Structural organization of Weibel-Palade bodies revealed by cryo-EM of vitrified endothelial cells

Human Umbilical Vein Endothelial Cells as a Versatile Cellular Model System in Diverse Experimental Paradigms: An Ultrastructural Perspective

Human umbilical vein endothelial cells (HUVECs) are primary cells isolated from the vein of an umbilical cord, extensively used in cardiovascular studies and medical research. These cells, retaining the characteristics of endothelial cells in vivo, serve as a valuable cellular model system for understanding vascular biology, endothelial dysfunction, pathophysiology of diseases such as atherosclerosis, and responses to different drugs or treatments. Transmission electron microscopy (TEM) has been a cornerstone in revealing the detailed architecture of multiple cellular model systems including HUVECs, allowing researchers to visualize subcellular organelles, membrane structures, and cytoskeletal elements. Among them, the endoplasmic reticulum, Golgi apparatus, mitochondria, and nucleus can be meticulously examined to recognize alterations indicative of cellular responses to various stimuli. Importantly, Weibel-Palade bodies are characteristic secretory organelles found in HUVECs, which can be easily distinguished in the TEM. These distinctive structures also dynamically react to different factors through regulated exocytosis, resulting in complete or selective release of their contents. This detailed review summarizes the ultrastructural features of HUVECs and highlights the utility of TEM as a pivotal tool for analyzing HUVECs in diverse research frameworks, contributing valuable insights into the comprehension of HUVEC behavior and enriching our knowledge into the complexity of vascular biology.

A new look at an old body: molecular determinants of Weibel-Palade body composition and von Willebrand factor exocytosis

Endothelial cells, forming a monolayer along blood vessels, intricately regulate vascular hemostasis, inflammatory responses, and angiogenesis. A key determinant of these functions is the controlled secretion of Weibel-Palade bodies (WPBs), which are specialized endothelial storage organelles housing a presynthesized pool of the hemostatic protein von Willebrand factor and various other hemostatic, inflammatory, angiogenic, and vasoactive mediators. This review delves into recent mechanistic insights into WPB biology, including the biogenesis that results in their unique morphology, the acquisition of intraluminal vesicles and other cargo, and the contribution of proton pumps to organelle acidification. Additionally, in light of a number of proteomic approaches to unravel the regulatory networks that control WPB formation and secretion, we provide a comprehensive overview of the WPB exocytotic machinery, including their molecular and cellular mechanisms.

Quantitative super-resolution imaging of platelet degranulation reveals differential release of von Willebrand factor and von Willebrand factor propeptide from alpha-granules

BACKGROUND

Von Willebrand factor (VWF) and VWF propeptide (VWFpp) are stored in eccentric nanodomains within platelet alpha-granules. VWF and VWFpp can undergo differential secretion following Weibel-Palade body exocytosis in endothelial cells; however, it is unclear if the same process occurs during platelet alpha-granule exocytosis. Using a high-throughput 3-dimensional super-resolution imaging workflow for quantification of individual platelet alpha-granule cargo, we studied alpha-granule cargo release in response to different physiological stimuli.

OBJECTIVES

To investigate how VWF and VWFpp are released from alpha-granules in response to physiological stimuli.

METHODS

Platelets were activated with protease-activated receptor 1 (PAR-1) activating peptide (PAR-1 ap) or collagen-related peptide (CRP-XL). Alpha-tubulin, VWF, VWFpp, secreted protein acidic and cysteine rich (SPARC), and fibrinogen were imaged using 3-dimensional structured illumination microscopy, followed by semiautomated analysis in FIJI. Uptake of anti-VWF nanobody during degranulation was used to identify alpha-granules that partially released content.

RESULTS

VWFpp overlapped with VWF in eccentric alpha-granule subdomains in resting platelets and showed a higher degree of overlap with VWF than SPARC or fibrinogen. Activation of PAR-1 (0.6-20 μM PAR-1 ap) or glycoprotein VI (GPVI) (0.25-1 μg/mL CRP-XL) signaling pathways caused a dose-dependent increase in alpha-granule exocytosis. More than 80% of alpha-granules remained positive for VWF, even at the highest agonist concentrations. In contrast, the residual fraction of alpha-granules containing VWFpp decreased in a dose-dependent manner to 23%, whereas SPARC and fibrinogen were detected in 60% to 70% of alpha-granules when stimulated with 20 μM PAR-1 ap. Similar results were obtained using CRP-XL. Using an extracellular anti-VWF nanobody, we identified VWF in postexocytotic alpha-granules.

CONCLUSION

We provide evidence for differential secretion of VWF and VWFpp from individual alpha-granules.

Altered Storage and Function of von Willebrand Factor in Human Cardiac Microvascular Endothelial Cells Isolated from Recipient Transplant Hearts

The assembly of von Willebrand factor (VWF) into ordered helical tubules within endothelial Weibel-Palade bodies (WPBs) is required for the efficient deployment of the protein at sites of vascular injury. VWF trafficking and storage are sensitive to cellular and environmental stresses that are associated with heart disease and heart failure. Altered storage of VWF manifests as a change in WPB morphology from a rod shape to a rounded shape and is associated with impaired VWF deployment during secretion. In this study, we examined the morphology, ultrastructure, molecular composition and kinetics of exocytosis of WPBs in cardiac microvascular endothelial cells isolated from explanted hearts of patients with a common form of heart failure, dilated cardiomyopathy (DCM; HCMEC_D), or from nominally healthy donors (controls; HCMEC_C). Using fluorescence microscopy, WPBs in HCMEC_C (n = 3 donors) showed the typical rod-shaped morphology containing VWF, P-selectin and tPA. In contrast, WPBs in primary cultures of HCMEC_D (n = 6 donors) were predominantly rounded in shape and lacked tissue plasminogen activator (t-PA). Ultrastructural analysis of HCMEC_D revealed a disordered arrangement of VWF tubules in nascent WPBs emerging from the trans-Golgi network. HCMEC_D WPBs still recruited Rab27A, Rab3B, Myosin-Rab Interacting Protein (MyRIP) and Synaptotagmin-like protein 4a (Slp4-a) and underwent regulated exocytosis with kinetics similar to that seen in HCMECc. However, secreted extracellular VWF strings from HCMEC_D were significantly shorter than for endothelial cells with rod-shaped WPBs, although VWF platelet binding was similar. Our observations suggest that VWF trafficking, storage and haemostatic potential are perturbed in HCMEC from DCM hearts.

Assembly of von Willebrand factor tubules with in vivo helical parameters requires A1 domain insertion

The von Willebrand factor (VWF) glycoprotein is stored in tubular form in Weibel-Palade bodies (WPBs) before secretion from endothelial cells into the bloodstream. The organization of VWF in the tubules promotes formation of covalently linked VWF polymers and enables orderly secretion without polymer tangling. Recent studies have described the high-resolution structure of helical tubular cores formed in vitro by the D1D2 and D'D3 amino-terminal protein segments of VWF. Here we show that formation of tubules with the helical geometry observed for VWF in intracellular WPBs requires also the VWA1 (A1) domain. We reconstituted VWF tubules from segments containing the A1 domain and discovered it to be inserted between helical turns of the tubule, altering helical parameters and explaining the increased robustness of tubule formation when A1 is present. The conclusion from this observation is that the A1 domain has a direct role in VWF assembly, along with its known activity in hemostasis after secretion.

Structures of VWF tubules before and after concatemerization reveal a mechanism of disulfide bond exchange

von Willebrand factor (VWF) is an adhesive glycoprotein that circulates in the blood as disulfide-linked concatemers and functions in primary hemostasis. The loss of long VWF concatemers is associated with the excessive bleeding of type 2A von Willebrand disease (VWD). Formation of the disulfide bonds that concatemerize VWF requires VWF to self-associate into helical tubules, yet how the helical tubules template intermolecular disulfide bonds is not known. Here, we report electron cryomicroscopy (cryo-EM) structures of VWF tubules before and after intermolecular disulfide bond formation. The structures provide evidence that VWF tubulates through a charge-neutralization mechanism and that the A1 domain enhances tubule length by crosslinking successive helical turns. In addition, the structures reveal disulfide states before and after disulfide bond-mediated concatemerization. The structures and proposed assembly mechanism provide a foundation to rationalize VWD-causing mutations.

Acidification of endothelial Weibel-Palade bodies is mediated by the vacuolar-type H+-ATPase

Weibel-Palade bodies (WPB) are unique secretory granules of endothelial cells that store the procoagulant von-Willebrand factor (VWF) in a highly compacted form. Upon exocytosis the densely packed VWF unfurls into long strands that expose binding sites for circulating platelets and thereby initiate the formation of a platelet plug at sites of blood vessel injury. Dense packing of VWF requires the establishment of an acidic pH in the lumen of maturing WPB but the mechanism responsible for this acidification has not yet been fully established. We show here that subunits of the vacuolar-type H⁺-ATPase are present on mature WPB and that interference with the proton pump activity of the ATPase employing inhibitors of different chemical nature blocks a reduction in the relative internal pH of WPB. Furthermore, depletion of the V-ATPase subunit V0d1 from primary endothelial cells prevents WPB pH reduction and the establishment of an elongated morphology of WPB that is dictated by the densely packed VWF tubules. Thus, the vacuolar-type H⁺-ATPase present on WPB is required for proper acidification and maturation of the organelle.

Tip-end fusion of a rod-shaped secretory organelle

Weibel–Palade bodies (WPB) are elongated, rod-like secretory organelles unique to endothelial cells that store the pro-coagulant von-Willebrand factor (VWF) and undergo regulated exocytosis upon stimulation with Ca2+- or cAMP-raising agonists. We show here that WPB preferentially initiate fusion with the plasma membrane at their tips and identify synaptotagmin-like protein 2-a (Slp2-a) as a positive regulator of VWF secretion most likely mediating this topological selectivity. Following secretagogue stimulation, Slp2-a accumulates at one WPB tip before fusion occurs at this site. Depletion of Slp2-a reduces Ca2+-dependent secretion of highly multimeric VWF and interferes with the formation of actin rings at WPB–plasma membrane fusion sites that support the expulsion of the VWF multimers and most likely require a tip-end fusion topology. Phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P2] binding via the C2A domain of Slp2-a is required for accumulation of Slp2-a at the tip ends of fusing WPB, suggesting that Slp2-a mediates polar exocytosis by initiating contacts between WPB tips and plasma membrane PI(4,5)P2.

Structural basis of von Willebrand factor multimerization and tubular storage

The von Willebrand factor (VWF) propeptide (domains D1D2) is essential for the assembly of VWF multimers and its tubular storage in Weibel-Palade bodies. However, detailed molecular mechanism underlying this propeptide dependence is unclear. Here, we prepared Weibel-Palade body-like tubules using the N-terminal fragment of VWF and solved the cryo-electron microscopy structures of the tubule at atomic resolution. Detailed structural and biochemical analysis indicate that the propeptide forms a homodimer at acidic pH through the D2:D2 binding interface and then recruits 2 D'D3 domains, forming an intertwined D1D2D'D3 homodimer in essence. Stacking of these homodimers by the intermolecular D1:D2 interfaces brings 2 D3 domains face-to-face and facilitates their disulfide linkages and multimerization of VWF. Sequential stacking of these homodimers leads to a right-hand helical tubule for VWF storage. The clinically identified VWF mutations in the propeptide disrupted different steps of the assembling process, leading to diminished VWF multimers in von Willebrand diseases (VWD). Overall, these results indicate that the propeptide serves as a pH-sensing template for VWF multimerization and tubular storage. This sheds light on delivering normal propeptide as a template to rectify the defects in multimerization of VWD mutants.

Trafficking in blood vessel development

Blood vessels demonstrate a multitude of complex signaling programs that work in concert to produce functional vasculature networks during development. A known, but less widely studied, area of endothelial cell regulation is vesicular trafficking, also termed sorting. After moving through the Golgi apparatus, proteins are shuttled to organelles, plugged into membranes, recycled, or degraded depending on the internal and extrinsic cues. A snapshot of these protein-sorting systems can be viewed as a trafficking signature that is not only unique to endothelial tissue, but critically important for blood vessel form and function. In this review, we will cover how vesicular trafficking impacts various aspects of angiogenesis, such as sprouting, lumen formation, vessel stabilization, and secretion, emphasizing the role of Rab GTPase family members and their various effectors.

Helical self-assembly of a mucin segment suggests an evolutionary origin for von Willebrand factor tubules

Extracellular proteins with mechanical functions often require specialized assembly processes to form covalent oligomers. Progress in tissue bioengineering and repair will benefit from an understanding of how to harness and manipulate these processes. Here, we show that a particular supramolecular assembly mode was pre-encoded in the ancient domain organization common to gel-forming mucins and von Willebrand factor, glycoproteins that are deceptively different due to their divergence for distinct mechanical tasks. This finding highlights symmetry principles and building blocks retooled in nature to construct polymers with wide-ranging properties. These building blocks and knowledge of their self-assembly can be used to design new polymeric structures.

The glycoprotein von Willebrand factor (VWF) contributes to hemostasis by stanching injuries in blood vessel walls. A distinctive feature of VWF is its assembly into long, helical tubules in endothelial cells prior to secretion. When VWF is released into the bloodstream, these tubules unfurl to release linear polymers that bind subendothelial collagen at wound sites, recruit platelets, and initiate the clotting cascade. VWF evolved from gel-forming mucins, the polymeric glycoproteins that coat and protect exposed epithelia. Despite the divergent function of VWF in blood vessel repair, sequence conservation and shared domain organization imply that VWF retained key aspects of the mucin bioassembly mechanism. Here, we show using cryo-electron microscopy that the ability to form tubules, a property hitherto thought to have arisen as a VWF adaptation to the vasculature, is a feature of the amino-terminal region of mucin. This segment of the human intestinal gel-forming mucin (MUC2) was found to self-assemble into tubules with a striking resemblance to those of VWF itself. To facilitate a comparison, we determined the residue-resolution structure of tubules formed by the homologous segment of VWF. The structures of the MUC2 and VWF tubules revealed the flexible joints and the intermolecular interactions required for tubule formation. Steric constraints in full-length MUC2 suggest that linear filaments, a previously observed supramolecular assembly form, are more likely than tubules to be the physiological mucin storage intermediate. Nevertheless, MUC2 tubules indicate a possible evolutionary origin for VWF tubules and elucidate design principles present in mucins and VWF.

V-ATPase V0a1 promotes Weibel-Palade body biogenesis through the regulation of membrane fission

Membrane fission, the division of a membrane-bound structure into two discrete compartments, is essential for diverse cellular events, such as endocytosis and vesicle/granule biogenesis; however, the process remains unclear. The hemostatic protein von Willebrand factor is produced in vascular endothelial cells and packaged into specialized secretory granules, Weibel–Palade bodies (WPBs) at the trans-Golgi network (TGN). Here, we reported that V0a1, a V-ATPase component, is required for the membrane fission of WPBs. We identified two V0a isoforms in distinct populations of WPBs in cultured endothelial cells, V0a1 and V0a2, on mature and nascent WPBs, respectively. Although WPB buds were formed, WPBs could not separate from the TGN in the absence of V0a1. Screening using dominant–negative forms of known membrane fission regulators revealed protein kinase D (PKD) as an essential factor in biogenesis of WPBs. Further, we showed that the induction of wild-type PKDs in V0a1-depleted cells does not support the segregation of WPBs from the TGN; suggesting a primary role of V0a1 in the membrane fission of WPBs. The identification of V0a1 as a new membrane fission regulator should facilitate the understanding of molecular events that enable membrane fission.

von Willebrand factor propeptide missense variants affect anterograde transport to Golgi resulting in ER retention

von Willebrand disease (VWD), the most prevalent congenital bleeding disorder, arises from a deficiency in von Willebrand factor (VWF), which has crucial roles in hemostasis. The present study investigated functional consequences and underlying pathomolecular mechanisms of several VWF propeptide (VWFpp) missense variants detected in our cohort of VWD patients for the first time. Transient expression experiments in HEK293T cells demonstrated that four out of the six investigated missense variants (p.Gly55Glu, p.Val86Glu, p.Trp191Arg, and p.Cys608Trp) severely impaired secretion. Their cotransfections with the wild-type partly corrected VWF secretion, displaying loss of large/intermediate multimers. Immunostaining of the transfected HEK293 cells illustrated the endoplasmic reticulum (ER) retention of the VWF variants. Docking of the COP I and COP II cargo recruitment proteins, ADP-ribosylation factor 1 and Sec24, onto the N-terminal VWF model (D1D2D'D3) revealed that these variants occur at VWFpp putative interfaces, which can hinder VWF loading at the ER exit quality control. Furthermore, quantitative and automated morphometric exploration of the three-dimensional immunofluorescence images showed changes in the number/size of the VWF storage organelles, Weibel-Palade body (WPB)-like vesicles. The result of this study highlighted the significance of the VWFpp variants on anterograde ER-Golgi trafficking of VWF as well as the biogenesis of WPB-like vesicles.

Sec22b determines Weibel-Palade body length by controlling anterograde ER-Golgi transport

Von Willebrand factor (VWF) is a multimeric hemostatic protein that is synthesized in endothelial cells, where it is stored for secretion in elongated secretory organelles called Weibel-Palade bodies (WPB). The hemostatic activity of VWF is strongly related to the length of these bodies, but how endothelial cells control the dimensions of their WPB is unclear. In this study, using a targeted short hairpin RNA screen, we identified longin-SNARE Sec22b as a novel determinant of WPB size and VWF trafficking. We found that Sec22b depletion resulted in loss of the typically elongated WPB morphology together with disintegration of the Golgi and dilation of rough endoplasmic reticulum cisternae. This was accompanied by reduced proteolytic processing of VWF, accumulation of VWF in the dilated rough endoplasmic reticulum and reduced basal and stimulated VWF secretion. Our data demonstrate that the elongation of WPB, and thus adhesive activity of their cargo VWF, is determined by the rate of anterograde transport between endoplasmic reticulum and Golgi, which depends on Sec22b-containing SNARE complexes.

Assembly Mechanism of Mucin and von Willebrand Factor Polymers

The respiratory and intestinal tracts are exposed to physical and biological hazards accompanying the intake of air and food. Likewise, the vasculature is threatened by inflammation and trauma. Mucin glycoproteins and the related von Willebrand factor guard the vulnerable cell layers in these diverse systems. Colon mucins additionally house and feed the gut microbiome. Here, we present an integrated structural analysis of the intestinal mucin MUC2. Our findings reveal the shared mechanism by which complex macromolecules responsible for blood clotting, mucociliary clearance, and the intestinal mucosal barrier form protective polymers and hydrogels. Specifically, cryo-electron microscopy and crystal structures show how disulfide-rich bridges and pH-tunable interfaces control successive assembly steps in the endoplasmic reticulum and Golgi apparatus. Remarkably, a densely O-glycosylated mucin domain performs an organizational role in MUC2. The mucin assembly mechanism and its adaptation for hemostasis provide the foundation for rational manipulation of barrier function and coagulation.

AutoCLEM: An Automated Workflow for Correlative Live-Cell Fluorescence Microscopy and Cryo-Electron Tomography

Correlative light and electron microscopy (CLEM) combines the strengths of both light and electron imaging modalities and enables linking of biological spatiotemporal information from live-cell fluorescence light microscopy (fLM) to high-resolution cellular ultra-structures from cryo-electron microscopy and tomography (cryoEM/ET). This has been previously achieved by using fLM signals to localize the regions of interest under cryogenic conditions. The correlation process, however, is often tedious and time-consuming with low throughput and limited accuracy, because multiple correlation steps at different length scales are largely carried out manually. Here, we present an experimental workflow, AutoCLEM, which overcomes the existing limitations and improves the performance and throughput of CLEM methods, and associated software. The AutoCLEM system encompasses a high-speed confocal live-cell imaging module to acquire an automated fLM grid atlas that is linked to the cryoEM grid atlas, followed by cryofLM imaging after freezing. The fLM coordinates of the targeted areas are automatically converted to cryoEM/ET and refined using fluorescent fiducial beads. This AutoCLEM workflow significantly accelerates the correlation efficiency between live-cell fluorescence imaging and cryoEM/ET structural analysis, as demonstrated by visualizing human immunodeficiency virus type 1 (HIV-1) interacting with host cells.

Mechanochemistry of von Willebrand factor

Von Willebrand factor (VWF), a blood multimeric protein with a very high molecular weight, plays a crucial role in the primary haemostasis, the physiological process characterized by the adhesion of blood platelets to the injured vessel wall. Hydrodynamic forces are responsible for extensive conformational transitions in the VWF multimers that change their structure from a globular form to a stretched linear conformation. This feature makes this protein particularly prone to be investigated by mechanochemistry, the branch of the biophysical chemistry devoted to investigating the effects of shear forces on protein conformation. This review describes the structural elements of the VWF molecule involved in the biochemical response to shear forces. The stretched VWF conformation favors the interaction with the platelet GpIb and at the same time with ADAMTS-13, the zinc-protease that cleaves VWF in the A2 domain, limiting its prothrombotic capacity. The shear-induced conformational transitions favor also a process of self-aggregation, responsible for the formation of a spider-web like network, particularly efficient in the trapping process of flowing platelets. The investigation of the biophysical effects of shear forces on VWF conformation contributes to unraveling the molecular mechanisms of many types of thrombotic and haemorrhagic syndromes.

WPBs: making a mark and leaving a trail

In this issue of Blood, Holthenrich et al used a proximity labeling approach to pull, from out of the crowded intracellular milieu, proteins that specifically interact with Weibel-Palade bodies (WPBs). From the resulting catalog of proteins, the authors identified Munc13-2 as a novel WPB-associated SNARE-interacting protein that positively regulates hormone-evoked WPB exocytosis.¹

Stimulated release of intraluminal vesicles from Weibel-Palade bodies

Weibel-Palade bodies (WPBs) are secretory granules that contain von Willebrand factor and P-selectin, molecules that regulate hemostasis and inflammation, respectively. The presence of CD63/LAMP3 in the limiting membrane of WPBs has led to their classification as lysosome-related organelles. Many lysosome-related organelles contain intraluminal vesicles (ILVs) enriched in CD63 that are secreted into the extracellular environment during cell activation to mediate intercellular communication. To date, there are no reports that WPBs contain or release ILVs. By light microscopy and live-cell imaging, we show that CD63 is enriched in microdomains within WPBs. Extracellular antibody recycling studies showed that CD63 in WPB microdomains can originate from the plasma membrane. By cryo-electron tomography of frozen-hydrated endothelial cells, we identify internal vesicles as novel structural features of the WPB lumen. By live-cell fluorescence microscopy, we directly observe the exocytotic release of EGFP-CD63 ILVs as discrete particles from individual WPBs. WPB exocytosis provides a novel route for release of ILVs during endothelial cell stimulation.

The von Willebrand factor D'D3 assembly and structural principles for factor VIII binding and concatemer biogenesis

D assemblies make up half of the von Willebrand factor (VWF), yet are of unknown structure. D1 and D2 in the prodomain and D'D3 in mature VWF at Golgi pH form helical VWF tubules in Weibel Palade bodies and template dimerization of D3 through disulfides to form ultralong VWF concatemers. D'D3 forms the binding site for factor VIII. The crystal structure of monomeric D'D3 with cysteine residues required for dimerization mutated to alanine was determined at an endoplasmic reticulum (ER)-like pH. The smaller C8-3, TIL3 (trypsin inhibitor-like 3), and E3 modules pack through specific interfaces as they wind around the larger, N-terminal, Ca2+-binding von Willebrand D domain (VWD) 3 module to form a wedge shape. D' with its TIL' and E' modules projects away from D3. The 2 mutated cysteines implicated in D3 dimerization are buried, providing a mechanism for protecting them against premature disulfide linkage in the ER, where intrachain disulfide linkages are formed. D3 dimerization requires co-association with D1 and D2, Ca2+, and Golgi-like acidic pH. Associated structural rearrangements in the C8-3 and TIL3 modules are required to expose cysteine residues for disulfide linkage. Our structure provides insight into many von Willebrand disease mutations, including those that diminish factor VIII binding, which suggest that factor VIII binds not only to the N-terminal TIL' domain of D' distal from D3 but also extends across 1 side of D3. The organizing principle for the D3 assembly has implications for other D assemblies and the construction of higher-order, disulfide-linked assemblies in the Golgi in both VWF and mucins.

Exocytosis of Weibel-Palade bodies: how to unpack a vascular emergency kit

The blood vessel wall has a number of self-healing properties, enabling it to minimize blood loss and prevent or overcome infections in the event of vascular trauma. Endothelial cells prepackage a cocktail of hemostatic, inflammatory and angiogenic mediators in their unique secretory organelles, the Weibel-Palade bodies (WPBs), which can be immediately released on demand. Secretion of their contents into the vascular lumen through a process called exocytosis enables the endothelium to actively participate in the arrest of bleeding and to slow down and direct leukocytes to areas of inflammation. Owing to their remarkable elongated morphology and their secretory contents, which span the entire size spectrum of small chemokines all the way up to ultralarge von Willebrand factor multimers, WPBs constitute an ideal model system for studying the molecular mechanisms of secretory organelle biogenesis, exocytosis, and content expulsion. Recent studies have now shown that, during exocytosis, WPBs can undergo several distinct modes of fusion, and can utilize fundamentally different mechanisms to expel their contents. In this article, we discuss recent advances in our understanding of the composition of the WPB exocytotic machinery and how, because of its configuration, it is able to support WPB release in its various forms.

RF/6A Chorioretinal Cells Do Not Display Key Endothelial Phenotypes

Purpose

The misuse of inauthentic cell lines is widely recognized as a major threat to the integrity of biomedical science. Whereas the majority of efforts to address this have focused on DNA profiling, we sought to anatomically, transcriptionally, and functionally authenticate the RF/6A chorioretinal cell line, which is widely used as an endothelial cell line to model retinal and choroidal angiogenesis.

Methods

Multiple vials of RF/6A cells obtained from different commercial distributors were studied to validate their genetic, transcriptomic, anatomic, and functional fidelity to bona fide endothelial cells.

Results

Transcriptomic profiles of RF/6A cells obtained either de novo or from a public data repository did not correspond to endothelial gene expression signatures. Expression of established endothelial markers were very low or undetectable in RF/6A compared to primary human endothelial cells. Importantly, RF/6A cells also did not display functional characteristics of endothelial cells such as uptake of acetylated LDL, expression of E-selectin in response to TNF-α exposure, alignment in the direction of shear stress, and AKT and ERK1/2 phosphorylation following VEGFA stimulation.

Conclusions

Multiple independent sources of RF/6A do not exhibit key endothelial cell phenotypes. Therefore, these cells appear unsuitable as surrogates for choroidal or retinal endothelial cells. Further, cell line authentication methods should extend beyond genomic profiling to include anatomic, transcriptional, and functional assessments.

Electron Tomography and Correlative Approaches in Platelet Studies

Blood platelets play a central role in the arrest of bleeding and the development of thrombosis. Unraveling the complex processes of platelet biogenesis from megakaryocytes, platelet adhesion, aggregation, and secretory responses are important topics in the field of hemostasis and thrombosis. Analysis of the ultrastructural changes that occur during these processes is essential for understanding the rapid membrane dynamics and has contributed substantially to our present knowledge of platelet formation and functioning. Recent developments in real-time imaging, correlative light and electron microscopy imaging (CLEM), and 3D (cryo) electron microscopy and tomography offer exciting opportunities to improve studies of the platelet adhesive responses and secretion at the ultrastructural level in a close to native environment. In this chapter we discuss and illustrate cryo preparation techniques (high-pressure freezing, vitrification), correlative LM and EM workflows, and 3D cryo-electron tomography that we apply in our current research projects.

Lifecycle of Weibel-Palade bodies

Weibel-Palade bodies (WPBs) are rod or cigar-shaped secretory organelles that are formed by the vascular endothelium. They contain a diverse set of proteins that either function in haemostasis, inflammation, or angiogenesis. Biogenesis of the WPB occurs at the Golgi apparatus in a process that is dependent on the main component of the WPB, the haemostatic protein von Willebrand Factor (VWF). During this process the organelle is directed towards the regulated secretion pathway by recruiting the machinery that responds to exocytosis stimulating agonists. Upon maturation in the periphery of the cell the WPB recruits Rab27A which regulates WPB secretion. To date several signaling pathways have been found to stimulate WPB release. These signaling pathways can trigger several secretion modes including single WPB release and multigranular exocytosis. In this review we will give an overview of the WPB lifecycle from biogenesis to secretion and we will discuss several deficiencies that affect the WPB lifecycle.

BLOC-2 subunit HPS6 deficiency affects the tubulation and secretion of von Willebrand factor from mouse endothelial cells

Hermansky-Pudlak syndrome (HPS) is a recessive disorder with bleeding diathesis, which has been linked to platelet granule defects. Both platelet granules and endothelial Weibel-Palade bodies (WPBs) are members of lysosome-related organelles (LROs) whose formation is regulated by HPS protein associated complexes such as BLOC (biogenesis of lysosome-related organelles complex) -1, -2, -3, AP-3 (adaptor protein complex-3) and HOPS (homotypic fusion and protein sorting complex). Von Willebrand factor (VWF) is critical to hemostasis, which is stored in a highly-multimerized form as tubules in the WPBs. In this study, we found the defective, but varying, release of VWF into plasma after desmopressin (DDAVP) stimulation in HPS1 (BLOC-3 subunit), HPS6 (BLOC-2 subunit), and HPS9 (BLOC-1 subunit) deficient mice. In particular, VWF tubulation, a critical step in VWF maturation, was impaired in HPS6 deficient WPBs. This likely reflects a defective endothelium, contributing to the bleeding tendency in HPS mice or patients. The differentially defective regulated release of VWF in these HPS mouse models suggests the need for precise HPS genotyping before DDAVP administration to HPS patients.

N-ethylmaleimide‑sensitive factor siRNA inhibits the release of Weibel-Palade bodies in endothelial cells

The aim of the present study was to examine the effect of small interfering RNA (siRNA) methods on the expression of N‑ethylmaleimide sensitive factor (NSF) and Weibel‑Palade body (WPB) release in endothelial cells. A small hairpin RNA (shRNA), mediated with an adenovirus vector, was designed to target the N‑terminal functional area of NSF. Subsequently, viruses were transfected into human aortic endothelial cells. The mRNA and protein expression levels of NSF were detected using reverse transcription‑quantitative polymerase chain reaction and Western blot analyses, respectively, and the release of WPBs in the endothelial cells was examined using immunofluorescence. The mRNA expression of NSF in the endothelial cells, which were transfected with the adenoviruses carrying the NSF‑shRNA was significantly decreased, compared with the negative control group (P=0.035) and blank control group (P=0.02). In addition, the mRNA expression of NSF was gradually decreased as duration increased; there were marked differences between the 24, 48 and 72 h groups (P<0.05). The protein expression of NSF was significantly decreased in the experimental group, compared with the negative control group (P=0.004) and blank control group (P=0.031), however, no difference was observed between the negative control and blank control groups (P=0.249). The immunofluorescence staining showed that the release of WPBs in the endothelial cells induced with thrombin was inhibited markedly following transfection with the virus carrying the NSF‑shRNA. Therefore NSF‑siRNA inhibited the mRNA and protein expression levels of NSF, and inhibited the release of WPBs in endothelial cells induced with thrombin. These results suggested that NSF-siRNA may be valuable for preventing and treating atherosclerosis and acute coronary syndrome.

Content delivery to newly forming Weibel-Palade bodies is facilitated by multiple connections with the Golgi apparatus

WPBs stay connected to the Golgi apparatus until vesicle formation is completed. During biogenesis at the Golgi, WPBs increase in size through the addition of nontubular VWF.

von Willebrand factor biosynthesis, secretion, and clearance: connecting the far ends

To understand the placement of a certain protein in a physiological system and the pathogenesis of related disorders, it is not only of interest to determine its function but also important to describe the sequential steps in its life cycle, from synthesis to secretion and ultimately its clearance. von Willebrand factor (VWF) is a particularly intriguing case in this regard because of its important auxiliary roles (both intra- and extracellular) that implicate a wide range of other proteins: its presence is required for the formation and regulated release of endothelial storage organelles, the Weibel-Palade bodies (WPBs), whereas VWF is also a key determinant in the clearance of coagulation factor VIII. Thus, understanding the molecular and cellular basis of the VWF life cycle will help us gain insight into the pathogenesis of von Willebrand disease, design alternative treatment options to prolong the factor VIII half-life, and delineate the role of VWF and coresidents of the WPBs in the prothrombotic and proinflammatory response of endothelial cells. In this review, an update on our current knowledge on VWF biosynthesis, secretion, and clearance is provided and we will discuss how they can be affected by the presence of protein defects.

Mechanism and functional impact of CD40 ligand-induced von Willebrand factor release from endothelial cells

Co-stimulation via CD154 binding to CD40, pivotal for both innate and adaptive immunity, may also link haemostasis to vascular remodelling. Here we demonstrate that human platelet-bound or recombinant soluble CD154 (sCD154) elicit the release from and tethering of ultra-large (UL) von Willebrand factor (vWF) multimers to the surface of human cultured endothelial cells (ECs) exposed to shear stress. This CD40-mediated ULVWF multimer release from the Weibel-Palade bodies was triggered by consecutive activation of TRAF6, the tyrosine kinase c-Src and phospholipase Cγ1 followed by inositol-1,4,5 trisphosphate-mediated calcium mobilisation. Subsequent exposure to human washed platelets caused ULVWF multimer-platelet string formation on the EC surface in a shear stress-dependent manner. Platelets tethered to these ULVWF multimers exhibited P-selectin on their surface and captured labelled monocytes from the superfusate. When exposed to shear stress and sCD154, native ECs from wild-type but not CD40 or vWF-deficient mice revealed a comparable release of ULVWF multimers to which murine washed platelets rapidly adhered, turning P-selectin-positive and subsequently capturing monocytes from the perfusate. This novel CD154-provoked ULVWF multimer-platelet string formation at normal to fast flow may contribute to vascular remodelling processes requiring the perivascular or intravascular accumulation of pro-inflammatory macrophages such as arteriogenesis or atherosclerosis.

Differential cargo mobilisation within Weibel-Palade bodies after transient fusion with the plasma membrane

Inflammatory chemokines can be selectively released from Weibel-Palade bodies (WPBs) during kiss-and-run exocytosis. Such selectivity may arise from molecular size filtering by the fusion pore, however differential intra-WPB cargo re-mobilisation following fusion-induced structural changes within the WPB may also contribute to this process. To determine whether WPB cargo molecules are differentially re-mobilised, we applied FRAP to residual post-fusion WPB structures formed after transient exocytosis in which some or all of the fluorescent cargo was retained. Transient fusion resulted in WPB collapse from a rod to a spheroid shape accompanied by substantial swelling (>2 times by surface area) and membrane mixing between the WPB and plasma membranes. Post-fusion WPBs supported cumulative WPB exocytosis. To quantify diffusion inside rounded organelles we developed a method of FRAP analysis based on image moments. FRAP analysis showed that von Willebrand factor-EGFP (VWF-EGFP) and the VWF-propolypeptide-EGFP (Pro-EGFP) were immobile in post-fusion WPBs. Because Eotaxin-3-EGFP and ssEGFP (small soluble cargo proteins) were largely depleted from post-fusion WPBs, we studied these molecules in cells preincubated in the weak base NH4Cl which caused WPB alkalinisation and rounding similar to that produced by plasma membrane fusion. In these cells we found a dramatic increase in mobilities of Eotaxin-3-EGFP and ssEGFP that exceeded the resolution of our method (∼ 2.4 µm2/s mean). In contrast, the membrane mobilities of EGFP-CD63 and EGFP-Rab27A in post-fusion WPBs were unchanged, while P-selectin-EGFP acquired mobility. Our data suggest that selective re-mobilisation of chemokines during transient fusion contributes to selective chemokine secretion during transient WPB exocytosis. Selective secretion provides a mechanism to regulate intravascular inflammatory processes with reduced risk of thrombosis.

Imaging endosomes and autophagosomes in whole mammalian cells using correlative cryo-fluorescence and cryo-soft X-ray microscopy (cryo-CLXM)

Cryo-soft X-ray tomography (cryo-SXT) is a powerful imaging technique that can extract ultrastructural information from whole, unstained mammalian cells as close to the living state as possible. Subcellular organelles including the nucleus, the Golgi apparatus and mitochondria have been identified by morphology alone, due to the similarity in contrast to transmission electron micrographs. In this study, we used cryo-SXT to image endosomes and autophagosomes, organelles that are particularly susceptible to chemical fixation artefacts during sample preparation for electron microscopy. We used two approaches to identify these compartments. For early and recycling endosomes, which are accessible to externally-loaded markers, we used an anti-transferrin receptor antibody conjugated to 10nm gold particles. For autophagosomes, which are not accessible to externally-applied markers, we developed a correlative cryo-fluorescence and cryo-SXT workflow (cryo-CLXM) to localise GFP-LC3 and RFP-Atg9. We used a stand-alone cryo-fluorescence stage in the home laboratory to localise the cloned fluorophores, followed by cryo-soft X-ray tomography at the synchrotron to analyse cellular ultrastructure. We mapped the 3D ultrastructure of the endocytic and autophagic structures, and discovered clusters of omegasomes arising from 'hotspots' on the ER. Thus, immunogold markers and cryo-CLXM can be used to analyse cellular processes that are inaccessible using other imaging modalities.

von Willebrand factor, Jedi knight of the bloodstream

When blood vessels are cut, the forces in the bloodstream increase and change character. The dark side of these forces causes hemorrhage and death. However, von Willebrand factor (VWF), with help from our circulatory system and platelets, harnesses the same forces to form a hemostatic plug. Force and VWF function are so closely intertwined that, like members of the Jedi Order in the movie Star Wars who learn to use "the Force" to do good, VWF may be considered the Jedi knight of the bloodstream. The long length of VWF enables responsiveness to flow. The shape of VWF is predicted to alter from irregularly coiled to extended thread-like in the transition from shear to elongational flow at sites of hemostasis and thrombosis. Elongational force propagated through the length of VWF in its thread-like shape exposes its monomers for multimeric binding to platelets and subendothelium and likely also increases affinity of the A1 domain for platelets. Specialized domains concatenate and compact VWF during biosynthesis. A2 domain unfolding by hydrodynamic force enables postsecretion regulation of VWF length. Mutations in VWF in von Willebrand disease contribute to and are illuminated by VWF biology. I attempt to integrate classic studies on the physiology of hemostatic plug formation into modern molecular understanding, and point out what remains to be learned.

The various states of von Willebrand factor and their function in physiology and pathophysiology

The specific interactions of von Willebrand factor (VWF) with the vessel wall, platelets or other interfaces strongly depend on (a shear-induced) VWF activation. Shear flow has been shown to induce a conformational transition of VWF, but is modulated by its thermodynamic state (state-function relationship). The state in turn is determined by physical (e.g. vessel geometry), physico-chemical (e.g. pH) and molecular-biological (e.g. mutants, binding) factors. Combining established results with recent insights, we reconstruct VWF biology and its state-function relationship from endothelial cell release to final degradation in the human vasculature. After VWF secretion, endothelial-anchored and shear activated VWF multimers can rapidly interact with surrounding colloids, typically with platelets. Simultaneously, this VWF activation enables ADAMTS13 to cleave VWF multimers thereby limiting VWF binding capacity. The subsequent cell-surface dissociation leads to a VWF recoiling to a globular conformation, shielding from further degradation by ADAMTS13. High local concentrations of these soluble VWF multimers, transported to the downstream vasculature, are capable for an immediate reactivation and re-polymerisation initiating colloid-binding or VWF-colloid aggregation at the site of inflamed endothelium, vessel injuries or pathological high-shear areas. Focusing on these functional steps in the lifecycle of VWF, its qualitative and quantitative deficiencies in the different VWD types will facilitate more precise diagnostics and reliable risk stratification for prophylactic therapies. The underlying biophysical principles are of general character, which broadens prospective studies on the physiological and pathophysiological impact of VWF and VWF-associated diseases and beares hope for a more universal understanding of an entire class of phenomena.

Correlative cryo-fluorescence and cryo-soft X-ray tomography of adherent cells at European synchrotrons

Cryo-soft X-ray tomography (cryo-SXT) is a synchrotron-hosted imaging technique used to analyze the ultrastructure of intact, cryo-prepared cells. Correlation of cryo-fluorescence microscopy and cryo-SXT can be used to localize fluorescent proteins to organelles preserved close to native state. Cryo-correlative light and X-ray microscopy (cryo-CLXM) is particularly useful for the study of organelles that are susceptible to chemical fixation artifacts during sample preparation for electron microscopy. In our recent work, we used cryo-CLXM to characterize GFP-LC3-positive early autophagosomes in nutrient-starved HEK293A cells (Duke et al., 2013). Cup-shaped omegasomes were found to form at "hot-spots" on the endoplasmic reticulum. Furthermore, cryo-SXT image stacks revealed the presence of large complex networks of tubulated mitochondria in the starved cells, which would be challenging to model at this scale and resolution using light or electron microscopy. In this chapter, we detail the cryo-CLXM workflow that we developed and optimized for studying adherent mammalian cells. We show examples of data collected at the three European synchrotrons that currently host cryo-SXT microscopes, and describe how raw cryo-SXT datasets are processed into tomoX stacks, modeled, and correlated with cryo-fluorescence data to identify structures of interest.

von Willebrand factor remodeling during exocytosis from vascular endothelial cells

BACKGROUND

In vascular endothelial cells, high molecular weight multimers of von Willebrand factor (VWF) are folded into tubular structures for storage in Weibel-Palade bodies. On stimulation, VWF is secreted and forms strings to induce primary hemostasis. The structural changes composing the transition of stored tubular VWF into secreted unfurled VWF strings are still unresolved even though they are vital for normal hemostasis. The secretory pod is a novel structure that we previously described in endothelial cells. It is formed on stimulation and has been postulated to function as a VWF release site. In this study, we investigated the actual formation of secretory pods and the subsequent remodeling of VWF into strings.

METHODS

Human umbilical vein endothelial cells were stimulated and studied using various imaging techniques such as live-cell imaging and correlative light and electron microscopy.

RESULTS

We found by using live-cell imaging that secretory pods are formed through the coalescence of multiple Weibel-Palade bodies without involvement of other large structures. Secreted VWF expelled from secretory pods was found to adopt a globular conformation. We visualized that VWF strings derive from those globular masses of VWF. Flow experiments showed that, on secretion, the globular masses of VWF move to the edge of the cell, where they anchor and generate VWF strings.

CONCLUSION

On secretion, VWF adopts a globular conformation that remodels into strings after translocation and anchoring at the edge of the cell. This finding reveals new pathophysiological mechanisms that could be affected in patients with von Willebrand disease.

The secretion of von Willebrand factor from endothelial cells; an increasingly complicated story

von Willebrand factor (VWF) plays key roles in both primary and secondary hemostasis by capturing platelets and chaperoning clotting factor VIII, respectively. It is stored within the Weibel-Palade bodies (WPBs) of endothelial cells as a highly prothrombotic protein, and its release is thus necessarily under tight control. Regulating the secretion of VWF involves multiple layers of cellular machinery that act together at different stages, leading to the exocytic fusion of WPBs with the plasma membrane and the consequent release of VWF. This review aims to provide a snapshot of the current understanding of those components, in particular the members of the Rab family, acting in the increasingly complex story of VWF secretion.

Lysosome-related organelles: unusual compartments become mainstream

Lysosome-related organelles (LROs) comprise a group of cell type-specific subcellular compartments with unique composition, morphology and structure that share some features with endosomes and lysosomes and that function in varied processes such as pigmentation, hemostasis, lung plasticity and immunity. In recent years, studies of genetic diseases in which LRO functions are compromised have provided new insights into the mechanisms of LRO biogenesis and the regulated secretion of LRO contents. These insights have revealed previously unappreciated specialized endosomal sorting processes in all cell types, and are expanding our views of the plasticity of the endosomal and secretory systems in adapting to cell type-specific needs.

Weibel-Palade bodies: a window to von Willebrand disease

Weibel-Palade bodies (WPBs) are the storage organelles for von Willebrand factor (VWF) in endothelial cells. VWF forms multimers that assemble into tubular structures in WPBs. Upon demand, VWF is secreted into the blood circulation, where it unfolds into strings that capture platelets during the onset of primary hemostasis. Numerous mutations affecting VWF lead to the bleeding disorder von Willebrand disease. This review reports the recent findings on the effects of VWF mutations on the biosynthetic pathway of VWF and its storage in WPBs. These new findings have deepened our understanding of VWF synthesis, storage, secretion, and function.

Proteomic screen identifies IGFBP7 as a novel component of endothelial cell-specific Weibel-Palade bodies

Vascular endothelial cells contain unique storage organelles, designated Weibel-Palade bodies (WPBs), that deliver inflammatory and hemostatic mediators to the vascular lumen in response to agonists like thrombin and vasopressin. The main component of WPBs is von Willebrand factor (VWF), a multimeric glycoprotein crucial for platelet plug formation. In addition to VWF, several other components are known to be stored in WPBs, like osteoprotegerin, monocyte chemoattractant protein-1 and angiopoetin-2 (Ang-2). Here, we used an unbiased proteomics approach to identify additional residents of WPBs. Mass spectrometry analysis of purified WPBs revealed the presence of several known components such as VWF, Ang-2, and P-selectin. Thirty-five novel candidate WPB residents were identified that included insulin-like growth factor binding protein-7 (IGFBP7), which has been proposed to regulate angiogenesis. Immunocytochemistry revealed that IGFBP7 is a bona fide WPB component. Cotransfection studies showed that IGFBP7 trafficked to pseudo-WPB in HEK293 cells. Using a series of deletion variants of VWF, we showed that targeting of IGFBP7 to pseudo-WPBs was dependent on the carboxy-terminal D4-C1-C2-C3-CK domains of VWF. IGFBP7 remained attached to ultralarge VWF strings released upon exocytosis of WPBs under flow. The presence of IGFBP7 in WPBs highlights the role of this subcellular compartment in regulation of angiogenesis.

Paraxial charge compensator for electron cryomicroscopy

We describe a multi-hole condenser aperture for the production of several electron beams in the transmission electron microscope (TEM) making it possible to simultaneously image and irradiate spatially separated regions of a specimen. When the specimen is a thin film of vitreous ice suspended over a holey carbon film, simultaneous irradiation of the adjacent carbon support with the off-axis beam compensates for some of the effects of charging in the image formed by a beam irradiating only the ice. Because the intervening region is not irradiated, charge-neutralization of frozen-hydrated specimens can occur by a through-space mechanism such as the emission of secondary electrons from a grounded carbon support film. We use paraxial charge compensation (PCC) to control the amount of charge build-up on the specimen and observe the effects of charge on images. The multi-hole aperture thus provides a tool for investigating the mechanism of charging and charge mitigation during the imaging of radiation sensitive biological specimens by cryomicroscopy.

Direct visualization of HIV-1 with correlative live-cell microscopy and cryo-electron tomography

Cryo-electron tomography (cryoET) allows 3D visualization of cellular structures at molecular resolution in a close-to-native state and therefore has the potential to help elucidate early events of HIV-1 infection in host cells. However, structural details of infecting HIV-1 have not been observed, due to technological challenges in working with rare and dynamic HIV-1 particles in human cells. Here, we report structural analysis of HIV-1 and host-cell interactions by means of a correlative high-speed 3D live-cell-imaging and cryoET method. Using this method, we showed under near-native conditions that intact hyperstable mutant HIV-1 cores are released into the cytoplasm of host cells. We further obtained direct evidence to suggest that a hyperstable mutant capsid, E45A, showed delayed capsid disassembly compared to the wild-type capsid. Together, these results demonstrate the advantages of our correlative live-cell and cryoET approach for imaging dynamic processes, such as viral infection.

Biogenesis of Weibel-Palade bodies in von Willebrand's disease variants with impaired von Willebrand factor intrachain or interchain disulfide bond formation

BACKGROUND

Mutations of cysteine residues in von Willebrand factor are known to reduce the storage and secretion of this factor, thus leading to reduced antigen levels. However, one cysteine mutation, p.Cys2773Ser, has been found in patients with type 2A(IID) von Willebrand's disease who have normal plasma levels of von Willebrand factor. We hypothesize that disruption of either intra- or interchain disulfide bonds by cysteine mutations in von Willebrand factor has different effects on the biogenesis of Weibel-Palade bodies.

DESIGN AND METHODS

The effect of specific cysteine mutations that either disrupt intrachain (p.Cys1130Phe and p.Cys2671Tyr) or interchain (p.Cys2773Ser) disulfide bonds on storage and secretion of von Willebrand factor was studied by transient transfection of human embryonic kidney cell line 293. Upon expression of von Willebrand factor these cells formed endothelial Weibel-Palade body-like organelles called pseudo-Weibel-Palade bodies. Storage of von Willebrand factor was analyzed with both confocal immunofluorescence and electron microscopy. Regulated secretion of von Willebrand factor was induced by phorbol 12-myristate 13-acetate.

RESULTS

p.Cys1130Phe and p.Cys2671Tyr reduced the storage of von Willebrand factor into pseudo-Weibel-Palade bodies with notable retention of von Willebrand factor in the endoplasmic reticulum, whereas p.Cys2773Ser-von Willebrand factor was stored normally. As expected, wild-type von Willebrand factor formed proteinaceous tubules that were seen under electron microscopy as longitudinal striations in pseudo-Weibel-Palade bodies. p.Cys2773Ser caused severe defects in von Willebrand factor multimerization but the factor formed normal tubules. Furthermore, the basal and regulated secretion of von Willebrand factor was drastically impaired by p.Cys1130Phe and p.Cys2671Tyr, but not by p.Cys2773Ser.

CONCLUSIONS

We postulate that natural mutations of cysteines involved in the formation of interchain disulfide bonds do not affect either the storage in Weibel-Palade bodies or secretion of von Willebrand factor, whereas mutations of cysteines forming intrachain disulfide bonds lead to reduced von Willebrand factor storage and secretion because the von Willebrand factor is retained in the endoplasmic reticulum.

Factor VIII alters tubular organization and functional properties of von Willebrand factor stored in Weibel-Palade bodies

In endothelial cells, von Willebrand factor (VWF) multimers are packaged into tubules that direct biogenesis of elongated Weibel-Palade bodies (WPBs). WPB release results in unfurling of VWF tubules and assembly into strings that serve to recruit platelets. By confocal microscopy, we have previously observed a rounded morphology of WPBs in blood outgrowth endothelial cells transduced to express factor VIII (FVIII). Using correlative light-electron microscopy and tomography, we now demonstrate that FVIII-containing WPBs have disorganized, short VWF tubules. Whereas normal FVIII and FVIII Y1680F interfered with formation of ultra-large VWF multimers, release of the WPBs resulted in VWF strings of equal length as those from nontransduced blood outgrowth endothelial cells. After release, both WPB-derived FVIII and FVIII Y1680F remained bound to VWF strings, which however had largely lost their ability to recruit platelets. Strings from nontransduced cells, however, were capable of simultaneously recruiting exogenous FVIII and platelets. These findings suggest that the interaction of FVIII with VWF during WPB formation is independent of Y1680, is maintained after WPB release in FVIII-covered VWF strings, and impairs recruitment of platelets. Apparently, intra-cellular and extracellular assembly of FVIII-VWF complex involves distinct mechanisms, which differ with regard to their implications for platelet binding to released VWF strings.

Temperature-dependence of Weibel-Palade body exocytosis and cell surface dispersal of von Willebrand factor and its propolypeptide

BACKGROUND

Weibel-Palade bodies (WPB) are endothelial cell (EC) specific secretory organelles containing Von Willebrand factor (VWF). The temperature-dependence of Ca(2+)-driven WPB exocytosis is not known, although indirect evidence suggests that WPB exocytosis may occur at very low temperatures. Here we quantitatively analyse the temperature-dependence of Ca(2+)-driven WPB exocytosis and release of secreted VWF from the cell surface of ECs using fluorescence microscopy of cultured human ECs containing fluorescent WPBs.

PRINCIPAL FINDINGS

Ca(2+)-driven WPB exocytosis occurred at all temperatures studied (7-37°C). The kinetics and extent of WPB exocytosis were strongly temperature-dependent: Delays in exocytosis increased from 0.92 s at 37°C to 134.2 s at 7°C, the maximum rate of WPB fusion decreased from 10.0±2.2 s(-1) (37°C) to 0.80±0.14 s(-1) (7°C) and the fractional extent of degranulation of WPBs in each cell from 67±3% (37°C) to 3.6±1.3% (7°C). A discrepancy was found between the reduction in Ca(2+)-driven VWF secretion and WPB exocytosis at reduced temperature; at 17°C VWF secretion was reduced by 95% but WPB exocytosis by 75-80%. This discrepancy arises because VWF dispersal from sites of WPB exocytosis is largely prevented at low temperature. In contrast VWF-propolypeptide (proregion) dispersal from WPBs, although slowed, was complete within 60-120 s. Novel antibodies to the cleaved and processed proregion were characterised and used to show that secreted proregion more accurately reports the secretion of WPBs at sub-physiological temperatures than assay of VWF itself.

CONCLUSIONS

We report the first quantitative analysis of the temperature-dependence of WPB exocytosis. We provide evidence; by comparison of biochemical data for VWF or proregion secretion with direct analysis of WPB exocytosis at reduced temperature, that proregion is a more reliable marker for WPB exocytosis at reduced temperature, where VWF-EC adhesion is increased.

A pH-regulated dimeric bouquet in the structure of von Willebrand factor

At the acidic pH of the trans-Golgi and Weibel-Palade bodies (WPBs), but not at the alkaline pH of secretion, the C-terminal ∼1350 residues of von Willebrand factor (VWF) zip up into an elongated, dimeric bouquet. Six small domains visualized here for the first time between the D4 and cystine-knot domains form a stem. The A2, A3, and D4 domains form a raceme with three pairs of opposed, large, flower-like domains. N-terminal VWF domains mediate helical tubule formation in WPBs and template N-terminal disulphide linkage between VWF dimers, to form ultralong VWF concatamers. The dimensions we measure in VWF at pH 6.2 and 7.4, and the distance between tubules in nascent WPB, suggest that dimeric bouquets are essential for correct VWF dimer incorporation into growing tubules and to prevent crosslinking between neighbouring tubules. Further insights into the structure of the domains and flexible segments in VWF provide an overall view of VWF structure important for understanding both the biogenesis of ultralong concatamers at acidic pH and flow-regulated changes in concatamer conformation in plasma at alkaline pH that trigger hemostasis.