FAM‐tastic phospho‐regulation of von Willebrand factor activity

Abstract

A recent investigation performed by Da et al provides evidence that von Willebrand factor (VWF) can be phosphorylated in the A2 domain, a modification that has never before been observed. 1 VWF is a multimeric glycoprotein that mediates platelet adhesion to the endothelium upon vascular injury, a process critical for hemostasis.Dysregulation of VWF can cause myriad bleeding and clotting diseases or complications including but not limited to von Willebrand disease; thrombotic thrombocytopenic purpura; and hemolysis, elevated liver enzymes, and low platelets (HELLP) syndrome. [2][3][4][5] VWF activity is regulated primarily by hydrodynamic shear forces in the bloodstream; however, VWF levels and activity are also influenced by other endogenous factors. For example, a disintegrinlike and metalloproteinase with thrombospondin type-1 motif member 13 (ADAMTS13) specifically targets a Tyr1605-Met1606 scissile bond within the A2 domain of VWF to generate shorter multimers, reducing the relative abundance of high-molecular-weight VWF multimers that more readily bind platelet receptor GPIbα to promote platelet adhesion. 6 Covalent modifications that regulate VWF activity and plasma levels include O-linked and N-linked glycosylation as well as oxidation. Mutagenesis of O-linked glycosylated residues on the N-terminal and C-terminal flanking regions of the VWF A1 domain leads to enhanced collagen and platelet binding. 7 N-linked glycosylation of VWF has been shown to modulate proteolysis of VWF multimers by ADAMTS13 as removal of these glycans undergoes enhanced cleavage. 8 Similarly, trimming of N-linked glycans and removal of negatively charged sialic acid (or desiaylation) on both N-linked and O-linked glycans result in accelerated clearance of VWF in mice. 9 The effects of desialylation of VWF are particularly important in the context of infection, in which neuraminidase from a pathogen desialylate and subsequently clear VWF in the bloodstream. 10 Oxidation of VWF may also pose an increased thrombotic risk. Release of neutrophil oxidants during inflammation enhances oxidation of methionine residues, particularly that of M1606 in the A2 domain, reducing ADAMTS13-mediated proteolysis of VWF multimers as evidenced through in vitro studies and studies with plasma from patients with sickle cell disease. 11 In this issue, Da et al demonstrate a potentially overlooked covalent modification of VWF by showing that the A2 domain can be phosphorylated by a secreted extracellular kinase family with sequence similarity 20, member C (FAM20c). This kinase phosphorylates proteins with an S-x-E motif and is critical for healthy bone and tooth development due to its role in tissue mineralization. 12Moreover, FAM20c was previously characterized to phosphorylate more than 100 extracellular proteins including kininogen, ADAM10, and von Willebrand factor A-domain containing protein 1, 13 lending support to the possibility that the A domains of VWF could also be phosphorylated.Da et al employed in vitro kinase studies and mass spectrometry ana...