Endothelial cells contain a storage organelle, Weibel-Palade Bodies (WPB), which contains key components of both inflammatory and haemostatic processes. As a rapid-response system, the WPB are of critical importance in initiating haemostatic and inflammatory responses within the rapidly-changing vascular environment.
For example, the leukocyte receptor P-selectin is found within the WPB membrane. A key event in the initiation of inflammation is the appearance of P-selectin on the apical surface of endothelial cells. Once exposed to the blood via exocytosis of the WPB, P-selectin initiates leukocyte recruitment by binding P-selectin glycoprotein ligand (PSGL)-1 on the leukocyte surface. This initial recruitment leads to leukocyte rolling which eventually progresses to their passage across the endothelial barrier into the damaged tissue.
Equally, the major content protein of WPB is von Willebrands factor (VWF), an adhesive glycoprotein that plays a major role in primary haemostasis. Mutations within VWF are the commonest cause of the inherited bleeding disorder Von Willebrands Disease. In addition to mutations causing a loss of platelet binding or other direct haemostatic problems, the presence of wild-type VWF is necessary to the formation of WPB. Mutations in VWF can therefore also lead to formation of damaged, poorly functional WPB. By expressing mutated VWF or human clinical variants that cause Von Willebrands disease, we can analyse the features of this very large highly multimerised protein that lead to its storage within the WPB as proteinacious tubules. We have recently shown that these tubules, which cause the characteristic cigar-shape of the WPB, are also essential to VWF's haemostatic function.
While the role of cargo such as VWF is necessary to drive the formation of WPB, cargo alone is not sufficient. We are also identifying the cellular machinery that underpins their formation. We have shown that a clathrin coat is also needed for WPB formation, and that this coat is recruited by the heterotetrameric adaptor AP-1, operating both to recruit the clathrin which acts as a scaffold during formation, but also to recruit other effectors that target the WPB into the regulated secretory pathway. Further, the small GTPase Rab27a acts as an exocytic brake to allow for complete maturation before exocytosis, preventing premature release of sub-functional contents.