PET imaging of transglutaminase type 2
Transglutaminase (Tgase) family contains eight enzymes, TG1-TG7 and Factor XIII. Transglutaminases can cross-link proteins by forming intermolecular isopeptide bonds between the side chains of glutamine and lysine residues, and participate in post-translational modification of proteins.
Transglutaminase type 2 (TG2, Tgase2) is the most ubiquitously expressed member of the Tgase family, but most tissues express also other TG types. TG2 is also found in erythrocytes.
Transglutaminase type 2
TG2 is mainly expressed intracellularly. TG2 has several functions: it assists in cell migration and adhesion, and during cell apoptosis it cross-links intracellular components, preventing the leakage of cellular debris. Extracellular TG2 (at cell surface) participates in deposition and stabilization of the extracellular matrix (ECM).
The cross-linking activity of TG2 is strictly regulated. TG2 can take two conformations depending on whether Ca2+ or GTP/GDP is bound to the TG2 molecule: high [Ca2+] and low [GTP/GDP] (typically in extracellular space) favours the open conformation, where the site of cross-linking activity is exposed; intracellular TG2 is mainly in the closed conformation, with hidden site of cross-linking activity, but involved in signalling processes as a G-protein. Redox state of the TG2 surroundings affects the cross-linking activity, since a disulphide bond between adjacent cysteine residues leads to inactivation of TG2. Formation of ternary complexes with integrins and fibronectin may reduce the cross-linking activity, while other molecular stimuli can increase the activity.
The open conformation of TG2 is involved in celiac disease. In the fibrotic diseases TG2 is activated and overexpressed, increasing the production of scar tissue by cross-linking and stabilizing ECM proteins.
TG2 is a difficult target for in vivo imaging because of its different conformations and intra/extracellular localization. Labelled antibodies can target either open or closed conformation. Open conformation may be targeted by reversible and irreversible inhibitors developed for the cross-linking activity. GTP-binding domain of TG2 may be another target suitable for both antibodies and small molecule ligands.
Labelled antibodies, antibody fragments, and small organic molecules for TG2 in vivo imaging are being developed (van der Wildt et al., 2016 and 2017).
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Updated at: 2017-09-14
Created at: 2017-03-05
Written by: Vesa Oikonen