Normally, vWF is synthesized as pre-pro-vWF in endothelial cells. This immature form of protein encompasses the signal peptide, pro-peptide and mature subunit. The protein goes through a process during which the signal peptide is removed, and dimerization and multimerization occur, respectively [24]. The pro-peptide, a tag for the protein, is stored in the storage granules and can be detached before releasing into blood, where it is known as von Willebrand antigen II (vWAgII). In the endothelial cells, vWF is stored in specialized rode-like granules identified as Weibel–Palade bodies (WPB), which are assumed to be a derivative of Golgi apparatus. It is worth noting that vWF multimerization itself is confirmed to trigger WPB formation. VWF, therefore, is responsible for the cigarette shape of WPBs [25]. These constructions are able to rapidly respond to alteration in the integrity of the cells covering the vessel walls. VWF, however, is synthesized in the vascular endothelial cell through both constitutive and stimulated release (Fig. 2). This major part of vWF (85 % of all vWF) may either constitutively be secreted in plasma or deposited in the WPBs. Moreover, circulating plasma vWF mostly originates from endothelial cells, while platelet vWF might be released through platelet activation. The role of plasma vWF in the hemostatic process is more dominant than platelet vWF (Figs. 2 and 3). The modification is a vital occasion that may affect different steps from biosynthesis to function and finally to degradation of a protein. VWF glycosylation as a posttranslational modification results in the covalent attachment of carbohydrate structures to the protein backbone.

Thus vWF goes through several modifications, including N-linked and O-linked glycosylation (Fig. 3). O-glycosylation proceeds in the Golgi following N-linked glycosylation, protein folding, and oligomerization. The process is initiated by transferring of N-acetylgalactosamine (GalNAc) from UDP-αGalNAc to a serine or threonine, followed by addition of monosaccharides via specific transferases [26]. In addition, the cellular origin of protein and the variability of enzymes mediating glycosylation in different cell types are the important elements of glycosylation.

The differences, however, between the vWF secreted constitutively by human endothelial cells and that released from Weibel-Palade bodies after stimulation was explained by Sporn et al. The majority of constitutively secreted molecules were dimeric and contained both pro-vWF and mature subunits as they described. In contrast the vWF released by the calcium ionophore A23187 or thrombin consisted of mature subunits of very large multimers. These large subunits have been recognized to play a very important task. They are more active in platelet binding assays in vitro, and their absence in vivo results in a bleeding disorder. Weibel-Palade bodies though contain a special concentrated subclass of very large and biologically potent vWF multimers [27].

VWF is also synthesized by the megakaryocyte, which includes 10–25 % of the total amount of vWF antigen present in plasma. However, analysis of reduced vWF purified from human megakaryocytes has demonstrated a subunit composition comparable to that of endothelial cell-derived vWF. Studies have also shown that platelet vWF is placed within the α-granules, as detected by immune-fluorescence study [28], which can be released upon platelet activation. Nevertheless the storage of α-granules may be provided via both biosynthesis at the megakaryocyte and uptake of plasma proteins. This may occur at the megakaryocyte and/or platelet stage [29]. Release of platelet vWF is prompted by a variety of agonists, including ADP, collagen and thrombin [30].

NETosis, an innate immune response mediated by neutrophils, has recently been recognized to immobilize microorganisms inside the vasculature in infectious and non-infectious diseases. In fact, NETs are extracellular DNA fibers comprising histones and neutrophil antimicrobial proteins, which might be detected by some markers. Since the markers of extracellular DNA traps were abundantly found in deep vein thrombosis (DVT) [38, 39], NETs were introduced as an exceptional link between inflammation and thrombosis, providing a stimulus and scaffold for thrombus formation. Further investigations have shown co-localization of the DNA network and vWF, indicating the involvement of NETosis mechanism in thrombus formation [40, 41]. A recent study revealed that vWF can bind and immobilize extracellular DNA released from leukocytes. Therefore, vWF might act as a linker for leukocyte adhesion to endothelial cells, but DNA–vWF binding does not affect vWF degradation by ADAMTS13. In contrast, DNA–vWF complexes diminish platelet binding to vWF [42]. These findings, however, have revealed a new feature of vWF biology, introducing vWF and NETs as prospective targets for the development of novel therapeutics for the treatment of venous thrombosis (Fig. 5).

Plasma vWF levels were determined in more than a thousand blood donors. The results showed that ABO blood group had a significant influence on vWF values, as investigated by quantitative immunoelectrophoresis. The findings also showed that the individuals with blood group O had the lowest mean vWF level than those of non O [43]. VWF antigen levels also showed significant differences between O heterozygotes and non-OO homozygotes [22, 44]. VWF and blood group, however, were related to thrombosis risk, which could be mediated through factor VIII [45].

The results of multiple regression analysis revealed that age was significantly correlated with vWF levels in each blood group [43]. In addition, G allele, which is linked with higher levels of vWF antigen, is more marked after the age of 40, as previously mentioned. Later studies also confirmed that vWF was increased with age increase [54]. In contrast, plasma levels of ADAMTS13 were reduced by aging in humans. This might indicate an increasing chance of thrombotic events, although the mechanism responsible for this is not accurately clarified [22, 55].

Since the active large vWF multimers were found to be degraded to smaller forms with less activity, Furlen et al. could reveal a proteolytic activity associated with a high molecular weight protein [56]. The weight of this protein, however, was about 300 kD. Purified vWF was treated with protease. Subsequently, the size, amino acid composition, and amino terminal sequence of the smaller fragments indicated that the cleavage of vWF by the plasma protease was dependent on its conformation and needed calcium ion [57, 58]. Eventually, a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13) was defined to mediate the breakdown of vWF. This takes place by the cleavage between tyrosine at position 842 and methionine at position 843 [56, 59]. Decreased vWF-cleaving activity has been reported in a wide range of conditions, including thrombotic thrombocytopenic purpura (TTP), idiopathic thrombocytopenic purpura (ITP), liver cirrhosis, chronic uremia, disseminated intravascular coagulation (DIC), systemic lupus erythematosus (SLE), leukemia, pregnancy, postoperative state, neonatal period, and ageing. Unconjugated bilirubin also inhibited proteolytic cleavage of vWF by ADAMTS13, as it was reported by a recent study [60]. This metalloprotease stretches the vWF A2 domain so that the cleavage site becomes available to hydrolyze the peptide bond within the A2 domain of vWF. Cleavage by ADAMTS13 facilitates vWF release into blood circulation [61]. Next cleavage by ADAMTS13 takes place after vWF release into the blood stream [62], where the elongated conformation occurs. This conformation not only exposes the binding site for platelets in the vWF A1 domain but also expands the vWF A2 domain. Consequently the cleavage site becomes accessible for ADAMTS13. The cleavage site, however, is bordered by the regions of vWF that bind platelet glycoprotein 1b in the A1 domain and collagen [63–65], resulting in two fragments of 176 and 140 kDa. ADAMTS13 cleavage site in vWF is enclosed by two N-linked glycosylation sites (asparagine 752 and asparagine 811) and five O-linked glycosylation sites (threonine 705, 714, 724 and 916, and serine 723). The presence of O, A or B blood groups bordering the site might be the reason for altered values of vWF in various ABO blood groups. It is suggested that this neighboring may influence proteolysis. However, individuals with the lowest levels of ADAMTS13 have shown twice the risk of ischemic stroke compared with individuals with the highest levels of ADAMTS13 [21]. In addition, reduced ADAMTS13 activity significantly exacerbates ischemic brain injury in murine stroke models [66]. Temporary appearance of ultra-large vWF (ULvWF) multimers is evident in the normal plasma of patients after therapeutic agent desmopressin (1-desamino-8-D-arginine vasopressin or DDAVP). ULvWF multimers are also present in the pathological state of diseases such as thrombotic thrombocytopenic purpura (TTP) in which the deficient ADAMTS-13 is a hallmark for the disease [67]. However, the relationship of both vWF and ADAMTS13 with arterial thrombosis has been reviewed. The most studies, however, have shown an association between high vWF levels and arterial thrombosis. Furthermore, a hydrophobic pocket in the Cys-rich domain of ADAMTS13 was recognized, which may interact with hydrophobic pocket in the A2 domain of vWF for its cleavage [68]. Although the infusion of recombinant human ADAMTS13 reduced the infarct size in mice [69], whether ADAMTS13 is a pivotal independent risk factor or not remains unclear.

Thrombotic thrombocytopenic purpura (TTP) is a disorder of systemic platelet aggregation and a life-threatening microangiopathic syndrome. The underlying pathophysiology of TTP is an extensive presence of platelet and vWF-rich microthrombi. However in hemolytic uremic syndrome (HUS) pathophysiology, microthrombi consist of platelet and fibrin, while in other thrombotic micro-angiopathies (TMAs), varying amounts of infiltrated lymphocytic or neutrophil are interconnected to the microthrombi [87]. Familial TTP/HUS is not common and usually occurs in the immediate postnatal period or infancy, but there are reported cases of delayed onset from the second to third decade of life, too. Nevertheless, more frequently TTP is either idiopathic or secondary to a variety of conditions.