The roles of Factor XIII-A and Transglutaminase 2 in the vasculature

Transglutaminases catalyse the formation of isopeptide bonds which covalently link proteins together and these enzymes have been previously implicated in cardiovascular repair processes. This thesis explores the transglutaminases, Factor (F)XIII-A and Transglutaminase (TG)2, and their roles within the vasculature. To do this, we bred and characterised single and double knockout mice deficient for FXIII A and/or TG2.
Previously, a protective role has been postulated for FXIII-A, as humans and mice lacking FXIII-A fail to maintain pregnancy potentially due to a failure of remodelling within the placental artery. This finding was also repeated in this study and further to this a significant decrease in the number of FXIII-A-/- pups born to FXIII-A+/- mothers was seen. FXIII-A-/- mice develop cardiac fibrosis and suffer significant mortality following injury and this phenotype is exacerbated with additional loss of TG2, highlighting the importance of FXIII A and TG2 as protective enzymes.
Interestingly, despite being present in the blood at μg/mL concentrations, the cellular source that maintains the plasma pool of FXIII-A has not been identified. FXIII-A is known to be produced by megakaryocytes, chondrocytes, osteoclasts, monocytes and macrophages and the prevailing view is that platelets are the most likely cellular source.
To delineate the cellular source of plasma FXIII-A, we commissioned a de novo FXIII A floxed mouse which was bred with tissue specific cre recombinase mice. These novel models were used to determine plasma and platelet FXIII-A activity, together with FXIII-A gene expression in the aorta and heart. The results presented here discount the platelet as the cellular source of plasma FXIII A. Mouse bone marrow transplants and gene expression in whole and fractionated organs instead implicate a resident tissue cell, possibly a macrophage, in the wall of the aorta (and perhaps the heart) as the cellular source of FXIII A.
Beyond its repair role, TG2 has been suggested to potentiate apoptosis in response to stress signalling. However, previous studies have utilised monodansylcadaverine and hence have not inhibited TG2 directly. Here, transglutaminase activity assays and characterisation of MAPK signalling in wildtype and TG2 /- derived vascular smooth muscle cells are used to demonstrate that TG2 is the main transglutaminase involved in calphostin C-induced apoptosis.
The findings shown here explore the common and distinct functions of these enzymes and details potential avenues for translation into therapy.