Factor VIII-bypassing activity of bovine tissue factor using the canine hemophilic model

Urinary extracellular vesicles carry multiple activators and regulators of coagulation

Cells shape their extracellular milieu by secreting intracellular products into the environment including extracellular vesicles which are lipid-bilayer limited membrane particles. These vesicles carry out a range of functions, including regulation of coagulation, via multiple contributor mechanisms. Urinary extracellular vesicles are secreted by various cells, lining the urinary space, including the nephron and bladder. They are known to have procoagulant properties, however, the details of this function, beyond tissue factor are not well known. The aim of the study was to access the role of urinary extracellular vesicles in impacting coagulation upon supplementation to plasma. This could indicate their physiological function upon kidney injury or pathology. Supplementation to standard human plasma and plasmas deficient in various coagulation factors was used for this purpose, and calibrated automated thrombogram (CAT^®) was the major technique applied. We found that these vesicles contain multiple coagulation-related factors, and their lipid composition affects coagulation activities of plasma upon supplementation. Remarkably, these vesicles can restore thrombin generation in FVII, FVIII, FIX and FXI -deficient plasmas. This study explores the multiple roles of urinary extracellular vesicles in coagulation in in vitro blood coagulation and implies their importance in its regulation by several mechanisms.

In vivo models of haemophilia - status on current knowledge of clinical phenotypes and therapeutic interventions

Animal models have contributed immensely to the understanding of and the improvement in treatment of haemophilia A and B. First, establishment of haemophilic dog colonies provided an invaluable opportunity to investigate the diseases and later, the advances in gene technologies resulting in small haemophilic animal models were a milestone in the preclinical research making it possible to address some of the many unanswered questions. This review provides an overview of animal models used in the study of haemophilia as well as a short overview of the contributions resulting from studies in these models.

Optimization of coagulometric tests that incorporate human plasma for determination of coagulation factor activities in canine plasma

OBJECTIVE

To optimize methods used to measure coagulation factor activities in canine plasma, define reference ranges in dogs, and compare activities between canine and human plasma.

SAMPLE POPULATION

Human plasma samples (n = 5) and plasma from healthy dogs (140) and dogs with low factor V activity (7), high factor V activity (7), and low factor VIII:C activity (6).

PROCEDURE

Coagulometric tests incorporated human plasma deficient in a single coagulation factor (human deficient plasma). Standard curves were generated with pooled plasma from 100 healthy dogs. Effect of sample dilution was evaluated, using plasma from dogs with high or low factor V activity and low factor VIII:C activity. Reference ranges for healthy dogs were established. Activities in human plasma were determined by comparison with standard curves obtained with canine plasma.

RESULTS

Activities of factors V and VIII:C in samples diluted < or = 1:20 influenced results of tests for other coagulation factors. Activities of factors V and VIII:C in human plasma were significantly less than in canine plasma. For the other coagulation factors, significant differences in human plasma-to-canine plasma activity ratios were detected among different sample dilutions.

CONCLUSIONS AND CLINICAL RELEVANCE

Accurate measurement of coagulation factor activities in canine plasma, using human deficient plasma, requires higher sample dilutions (ie, > 1:20) than typically used for human plasma. Differences in activities between human and canine plasma and nonparallelism of the standard curves emphasize the necessity for use of species-specific standard curves for accurate determination of coagulation factor activity.

Formation of tissue factor activity following incubation of recombinant human tissue factor apoprotein with plasma lipoproteins

Incubation of recombinant human tissue factor apoprotein (Apo-TF) with human plasma decreased the recalcified clotting time of this plasma in a time-and dose-dependent manner suggesting relipidation of the Apo-TF by plasma lipoproteins. Incubation of Apo-TF with purified preparations of human very low density, low density and high density lipoproteins resulted in tissue factor activity in a clotting assay. The order of effectiveness was VLDL greater than LDL much greater than HDL. Tissue factor activity generated by incubation of a fixed amount of Apo-TF with plasma lipoproteins was lipoprotein concentration-dependent and saturable. The association of Apo-TF with lipoprotein particles was supported by gel filtration studies in which 125I-Apo-TF coeluted with the plasma lipoprotein in the void volume of a Superose 6 column in the presence and absence of calcium ions. In addition, void-volume Apo-TF-lipoprotein fractions exhibited tissue factor activity. These results suggest that the factor VIII-bypassing activity of bovine Apo-TF observed in a canine hemophilic model may be due, in part, to its association with plasma lipoproteins and expression of functional tissue factor activity.

Purification of recombinant human tissue factor

Tissue factor (TF) is a 263 amino acid membrane-bound procoagulant protein that serves as a cofactor for the serine protease factor VII (fVII). Recombinant human TF (rTF) produced in both human kidney 293 cells and Escherichia coli has been immunoaffinity purified by using a TF-specific monoclonal antibody. Recombinant TF produced in 293 cells is glycosylated and migrates on reducing SDS-PAGE with an apparent molecular weight (Mr) of 45K. Some interchain disulfide-bonded rTF dimers are observed under nonreducing conditions. The E. coli produced rTF has a molecular weight of 33K and 35K, with the 33K band missing nine amino acids at the carboxy terminus. Although the E. coli produced rTF does not contain any carbohydrate, it is fully functional in both a chromogenic assay and a one-stage prothrombin time assay. A variant has been constructed wherein the cytoplasmic cysteine (residue 245) has been mutagenized to a serine residue. The amount of disulfide-linked aggregates is dramatically reduced following immunoaffinity purification of this four-cysteine variant (C2455), which is active in the chromogenic and prothrombin time assays.

The molecular biology and biochemistry of tissue factor

The cloning of tissue factor (TF) cDNAs has recently been carried out which has allowed the characterization of this cellular receptor for factor VII. The predicted primary structure indicates that it is a multidomain protein comprised of a hydrophilic extracellular domain, a membrane spanning structure and a cytoplasmic region. Post-translational modifications include four potential N-linked glycosylation sites of which three are occupied. There is evidence that the intracellular cysteine residue is thioester-linked to palmitic acid, and that this could enhance the anchoring of the receptor to the membrane. The four remaining cysteine residues are located in the putative extracellular domain and appear to form two intramolecular disulphide bonds. There is no significant homology between TF and any other published protein sequence in current databases. There is evidence that TF is the primary initiator of the coagulation cascade. Early models of coagulation tended to assign TF to a subordinate role, but now it is widely accepted that the extrinsic pathway is critical as TF/FVIIa activates both FX and FIX. It has been reported that zymogen FVII has catalytic activity and this would indicate that the simple complexing of FVII and TF is sufficient to initiate coagulation without an infinite regression of proteolytic events. Many cell types synthesize TF constitutively but, significantly, endothelial cells do not normally express TF on their surfaces, consistent with a quiescent haemostatic system. Recently a number of agonists known to be thrombogenic in vivo have been shown to induce de novo synthesis of TF by endothelial cells. TF thus operates at two levels: exposure of the subendothelium to blood results in the binding of FVII to TF and consequently to clot formation, and induction of TF synthesis by a variety of agonists results in the intact endothelium becoming procoagulant. In concert with TF expressed by monocytes and macrophages this endothelial cell procoagulant activity may play a role in the pathogenesis of thrombotic disease.