Polyphosphate binds to human von Willebrand factor in vivo and modulates its interaction with glycoprotein Ib

BACKGROUND

Polyphosphate, a phosphate polymer released by activated platelets, has recently been described as a potent modulator of blood coagulation and fibrinolysis. In blood plasma, polyphosphate binds to and alters the biological functions of factor XII, fibrin(ogen), thrombin and factor VII activating protease.

OBJECTIVES

The aim of the present study is to investigate whether polyphosphate also binds to von Willebrand factor (VWF) and alters some of its activities.

METHODS/RESULTS

When studying patients with type 1 von Willebrand disease (VWD) and their healthy relatives, we discovered a significant correlation between von Willebrand factor (VWF) and platelet polyphosphate levels. We have also found polyphosphate in preparations of VWF isolated from normal platelets and plasma. Surface plasmon resonance and electrophoretic mobility assays indicated that polyphosphate interacts with VWF in a dose- and time-dependent manner. Treatment of normal plasma with active exopolyphosphatase decreased the VWF ristocetin cofactor (VWF:RCo) activity, a functional measure of VWF binding to platelet glycoprotein receptor Ib. VWF collagen binding and multimerization were unaltered after polyphosphate depletion. Moreover, addition of polyphosphate increased the deficient VWF:RCo activity presented by plasma from patients with type 1 VWD.

CONCLUSIONS

Our results reveal that a new role is played by polyphosphate in hemostasis by its interaction with VWF, and suggest that this polymer may be effective in the treatment of some types of VWD.

Organellar proteomics of human platelet dense granules reveals that 14-3-3zeta is a granule protein related to atherosclerosis

Dense granules, a type of platelet secretory organelle, are known to accumulate high concentrations of small molecules such as calcium, adenine nucleotides, serotonin, pyrophosphate, and polyphosphate. Protein composition of these granules has been obscure, however. In this paper, we use proteomics techniques to describe, for the first time, the soluble protein composition of platelet dense granules. We have isolated highly enriched human platelet dense granule fractions that have been analyzed using two proteomics methods. Using this approach, we have identified 40 proteins, and most of them, such as actin-associated proteins, glycolytic enzymes, and regulatory proteins, have not previously been related to the organelle. We have focused our efforts on studying 14-3-3zeta, a member of a conserved family of proteins that interact with hundreds of different proteins. We have demonstrated that 14-3-3zeta is localized mostly on dense granules and that it is secreted after platelet activation. As some proteins secreted from activated platelets could promote the development of atherosclerosis and thrombosis, we have studied the expression of 14-3-3zeta in sections of human abdominal aorta of patients with aneurysm, identifying it at the atherosclerotic plaques. Together, our results reveal new details of the composition of the platelet dense granule and suggest an extracellular function for 14-3-3zeta associated with atherosclerosis.

Antiphospholipid syndrome and asymptomatic carriers of antiphospholipid antibody: prospective analysis of 404 individuals

OBJECTIVE

We carried out a prospective analysis of clinical and analytical findings in individuals with antiphospholipid antibodies (aPL).

METHODS

We prospectively studied 404 individuals, classified in 2 groups: (1) patients with primary or secondary antiphospholipid syndrome (APS, n = 226); and (2) asymptomatic carriers of aPL (n = 178). Patients with APS and thrombosis were treated with dicumarin, and an international normalized ratio around 3.0 (range 2.5-3.5) was targeted. Asymptomatic carriers were not treated, but specific prophylaxis with low molecular weight heparin or aspirin was prescribed for the periods when individuals were at increased risk of thrombosis. Both groups of individuals were followed up at semester intervals for 36 months.

RESULTS

Patients with APS presented with venous (n = 106, 46.9%) and/or arterial (n = 70. 31%) thrombosis or fetal loss (n = 58 out of 112 women of fertility age, 51.8%). At the time of the first thrombotic event, 50.0% of patients with APS had coincident risk factors for thrombosis (previous surgery and prolonged immobilization were significantly associated with venous thrombosis, and hypercholesterolemia and arterial hypertension with arterial thrombosis). Eighteen patients with APS died during the study period. Recurrence of thrombosis in patients with APS was linked to insufficient anticoagulation. During the followup, no episode of thrombosis was detected in any asymptomatic carrier. The proportion of subjects with aPL was similar in patients and in asymptomatic carriers. The proportion of subjects with aPL decreased during the followup, in both patients and carriers.

CONCLUSION

Differences between patients and asymptomatic carriers with aPL are at least partially dependent on the proportion of coincident vascular risk factors. The decline in aPL during the followup defines a subgroup in which an anticoagulation suppression assay could be tried.