Analyses on Mechanisms for Biosynthesis and Release of Human Coagulation Factor XIII at Molecular, Cellular, and Individual Levels.

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At the molecular level: Full-length recombinant factor XIIIB subunit (rXIIIB) and truncated human XIIIBs with various numbers of Sushi domains (rXIIIBx-y) were expressed. The rXIIIB was in dimer form and produced a heterotetramer complex with XIIIA. Gel-filtration and XIIIA-binding analysis revealed that the first Sushi domain was responsible for the binding of XIIIB to XIIIA, and that the 4th and 9th Sushi domains were involved in the XIIIB homodimer assembly. Fibrin crosslinking in XIIIB-deficient plasma was accelerated by the addition of rXIIIB, while no effect was observed in a reconstitution system using purified fibrinogen, implying the presence of an unknown factor(s) that mediate the XIIIB-dependent modificaton of fibrin crosslinking in plasma.At the cellular level: The interaction between XIIIA and actin or a trimeric G protein β-subunit (G_<β2>)was confirmed by co-immunoprecipitation and by con-focal laser microscopy. The incorporation of amine substrates into nuclear proteins was detected and confirmed as a transglutaminase reaction. G_<β2> was found to be co-localized with XIIIA in lipid rafts on the cell surface, implying that the newly synthesized XIIIA in the cytoplasm was transported outside the cell membrane. This is an epoch-making new finding in this research field.At the individual level: We generated mice lacking either XIIIA or XIIIB. Survival rates of XIIIA null males decreased to approximately 50% at 10 months from birth. Four XIIIA null males died of severe intra-thoracic hemorrhage, and a large hematoma was found in their hearts. Hemorrhage, hemosiderin deposition, and/or fibrosis were observed in the hearts of other dead XIIIA null males. Fibrosis together with hemosiderin deposition was also found in the hearts of XIIIA null and XIIIB null males sacrificed at 6-8 months from birth, it is important therefore to examine the possible existence of cardiac complications in human patients with congenital XIII deficiency.