Microparticles are membrane vesicles released from many different cell types. There are two mechanisms that can result in their formation, cell activation and apoptosis. In these two mechanisms, ...different pathways are involved in microparticle generation. Microparticle generation seems to be a well regulated process. Microparticles vary in size, phospholipid and protein composition. They have a potent pro-inflammatory effect, promote coagulation and affect vascular function. Since these processes are all involved in the pathogenesis of cardiovascular disease and circulating microparticle numbers are altered in many cardiovascular diseases, a role for microparticles in the pathogenesis of cardiovascular diseases is likely. Although hard evidence for a role of microparticles in cardiovascular diseases at present is still only limited, new evidence is accumulating rapidly to support this theory. Elucidation of the microparticle composition and the mechanisms involved in exertion of their effects will supply this evidence and enable us to develop additional intervention strategies for prevention and treatment of cardiovascular diseases.
Extracellular vesicles (EVs) in plasma are commonly identified by staining with antibodies and generic dyes, but the specificity of antibodies and dyes to stain EVs is often unknown. Previously, we ...showed that platelet-depleted platelet concentrate contains two populations of particles >200 nm, one population with a refractive index (RI) < 1.42 that included the majority of EVs, and a second population with an RI > 1.42, which was thought to include lipoproteins. In this study, we investigated whether EVs can be distinguished from lipoproteins by the RI and whether the RI can be used to determine the specificity of antibodies and generic dyes used to stain plasma EVs. EVs and lipoproteins present in platelet-depleted platelet concentrate were separated by density gradient centrifugation. The density fractions were analyzed by Western blot and transmission electron microscopy, the RI of particles was determined by Flow-SR. The RI was used to evaluate the staining specificity of an antibody against platelet glycoprotein IIIa (CD61) and the commonly used generic dyes calcein AM, calcein violet, di-8-ANEPPS, and lactadherin in plasma. After density gradient centrifugation, EV-enriched fractions (1.12 to 1.07 g/mL) contained the highest concentration of particles with an RI < 1.42, and the lipoprotein-enriched fractions (1.04 to 1.03 g/mL) contained the highest concentration of particles with an RI > 1.42. Application of the RI showed that CD61-APC had the highest staining specificity for EVs, followed by lactadherin and calcein violet. Di-8-ANEPPS stained mainly lipoproteins and calcein AM stained neither lipoproteins nor EVs. Taken together, the RI can be used to distinguish EVs and lipoproteins, and thus allows evaluation of the specificity of antibodies and generic dyes to stain EVs.
At present, little is known about the clearance of platelet-derived microparticles (PMP) in human blood, as due to ethical considerations infusion experiments with labeled microparticles are ...delicate. Therefore, we investigated the kinetics of PMP, which are abundantly present in apheresis platelet concentrates (PC), following platelet transfusion in severe thrombocytopenic patients (n = 11). PMP were double-stained with annexin V and cell-specific antibodies (anti-CD61, anti-CD63 or anti-CD62P, respectively) and detected by flow cytometry before and after transfusion of a single PC at fixed time intervals. Upon transfusion, the plasma levels of MP binding annexin V (2.5-fold), PMP (CD61+; 2.9-fold), and PMP from activated platelets (CD63+; 1.9-fold) or P-selectin (2.5-fold) increased immediately. The plasma levels of MP decreased with a half life of 5.8 hours (annexin V; 95% CI: 1.8-18.3) and 5.3 hours (CD61; 95% CI: 2.0-14.2). This is the first report in which the half life time of transfused PMP has been investigated in humans.
Extracellular vesicles (EVs) have great potential as biomarkers since their composition and concentration in biofluids are disease state dependent and their cargo can contain disease-related ...information. Large tumor-derived EVs (tdEVs, >1 μm) in blood from cancer patients are associated with poor outcome, and changes in their number can be used to monitor therapy effectiveness. Whereas, small tumor-derived EVs (<1 μm) are likely to outnumber their larger counterparts, thereby offering better statistical significance, identification and quantification of small tdEVs are more challenging. In the blood of cancer patients, a subpopulation of EVs originate from tumor cells, but these EVs are outnumbered by non-EV particles and EVs from other origin. In the Dutch NWO Perspectief Cancer-ID program, we developed and evaluated detection and characterization techniques to distinguish EVs from non-EV particles and other EVs. Despite low signal amplitudes, we identified characteristics of these small tdEVs that may enable the enumeration of small tdEVs and extract relevant information. The insights obtained from Cancer-ID can help to explore the full potential of tdEVs in the clinic.
Essentials
Human salivary extracellular vesicles (EVs) expose coagulant tissue factor (TF).
Salivary EVs expose CD24, a ligand of P‐selectin.
CD24 and coagulant TF co‐localize on salivary EVs.
...TF+/CD24+ salivary EVs bind to activated platelets and trigger coagulation.
Summary
Background
Extracellular vesicles (EVs) from human saliva expose coagulant tissue factor (TF). Whether such TF‐exposing EVs contribute to hemostasis, however, is unknown. Recently, in a mice model, tumor cell‐derived EVs were shown to deliver coagulant TF to activated platelets at a site of vascular injury via interaction between P‐selectin glycoprotein ligand‐1 (PSGL‐1) and P‐selectin.
Objectives
We hypothesized that salivary EVs may deliver coagulant TF to activated platelets via interaction with P‐selectin.
Methods
We investigated the presence of two ligands of P‐selectin on salivary EVs, PSGL‐1 and CD24.
Results
Salivary EVs expose CD24 but PSGL‐1 was not detected. Immune depletion of CD24‐exposing EVs completely abolished the TF‐dependent coagulant activity of cell‐free saliva, showing that coagulant TF and CD24 co‐localize on salivary EVs. In a whole blood perfusion model, salivary EVs accumulated at the surface of activated platelets and promoted fibrin generation, which was abolished by an inhibitory antibody against human CD24.
Conclusions
A subset of EVs in human saliva expose coagulant TF and CD24, a ligand of P‐selectin, suggesting that such EVs may facilitate hemostasis at a site of skin injury where the wound is licked in a reflex action.
We determined the numbers, cellular origin and thrombin-generating properties of microparticles in healthy individuals (n = 15). Microparticles, isolated from fresh blood samples and identified by ...flow cytometry, originated from platelets 237 x 10(6)/L (median; range 116-565), erythrocytes (28 x 10(6)/L; 13-46), granulocytes (46 x 10(6)/L; 16-94) and endothelial cells (64 x 10(6)/L; 16-136). They bound annexin V, indicating surface exposure of phosphatidylserine, and supported coagulation in vitro. Interestingly, coagulation occurred via tissue factor (TF)-independent pathways, because antibodies against TF or factor (F)VII were ineffective. In contrast, in our in vitro experiments coagulation was partially inhibited by antibodies against FXII (12%, p = 0.006), FXI (36%, p <0.001), FIX (28%, p <0.001) or FVIII (32%, p <0.001). Both the number of annexin V-positive microparticles present in plasma and the thrombin-generating capacity inversely correlated to the plasma concentrations of thrombin-antithrombin complex (r = -0.49, p = 0.072 and r = -0.77, p = 0.001, respectively), but did not correlate to prothrombin fragment F1+2 (r = -0.002, p = 0.99). The inverse correlations between the number of microparticles and their thrombin-forming capacity and the levels of thrombin-antithrombin complex in plasma may indicate that microparticles present in the circulation of healthy individuals have an anticoagulant function by promoting the generation of low amounts of thrombin that activate protein C. We conclude that microparticles in blood from healthy individuals support thrombin generation via TF- and FVII-independent pathways, and which may have an anticoagulant function.
Cleavage of Rho associated Coiled Coil kinase I (ROCK I) by caspase-3 contributes to membrane blebbing. Whether caspase-3 and ROCK I also play a role in the release of membrane vesicles is unknown. ...Therefore, we transfected a human breast cancer cell line (MCF-7) that is caspase-3 deficient, lacks membrane blebbing, and does not release membrane vesicles, with caspase-3. Cells expressing caspase-3 demonstrate both ROCK I-mediated membrane blebbing, and release of small (400–600nm) membrane vesicles in a ROCK I-independent manner. These membrane vesicles contain caspase-3, and are enriched in caspase-3 activity compared to the releasing cells. Caspase-3-containing vesicles are taken up by untransfected cells but the cells do not show any sign of apoptosis. In conclusion, we show that the release of caspase-3-enriched membrane vesicles and membrane blebbing are two differentially regulated processes. Furthermore, we hypothesize that packaging of caspase-3 into membrane vesicles contributes to cellular homeostasis by the removal of caspase-3, and concurrently, protects the cells' environment from direct exposure to caspase-3 activity.
•Expression of caspase-3 results in ROCK I independent release of membrane vesicles.•Release of vesicles and membrane blebbing are two different processes.•The released vesicles are enriched in caspase-3 activity, compared to the cells.•Addition of caspase-3 enriched vesicles to cells does not result in cellular apoptosis.
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