The high molecular weight form of endothelial cell von Willebrand factor is released by the regulated pathway

Abstract

We have previously reported that two forms of von Willebrand factor (vWf) exist in cultured human umbilical vein endothelial cells: a high molecular weight (HMW) form that is released and can be proteolytically cleaved into a series of plasma-like multimers, and a non-secreted low molecular weight (LMW) form. In this study, the mode of vWf release and the relationship between the two forms were examined. As determined by two-dimensional analysis as well as by immunoreactivity with an antibody to the propolypeptide, the LMW form of endothelial cell vWf consisted of a 260 kD pro-vWf polypeptide, while the HMW form consisted of a 225 kD mature polypeptide. Only the 260 kD polypeptide was susceptible to digestion with endoglycosidase H. Release of the HMW form into the culture media was accompanied by a decrease in cellular vWf. Treatment of endothelial cells with cycloheximide or tunicamycin caused a decrease in the LMW form but did not affect the secretion of the HMW form. These results suggest that two pools of vWf exist in endothelial cells--a LMW form of pro-vWf in the endoplasmic reticulum and a HMW form of mature vWf in the storage compartment. Released vWf derives only from the storage pool.

von Willebrand factor multimerization and the polarity of secretory pathways in endothelial cells

“…6 WPBs store VWF for regulated secretion upon endothelial activation, but they can also fuse and release VWF in the absence of activation, a process termed "basal" secretion to distinguish it from "constitutive" secretion, 7,8 although both collect together in the supernatant of unstimulated cells. Low-molecularweight-VWF (LMW-VWF) is secreted by the constitutive pathway, in contrast to the HMW-VWF released via stimulated WPBs, 6,9 consistent with the need to control release of highly prothrombotic material.…”

High-pressure freezing provides insights into Weibel-Palade body biogenesis

“…Possibly these precursors will never form mature WPBs but are destined for constitutive release as low molecular weight multimers, or for degradation. Indeed, it has been reported that between 90% (Tsai et al, 1991) and 10% (Sporn et al, 1986) of VWF is secreted constitutively, and that such material is of lower molecular weight than that released from WPBs by regulated exocytosis. Subunits may, alternatively, be able to organise themselves into tubules after they have left the Golgi.…”

VWF secretion: what's in a name?

“…1 Tregs appear to be a relatively rare CD4 ϩ T-cell subset that comprise many subpopulations, including IL-10 -producing "Tr1Ј cells10, TGF-␤-producing T helper type 3 cells, CD8 ϩ T suppressor cells, natural killer T cells, CD4-CD8 -T cells and ␥␦ T cells." [2][3][4][5] Some of these cells originate in the thymus during ontogeny and are referred to as "natural" Tregs. Some Tregs can also be induced from naive T cells in the periphery.…”

“…They are critical to our survival, and absence of them can lead to either autoimmunity or inflammatory disorders, with fatal consequences in both mice and humans. [1][2][3][4][5] Although defects of these cell types have previously been described in immune thrombocytopenic purpura (ITP), 7-9 3 papers in this issue collectively suggest that a deficient Treg compartment allows enhanced T-cell and B-cell autoreactivity in ITP, and that therapies like rituximab actually correct the deficiency and reverse autoimmune platelet reactivity, thus leading to increased platelet counts.…”