Claudin

Claudine (Latin claudere " close " ) are a group of proteins that are the most important part of the deck in tissues ( epithelia ) occurring cell connections that are so-called tight junctions. Epithelia cover the body surfaces of multicellular animal organisms and also limit the lumen of the organs. Claudine close to the epithelium, the spaces between cells and allow for controlling the flow of substances through the cell gap. They thus form a kind of seal between the cells ( " paracellular barrier "), which is required in the epithelium for ions and molecules to organs can not pass freely. A maintenance of a certain milieu of individual compartments ( such as pH values ​​1-2 in the stomach ) would not be possible without these paracellular barriers.

Construction

Claudine are small transmembrane proteins that occur in many organisms and the nematode Caenorhabditis elegans to humans in their structure very similar ( conserved ) are. They have a size of 20-27 kilodaltons ( kDa). At the DNA sequence level, however, the preservation is not very high. They traverse the cell membrane four times ( tetraspanin ), the N-terminus and the C-terminus are both in the cytoplasm. A claudin has two extracellular ( outside the cell location ) areas, which are called the small and large extracellular loop. The first extracellular loop consists of an average of 53 amino acids, the second is slightly smaller and has an average of 24 amino acids. The N-terminus is usually very short (4-10 amino acids), the C-terminus is variable in its length ( 21-63 amino acids). In the first extracellular loop is an amino acid motif that is found in all claudins and consists of amino acids W - GLW -CC. It is believed that form between the cysteines of Claudins or the cysteines of different Claudine disulfide bridges. The most conserved regions are the transmembrane domains, the most variable region of the C -terminus. Up to 12 claudin all human Claudine possess a PDZ- binding motif at the C -terminus, with which they bind to PDZ- domain proteins. For the C-terminus was also shown that it is required for the localization of claudin molecules in the tight junctions.

To date, 24 different human Claudine were identified and referred to as " claudin 1" " Claudin 24 ". They do not lie in a cluster before, but found distributed on different chromosomes.

History

Since then, the tight junctions were described and their function was recognized for the cells searched for factors or structural components of the tight junctions, which were needed for the sealing of the spaces between the cells. At the beginning of the 1990s succeeded in the Japanese research group led by Mikio Furuse and Shoichiro Tsukita at the University of Kyoto, to identify the transmembrane protein occludin junction as the first integral membrane protein of tight. However, it turned out that occludin is not responsible for the formation and maintenance of the paracellular barrier, since cells lacking occludin, still can form tight- junction strands. The same group discovered a few years later the first two actually for the formation of tight junctions competent proteins and gave them 1998 the name " Claudin ," from the Latin claudere " close " for. Some claudin proteins have been previously described in other contexts, its function as a sealing component of the tight junctions, however, was not initially recognized. When classifying in the group of Claudine they were renamed accordingly.

Targeted search in the genome of human and mouse total of 24 proteins have been found that could be classified in the group of Claudine basis of their sequence, structure and expression.

Also in other animal species Claudine were discovered, such as in all then mammals studied, in various species of fish ( Fugu rubripes, Danio rerio), in amphibians ( Xenopus laevis and Xenopus tropicalis ) and 2003 unexpectedly even in the nematode Caenorhabditis elegans and the fruit fly Drosophila melanogaster. Unexpectedly, this discovery was because both C. elegans and Drosophila, have no tight junctions, but analogous structures, the septate junctions in cells of the epithelium.

Since then, over 500 studies have been published that deal with Claudine, their expression, function and regulation. Since they are the core components of tight junctions, their manipulation would be a great advance, for example, in overcoming the blood -brain barrier, which makes it almost impossible to time a drug treatment of brain tumors.

Expression

The genetic activity of various Claudine in the various tissues was in many model organisms such as mouse, C. elegans, zebrafish (D. rerio ), D. melanogaster and partly also in humans studied. Each of the 24 known Claudine has a specific expression pattern. Various studies on ectopically expressed claudins in cell cultures led to the assumption that each Claudin has different properties in terms of selectivity, the charge and probably also to the size selectivity of the barrier. The combination of various Claudine, which are expressed in epithelia, determines the properties of the barrier. Depending on the requirements of the epithelium Claudine are expressed, leading to a more or less dense barrier. There are Claudine, which are ubiquitous, almost all epithelial tissues, active, such as claudin 1, and others that are very specific expimiert, either spatially, as claudin 16, which is found exclusively in the ascending limb of Henle 's loop, or temporally as claudin 6, which is expressed only in mice during embryogenesis.

Paracellular barrier

How interact Claudine each other and how the structure of the tight junctions is built at the molecular level is not yet known. In cell culture experiments, it was shown that Claudine both homophilic and specific heterophilic bonds can form: cells capable of expressing claudin 1 with cells expressing claudin 3, which form tight- junction - like structures, as well as claudin 2 with claudin 3, but not claudin 1 with claudin 2 a model suggests that Claudine aqueous pores ( aqueous pores ) form which certain ions and molecules up to a certain size to pass through, depending on the type of Claudine ( charge selectivity and size selectivity). These properties are determined by charged amino acid residues in the first extracellular loop and are calcium - independent. Mutagenesis experiments in cell culture showed that the reversal of the charge of certain residues in the first extracellular loop leads to a reversal of the loading selectivity of the barrier. This will be measured via the TER ( transepithelial electrical resistance, the transepithelial resistance).

Claudin superfamily

The Claudine family is providing from the Pfam protein database at the Sanger Institute in Cambridge, England, the alignments of protein sequences, classified in the PMP22/EMP/MP20/Claudin-Superfamilie ( pfam00822 ). This consists of a total of 450 proteins of various species, all of which have a similar structure. However, completely different functions than the actual Claudine been described for some of these proteins.

Associated diseases

Some diseases are associated with the change of claudin: claudin 3 and 4, for example, are receptors for the Clostridium perfringens enterotoxin ( CPE). They were not initially described as Claudine, but as Rvp.1 (rat ventral prostrate, later renamed claudin 3) and CPE -R ( Clostridium perfringens enterotoxin receptor, later renamed claudin 4). The binding of CPE to claudin 3 or 4 shall, within ten to twenty minutes in lysis of claudin 3/Claudin 4-expressing cells and thereby damage to the intestinal epithelium, which expresses itself in a severe diarrhea.

Several human diseases are due to mutations in claudin genes. Thus lead, for example, mutations in claudin 16, which also means Paracellin to an increased calcium excretion in the urine ( hypercalciuria ) and to a decrease in magnesium content in the blood ( hypomagnesemia ).

Claudin 14 is expressed in the Vertebrata ( vertebrates ) in liver and kidney, pancreas and in the inner ear. In mice, it was shown that the expression of claudin 14 begins only after birth in the inner ear. In two families living in Pakistan mutations have been identified which produce a shift of the open reading frame and a stop codon. These mutations cause recessive deafness, whereas renal and liver functions seem to be normal. The corresponding mouse mutants have no defects in kidney and liver function. The one in the mouse mutant phenotype was analyzed in detail showed that the hair cells of the inner ear degenerate when claudin 14 junctions of the inner ear is missing in the Tight, resulting in loss of hearing function. However, the tight- junction strands between the outer hair cells and the organ of Corti are still present and the cell polarity of the epithelium is not disturbed. The degeneration of hair cells is presumably due to the high potassium concentrations during the early development of hearing.

Claudin 11, which was initially referred to as OSP ( Oligodendrocyte -specific protein), is mainly in the myelin of the central nervous system ( CNS) and expressed in the testis. The mouse mutant has a delayed impulse conduction in the nerve and the hind legs are weak. The males are sterile, but survived the mutant. Show electron microscopic freeze-fracture images show that no Tight junctions are found in CNS myelin and Sertoli cells. The blood- testis barrier is defective in these mutants. Claudin 11 interacts with OAP (OSP / Claudin 11 - associated protein ) and beta-1 integrin in a complex, which plays a role in cell adhesion and integrin signaling.

Claudin 19 is expressed in the peripheral nervous system in the Schwann cells, where it forms tight- junction -like structures. Mice 19 have a deletion in the genomic region of claudin, lack these structures, and they have impaired impulse conduction, causing the musculoskeletal system is affected.

Mice lacking claudin 1 is missing, die a few hours after birth because they dry out. A mutation in the human claudin 1 gene leads to severe skin lesions. Mice lacking claudin 5 is missing, have a specific phenotype: their blood -brain barrier is permeable to small molecules. Overexpression of Claudin 6 leads to a malfunction of the epidermis.

In these examples, it is clear that the Claudine critical functions to form a functional paracellular barrier to exercise in various epithelia, and are thereby divided into different groups: "housekeeping " Claudine, who are largely expressed ubiquitously and basic functions for the formation of the tight - junction belts transfer, and specialized Claudine, which are expressed only in certain tissues or in all tissues in which they occur, exert an essential function.