The human transglutaminase (TG) family consists of a structural protein, protein 4.2, that lacks catalytic activity, and eight zymogens/enzymes, designated factor XIII-A (FXIII-A) and TG1-7, that catalyze three types of posttranslational modification reactions: transamidation, esterification, and hydrolysis. These reactions are essential for biological processes such as blood coagulation, skin barrier formation, and extracellular matrix assembly but can also contribute to the pathophysiology of various inflammatory, autoimmune, and degenerative conditions. Some members of the TG family, for example, TG2, can participate in biological processes through actions unrelated to transamidase catalytic activity. We present here a comprehensive review of recent insights into the physiology and pathophysiology of TG family members that have come from studies of genetically engineered mouse models and/or inherited disorders. The review focuses on FXIII-A, TG1, TG2, TG5, and protein 4.2, as mice deficient in TG3, TG4, TG6, or TG7 have not yet been reported, nor have mutations in these proteins been linked to human disease.

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Several active transglutaminase (TGase) isoforms are known to be present in human and rodent tissues, at least three of which, namely, TGase 1, TGase 2 (tissue transglutaminase), and TGase 3, are present in the brain. TGase activity is known to be present in the cytosolic, nuclear, and extracellular compartments of the brain. Here, we show that highly purified mouse brain nonsynaptosomal mitochondria and mouse liver mitochondria and mitoplast fractions derived from these preparations possess TGase activity. Western blotting and experiments with TGase 2 knockout (KO) mice ruled out the possibility that most of the mitochondrial/mitoplast TGase activity is due to TGase 2, the TGase isoform responsible for the majority of the activity ([ 14 C]putrescinebinding assay) in whole brain and liver homogenates. The identity of the mitochondrial/mitoplast TGase(s) is not yet known. Possibly, the activity may be due to one of the other TGase isoforms or perhaps to a protein that does not belong to the classical TGase family. This activity may play a role in regulation of mitochondrial function both in normal physiology and in disease. Its nature and regulation deserve further study.Human and rodent tissues contain eight active transglutaminases (TGases; 1 EC 2.3.2.13) that catalyze the Ca 2+ -dependent covalent linkage of the carboxamide moiety of a Q residue (acyl donor) in a protein/peptide substrate to the ∊-amino group of a K residue (acyl acceptor) in a protein/peptide cosubstrate. The reaction results in the formation of an N ∊ -(γ-L-glutamyl)-Llysine (GGEL) cross-link and ammonia (1). TGases can also utilize amines, diamines, and polyamines as acyl acceptors (2). Human brain (3) and rat brain (4) possess at least three TGase proteins, namely, TGase 1, 2 (tissue transglutaminase, tTGase), and 3. In addition, the human † This work was supported in part by the National Institutes of Health Grants PO1 AG14930 and NS38741. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript brain also contains mRNAs for TGases 5, 6, and 7 (5). Increased brain TGase activity and increased GGEL immunoreactivity have been reported for Alzheimer disease (AD) (6,7), Huntington disease (HD) (8-13), Parkinson disease (PD) (14), and progressive supranuclear palsy (15).Considerable evidence suggests that increased protein cross-linking occurs in diseased brain. For example, free GGEL is increased in HD CSF and brain (11,16) and in AD CSF (17). Free GGEL is also increased about 30-fold in digests of protein obtained from AD brain (3,18). While TGase activity is not causative of neurodegeneration, it is possible that TGase(s) contribute(s) to the disease progression (e.g., refs 19 and 20). Circumstantial evidence showing that the in vitro TGase inhibitor cystamine (9,11) increases the life expectancy of HD transgenic mice and more direct evidence from experiments showing the prolongation of life expectancy in HD transgenic mice exhibiting a TGase 2 KO (21) are consistent with this possibility. Under standard assay con...