Tight junctions ( English for “tight connection”, lat. Zonula occludens, in German literature also “final ridge”) are narrow bands of membrane proteins that completely girdle the epithelial cells of vertebrates and are closely connected to the ligaments of neighboring cells. In this way, the tight junctions close the intercellular space and form a paracellular barrier, called a diffusion barrier , which controls the flow of molecules across the epithelium. They also have the task of maintaining the polarity of the epithelial cells: They prevent membrane components from diffusing laterally from the apical area and vice versa.
Tight junctions are located in the apical area (closer to the outside) of the lateral cell membrane and, by tying the ligaments of neighboring cells together, create a very close contact between the membranes of two neighboring cells. These contacts form a diffusion barrier that prevents or controls the paracellular transport of molecules across the epithelium ( barrier function ). If several tight junction strands lie one behind the other, the impermeability increases logarithmically. In this way, food intake and material transport can take place in a controlled manner through the epithelial cells.
In addition, tight junctions have a so-called "fence-function" (fence-function) : they prevent the free movement of membrane components and thus obtain the cell polarity of epithelial cells by placing them in apical and basal divide areas. This is important because e.g. B. the apical cell membrane of the epithelial cells with the microvilli has a completely different biochemical composition than the basolateral membrane. These are separated from each other by the tight junctions - a necessary prerequisite for directed material transport.
Tight junctions occur in epithelial cells (e.g. kidney, urinary bladder and intestinal epithelium) and cerebral capillary endothelial cells ( blood-brain barrier ) of vertebrates . In the invertebrates , however, septate junctions have an analogous function.
The most important previously known membrane proteins of the tight junctions are the family of Claudine and the occludin . So far, 24 different types of the Claudine family are known in vertebrates, four in other eukaryotes such as the nematode Caenorhabditis elegans and seven in the fruit fly Drosophila melanogaster . These integral membrane proteins are arranged in a network and bring the membranes of two cells directly to one another in a kind of head-to-head contact. They form aqueous pores that selectively allow certain ions and molecules to pass while others are retained. The specificity of the diffusion barrier is adapted to the respective requirements that the epithelium must meet through the different composition of the claudins. A special claudin, claudin 16, which was found in the renal epithelium, is necessary to absorb Mg 2+ ions from the kidneys into the blood. A mutation in the gene that codes for this protein can lead to excessive Mg 2+ loss in the urine. Tight junctions sometimes form looser (example: intestines ), sometimes denser barriers (example: blood-brain barrier ). The tight junctions of the small intestine are z. B. 10,000 times more permeable to ions such as Na + than those in the urinary bladder . However, the permeability between epithelial cells can also be changed temporarily if increased paracellular transport is desired. Claudine and occludins associate with intracellular membrane proteins ZO-proteins according to the zonula occludens named which the connection with the actin - cytoskeleton prepared.
- Cell contact Overview of the different types of cell contact
- Adherens junction
- Focal adhesion
- Gap junction
- Tight Junction Biology ( Memento from December 15, 2007 in the Internet Archive ) - pictures and texts (English)
- Good reception of a junctional complex
- Alberts, B. et al .: Molecular Biology of the Cell . Garland Science, 4th Edition, 2002. ISBN 0-8153-4072-9 .