Characterization of a full-length cDNA for rabbit factor X

Ultrastructural morphology of platelets and fibrin networks of lactating and non-pregnant rabbits

Platelets and fibrin play an important role in the coagulation process where they are involved in the maintenance of haemostasis. Fibrin dysfunction is associated with the development of vascular complications, while proneness to the formation of tight and rigid fibrin networks is independently associated with thrombotic disease. Rabbits have long been used successfully as animal models, and are often the species of choice for models of antithrombotic efficacy. It was previously shown that rabbit and human platelet and fibrin morphology are very similar in ultrastructure and fibrin fibre thickness. It was also previously reported that the thin minor fibres forms a thick fine network cover over the major fibres during pregnancy. According to research, white blood cell counts also changes during pregnancy and stays changed for up to 6 weeks post-partum; where the number of neutrophils increased, and the number of lymphocytes, basophils and eosinophils decreased. Here, we show that the same ultrastructure and white blood cell count changes occur in lactating rabbits (4 weeks post-partum). We therefore suggest that a rabbit morphology model studying platelet and fibrin morphology can be used successfully, either to study the effect of pharmaceutical products to be used during lactation and pregnancy in humans, or used in veterinary research. Furthermore, the effects of pharmaceutical products on immunology and white blood cell counts can possibly also be used successfully.

Differences between human and rabbit coagulation factor X-implications for in vivo models of thrombosis

The activation of factor X (fX) to factor Xa (fXa) marks the penultimate step in the coagulation cascade and modulating fXa activity may be effective for antithrombotic therapy. Even though fXa inhibitors are screened using in vitro inhibition of human fXa (HfXa) while subsequent evaluation uses in vivo rabbit models, there is limited knowledge of species differences between the coagulation proteins. When comparing amino acid sequences for the human (HfX) and rabbit (RafX) protein, differences are found in the activation peptide and active site regions. In order to study the relative functional characteristics of HfX and RafX, we asked (1) whether fX from the two species is immunologically related, (2) whether the two proteins are activated to fXa in a similar manner, (3) whether HfXa and rabbit factor Xa (RafXa) have similar catalytic activities toward tripeptide substrates. To answer (1), we expressed RafX-glutathione S-transferase (RafX-GST) fusion protein in bacteria and purified the protein for use as an antigen. The resulting monoclonal antibodies were suitable for affinity purification of plasma RafX and for effective anticoagulation in rabbit plasma clotting assays. We found two antibodies (mAb 214 and mAb 290) that anticoagulated rabbit plasma in a dose responsive manner but did not cross-react with human plasma. At a concentration of 500 nM, mAb 214 attained a two-fold extension of rabbit plasma activated partial thromboplastin time (aPTT). To answer (2), we purified plasma RafX and compared the activation of HfX and RafX with Russell's viper venom (RVV-X). Under equivalent reaction conditions, conversion was 30% slower for the rabbit protein. To answer (3), amidolytic activity of HfXa and RafXa were assayed by cleavage of three para-nitroanilide (pNA) substrates (S2222 [Bz-Ile-Glu(gamma-OR)-Gly-Arg-pNA.HCl], S2765 [Z-D-Arg-Gly-Arg-pNA.HCl] and Spectrozyme Xa [MeO-CO-D-CHG-Gly-Arg-pNA.AcOH]). Michaelis constants (K(m)) for the rabbit protein were 187, 72 and 69 microM, respectively, and for the human analog, 255, 63 and 135 microM, respectively. Comparing the extent of substrate turnover (V(max)) for HfXa and RafXa, the latter was shown to cleave all three substrates at a reduced rate. Based on these observations, it can be speculated that the relative antithrombotic potency of active site directed fXa inhibitors might be different between the two species. Predicted human therapeutic doses derived from in vivo results in rabbit models should therefore take species variation into consideration.

Cloning and recombinant expression of mouse coagulation factor X

Engineering of recombinant coagulation factor X variants, which can be activated by tumor-associated proteinases may lead to the development of new therapeutic molecules. However, the evaluation of such variants requires an appropriate animal model. Therefore, we isolated the complete coding sequence of mouse coagulation factor X from mouse liver cDNA by polymerase chain reaction. The deduced amino acid sequence codes for a prepro protein of 481 amino acids homologous to factor X sequences from various species. Recombinant mouse factor X was expressed in human embryonic kidney cells and secreted into cell culture supernatant as zymogen, which could be converted to catalytically active factor Xa by Russell's viper venom. Purified recombinant mouse factor X restored coagulation in human factor X deficient plasma, demonstrating that mouse factor X is able to functionally interact with the human blood coagulation system. Recombinant mouse factor X opens the possibility to analyze therapeutically useful variants in the mouse system.