Factor XIII (FXIII) is a plasma transglutaminase that cross-links fibrin monomers, ␣ 2 -plasmin inhibitor, and so forth. Congenital FXIII deficiency causes lifelong bleeding symptoms. To understand the molecular pathology of FXIII deficiency in vivo, its knockout mice have been functionally analyzed. Because prolonged bleeding times, a sign of defective/abnormal primary hemostasis, were commonly observed in 2 separate lines of FXIII A subunit (FXIII-A) knockout mice, a possible role or roles of FXIII in platelet-related function was investigated in the present study. Although platelet aggregation induced by adenosine diphosphate or collagen was normal, clot retraction (CR) was lost in the platelet-rich plasma (PRP) of FXIII-A knockout mice.
IntroductionCoagulation factor XIII (FXIII) is a pro-enzyme of plasma transglutaminase (TGase) consisting of 2 enzymatic A subunits (FXIII-A) and 2 noncatalytic B subunits, and plays a critical role in the generation of a stable hemostatic plug. 1-3 FXIII catalyzes intermolecular cross-linking reactions between fibrin monomers, ␣ 2 -plasmin inhibitor, fibronectin, etc. These reactions increase the mechanical strength of the fibrin clot and its resistance to proteolytic degradation, and enhance the assembly of the extracellular matrix.Congenital FXIII deficiency is a rare autosomal recessive disorder, the cases of most of which are caused by defects in the FXIII-A gene. 3 A lifelong bleeding tendency, abnormal wound healing, and recurrent spontaneous miscarriage are common symptoms of FXIII deficiency. 1,4 FXIII-A exists extracellularly in plasma as well as intracellularly as a cytosolic protein in megakaryocytes/platelets and monocytes/macrophages, although the function(s) of intracellular FXIII-A remain(s) unknown. 5,6 In particular, platelets cause clot retraction (CR) by retracting extended filopodia along fibrin strands. 7 There have been conflicting reports about the effects of FXIII deficiency on CR; investigators reported that the absence of FXIII abolished, 7-9 did not affect, 10,11 or rather enhanced 12 CR in patients with congenital FXIII deficiency. However, platelet aggregation induced by various agents is uniformly normal in patients with congenital FXIII deficiency. 8,9,13,14 To understand the precise molecular pathology of FXIII deficiency in vivo, FXIII-A knockout (KO) mice have been analyzed. FXIII-A KO mice demonstrated a severe bleeding tendency. 15 We also reported that FXIII-A KO mice developed severe uterine bleeding, resulting in spontaneous miscarriage in females, and male-specific intrathoracic hemorrhage as well as excessive cardiac fibrosis. 16,17 Because bleeding times in FXIII-A KO mice were longer than in wild-type, 15,18 we hypothesized that FXIII-A KO mice might have a defective platelet-related function(s). Accordingly, we have explored the contribution of FXIII to the process of CR in the present study.
Methods
AnimalWild-type C57BL/6J mice were obtained from Japan SLC Inc. Genetargeted mice of FXIII-A were generated as previously reported, ...