Transglutaminases

Enzyme family belonging to the transferases. From a biochemical point of view, transglutaminases catalyze the acyl transfer from protein-bound glutamine residues to primary amines. Dysfunctions of transglutaminases in vivo have clinical relevance with regard to various dermatoses. Mutations leading to hereditary diseases are known from coding genes: TGM1, TGM3 and TGM5 .

TG1 (keratinocyte TG) is expressed in the stratified squamous epithelia of the skin and upper digestive tract as well as in the lower female genital tract. The promoter of the TGM1 gene contains three activator protein AP2-like response elements located ∼0.5 kb from the transcription initiation site (238). Proteolytic cleavage, elevated Ca2+ levels, and interaction with tazarotene-induced gene 3 (TIG3) are known to activate the catalytic activity of TG1 . Phorbol esters induce and retinoic acid reduces mRNA and protein expression of TG1. The TG1 protein associates with the plasma membrane via a fatty acyl bond in the NH2-terminal cysteine residue and is released by proteolysis as 10-, 33- and 66-kDa fragments .

Autosomal recessive lamellar ichthyosis is caused by mutation of the gene encoding TG1. Common mutations include a C-to-T change at the binding site for the transcription factor Sp1 in the promoter region, a Gly143-to-Glu mutation in exon 3 and a Val382-to-Met mutation in exon 7. Lamellar ichthyosis is a rare keratinization disorder of the skin characterized by abnormal keratinization of the epidermis. Individuals with ichthyosis exhibit drastically reduced TG1 activity and the absence of detectable TG1 protein. TG1 knockout mice exhibit the phenotype of lamellar ichthyosis.

TG2 (transglutaminase 2) Tissue transglutaminase (transglutaminase 2, TG2) has become extremely important for the diagnosis of coeliac disease. This food intolerance to cereal proteins leads to the formation of autoantibodies against the body's own tissue transglutaminase. The detection of this autoantibody (IgA anti-TG2 antibody) is considered the most important laboratory test for coeliac disease (alongside the detection of the IgA-type endomysium antibody (EMA)); it is almost 100 % negative predictive, its positive predictive value is approx. 72 %.

Transglutaminase 3 (TG3) or epidermal TG is found in hair follicles, the epidermis and in the brain. The promoter of the TG3 gene (TGM3) contains Sp1 and Ets motifs (128 and 91 bp upstream of the initiation site, respectively), and the expression of pro-transglutaminase 3 mRNA is increased by Ca2+. The TG3 protein is encoded as two polypeptide chains formed by proteolysis from a single precursor protein. Like TG2, TG3 binds to GTP and hydrolyzes it. It catalyzes the cross-linking of trichohyalin and keratin intermediate filaments to harden the inner root sheath of a hair follicle, which is crucial for hair fiber morphogenesis. The enzyme is also involved in the formation of the cell envelope in the final stages of differentiation. TG3 knockout mice show impaired hair development and reduced skin barrier function.

Transglutaminase 4 (TG4) or prostate TG is present in the prostate, prostatic fluids and seminal plasma. An Sp1 binding site located -96 to -87 bp upstream of the transcription initiation site is critical for transcriptional regulation of TG4 gene expression, and androgen treatment increases TG4 mRNA levels in human prostate cancer cells. In rats, the enzyme is involved in the formation of the copulatory plug in the female genital tract and in masking the antigenicity of the male gamete. TG4 knockout mice exhibit reduced fertility due to defects in the formation of the copulatory plug. The exact function of TG4 in humans is not known, but some recent reports suggest a link between increased expression of TG4 and the promotion of an aggressive prostate cancer phenotype.

Transglutaminase 5 (TG5) is mainly expressed in keratinocytes of the foreskin, in the epithelial barrier layer and in skeletal muscle. The TG5 gene (TGM5) has a TATA-less promoter, but contains putative binding sites for several transcription factors, including C-Myb, AP-1, NF-κB and NF-1. GTP and ATP inhibit the protein cross-linking activity of TG5, while Ca2+ abolishes this inhibition. In addition to the full-length TG5 protein, three alternatively spliced isoforms of TG5 have been described: delta3 (deletion of exon 3), delta11 (deletion of exon 11) and delta3-delta11 (deletion of both exons). Full-length TG5 and the isoform delta11 are active, while delta3 and delta3-delta11 show low activity. TG5 cross-links loricrin, involucrin and SPR3 in the epidermis and contributes to hyperkeratosis in patients with ichthyosis and psoriasis. TG5 inactivating mutations lead to skin peeling syndrome. TG5 knockout mice have not yet been generated.

Transglutaminase 6 (TG6): The expression of transglutaminase 6 (TG6) is localized in the human testis and lung as well as in the brain of mice. Human carcinoma cells with neuronal characteristics also express TG6. In addition to the full-length protein, alternative splicing results in a short variant that lacks the second β-barrel domain. The catalytic function of TG6 is activated after proteolytic cleavage of the proenzyme; TG6 thus consists of two polypeptide chains that are cleaved from a single precursor. TG6 knockout mice have not yet been generated. The clinical picture of gluten ataxia is associated with anti-transglutaminase 6 (TG6) antibodies (Sato K et al. 2017).

Transglutaminase 7 (TG7): Not much is known about the regulation or function of the TG7 gene and its gene product. Like TG6, TG7 expression is restricted to the testis, lung and brain. One report suggests that TG7 transcript levels are elevated in breast cancer cells from patients with poor prognosis. TG7 knockout mice are not available.

Transglutaminase 8 (TG8): Little is known about the regulation or function of the TG8 gene and its gene product.

Transglutaminase 9 (TG9): Little is known about the regulation or function of the TG9 gene and its gene product.

The transglutaminase family is a family of enzymes found in every prokaryotic and eukaryotic cell. In humans, 9 isoenzymes are currently known. They are referred to as TG1 to TG9. Transglutaminases have several thousand substrates and catalyze numerous vital functions.

Transglutaminases are involved in the control of apoptosis, blood clotting, cell adhesion, as well as in the assembly of the extracellular matrix, among other functions.

6 isoenzymes can be detected in the skin. They play a role in cornification and are responsible for the formation of the " cornified cell envelope" (see below: ichthyosis lamellosa, autosomal recessive with transglutaminase deficiency;bathing-suit-ichthyosis, syndrome of uncombed hair).

In the gluten-sensitive diseases (see dermatitis herpetiformis Duhring and celiac disease below), 3 transglutaminase isozymes play a role.

• In dermatitis herpetiformis, the epidermal isoenzyme TG3 (epidermal transglutaminase) is mainly involved. The coding TGM3 gene is a protein coding gene located on chromosome 20p13. The encoded gene product, transglutaminase 3, also known as epidermal TGM, is one of the central enzymes responsible for the formation of protein polymers in the epidermis and hair follicle. TGM3 catalyzes the calcium-dependent formation of isopeptide crosslinks between glutamine and lysine residues in various proteins, as well as the conjugation of polyamines to proteins.
• Auto-antibodies against the transglutaminase TG6 (neuronal transglutaminase . see below TGM6 gene) have been detected in patients with gluten ataxia . However, it is likely that they also occur in other gluten-sensitive neurological symptoms.
• In celiac disease, the tissue transglutaminase TG2 (cTG), as an auto-antigen, plays a central role in the disease process. The harmful polypeptides of the gluten complex cannot pass through the small intestine epithelium under physiological conditions. This only occurs under pathological conditions (e.g. intestinal infections). However, the immunity of the glutamine-rich gluten peptides is low.
• In celiac disease patients, an increased concentration of transglutaminases (cTG) is present in the cells of the small intestine. This causes them to react with the glutamine-rich gluten peptides (deamidation). As a result of the deamidation, numerous glutamic acid residues are released and become active, which in turn form a complex with HLA-DQ2 and -DQ8 (see below Dermatitis herpetiformis). These complexes activate CD4 cells and trigger a specific Ak response.

An industrial application of transglutaminase is the cross-linking of proteins in sausages and "restructured" meat (moulded meat) as well as fish and dairy products; for this purpose, the Ca2+-independent transglutaminase is obtained from bacterial strains (Streptomyces mobaraensis). As it is a processing aid, this additive is not declared in the list of ingredients in foodstuffs.

In Switzerland, stricter regulations apply according to current food law. Transglutaminase must be declared in the list of ingredients for all foods. In addition, manufacturers are obliged to add the words "made from pieces of meat" to the name of the product.

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