Actomyosin II contractility expels von Willebrand factor from Weibel-Palade bodies during exocytosis

2-Chlorofatty acids induce Weibel-Palade body mobilization

Endothelial dysfunction is a hallmark of multiple inflammatory diseases. Leukocyte interactions with the endothelium have significant effects on vascular wall biology and pathophysiology. Myeloperoxidase (MPO)-derived oxidant products released from leukocytes are potential mediators of inflammation and endothelial dysfunction. 2-Chlorofatty acids (2-ClFAs) are produced as a result of MPO-derived HOCl targeting plasmalogen phospholipids. Chlorinated lipids have been shown to be associated with multiple inflammatory diseases, but their impact on surrounding endothelial cells has not been examined. This study tested the biological properties of the 2-ClFA molecular species 2-chlorohexadecanoic acid (2-ClHA) on endothelial cells. A synthetic alkyne analog of 2-ClHA, 2-chlorohexadec-15-ynoic acid (2-ClHyA), was used to examine the subcellular localization of 2-ClFA in human coronary artery endothelial cells. Click chemistry experiments revealed that 2-ClHyA localizes to Weibel-Palade bodies. 2-ClHA and 2-ClHyA promote the release of P-selectin, von Willebrand factor, and angiopoietin-2 from endothelial cells. Functionally, 2-ClHA and 2-ClHyA cause neutrophils to adhere to and platelets to aggregate on the endothelium, as well as increase permeability of the endothelial barrier which has been tied to the release of angiopoietin-2. These findings suggest that 2-ClFAs promote endothelial cell dysfunction, which may lead to broad implications in inflammation, thrombosis, and blood vessel stability.

Myosin IIa is critical for cAMP-mediated endothelial secretion of von Willebrand factor

Nonmuscle myosin II has been implicated in regulation of von Willebrand factor (VWF) release from endothelial Weibel-Palade bodies (WPBs), but the specific role of myosin IIa isoform is poorly defined. Here, we report that myosin IIa is expressed both in primary human endothelial cells and intact mouse vessels, essential for cyclic adenosine monophosphate (cAMP)-mediated endothelial VWF secretion. Downregulation of myosin IIa by shRNAs significantly suppressed both forskolin- and epinephrine-induced VWF secretion. Endothelium-specific myosin IIa knockout mice exhibited impaired epinephrine-stimulated VWF release, prolonged bleeding time, and thrombosis. Further study showed that in resting cells, myosin IIa deficiency disrupted the peripheral localization of Rab27-positive WPBs along stress fibers; on stimulation by cAMP agonists, myosin IIa in synergy with zyxin promotes the formation of a functional actin framework, which is derived from preexisting cortical actin filaments, around WPBs, facilitating fusion and subsequent exocytosis. In summary, our findings not only identify new functions of myosin IIa in regulation of WPB positioning and the interaction between preexisting cortical actin filaments and exocytosing vesicles before fusion but also reveal myosin IIa as a physiological regulator of endothelial VWF secretion in stress-induced hemostasis and thrombosis.

Weibel-Palade bodies at a glance

The vascular environment can rapidly alter, and the speed with which responses to both physiological and pathological changes are required necessitates the existence of a highly responsive system. The endothelium can quickly deliver bioactive molecules by regulated exocytosis of its secretory granules, the Weibel-Palade bodies (WPBs). WPBs include proteins that initiate both haemostasis and inflammation, as well those that modulate blood pressure and angiogenesis. WPB formation is driven by von Willebrand factor, their most abundant protein, which controls both shape and size of WPBs. WPB are generated in a range of sizes, with the largest granules over ten times the size of the smallest. In this Cell Science at a Glance and the accompanying poster, we discuss the emerging mechanisms by which WPB size is controlled and how this affects the ability of this organelle to modulate haemostasis. We will also outline the different modes of exocytosis and their polarity that are currently being explored, and illustrate that these large secretory organelles provide a model for how elements of secretory granule biogenesis and exocytosis cooperate to support a complex and diverse set of functions.

Clathrin-mediated post-fusion membrane retrieval influences the exocytic mode of endothelial Weibel-Palade bodies

Weibel-Palade bodies (WPBs), the storage organelles of endothelial cells, are essential to normal haemostatic and inflammatory responses. Their major constituent protein is von Willebrand factor (VWF) which, following stimulation with secretagogues, is released into the blood vessel lumen as large platelet-catching strings. This exocytosis changes the protein composition of the cell surface and also results in a net increase in the amount of plasma membrane. Compensatory endocytosis is thought to limit changes in cell size and retrieve fusion machinery and other misplaced integral membrane proteins following exocytosis; however, little is known about the extent, timing, mechanism and precise function of compensatory endocytosis in endothelial cells. Using biochemical assays, live-cell imaging and correlative spinning-disk microscopy and transmission electron microscopy assays we provide the first in-depth high-resolution characterisation of this process. We provide a model of compensatory endocytosis based on rapid clathrin- and dynamin-mediated retrieval. Inhibition of this process results in a change of exocytic mode: WPBs then fuse with previously fused WPBs rather than the plasma membrane, leading, in turn, to the formation of structurally impaired tangled VWF strings.

This article has an associated First Person interview with the first authors of the paper.

Summary: Compensatory endocytosis plays key roles in Weibel-Palade body exocytosis. Inhibition of this process results in a change of exocytic mode and the release of von Willebrand factor as tangled strings.

Concerted actions of distinct nonmuscle myosin II isoforms drive intracellular membrane remodeling in live animals

Membrane remodeling plays a fundamental role during a variety of biological events. However, the dynamics and the molecular mechanisms regulating this process within cells in mammalian tissues in situ remain largely unknown. In this study, we use intravital subcellular microscopy in live mice to study the role of the actomyosin cytoskeleton in driving the remodeling of membranes of large secretory granules, which are integrated into the plasma membrane during regulated exocytosis. We show that two isoforms of nonmuscle myosin II, NMIIA and NMIIB, control distinct steps of the integration process. Furthermore, we find that F-actin is not essential for the recruitment of NMII to the secretory granules but plays a key role in the assembly and activation of NMII into contractile filaments. Our data support a dual role for the actomyosin cytoskeleton in providing the mechanical forces required to remodel the lipid bilayer and serving as a scaffold to recruit key regulatory molecules.

A role for tropomyosins in activity-dependent bulk endocytosis?

Bulk endocytosis allows stimulated neurons to take up a large portion of the presynaptic plasma membrane in order to regenerate synaptic vesicle pools. Actin, one of the most abundant proteins in eukaryotic cells, plays an important role in this process, but a detailed mechanistic understanding of the involvement of the cortical actin network is still lacking, in part due to the relatively small size of nerve terminals and the limitation of optical microscopy. We recently discovered that neurosecretory cells display a similar, albeit much larger, form of bulk endocytosis in response to secretagogue stimulation. This allowed us to identify a novel highly dynamic role for the acto-myosin II cortex in generating constricting rings that precede the fission of nascent bulk endosomes. In this review we focus on the mechanism underpinning this dramatic switch in the organization and function of the cortical actin network. We provide additional experimental data that suggest a role of tropomyosin Tpm3.1 and Tpm4.2 in this process, together with an emerging model of how actin controls bulk endocytosis.

Real-time insights into regulated exocytosis

Real-time imaging of regulated exocytosis in secreting organs can provide unprecedented temporal and spatial detail. Here, we highlight recent advances in 3D time-lapse imaging in Drosophila salivary glands at single-granule resolution. Using fluorescently labeled proteins expressed in the fly, it is now possible to image the dynamics of vesicle biogenesis and the cytoskeletal factors involved in secretion. 3D imaging over time allows one to visualize and define the temporal sequence of events, including clearance of cortical actin, fusion pore formation, mixing of the vesicular and plasma membranes and recruitment of components of the cytoskeleton. We will also discuss the genetic tools available in the fly that allow one to interrogate the essential factors involved in secretory vesicle formation, cargo secretion and the ultimate integration of the vesicular and plasma membranes. We argue that the combination of high-resolution real-time imaging and powerful genetics provides a platform to investigate the role of any factor in regulated secretion.

Zyxin regulates endothelial von Willebrand factor secretion by reorganizing actin filaments around exocytic granules

Endothelial exocytosis of Weibel-Palade body (WPB) is one of the first lines of defence against vascular injury. However, the mechanisms that control WPB exocytosis in the final stages (including the docking, priming and fusion of granules) are poorly understood. Here we show that the focal adhesion protein zyxin is crucial in this process. Zyxin downregulation inhibits the secretion of von Willebrand factor (VWF), the most abundant cargo in WPBs, from human primary endothelial cells (ECs) induced by cAMP agonists. Zyxin-deficient mice exhibit impaired epinephrine-stimulated VWF release, prolonged bleeding time and thrombosis, largely due to defective endothelial secretion of VWF. Using live-cell super-resolution microscopy, we visualize previously unappreciated reorganization of pre-existing actin filaments around WPBs before fusion, dependent on zyxin and an interaction with the actin crosslinker α-actinin. Our findings identify zyxin as a physiological regulator of endothelial exocytosis through reorganizing local actin network in the final stage of exocytosis.

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.

Von Willebrand factor processing

Von Willebrand factor (VWF) is a multimeric glycoprotein essential for primary haemostasis that is produced only in endothelial cells and megakaryocytes. Key to VWF's function in recruitment of platelets to the site of vascular injury is its multimeric structure. The individual steps of VWF multimer biosynthesis rely on distinct posttranslational modifications at specific pH conditions, which are realized by spatial separation of the involved processes to different cell organelles. Production of multimers starts with translocation and modification of the VWF prepropolypeptide in the endoplasmic reticulum to produce dimers primed for glycosylation. In the Golgi apparatus they are further processed to multimers that carry more than 300 complex glycan structures functionalized by sialylation, sulfation and blood group determinants. Of special importance is the sequential formation of disulfide bonds with different functions in structural support of VWF multimers, which are packaged, stored and further processed after secretion. Here, all these processes are being reviewed in detail including background information on the occurring biochemical reactions.

Actin dynamics provides membrane tension to merge fusing vesicles into the plasma membrane

Vesicle fusion is executed via formation of an Ω-shaped structure (Ω-profile), followed by closure (kiss-and-run) or merging of the Ω-profile into the plasma membrane (full fusion). Although Ω-profile closure limits release but recycles vesicles economically, Ω-profile merging facilitates release but couples to classical endocytosis for recycling. Despite its crucial role in determining exocytosis/endocytosis modes, how Ω-profile merging is mediated is poorly understood in endocrine cells and neurons containing small ∼30–300 nm vesicles. Here, using confocal and super-resolution STED imaging, force measurements, pharmacology and gene knockout, we show that dynamic assembly of filamentous actin, involving ATP hydrolysis, N-WASP and formin, mediates Ω-profile merging by providing sufficient plasma membrane tension to shrink the Ω-profile in neuroendocrine chromaffin cells containing ∼300 nm vesicles. Actin-directed compounds also induce Ω-profile accumulation at lamprey synaptic active zones, suggesting that actin may mediate Ω-profile merging at synapses. These results uncover molecular and biophysical mechanisms underlying Ω-profile merging.

As vesicles fuse to the plasma membrane, they form intermediate Ω-shaped structures followed by either closure of the pore or full merging with the plasma membrane. Here Wen et al. show that dynamic actin assembly provides membrane tension to promote Ω merging in neuroendocrine cells and synapses.

Super-resolution imaging for monitoring cytoskeleton dynamics

The cytoskeleton is a key cellular structure that is important in the control of cellular movement, structure, and sensing. To successfully image the individual cytoskeleton components, high resolution and super-resolution fluorescence imaging methods are needed. This review covers the three basic cytoskeletal elements and the relative benefits and drawbacks of fixed versus live cell imaging before moving on to recent studies using high resolution and super-resolution techniques. The techniques covered include the near-diffraction limited imaging methods of confocal microscopy and TIRF microscopy and the super-resolution fluorescence imaging methods of STORM, PALM, and STED.

Actin puts the squeeze on Drosophila glue secretion

An actin filament coat promotes cargo expulsion from large exocytosing vesicles, but the mechanisms of coat formation and force generation have been poorly characterized. Elegant imaging studies of the Drosophila melanogaster salivary gland now reveal how actin and myosin are recruited, and show that myosin II forms a contractile 'cage' that facilitates exocytosis.

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

The von Willebrand factor (VWF) synthesized and secreted by endothelial cells is central to hemostasis and thrombosis, providing a multifunctional adhesive platform that brings together components needed for these processes. VWF secretion can occur from both apical and basolateral sides of endothelial cells, and from constitutive, basal, and regulated secretory pathways, the latter two via Weibel-Palade bodies (WPB). Although the amount and structure of VWF is crucial to its function, the extent of VWF release, multimerization, and polarity of the 3 secretory pathways have only been addressed separately, and with conflicting results. We set out to clarify these relationships using polarized human umbilical vein endothelial cells (HUVECs) grown on Transwell membranes. We found that regulated secretion of ultra-large (UL)-molecular-weight VWF predominantly occurred apically, consistent with a role in localized platelet capture in the vessel lumen. We found that constitutive secretion of low-molecular-weight (LMW) VWF is targeted basolaterally, toward the subendothelial matrix, using the adaptor protein complex 1 (AP-1), where it may provide the bulk of collagen-bound subendothelial VWF. We also found that basally-secreted VWF is composed of UL-VWF, released continuously from WPBs in the absence of stimuli, and occurs predominantly apically, suggesting this could be the main source of circulating plasma VWF. Together, we provide a unified dataset reporting the amount and multimeric state of VWF secreted from the constitutive, basal, and regulated pathways in polarized HUVECs, and have established a new role for AP-1 in the basolateral constitutive secretion of VWF.

The role of myosin 1c and myosin 1b in surfactant exocytosis

Actin and actin-associated proteins have a pivotal effect on regulated exocytosis in secretory cells and influence pre-fusion as well as post-fusion stages of exocytosis. Actin polymerization on secretory granules during the post-fusion phase (formation of an actin coat) is especially important in cells with large secretory vesicles or poorly soluble secretions. Alveolar type II (ATII) cells secrete hydrophobic lipo-protein surfactant, which does not easily diffuse from fused vesicles. Previous work showed that compression of actin coat is necessary for surfactant extrusion. Here, we investigate the role of class 1 myosins as possible linkers between actin and membranes during exocytosis. Live-cell microscopy showed translocation of fluorescently labeled myosin 1b and myosin 1c to the secretory vesicle membrane after fusion. Myosin 1c translocation was dependent on its pleckstrin homology domain. Expression of myosin 1b and myosin 1c constructs influenced vesicle compression rate, whereas only the inhibition of myosin 1c reduced exocytosis. These findings suggest that class 1 myosins participate in several stages of ATII cell exocytosis and link actin coats to the secretory vesicle membrane to influence vesicle compression.

Cdc42-dependent actin dynamics controls maturation and secretory activity of dendritic cells

Cdc42 control of actin dynamics keeps DCs in an immature state, and loss of Cdc42 activity facilitates secretion and rapid up-regulation of intracellular molecules to the cell surface, which shows that Cdc42 contributes to DC immunogenicity by regulating the DC actin cytoskeleton.

Cell division cycle 42 (Cdc42) is a member of the Rho guanosine triphosphatase family and has pivotal functions in actin organization, cell migration, and proliferation. To further study the molecular mechanisms of dendritic cell (DC) regulation by Cdc42, we used Cdc42-deficient DCs. Cdc42 deficiency renders DCs phenotypically mature as they up-regulate the co-stimulatory molecule CD86 from intracellular storages to the cell surface. Cdc42 knockout DCs also accumulate high amounts of invariant chain–major histocompatibility complex (MHC) class II complexes at the cell surface, which cannot efficiently present peptide antigens (Ag’s) for priming of Ag-specific CD4 T cells. Proteome analyses showed a significant reduction in lysosomal MHC class II–processing proteins, such as cathepsins, which are lost from DCs by enhanced secretion. As these effects on DCs can be mimicked by chemical actin disruption, our results propose that Cdc42 control of actin dynamics keeps DCs in an immature state, and cessation of Cdc42 activity during DC maturation facilitates secretion as well as rapid up-regulation of intracellular molecules to the cell surface.

Interaction between MyRIP and the actin cytoskeleton regulates Weibel-Palade body trafficking and exocytosis

Weibel-Palade body (WPB)-actin interactions are essential for the trafficking and secretion of von Willebrand factor; however, the molecular basis for this interaction remains poorly defined. Myosin Va (MyoVa or MYO5A) is recruited to WPBs by a Rab27A-MyRIP complex and is thought to be the prime mediator of actin binding, but direct MyRIP-actin interactions can also occur. To evaluate the specific contribution of MyRIP-actin and MyRIP-MyoVa binding in WPB trafficking and Ca(2+)-driven exocytosis, we used EGFP-MyRIP point mutants with disrupted MyoVa and/or actin binding and high-speed live-cell fluorescence microscopy. We now show that the ability of MyRIP to restrict WPB movement depends upon its actin-binding rather than its MyoVa-binding properties. We also show that, although the role of MyRIP in Ca(2+)-driven exocytosis requires both MyoVa- and actin-binding potential, it is the latter that plays a dominant role. In view of these results and together with the analysis of actin disruption or stabilisation experiments, we propose that the role of MyRIP in regulating WPB trafficking and exocytosis is mediated largely through its interaction with actin rather than with MyoVa.

Arp2/3-mediated F-actin formation controls regulated exocytosis in vivo

The actin cytoskeleton plays crucial roles in many cellular processes, including regulated secretion. However, the mechanisms controlling F-actin dynamics in this process are largely unknown. Through 3D time-lapse imaging in a secreting organ, we show that F-actin is actively disassembled along the apical plasma membrane at the site of secretory vesicle fusion and re-assembled directionally on vesicle membranes. Moreover, we show that fusion pore formation and PIP₂ redistribution precedes actin and myosin recruitment to secretory vesicle membranes. Finally, we show essential roles for the branched actin nucleators Arp2/3- and WASp in the process of secretory cargo expulsion and integration of vesicular membranes with the apical plasma membrane. Our results highlight previously unknown roles for branched actin in exocytosis and provide a genetically tractable system to image the temporal and spatial dynamics of polarized secretion in vivo.

The cytoskeleton plays a crucial role in secretion. Here Tran et al. demonstrate that cortical actin is rearranged at the site of vesicle fusion and recruited to fused secretory granules in Drosophila salivary glands, and show that branched actin nucleators are required for cargo expulsion.

Small GTPases promote actin coat formation on microsporidian pathogens traversing the apical membrane of Caenorhabditis elegans intestinal cells

Many intracellular pathogens co-opt actin in host cells, but little is known about these interactions in vivo. We study the in vivo trafficking and exit of the microsporidian Nematocida parisii, which is an intracellular pathogen that infects intestinal cells of the nematode Caenorhabditis elegans. We recently demonstrated that N. parisii uses directional exocytosis to escape out of intestinal cells into the intestinal tract. Here, we show that an intestinal-specific isoform of C. elegans actin called ACT-5 forms coats around membrane compartments that contain single exocytosing spores, and that these coats appear to form after fusion with the apical membrane. We performed a genetic screen for host factors required for actin coat formation and identified small GTPases important for this process. Through analysis of animals defective in these factors, we found that actin coats are not required for pathogen exit although they may boost exocytic output. Later during infection, we find that ACT-5 also forms coats around membrane-bound vesicles that contain multiple spores. These vesicles are likely formed by clathrin-dependent compensatory endocytosis to retrieve membrane material that has been trafficked to the apical membrane as part of the exocytosis process. These findings provide insight into microsporidia interaction with host cells, and provide novel in vivo examples of the manner in which intracellular pathogens co-opt host actin during their life cycle.

Imaging membrane remodeling during regulated exocytosis in live mice

In this mini-review we focus on the use of time-lapse light microscopy to study membrane remodeling during protein secretion in live animals. In particular, we highlight how subcellular intravital microscopy has enabled imaging the dynamics of both individual secretory vesicles and the plasma membrane, during different steps in the exocytic process. This powerful approach has provided us with the unique opportunity to unravel the role of the actin cytoskeleton in regulating this process under physiological conditions, and to overcome the shortcomings of more reductionist model systems.

The contractome – a systems view of actomyosin contractility in non-muscle cells

Actomyosin contractility is a highly regulated process that affects many fundamental biological processes in each and every cell in our body. In this Cell Science at a Glance article and the accompanying poster, we mined the literature and databases to map the contractome of non-muscle cells. Actomyosin contractility is involved in at least 49 distinct cellular functions that range from providing cell architecture to signal transduction and nuclear activity. Containing over 100 scaffolding and regulatory proteins, the contractome forms a highly complex network with more than 230 direct interactions between its components, 86 of them involving phosphorylation. Mapping these interactions, we identify the key regulatory pathways involved in the assembly of actomyosin structures and in activating myosin to produce contractile forces within non-muscle cells at the exact time and place necessary for cellular function.

An acto-myosin II constricting ring initiates the fission of activity-dependent bulk endosomes in neurosecretory cells

Activity-dependent bulk endocytosis allows neurons to internalize large portions of the plasma membrane in response to stimulation. However, whether this critical type of compensatory endocytosis is unique to neurons or also occurs in other excitable cells is currently unknown. Here we used fluorescent 70 kDa dextran to demonstrate that secretagogue-induced bulk endocytosis also occurs in bovine chromaffin cells. The relatively large size of the bulk endosomes found in this model allowed us to investigate how the neck of the budding endosomes constricts to allow efficient recruitment of the fission machinery. Using time-lapse imaging of Lifeact-GFP-transfected chromaffin cells in combination with fluorescent 70 kDa dextran, we detected acto-myosin II rings surrounding dextran-positive budding endosomes. Importantly, these rings were transient and contracted before disappearing, suggesting that they might be involved in restricting the size of the budding endosome neck. Based on the complete recovery of dextran fluorescence after photobleaching, we demonstrated that the actin ring-associated budding endosomes were still connected with the extracellular fluid. In contrast, no such recovery was observed following the constriction and disappearance of the actin rings, suggesting that these structures were pinched-off endosomes. Finally, we showed that the rings were initiated by a circular array of phosphatidylinositol(4,5)bisphosphate microdomains, and that their constriction was sensitive to both myosin II and dynamin inhibition. The acto-myosin II rings therefore play a key role in constricting the neck of budding bulk endosomes before dynamin-dependent fission from the plasma membrane of neurosecretory cells.

Actin depolymerisation and crosslinking join forces with myosin II to contract actin coats on fused secretory vesicles

In many secretory cells actin and myosin are specifically recruited to the surface of secretory granules following their fusion with the plasma membrane. Actomyosin-dependent compression of fused granules is essential to promote active extrusion of cargo. However, little is known about molecular mechanisms regulating actin coat formation and contraction. Here, we provide a detailed kinetic analysis of the molecules regulating actin coat contraction on fused lamellar bodies in primary alveolar type II cells. We demonstrate that ROCK1 and myosin light chain kinase 1 (MLCK1, also known as MYLK) translocate to fused lamellar bodies and activate myosin II on actin coats. However, myosin II activity is not sufficient for efficient actin coat contraction. In addition, cofilin-1 and α-actinin translocate to actin coats. ROCK1-dependent regulated actin depolymerisation by cofilin-1 in cooperation with actin crosslinking by α-actinin is essential for complete coat contraction. In summary, our data suggest a complementary role for regulated actin depolymerisation and crosslinking, and myosin II activity, to contract actin coats and drive secretion.

Yeast translation elongation factor-1A binds vacuole-localized Rho1p to facilitate membrane integrity through F-actin remodeling

Rho GTPases are molecular switches that modulate a variety of cellular processes, most notably those involving actin dynamics. We have previously shown that yeast vacuolar membrane fusion requires re-organization of actin filaments mediated by two Rho GTPases, Rho1p and Cdc42p. Cdc42p initiates actin polymerization to facilitate membrane tethering; Rho1p has a role in the late stages of vacuolar fusion, but its mode of action is unknown. Here, we identified eEF1A as a vacuolar Rho1p-interacting protein. eEF1A (encoded by the TEF1 and TEF2 genes in yeast) is an aminoacyl-tRNA transferase needed during protein translation. eEF1A also has a second function that is independent of translation; it binds and organizes actin filaments into ordered cable structures. Here, we report that eEF1A interacts with Rho1p via a C-terminal subdomain. This interaction occurs predominantly when both proteins are in the GDP-bound state. Therefore, eEF1A is an atypical downstream effector of Rho1p. eEF1A does not promote vacuolar fusion; however, overexpression of the Rho1p-interacting subdomain affects vacuolar morphology. Vacuoles were destabilized and prone to leakage when treated with the eEF1A inhibitor narciclasine. We propose a model whereby eEF1A binds to Rho1p-GDP on the vacuolar membrane; it is released upon Rho1p activation and then bundles actin filaments to stabilize fused vacuoles. Therefore, the Rho1p-eEF1A complex acts to spatially localize a pool of eEF1A to vacuoles where it can readily organize F-actin.

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.

Cdc42 controls the dilation of the exocytotic fusion pore by regulating membrane tension

On exocytosis, membrane fusion starts with the formation of a narrow fusion pore that must expand to allow the release of secretory compounds. The GTPase Cdc42 promotes fusion pore dilation in neuroendocrine cells by controlling membrane tension.

Membrane fusion underlies multiple processes, including exocytosis of hormones and neurotransmitters. Membrane fusion starts with the formation of a narrow fusion pore. Radial expansion of this pore completes the process and allows fast release of secretory compounds, but this step remains poorly understood. Here we show that inhibiting the expression of the small GTPase Cdc42 or preventing its activation with a dominant negative Cdc42 construct in human neuroendocrine cells impaired the release process by compromising fusion pore enlargement. Consequently the mode of vesicle exocytosis was shifted from full-collapse fusion to kiss-and-run. Remarkably, Cdc42-knockdown cells showed reduced membrane tension, and the artificial increase of membrane tension restored fusion pore enlargement. Moreover, inhibiting the motor protein myosin II by blebbistatin decreased membrane tension, as well as fusion pore dilation. We conclude that membrane tension is the driving force for fusion pore dilation and that Cdc42 is a key regulator of this force.

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.

Role of MARCKS in regulated secretion from mast cells and airway goblet cells

MARCKS (myristoylated alanine-rich C kinase substrate) is postulated to regulate the passage of secretory granules through cortical actin in the early phase of exocytosis. There are, however, three proposed mechanisms of action, all of which were derived from studies using synthetic peptides representing either the central phosphorylation site domain or the upstream, NH2-terminal domain: it tethers actin to the plasma membrane and/or to secretory granules, and/or it sequesters PIP2. Using MARCKS-null mice, we probed for a loss of function secretory phenotype in mast cells harvested from embryonic livers and maturated in vivo [embryonic hepatic-derived mast cells (eHMCs)]. Both wild-type (WT) and MARCKS-null eHMCs exhibited full exocytic responses upon FcϵRI receptor activation with DNP-BSA (2,4-dinitrophenyl-BSA), whether they were in suspension or adherent. The secretory responses of MARCKS-null eHMCs were consistently higher than those of WT cells, but the differences had sporadic statistical significance. The MARCKS-null cells exhibited faster secretory kinetics, however, achieving the plateau phase of the response with a t½ ∼2.5-fold faster. Hence, MARCKS appears to be a nonessential regulatory protein in mast cell exocytosis but exerts a negative modulation. Surprisingly, the MARCKS NH2-terminal peptide, MANS, which has been reported to inhibit mucin secretion from airway goblet cells (Li Y, Martin LD, Spizz G, Adler KB. J Biol Chem 276: 40982-40990, 2001), inhibited hexosaminidase secretion from WT and MARCKS-null eHMCs, leading us to reexamine its effects on mucin secretion. Results from studies using peptide inhibitors with human bronchial epithelial cells and with binding assays using purified mucins suggested that MANS inhibited the mucin binding assay, rather than the secretory response.

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.

Phosphatidylinositol-3,4,5-triphosphate-dependent Rac exchange factor 1 regulates epinephrine-induced exocytosis of Weibel-Palade bodies

BACKGROUND

Weibel-Palade bodies (WPBs) function as storage vesicles for von Willebrand factor (VWF) and a number of other bioactive compounds, including angiopoietin-2 and insulin-like growth factor-binding protein 7. WPBs release their content following stimulation with agonists that increase the level of intracellular Ca²⁺, such as thrombin, or agonists that increase intracellular levels of cAMP, such as epinephrine.

OBJECTIVE

Previously, we have shown that the exchange protein activated by cAMP, exchange protein activated by cAMP, and the small GTPase Rap1 are involved in cAMP-mediated release of WPBs. In this study, we explored potential downstream effectors of Rap1 in cAMP-mediated WPB release.

METHODS

Studies were performed in primary human umbilical vein endothelial cells. Activation of the small GTP-binding protein Rac1 was monitored by its ability to bind to the CRIB domain of the serine/threonine kinase P21-activated kinase (PAK)1. Downstream effectors of Rap1 were identified with a proteomic screen using a glutathione-S-transferase fusion of the Ras-binding domain of RalGDS. Functional involvement of candidate proteins in WPB release was determined by RNA interference (RNAi)-mediated knockdown of gene expression.

RESULTS

Depletion of Rac1 by RNAi prevented epinephrine-induced VWF secretion. Also, the Rac1 inhibitor EHT1864 reduced epinephrine-induced WPB release. We identified the phosphatidylinositol-3,4,5-triphosphate-dependent Rac exchange factor 1 (PREX1) and the regulatory β-subunit of phosphatidylinositol 3-kinase (PI3K) as downstream targets of Rap1. The PI3K inhibitor LY294002 reduced epinephrine-induced release of VWF. RNAi-mediated downregulation of PREX1 abolished epinephrine-induced but not thrombin-induced release of WPBs.

CONCLUSION

Our findings show that PREX1 regulates epinephrine-induced release of WPBs.

Measurement of dynamic F-actin changes during exocytosis

Exocytosis requires the fusion of vesicle membrane to the cell membrane. It is tightly regulated and orchestrated in space and time by diverse cellular mechanisms. It has long been recognized that one of these mechanisms is an essential role played by the cytoskeleton. In particular, accumulating evidence shows that the F-actin network is engaged during the final stages of vesicle interactions with the cell membrane. Using a combination of methods it is now possible to gain insights into F-actin dynamics and reveal its role during exocytosis. Here, we describe the use of two-photon and confocal microscopy to visualize F-actin changes at the cell membrane during exocytosis.

Omega-3 fatty acids modulate Weibel-Palade body degranulation and actin cytoskeleton rearrangement in PMA-stimulated human umbilical vein endothelial cells

Long chain omega-3 polyunsaturated fatty acids (LC n-3 PUFAs) produce cardiovascular benefits by improving endothelial function. Endothelial cells store von Willebrand factor (vWF) in cytoplasmic Weibel-Palade bodies (WPBs). We examined whether LC n-3 PUFAs regulate WPB degranulation using cultured human umbilical vein endothelial cells (HUVECs). HUVECs were incubated with or without 75 or 120 µM docosahexaenoic acid or eicosapentaenoic acid for 5 days at 37 °C. WPB degranulation was stimulated using phorbol 12-myristate 13-acetate (PMA), and this was assessed by immunocytochemical staining for vWF. Actin reorganization was determined using phalloidin-TRITC staining. We found that PMA stimulated WPB degranulation, and that this was significantly reduced by prior incubation of cells with LC n-3 PUFAs. In these cells, WPBs had rounded rather than rod-shaped morphology and localized to the perinuclear region, suggesting interference with cytoskeletal remodeling that is necessary for complete WPB degranulation. In line with this, actin rearrangement was altered in cells containing perinuclear WPBs, where cells exhibited a thickened actin rim in the absence of prominent cytoplasmic stress fibers. These findings indicate that LC n-3 PUFAs provide some protection against WBP degranulation, and may contribute to an improved understanding of the anti-thrombotic effects previously attributed to LC n-3 PUFAs.

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.

G protein-dependent basal and evoked endothelial cell vWF secretion

von Willebrand factor (vWF) secretion by endothelial cells (ECs) is essential for hemostasis and thrombosis; however, the molecular mechanisms are poorly understood. Interestingly, we observed increased bleeding in EC-Gα13(-/-);Gα12(-/-) mice that could be normalized by infusion of human vWF. Blood from Gα12(-/-) mice exhibited significantly reduced vWF levels but normal vWF multimers and impaired laser-induced thrombus formation, indicating that Gα12 plays a prominent role in EC vWF secretion required for hemostasis and thrombosis. In isolated buffer-perfused mouse lungs, basal vWF levels were significantly reduced in Gα12(-/-), whereas thrombin-induced vWF secretion was defective in both EC-Gαq(-/-);Gα11(-/-) and Gα12(-/-) mice. Using siRNA in cultured human umbilical vein ECs and human pulmonary artery ECs, depletion of Gα12 and soluble N-ethylmaleimide-sensitive-fusion factor attachment protein α (α-SNAP), but not Gα13, inhibited both basal and thrombin-induced vWF secretion, whereas overexpression of activated Gα12 promoted vWF secretion. In Gαq, p115 RhoGEF, and RhoA-depleted human umbilical vein ECs, thrombin-induced vWF secretion was reduced by 40%, whereas basal secretion was unchanged. Finally, in vitro binding assays revealed that Gα12 N-terminal residues 10-15 mediated the binding of Gα12 to α-SNAP, and an engineered α-SNAP binding-domain minigene peptide blocked basal and evoked vWF secretion. Discovery of obligatory and complementary roles of Gα12 and Gαq/11 in basal vs evoked EC vWF secretion may provide promising new therapeutic strategies for treatment of thrombotic disease.

Delivery of chemotherapeutic drugs in tumour cell-derived microparticles

Cellular microparticles are vesicular plasma membrane fragments with a diameter of 100-1,000 nanometres that are shed by cells in response to various physiological and artificial stimuli. Here we demonstrate that tumour cell-derived microparticles can be used as vectors to deliver chemotherapeutic drugs. We show that tumour cells incubated with chemotherapeutic drugs package these drugs into microparticles, which can be collected and used to effectively kill tumour cells in murine tumour models without typical side effects. We describe several mechanisms involved in this process, including uptake of drug-containing microparticles by tumour cells, synthesis of additional drug-packaging microparticles by these cells that contribute to the cytotoxic effect and the inhibition of drug efflux from tumour cells. This study highlights a novel drug delivery strategy with potential clinical application.

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.

Coordinated oscillations in cortical actin and Ca2+ correlate with cycles of vesicle secretion

The actin cortex both facilitates and hinders exocytosis of secretory granules. How cells consolidate these two opposing roles was not well understood. Here we show that antigen activation of mast cells induces oscillations in Ca2+ and PI(4,5)P2 lipids that in turn drive cyclic recruitment of N-WASP and cortical actin oscillations. Experimental and computational analysis argues that vesicle fusion correlates with the observed actin and Ca2+ oscillations. A vesicle secretion cycle starts with the capture of vesicles by actin when cortical F-actin is high, followed by vesicle passage through the cortex when F-actin levels are low, and vesicle fusion with the plasma membrane when Ca2+ levels subsequently increase. Thus, cells employ oscillating levels of Ca2+, PI(4,5)P2 and cortical F-actin to increase secretion efficiency, explaining how the actin cortex can function as a carrier as well as barrier for vesicle secretion.

Linking differences in membrane tension with the requirement for a contractile actomyosin scaffold during exocytosis in salivary glands

In all the major secretory organs regulated exocytosis is a fundamental process that is used for releasing molecules in the extracellular space. Molecules destined for secretion are packaged into secretory vesicles that fuse with the plasma membrane upon the appropriate stimulus. In exocrine glands, large secretory vesicles fuse with specialized domains of the plasma membrane, which form ductal structures that are in direct continuity with the external environment and exhibit various architectures and diameters. In a recent study, we used intravital microscopy to analyze in detail the dynamics of exocytic events in the salivary glands of live rodents under conditions that cannot be reproduced in in vitro or ex vivo model systems. We found that after the opening of the fusion pore large secretory vesicles gradually collapse with their limiting membranes being completely absorbed into the apical plasma membrane canaliculi within 40-60 sec. Moreover, we observed that this controlled collapse requires the contractile activity of actin and its motor myosin II, which are recruited onto the large secretory vesicles immediately after their fusion with the plasma membrane. Here we suggest that the actomyosin complex may be required to facilitate exocytosis in those systems, such as the salivary glands, in which the full collapse of the vesicles is not energetically favorable due to a difference in membrane tension between the large secretory vesicles and the canaliculi.

Formation of platelet-binding von Willebrand factor strings on non-endothelial cells

BACKGROUND AND OBJECTIVE

Von Willebrand factor (VWF) forms strings on activated vascular endothelial cells that recruit platelets and initiate clot formation. Alterations in VWF strings may disturb hemostasis. This study was aimed at developing a flexible model system for structure-function studies of VWF strings.

METHODS

VWF strings were generated by inducing exocytosis of pseudo-Weibel-Palade bodies from VWF-transfected HEK293 cells, and the properties of these strings under static conditions and under flow were characterized.

RESULTS

Upon exocytosis, VWF unfurled into strings several hundred micrometers in length. These strings could form bundles and networks, and bound platelets under flow, resembling authentic endothelial VWF strings. Anchorage of the platelet-decorated VWF strings was independent of P-selectin and integrin α(V) β(3). Translocation of platelets along the strings, elongation and fragmentation of the strings frequently occurred under flow. Furthermore, VWF variants with the p.Tyr87Ser and p.Cys2773Ser mutations, which are defective in multimer assembly, did not give rise to VWF strings. Also, insertion of the green fluorescent protein into VWF inhibited string formation.

CONCLUSIONS

HEK293 cells provide a flexible and useful model system for the study of VWF string formation. Our results suggest that structural changes in VWF may modulate string formation and function, and contribute to hemostatic disorders.

The VPS33B-binding protein VPS16B is required in megakaryocyte and platelet α-granule biogenesis

Patients with platelet α or dense δ-granule defects have bleeding problems. Although several proteins are known to be required for δ-granule development, less is known about α-granule biogenesis. Our previous work showed that the BEACH protein NBEAL2 and the Sec1/Munc18 protein VPS33B are required for α-granule biogenesis. Using a yeast two-hybrid screen, mass spectrometry, coimmunoprecipitation, and bioinformatics studies, we identified VPS16B as a VPS33B-binding protein. Immunoblotting confirmed VPS16B expression in various human tissues and cells including megakaryocytes and platelets, and also in megakaryocytic Dami cells. Characterization of platelets from a patient with arthrogryposis, renal dysfunction, and cholestasis (ARC) syndrome containing mutations in C14orf133 encoding VPS16B revealed pale-appearing platelets in blood films and electron microscopy revealed a complete absence of α-granules, whereas δ-granules were observed. Soluble and membrane-bound α-granule proteins were reduced or undetectable, suggesting that both releasable and membrane-bound α-granule constituents were absent. Immunofluorescence microscopy of Dami cells stably expressing GFP-VPS16B revealed that similar to VPS33B, GFP-VPS16B colocalized with markers of the trans-Golgi network, late endosomes and α-granules. We conclude that VPS16B, similar to its binding partner VPS33B, is essential for megakaryocyte and platelet α-granule biogenesis.

Multiple roles for the actin cytoskeleton during regulated exocytosis

Regulated exocytosis is the main mechanism utilized by specialized secretory cells to deliver molecules to the cell surface by virtue of membranous containers (i.e., secretory vesicles). The process involves a series of highly coordinated and sequential steps, which include the biogenesis of the vesicles, their delivery to the cell periphery, their fusion with the plasma membrane, and the release of their content into the extracellular space. Each of these steps is regulated by the actin cytoskeleton. In this review, we summarize the current knowledge regarding the involvement of actin and its associated molecules during each of the exocytic steps in vertebrates, and suggest that the overall role of the actin cytoskeleton during regulated exocytosis is linked to the architecture and the physiology of the secretory cells under examination. Specifically, in neurons, neuroendocrine, endocrine, and hematopoietic cells, which contain small secretory vesicles that undergo rapid exocytosis (on the order of milliseconds), the actin cytoskeleton plays a role in pre-fusion events, where it acts primarily as a functional barrier and facilitates docking. In exocrine and other secretory cells, which contain large secretory vesicles that undergo slow exocytosis (seconds to minutes), the actin cytoskeleton plays a role in post-fusion events, where it regulates the dynamics of the fusion pore, facilitates the integration of the vesicles into the plasma membrane, provides structural support, and promotes the expulsion of large cargo molecules.

Cell polarisation and the immunological synapse

Directed secretion by immune cells requires formation of the immunological synapse at the site of cell-cell contact, concomitant with a dramatic induction of cell polarity. Recent findings provide us with insights into the various steps that are required for these processes: for example, the first identification of a protein at the centrosome that regulates its relocation to the plasma membrane; the use of super-resolution imaging techniques to reveal a residual actin network at the immunological synapse that may permit secretory granule exocytosis; and the drawing of parallels between primary cilia and IS architecture. Here we discuss these and other novel findings that have advanced our understanding of the complex process of immunological synapse formation and subsequent induced cell polarity in immune cells.

Effect of laminar shear stress on the distribution of Weibel-Palade bodies in endothelial cells

BACKGROUND

Vascular endothelial cells (ECs) provide a highly interactive barrier between blood and the underlying tissues. It is well established that ECs exposed to laminar flow align in the direction of flow and also arrange their actin stress fibers in a parallel manner in the direction of flow. Also the organization of the microtubule network is altered in response to flow with repositioning of the microtubule-organizing centre (MTOC) in the direction of flow. Weibel-Palade bodies (WPBs) are endothelial cell specific storage organelles that contain a number of important homeostatic and inflammatory components. Dynamics of WPBs are controlled by microtubules and the actin cytoskeleton.

OBJECTIVES

Here, we monitored flow-induced changes in distribution of WPBs.

METHODS

ECs were exposed for five days to laminar shear stress of 10 dyne/cm(2). Subsequently we measured the distance of individual WPBs with respect to the centre of the nucleus using Image Pro Plus.

RESULTS

ECs aligned in the direction of flow under these conditions. After 5 days the MTOC was positioned downstream of the nucleus in the direction of the flow. The number of WPBs per cell was slightly reduced as a result of the application of flow. Unexpectedly, only minor differences in the distribution of WPBs in ECs cultured under laminar flow were observed when compared to that of cells grown under static conditions.

CONCLUSIONS

Our findings suggest that laminar flow does not induce major changes in number and distribution of WPBs in ECs.

Real-time measurement of F-actin remodelling during exocytosis using Lifeact-EGFP transgenic animals

F-actin remodelling is essential for a wide variety of cell processes. It is important in exocytosis, where F-actin coats fusing exocytic granules. The purpose of these F-actin coats is unknown. They may be important in stabilizing the fused granules, they may play a contractile role and promote expulsion of granule content and finally may be important in endocytosis. To elucidate these functions of F-actin remodelling requires a reliable method to visualize F-actin dynamics in living cells. The recent development of Lifeact-EGFP transgenic animals offers such an opportunity. Here, we studied the characteristics of exocytosis in pancreatic acinar cells obtained from the Lifeact-EGFP transgenic mice. We show that the time-course of agonist-evoked exocytic events and the kinetics of each single exocytic event are the same for wild type and Lifeact-EGFP transgenic animals. We conclude that Lifeact-EGFP animals are a good model to study of exocytosis and reveal that F-actin coating is dependent on the de novo synthesis of F-actin and that development of actin polymerization occurs simultaneously in all regions of the granule. Our insights using the Lifeact-EGFP mice demonstrate that F-actin coating occurs after granule fusion and is a granule-wide event.

Fifty years of Weibel-Palade bodies: the discovery and early history of an enigmatic organelle of endothelial cells

In 1962, a rod-shaped cytoplasmic organelle of endothelial cells, later called the Weibel-Palade body, was serendipitously discovered by electron microscopy. It contains a set of parallel tubules and is wrapped in a membrane. Subsequent studies in the following decades established the unique localization of this organelle in endothelial cells of all vertebrates studied, meaning that it could serve as a marker of endothelial cells in tissue cultures. However, these studies did not reveal its functional significance, except for an indication that it could be related to an undefined thromboplastic substance. Twenty years after its discovery as a structural entity, it was shown by others that it houses von Willebrand factor and is thus clearly related to the coagulation system. In this review, I provide a personal historical account of the discovery and the subsequent limited work that I carried out on the organelle, putting it in the perspective of the current state of knowledge after half a century of research by many scientists.