Abstract Number: PB0910
Meeting: ISTH 2021 Congress
Background: Platelets bud off from megakaryocytes into the circulation and contain different types of granules. Alpha-granules contain many hemostatic proteins, including Von Willebrand Factor (VWF) and a processed part of the protein, VWF-propeptide (VWFpp). While multimerization, storage and release of VWF has been extensively studied in endothelial cells, regulation in megakaryocytes and platelets is unclear. Studying these processes in platelets will help us better understand how this key hemostatic protein contributes to adequate hemostasis from different compartments.
Aims: To characterize the storage and release of VWF and VWFpp in platelet alpha-granules.
Methods: Healthy platelets were stimulated with PAR-1 activating peptide (PAR-1-ap). We employed super-resolution light microscopy and image analysis to generate quantitative imaging data. Slides were stained for alpha-tubulin, VWF and VWFpp, SPARC or fibrinogen. Data are normalized to resting platelets as percentage of granule numbers ± SEM.
Results: We observed extensive, but not perfect (~85-95%) overlap in VWFpp+ and VWF+ granules in hundreds of resting platelets, implying that these proteins are stored in similar eccentric fashion in platelet alpha-granules (Figure 1).
In comparison to unstimulated platelets (100±1.7% granules over 3 donors), we observed less VWFpp+ (64.3±3.1%) and VWF+ granules (75.3±1.4%) at 0.6 µM of PAR-1-ap, suggesting rapid release of a subset of granules (Figure 2). Higher concentrations of PAR-1-ap triggered more pronounced differential release of VWFpp (14.7±1.6% at 20 µM, p<0.0001) compared to VWF (62.4±1.4%, p=0.03). Release of other alpha-granule proteins was intermediate at 20 µM PAR-1-ap (SPARC: 37.8±1.4%, fibrinogen 48.1±2.9%; p<0.001), providing further evidence for differential exocytosis of alpha-granule cargo.
Conclusions: Our findings show that VWF and VWFpp, in contrast to endothelial cells, are differentially released from activated platelets. This may affect how platelet-derived VWF and VWFpp contribute to hemostatic clots.
To cite this abstract in AMA style:Swinkels M, Slotman J, Houtsmuller A, Leebeek F, Voorberg J, Jansen G, Bierings R. Differential Release of VWF and VWF-propeptide from Platelet Alpha-granules [abstract]. Res Pract Thromb Haemost. 2021; 5 (Suppl 1). https://abstracts.isth.org/abstract/differential-release-of-vwf-and-vwf-propeptide-from-platelet-alpha-granules/. Accessed September 24, 2021.
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