2.2.2. Regulated vWF secretion

2.2. Basal versus regulated vWF secretion

150 Endothelial Dysfunction - Old Concepts and New Challenges

upon vascular injury [51, 52];

WPBs [7];

2.2.1. Basal vWF secretion

highly multimeric vWF in plasma.

Secretion of vWF occurs by one of the following three modes:

1 regulated secretion from WPBs, in response to a specific agonist (secretagogue) on-demand

2 basal (constitutive-like or continuous) secretagogue-independent secretion of vWF from

3 constitutive, secretagogue-independent release of vWF from non-WPB compartments [34, 40]. Over the years, there have been in the field opposite views about how much vWF is released through each of these three pathways and what determines the balance between basal

In resting ECs, secretion of vWF was thought to take place predominantly via conventional constitutive vWF secretion which is susceptible to protein synthesis inhibitor cycloheximide [34]. However, the importance of this pathway was exaggerated initially, as it could be drawn from the more recently published studies listed below. A vWF multimeric analysis in ECs challenged this view, indicating that highly multimeric vWF might be released solely from WPB storage pool [53]. Most recently, the issue was revisited when a study using metabolic labeling of vWF in human umbilical vein ECs (HUVECs) showed that most of the vWF secreted from unstimulated cells is not mediated by conventional constitutive secretion, but rather by basal release from the WPBs [7, 55]. Giblin and colleagues [7] recently brought more evidence to dismiss the belief that multimeric vWF passes uninterrupted from the Golgi to plasma membrane. They showed that freshly synthetized vWF is held for at least 24 h in WPBs before unstimulated release occurred, escaping cycloheximide-induced inhibition of protein synthesis, and, thus, consistent with release from the storage pool [7]. Similarly, Romani de Wit and coworkers [56] studied the dynamics of GFP-vWF-positive WPBs in resting HUVECs over time, and they were able to capture multiple image frames of WPBs traveling long distances toward the cell periphery. Intriguingly, resting ECs displayed membrane-apposed accumulation of GFP-vWF-containing vesicles [33, 56, 57]. It is worth mentioning here that only whenever a component of the cytoplasmic machinery is ablated the constitutive vWF secretion becomes predominant [40]. Taken together, these studies of WPBs behavior suggest that only the multimeric form of vWF is stored and that ECs secrete multimeric vWF via a continuous secretory pathway, which is a source of vWF in the bloodstream. If the WPB storage compartment is the source of the continuously secreted form of vWF that travels in the bloodstream, then the ability to regulate this constitutive vWF secretory route has significant clinical implications. Disruption of continuous vWF secretion may be implicated in the pathophysiology of type 2 vWD, which is due to both reduced vWF-dependent platelet function and reduced

(constitutive-like) and regulated vWF secretion is still under debate [34, 53, 54].

The release of vWF from the regulated pathway occurs only following stimulation of ECs with an appropriate agonist, providing the endothelium with the means to react to its microenvironment by finely tuning the rate of release.

The notion that blood collected under stress clots faster was known for a while. In the 1970s, it was found that desmopressin (a synthetic analog of vassopresin) can elevate vWF level twofolds in healthy individuals [58] The follow-up studies showed that desmopressin activates vasopressin receptor 2 (V2R) on ECs and triggers cyclic adenosine monophosphate (cAMP)-mediated release of vWF from EC WPBs [58]. A multitude of agonists have been reported to induce vWF release via regulated secretion. These include thrombin [51], calcium ionophores [34], hypoxia [59], vasopressin [58], histamine [14], complement [60], platelet-activating factor [61] and interleukins [62]. Importantly, high shear stress is modulating regulated vWF release [63].
