Inhibition of disulfide bonding of von Willebrand protein by monensin results in small, functionally defective multimers

The significant role of the Golgi apparatus in cardiovascular diseases

The Golgi apparatus (GA) is a ribbon-like system of stacks which consist of multiple closely apposed flattened cisternae and vesicles usually localized in the juxta-nuclear area. As for the biological functions, the GA plays a major role in protein biosynthesis, post-translational modification, and sorting protein from ER to plasma membrane and other destinations. Structural changes and functional disorder of the GA is associated with various diseases. Moreover, increasing evidence revealed that swelling, poor development, and other morphological alterations of the GA are linked to cardiovascular diseases such as heart failure (HF), arrhythmia, and dilated cardiomyopathy. Furthermore, dysfunction of the GA is also related to cardiovascular diseases since the GA is extremely responsible for transport, glycosylation, biosynthesis, and subcellular distribution of cardiovascular proteins. This review gives a brief overview of the intricate relationship between the GA and cardiovascular diseases. In addition, we provide a further prospective that the GA may provide diagnosis reference for cardiovascular diseases, and changes in the ultrastructure and morphology of the GA such as swelling, poor development, and fragmentation may serve as a reliable index for cardiovascular diseases.

Gut microbiota regulate hepatic von Willebrand factor synthesis and arterial thrombus formation via Toll-like receptor-2

The symbiotic gut microbiota play pivotal roles in host physiology and the development of cardiovascular diseases, but the microbiota-triggered pattern recognition signaling mechanisms that impact thrombosis are poorly defined. In this article, we show that germ-free (GF) and Toll-like receptor-2 (Tlr2)-deficient mice have reduced thrombus growth after carotid artery injury relative to conventionally raised controls. GF Tlr2^-/- and wild-type (WT) mice were indistinguishable, but colonization with microbiota restored a significant difference in thrombus growth between the genotypes. We identify reduced plasma levels of von Willebrand factor (VWF) and reduced VWF synthesis, specifically in hepatic endothelial cells, as a critical factor that is regulated by gut microbiota and determines thrombus growth in Tlr2^-/- mice. Static platelet aggregate formation on extracellular matrix was similarly reduced in GF WT, Tlr2^-/- , and heterozygous Vwf^+/- mice that are all characterized by a modest reduction in plasma VWF levels. Defective platelet matrix interaction can be restored by exposure to WT plasma or to purified VWF depending on the VWF integrin binding site. Moreover, administration of VWF rescues defective thrombus growth in Tlr2^-/- mice in vivo. These experiments delineate an unexpected pathway in which microbiota-triggered TLR2 signaling alters the synthesis of proadhesive VWF by the liver endothelium and favors platelet integrin-dependent thrombus growth.

Autophagy regulates endothelial cell processing, maturation and secretion of von Willebrand factor

Endothelial secretion of von Willebrand factor (VWF) from intracellular organelles known as Weibel-Palade bodies (WPBs) is required for platelet adhesion to the injured vessel wall. Here we demonstrate that WPBs are often found near or within autophagosomes and that endothelial autophagosomes contain abundant VWF protein. Pharmacological inhibitors of autophagy or knockdown of the essential autophagy genes Atg5 or Atg7 inhibits the in vitro secretion of VWF. Furthermore, although mice with endothelial-specific deletion of Atg7 have normal vessel architecture and capillary density, they exhibit impaired epinephrine-stimulated VWF release, reduced levels of high-molecular weight VWF multimers and a corresponding prolongation of bleeding times. Endothelial-specific deletion of Atg5 or pharmacological inhibition of autophagic flux results in a similar in vivo alteration of hemostasis. Thus, autophagy regulates endothelial VWF secretion, and transient pharmacological inhibition of autophagic flux may be a useful strategy to prevent thrombotic events.

Assembly of Weibel-Palade body-like tubules from N-terminal domains of von Willebrand factor

Endothelial cells assemble von Willebrand factor (VWF) multimers into ordered tubules within storage organelles called Weibel-Palade bodies, and tubular packing is necessary for the secretion of VWF filaments that can bind connective tissue and recruit platelets to sites of vascular injury. We now have recreated VWF tubule assembly in vitro, starting with only pure VWF propeptide (domains D1D2) and disulfide-linked dimers of adjacent N-terminal D'D3 domains. Assembly requires low pH and calcium ions and is reversed at neutral pH. Quick-freeze deep-etch electron microscopy and three-dimensional reconstruction of negatively stained images show that tubules contain a repeating unit of one D'D3 dimer and two propeptides arranged in a right-handed helix with 4.2 units per turn. The symmetry and location of interdomain contacts suggest that decreasing pH along the secretory pathway coordinates the disulfide-linked assembly of VWF multimers with their tubular packaging.

The Physiological Function of von Willebrand's Factor Depends on Its Tubular Storage in Endothelial Weibel-Palade Bodies

Weibel-Palade bodies are the 1-5 microm long rod-shaped storage organelles of endothelial cells. We have investigated the determinants and functional significance of this shape. We find that the folding of the hemostatic protein von Willebrand's factor (VWF) into tubules underpins the rod-like shape of Weibel-Palade bodies. Further, while the propeptide and the N-terminal domains of mature VWF are sufficient to form tubules, their maintenance relies on a pH-dependent interaction between the two. We show that the tubular conformation of VWF is essential for a rapid unfurling of 100 microm long, platelet-catching VWF filaments when exposed to neutral pH after exocytosis in cell culture and in living blood vessels. If tubules are disassembled prior to exocytosis, then short or tangled filaments are released and platelet recruitment is reduced. Thus, a 100-fold compaction of VWF into tubules determines the unique shape of Weibel-Palade bodies and is critical to this protein's hemostatic function.

A Covalent Oxidoreductase Intermediate in Propeptide-dependent von Willebrand Factor Multimerization

The assembly of von Willebrand factor multimers in the Golgi apparatus requires D1D2 domains of the von Willebrand factor propeptide, which may act as an oxidoreductase to promote disulfide bond formation or rearrangement between two D3 domains in the mature subunit. This mechanism predicts that the propeptide should form a transient intrachain disulfide bond with the D3 domain before multimerization. Such an intermediate was detected using truncated subunits that simplify the analysis of the multimerization process. When only the D1D2D'D3 region of von Willebrand factor was expressed in baby hamster kidney cells, the propeptide and D'D3 formed an intrachain disulfide-linked species in the endoplasmic reticulum that could be identified by two-dimensional gel electrophoresis after cleavage with thrombin or furin. This intermediate rearranged in the Golgi to form free propeptide and D'D3 dimers that were secreted. A similar intracellular disulfide-linked species was identified in cells expressing the propeptide and D'D3 as separate proteins and in cells expressing full-length von Willebrand factor. These results support a model in which the propeptide acts as an oxidoreductase to promote von Willebrand factor multimerization in the Golgi apparatus.

How to roll an endothelial cigar: the biogenesis of Weibel-Palade bodies

Weibel-Palade bodies (WPB) are the regulated secretory organelles of endothelial cells. These cigar-shaped membrane-bound structures function in both hemostasis and inflammation but their biogenesis is poorly understood. Here, we review what is currently known about their formation. The content of WPBs is dominated by the hemostatic factor von Willebrand factor (VWF), whose complex biogenesis ends in the formation of high molecular weight multimers. VWF is also organized into proteinaceous tubules which underlie the striated interior of WPBs as seen in the EM. VWF expression is necessary for formation of WPBs, and its heterologous expression can even lead to the specific recruitment of WPB membrane proteins, including the leukocyte receptor P-selectin, the tetraspanin CD63, and Rab27a. Unusually, the VWF propeptide is implicated in the biogenesis of WPBs, being essential for formation of the storage compartment. The elongation of the cigars and the formation of the tubules are determined by non-covalent interactions between pro- and mature VWF proteins. Surprisingly, high molecular weight multimers seem neither necessary nor sufficient to trigger formation of a storage compartment, and do not seem to have any role in WPB biogenesis. Von Willebrand's disease, usually caused by mutations within VWF, has provided many of the insights into the way in which VWF drives the formation of these organelles.

Dithiothreitol prevents membrane fusion but not centrosome or microtubule organization during the first cell cycles in sea urchins

Dithiothreitol (DTT), a disulfide reducing agent, inhibits the fusion of male and female pronuclei within the activated cytoplasm of sea urchin eggs. The migrations of the pronuclei are not affected by DTT, indicating that microtubule function is not impaired. Centrosomal antigens are detected in the sperm aster and in all subsequent microtubule-based configurations. Nuclear membranes never fuse and the chromatin of male and female pronuclei never mix in the DTT-treated cells. During prophase, when nuclear envelopes break down to undergo mitosis, both sets of chromosomes undergo condensation cycles independent from each other. Both pronuclei initially stain for centrosomal material and surrounding microtubules. With time, the female's centrosomal material as well as the microtubules disappear while the male forms a bipolar spindle. Interestingly, one pole of the paternal mitotic apparatus communicates with the separate maternal chromatin, forming a half spindle which moves the egg-derived chromatin towards its pole. At the time for cell division, the individual karyomeres are not able to fuse their nuclear membranes to reconstitute the blastomere nuclei. When DTT is applied at prometaphase of the first cell cycle, the chromosome cycle continues until next metaphase. Centrosomes also continue their cycle and undergo somewhat atypical splitting during the time for second telophase. Division furrows are initiated but aborted. These results support the hypothesis that disulfide groups are required for membrane fusion of the pronuclei, for membrane fusion of the karyomeres, and for the completion of the division furrow to achieve successful cell division.

The effects of monensin on blood-borne arrest and glycosylation of BL/VL3 lymphoma cells

We have previously shown that inhibitors of N-glycan processing alter both the cell surface carbohydrates and the homing properties in lymphoid cells. We have now studied the effects of the ionophore monensin (MON) on these parameters. Arrest in the spleen of [111In]-labelled BL/VL3 murine T lymphoma cells, injected intravenously was clearly reduced if the cells had been cultured for 24 h in the presence of monensin (0.1-1.0 microgram ml-1). We have characterized glycopeptides from BL/VL3 murine T lymphoma cells. Following labelling with tritiated precursors (fucose, mannose, galactose, glucosamine), surface glycopeptides from BL/VL3 murine T lymphoma cells, were released by trypsin and separated by gel filtration on Bio-Gel P6 and by affinity chromatography on immobilized lectins. After treatment with MON, a class of high molecular mass glycopeptides was no longer found. There were less complex and more high mannose glycans, as a consequence of a reduction of terminal glycosylation (sialylation, fucosylation or incorporation of N-acetyl-glucosamine). Similar findings were obtained with immunoprecipitated Thy-1 antigen. However, as estimated by flow cytometry analysis, the cell surface expression of Thy-1 was not reduced in MON-treated cells. Taken together our results show that cell surface oligosaccharides are modified dramatically, but that at least, certain cell surface antigens are present in normal amounts. It is tempting to speculate that changes in glycosylation account for the abnormal homing properties of MON-treated cells.

von Willebrand factor and platelet function

vWF is an adhesive protein that binds to two distinct platelet glycoproteins, GP Ib and GP IIb-IIa complex. Its interaction with GP Ib is primarily responsible for platelet adhesion to the subendothelium. The current model is that vWF binds to collagen and/or another component of the subendothelium, after which a conformational change in the vWF molecule exposes the GP Ib binding site. This interaction may not only promote the initial attachment of platelets to the subendothelium but also play a role in thrombus formation through exposure of GP IIb-IIIa to which vWF and fibrinogen can bind. The second important function of vWF is to be a carrier for F. VIII, protecting it from degradation and playing a role in its activation by thrombin. Circulating vWF has a complex multimeric structure that ranges in Mrs from 0.5 to 20 x 10(6) Daltons. The basic subunit has a Mr of 270 kDa. Amino acid sequencing of vWF demonstrated that the basic subunit or mature vWF is made up of 2050 amino acids. Molecular cloning of the vWF cDNA revealed that the primary transcript consists of 8900 base pairs that encode for 2813 amino acids, including a 22 amino acid signal peptide and a propolypeptide of 741 amino acids, called vWF antigen II. Recent studies on the expression of recombinant vWF molecules indicate that the propolypeptide is involved in the multimerization of vWF. The domains on the vWF molecule involved in the interactions of vWF with GP Ib, GP IIb-IIIa, collagen, F. VIII and heparin have been localized to varying extents. It is anticipated that peptide analysis and recombinant DNA techniques, such as in vitro mutagenesis, will further define the structural requirements of these binding domains. vWF is synthesized in a cell-specific manner by endothelial cells and megakaryocytes. It undergoes a complex intracellular biosynthesis involving transcription of a 200 kb gene, splicing out more than 42 introns, translation of a 8900 bp mRNA, glycosylation, disulphide bond formation, sulphatation, multimerization and proteolytic cleavage. The molecule can be secreted in a constitutive or regulated manner upon perturbation of the endothelial cells with physiological and non-physiological secretagogues. The mechanisms that control the synthesis of vWF should be an exciting area of further research. vWD is probably the most common of all congenital disorders of haemostasis. It is an extremely heterogeneous syndrome involving quantitative or qualitative disorders of vWF.(ABSTRACT TRUNCATED AT 400 WORDS)

Anti-invasive activities of experimental chemotherapeutic agents

We have discussed a number of agents that affect invasion and we have grouped them according to their most probable targets. This strategy is based on the following hypothesis. Invasion is the result of cellular responses to extracellular signals. Candidate signals are components of the extracellular matrix, which are rendered inactive by the flavonoid (+)-catechin (see Section III). Signals are recognized by receptors on the plasma membrane, possibly glycoproteins, that may lose their recognition function through alteration of the oligosaccharide side chains by inhibitors of protein glycosylation (see Section IV) and possibly also by alkyllysophospholipids (see Section V). Synthetic oligopeptides reflecting sequences from cell-binding domains of extracellular matrix molecules are also effective tools for blocking specific receptors (see Section VI). GTP-binding proteins (G proteins) act as signal transducers and can be inactivated by pertussis toxin (see Section VII). An intriguing aspect of both alkyllysophospholipids and pertussis toxin is that they can either inhibit the invasion of constitutively invasive cells or induce invasion of constitutively noninvasive cells. Without doubt, cellular responses implicated in invasion are many-fold. Discussed here are cell motility and directional migration with inhibition through dipyridamole and its analogs and through microtubule inhibitors, respectively (see Section VIII). Alternative hypotheses and alternative strategies for the dissection of the invasion process do exist, and alternative cellular and molecular mechanisms of action may explain the anti-invasive activity of the agents discussed earlier. The latter are mentioned in each section. It is the authors' opinion that the possibilities for exploiting the battery of anti-invasive agents have by no means been exhausted. Introducing researchers to experiments that may lead to an understanding of the mechanisms of invasion and metastasis and to new rationales for cancer treatment has been the purpose of our review.

Genetic linkage of two intragenic restriction fragment length polymorphisms with von Willebrand's disease type IIA. Evidence for a defect in the von Willebrand factor gene

Restriction fragment length polymorphisms (RFLPs), using the enzymes Bgl II and Xba I in conjunction with human von Willebrand factor (vWF) cDNA probes, have been described previously. In the present study we demonstrate the localization of both genetic markers within the vWF gene. The RFLPs were used to study the segregation of alleles associated with von Willebrand's disease (vWD) type IIA in a comprehensive, affected family. Individuals of this family were tested for their bleeding time and their plasma was analyzed for vWF antigen concentration and vWF ristocetin-cofactor activity. Based on these data, the affected members were diagnosed as vWD type-IIA patients; this conclusion was confirmed by the analysis of the multimeric vWF pattern of some of the patients. It was demonstrated that both RFLPs are completely linked with the vWD type-IIA trait. From this finding, we conclude that the defect that causes the vWD type IIA is most likely due to a mutation in the vWF gene and not to a mutation in a gene involved in posttranslational processing of the vWF protein.

Endothelial cell processing of von Willebrand proteins

The endothelial regulation of vWF synthesis, storage, regulated and constitutive release, and now binding are the focus of numerous research laboratories. The understanding of these phenomena is important to the pathophysiology of local hemostasis. Although the structural understanding of the relationship of vW AgII to vWF is now known, its functional role in synthesis, release, or function is yet to be clearly defined.

Perturbation of cultured human vascular endothelial cells by phorbol ester or thrombin alters the cellular von Willebrand factor distribution

We have studied the influence of perturbation of cultured human umbilical vein endothelial cells on the distribution of the von Willebrand factor. As shown previously, short-term (less than 1 hr) treatment of endothelial cells with the phorbol ester 4 beta-phorbol 12-myristate 13-acetate (PMA) or thrombin resulted in the release of cellular stored von Willebrand factor. Long-term treatment with PMA or thrombin evoked a distinct change in the endothelial cell distribution of von Willebrand factor, evident 24 to 48 hrs after exposure. Whereas the contents of the von Willebrand factor storage sites in the cells were gradually restored within 48 hrs, enhanced amounts of von Willebrand factor were secreted into the medium. However, PMA did not increase the endothelial cell contents of mRNA encoding for von Willebrand factor. The number as well as the size of von Willebrand factor storage granules in the endothelial cells increased after exposure to the phorbol ester, as determined by immunofluorescence microscopy. A second treatment with PMA or thrombin, 48 hrs after cells had been stimulated with these agents, resulted again in the instantaneous release of von Willebrand factor. PMA and thrombin caused a decrease in the von Willebrand factor contents of the extracellular matrix. Pulse-chase experiments revealed that PMA blocked the deposition of von Willebrand factor in the subendothelium, whereas PMA did not affect the degradation of matrix von Willebrand factor. Thus, perturbation of endothelial cells changes the cellular distribution of von Willebrand factor.

Expression of abnormal von Willebrand factor by endothelial cells from a patient with type IIA von Willebrand disease

Studies were conducted to characterize the biosynthesis of von Willebrand factor (vWf) by cultured endothelial cells (EC) derived from the umbilical vein of a patient with type IIA von Willebrand disease. The patient's EC, compared with those from normal individuals, produced vWf that had decreased amounts of large multimers and an increase in rapidly migrating satellite species, features characteristic of plasma vWf from patients with type IIA von Willebrand disease. The type IIA EC did produce a full spectrum of vWf multimers in both cell lysates and postculture medium, although the relative amounts of the largest species were decreased. The large multimers were degraded in conjunction with the appearance of rapidly migrating satellites that contained approximately equal to 170-kDa proteolytic fragments, suggesting that this patient's functional defect is due to abnormal proteolysis and not to a primary failure of vWf subunit oligomerization. Moreover, the observed degradation appears to result from an abnormal vWf molecule and not elevated protease levels. These results suggest that this patient's von Willebrand disease phenotype is caused by increased proteolytic sensitivity of his vWf protein.

Initial glycosylation and acidic pH in the Golgi apparatus are required for multimerization of von Willebrand factor

Two conditions were identified that interfered with the complex polymerization process in biosynthesis of von Willebrand factor (vWf). Treatment of human umbilical vein endothelial cells with tunicamycin inhibited N-linked glycosylation of nascent vWf and the resulting pro-vWf monomers failed to dimerize. The single subunits accumulated in the endoplasmic reticulum and were neither processed further nor secreted. In the presence of a weak base (ammonium chloride or chloroquine), interdimer disulfide bond formation was inhibited in a dose-dependent manner. This process appeared therefore to be pH sensitive and likely to be initiated in the acidic trans-Golgi apparatus (Anderson, R. G. W., and R. K. Pathak, 1985, Cell, 40: 635-643). The weak base had no obvious effect on the other processing steps, i.e. dimerization, complex carbohydrate formation and sulfation, and produced only slight inhibition of prosequence cleavage. On the other hand, the weak base interfered with the targeting of newly synthesized vWf into Weibel-Palade bodies, with all of the vWf being secreted constitutively and none stored in the Weibel-Palade bodies. In summary, initial glycosylation of the nascent vWf protein and low pH in the trans-Golgi apparatus were important conditions for the successful polymerization of human vWf. Genetic defects disrupting any one of these conditions could result in the phenotype of von Willebrand disease.