Endothelial Cell Biology: Membrane traffic, Inflammation and Haemostasis
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 by exocytosis of WPB. 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. We recently discovered that CD63, of previously unknown function, clusters the P-selectin into macromolecular assemblies that are essential to its function. This initial recruitment leukocytes leads to rolling which eventually progresses to their passage across the endothelial barrier into the damaged tissue.
The major content protein of WPB is von Willebrands factor (VWF), an adhesive glycoprotein that plays a major role in primary haemostasis. This very large highly multimerised protein is stored within the WPB coiled up as proteinacious tubules 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, mutations in VWF can affect the formation of the protein tubules, leading to formation of damaged, poorly functional WPB.
Recent advances in microscopy and in bioinformatics allowed us to measure the length of millions of WPB. This revealed that their size increases in half-micron steps. These in turn arise from the assembly of the WPB from 500nm subunits. We discovered that the subunits are brought together in the trans-Golgi where they assemble into the wide distribution of lengths that we find in human cells. Finally, we can then manipulate the length of WPB and show that short WPB are much less efficient at supporting platelet recruitment, and thus initiating primary haemostasis.
Organelle sizing by Golgi architecture: Weibel-Palade bodies (in green) are cigar-shaped endothelial specific secretory granules with a central role in hemostasis and inflammation.The architectural status of the Golgi apparatus (red, GM130, and blue, TGN46) determines the size of the WPBs produced. The vertebrate ribbon format permits the generation of large WPBs (center of the figure); its unlinking into separate units, the ministacks (disposed in a circle), results in small organelles being made.