In this video lecture, Dr. Ferdows Atiq discusses:
- The diverse physiologic functions of von Willebrand factor in hemostasis, factor VIII stabilization, and vascular biology.
- Emerging evidence that von Willebrand factor regulates inflammation, angiogenesis, immune responses, and multiple non-hemostatic disease processes.
- How expanding knowledge of von Willebrand factor biology may identify new therapeutic targets beyond inherited bleeding disorders.
Dr. Ferdows Atiq obtained his medical degree from Erasmus University Medical Center in Rotterdam, the Netherlands. He subsequently completed a PhD on von Willebrand disease at the same institution, during which he also earned a Master’s degree in Clinical Epidemiology. After completing part of his specialist training in Internal Medicine and Hematology, he undertook a two-and-a-half-year postdoctoral fellowship at the Royal College of Surgeons in Ireland. He currently combines the final phase of his specialist training with leading his own research group at Erasmus University Medical Center, where his research focuses on von Willebrand disease. In addition, he currently serves as Chair of the International Society on Thrombosis and Haemostasis (ISTH) Scientific and Standardization Subcommittee (SSC) on von Willebrand Factor (VWF).
(Video Lecture Summary)
Introduction
Dr. Ferdows Atiq explores the diverse biological functions of von Willebrand factor (VWF), highlighting its expanding role beyond hemostasis. While VWF has long been recognized for its essential contributions to platelet adhesion and factor VIII stabilization, growing evidence demonstrates that it interacts with numerous ligands and cell types involved in vascular biology, inflammation, immunity, and disease. These emerging functions provide new insights into both von Willebrand disease (vWD) and a wide range of non-hemostatic disorders.
The Classical Functions of von Willebrand Factor
Dr. Atiq begins by reviewing the well-established functions of VWF in normal hemostasis. Under physiologic conditions, circulating VWF remains in an inactive globular conformation. Following vascular injury, VWF unfolds, binds platelets, promotes platelet adhesion and aggregation, and contributes to thrombus formation.
VWF also serves as the carrier protein for factor VIII, protecting it from degradation and maintaining normal plasma factor VIII levels.
He reviews VWF biosynthesis, explaining that approximately 85% is produced by endothelial cells and 15% by megakaryocytes. Newly synthesized monomers undergo glycosylation, dimerization, multimerization, and packaging into Weibel-Palade bodies before secretion into the circulation.
Following secretion, VWF activity is regulated by ADAMTS13, which cleaves unfolded VWF multimers into smaller, less active forms.
VWF Deficiency and Excess
Because VWF participates in multiple hemostatic interactions, both deficiency and excess can lead to disease.
Dr. Atiq reviews the spectrum of von Willebrand disease, including quantitative deficiencies in type 1 and type 3 disease and qualitative defects in the various type 2 subtypes. He explains how mutations affecting specific functional domains of the VWF molecule produce distinct abnormalities in platelet binding, collagen binding, multimer formation, or factor VIII binding.
At the opposite end of the spectrum, elevated VWF levels are associated with thrombosis. He cites evidence from animal models, epidemiologic studies, and thrombotic thrombocytopenic purpura (TTP), where severe ADAMTS13 deficiency allows ultra-large VWF multimers to persist, promoting widespread microvascular thrombosis.
Beyond Hemostasis: More Than 60 VWF Ligands
Dr. Atiq emphasizes that VWF biology extends far beyond its traditional hemostatic role. More than 60 distinct VWF ligands have now been identified, raising important questions about the biological significance of these numerous interactions.
He first discusses how ligand interactions regulate VWF itself. Various proteins participate in VWF synthesis, secretion, proteolysis, and clearance. Interactions with endothelial receptors influence VWF localization, while binding partners modify cleavage by ADAMTS13. Multiple cellular clearance receptors regulate removal of VWF from the circulation.
These mechanisms have direct therapeutic implications. Dr. Atiq highlights emerging therapies designed to inhibit macrophage-mediated VWF clearance, thereby increasing circulating VWF levels. He also notes that differences in glycosylation contribute to distinct clearance rates between plasma-derived and recombinant VWF products.
Emerging Non-Hemostatic Functions
The lecture then shifts to the expanding understanding of VWF as a regulator of multiple biological processes.
Dr. Atiq reviews evidence demonstrating that VWF deficiency impairs angiogenesis. Animal studies and investigations using endothelial colony-forming cells from patients with type 3 vWD show abnormal blood vessel formation, suggesting that VWF plays an important role in vascular development and repair.
He also discusses studies demonstrating effects of VWF on vascular smooth muscle cell proliferation and migration through specific receptor-mediated signaling pathways.
Beyond the vasculature, VWF influences numerous components of the immune system. It affects macrophage polarization toward pro-inflammatory phenotypes, promotes cytokine and chemokine release, facilitates neutrophil recruitment and NET formation, regulates leukocyte adhesion and extravasation, and influences dendritic cell biology.
Additional emerging functions include regulation of endothelial permeability, maintenance of Weibel-Palade body formation, osteoclast activity and bone resorption, and interactions with tumor cells.
VWF in Human Disease Beyond von Willebrand Disease
Dr. Atiq reviews growing evidence that VWF contributes directly to the pathophysiology of numerous disorders outside inherited bleeding disease.
In sepsis, several bacterial pathogens, including Staphylococcus aureus, exploit VWF to facilitate movement within the circulation and evade host defenses. In atherosclerosis, VWF promotes macrophage infiltration into atherosclerotic plaques.
Experimental models also demonstrate an important role for VWF in sickle cell disease. Studies using humanized sickle cell mouse models show that either deletion of VWF or administration of ADAMTS13 reduces vaso-occlusion in multiple organs, supporting a pathogenic role for VWF during vaso-occlusive crises.
He further reviews evidence implicating VWF in COVID-19, liver disease, acute kidney injury, vasculitis, malaria, and several malignancies, where VWF may influence tumor survival, migration, metastasis, or apoptosis depending on the cancer type.
Implications for von Willebrand Disease
The expanding understanding of VWF biology also provides new perspectives on clinical manifestations of von Willebrand disease.
Dr. Atiq highlights angiodysplasia and gastrointestinal bleeding as potential consequences of impaired angiogenesis in patients with VWF deficiency.
He also discusses heavy menstrual bleeding, noting that its severity cannot always be explained solely by reduced hemostasis. Because VWF also influences inflammation, wound repair, and angiogenesis during the menstrual cycle, even modest reductions in VWF may contribute substantially to menstrual bleeding symptoms.
Future Directions
Dr. Atiq concludes that many non-hemostatic functions of VWF remain incompletely understood. As knowledge of these pathways expands, VWF may become an increasingly important therapeutic target not only in von Willebrand disease but also in inflammatory disorders, sickle cell disease, liver disease, cancer, and other conditions in which VWF contributes directly to disease pathophysiology.
He suggests that continued investigation into these emerging functions will likely lead to novel treatment strategies that extend well beyond the traditional management of bleeding disorders.