry the charge through a cell, either reusing it or breaking it down.

Endocytosis enables controlled uptake, and it also plays an important role in immune response, tissue and cell development, cell communication and signal transduction. It is also involved in neuronal signal transmission.

With the help of endocytosis, microorganisms can be warded off, but it is also possible that viruses or undesired microorganisms enter the cell by endocytic means.

Altogether, two different forms of endocytosis can be distinguished: pinocytosis and phagocytosis. By means of phagocytosis, larger particles can be internalized, including macrophages or leukocytes, also called phagocytes.

Phagocytosis is primarily used for food intake and removes degenerated cells and extracellular deposits. Phagocytosis is mediated by the Fc receptor, which recognizes particles that are marked with IgG molecules.

Phagocytosis is also referred to as "foreign body uptake" because the cell incorporates foreign material. Eukaryotes with one or few cells, such as algae or fungi, have this ability.

With the help of phagocytosis, the body can fight exogenous antigens such as bacteria. The MHC-II receptors remember the particles that have been broken up so that they can be repelled in the event of further infestation.

There are several types of cells in the human body that are capable of phagocytosis. These include:

• dendritic cells
• Macrophages contained in the tissue
• Monocytes
• Granulocytes

The process of phagocytosis is closely related to human immunity. Therefore, cells that have the ability to phagocytose play an important role in the defense against viral or bacterial diseases.

Pinocytosis absorbs extracellular fluid, and the cell internalizes the extracellular fluid and the substances dissolved in it within a very short time. This process is also known as fluid phase endocytosis.

There are four different forms of pinocytosis in eukaryotic cells: macropinocytosis, clathrin-dependent endocytosis, caveolae-mediated endocytosis, and clathrin- and caveolae-independent endocytosis.

As part of macropinocytosis, the plasma membrane fuses with the long membrane protuberances, trapping a lot of extracellular fluid.

Extracellular molecules are internalized by means of clathrin-dependent endocytosis. This means that important substances such as iron can be continuously absorbed.

Caveolae are invaginations of the plasma membrane that have the shape of a bottle and fulfill numerous functions in the cell. For example, they are responsible for signal transduction. However, caveolae are internalized very slowly in the cells, so that no large amounts of extracellular fluid are absorbed by caveolea-mediated endocytosis.

Clathrin-independent mechanisms are found in neuroendocrine cells and in neurons, where they are involved in the reuptake of proteins into the plasma membrane.

Illnesses & ailments

Endocytosis is a cellular process through which signals are transmitted and food is ingested. If this process is disturbed, diseases can occur.

A number of diseases can be traced back to a defect in membrane transport, including, for example, tumors, infections or neurogenerative diseases. Charcot-Marie-Tooth neuropathy, for example, is triggered by mutations in the genes of the Rab family.

This syndrome is a disease of the peripheral nervous system in which walking is impaired. Foot deformities occur and the muscles tire very quickly. Muscle wasting can also occur in the feet and lower legs or in the forearms and hands. In addition, the nerve conduction speed is reduced and those affected suffer from sensory disorders. Muscle reflexes are weakened or completely absent, and skeletal deformations can also occur in the course of life.

Endocytosis is also disturbed in Huntington's disease. Huntington's disease is also a neurodegenerative disease in which nerve cells die and symptoms such as dementia, movement disorders or changes in personality appear.

Huntington's is an inherited disease caused by the protein huntingtin. The base triplet CAG occurs up to 250 times in those affected, whereas it occurs only 9 to 35 times in healthy people. The first symptoms usually become noticeable between the ages of 30 and 40, although the disease can last up to 20 years and ultimately have a fatal course.