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Endogenous factor V synthesis in megakaryocytes contributes negligibly to the platelet factor V pool
M Christella L G D Thomassen1, Elisabetta Castoldi, Guido Tans
1Department of Biochemistry, Cardiovascular Research Institute Maastricht, Maastricht University, Maastricht, The Netherlands.
Background And Objectives:
Coagulation factor V (FV) is distributed between two pools: 80% circulates in plasma and 20% is stored in platelets. The aim of the study was to determine the origin of platelet FV.
Design And Methods:
We investigated a FV Leiden heterozygous patient who had received an allogeneic bone marrow transplant from a normal donor. The patient had been referred to our laboratory for his marked activated protein C (APC) resistance in the apparent absence of FV Leiden. Analysis of the DNA from a buccal swab showed that the patient was indeed a heterozygous carrier of FV Leiden. The difference in FV genotype between the hepatocytes (heterozygous FV Leiden) and the blood cells (homozygous normal) of the patient provided a good model to investigate the origin of platelet FV. Platelets were isolated from the patient and the bone marrow donor and activated with thrombin and ionomycin to release and activate FV. APC was then added and the inactivation of platelet FVa was followed over time with a highly sensitive prothrombinase-based assay.
Results:
While the donor's platelet FVa showed a normal inactivation time course, the patient's platelet FVa was considerably resistant to APC. The kinetic pattern of APC-catalyzed inactivation of the patient's platelet FVa was indistinguishable from that of plasma FVa from a FV Leiden heterozygote.
Interpretation And Conclusions:
These data indicate that platelet FV is derived from plasma and that endogenous FV synthesis by megakaryocytes contributes negligibly to the platelet FV pool.
Insights
Platelet coagulation factor V (FV) originates from plasma, not from megakaryocyte synthesis. This finding clarifies the source of FV in platelets, impacting our understanding of coagulation.
Area of Science:
- Hematology
- Coagulation Science
- Molecular Biology
Background:
- Coagulation factor V (FV) exists in plasma (80%) and platelets (20%).
- The origin of platelet-derived FV has been debated.
- Understanding FV origin is crucial for hemostasis and thrombosis research.
Purpose of the Study:
- To determine the origin of coagulation factor V (FV) stored in platelets.
- To investigate whether platelet FV is synthesized by megakaryocytes or sourced from plasma.
Main Methods:
- Utilized a unique patient model with differing FV genotypes in hepatocytes and blood cells post-bone marrow transplant.
- Isolated platelets from the patient and donor.
- Assessed the APC-mediated inactivation kinetics of platelet-derived factor Va (FVa).
Main Results:
- Patient's platelet FVa exhibited significant resistance to APC inactivation, similar to plasma FVa from a FV Leiden heterozygote.
- Donor's platelet FVa showed normal APC inactivation kinetics.
- This differential inactivation pattern strongly suggests the origin of platelet FV.
Conclusions:
- Platelet-bound coagulation factor V is primarily derived from circulating plasma.
- Endogenous synthesis of FV by megakaryocytes contributes minimally to the platelet FV pool.
- This clarifies a fundamental aspect of FV biology and platelet function.