Characterisation and properties of ectosomes released by human polymorphonuclear neutrophils

Emission of membrane vesicles: roles in complement resistance, immunity and cancer

Complement-mediated cell death is caused by C5b-9, the membrane attack complex (MAC) composed of the five complement proteins C5b, C6, C7, C8, and C9. Assembly of the C5b-9 complex initiates oligomerization of C9 and production of a transmembrane protein channel that inflicts damage to target cells. For protection, cells eliminate the MAC from their surface either by ectocytosis (direct emission of membrane vesicles) or by endocytosis (internalization). The process of ectosome release is rapid and involves cytosolic Ca(2+) and activation of protein kinases, such as protein kinase C (PKC) and extracellular signal-regulated protein kinase (ERK). Recently, the involvement of mortalin (also known as GRP75 and mitochondrial hsp70) in MAC elimination has been suggested. Extracellular application of antibodies directed to mortalin increases cell sensitivity to MAC-mediated lysis. Release of membrane vesicles is ubiquitous and enhanced in apoptotic or tumor cells and upon cell activation. Composition of the ectosomes (also often referred to as microparticles) membrane proteins and lipids appears to be different from those of the original plasma membrane, indicating involvement of a selective sorting process during ectosome formation. Exosomes (unlike ectosomes) are membrane vesicles generated by endocytosis, endosome sorting into perinuclear multivesicular bodies (MVB) and exocytosis of MVBs. Exosomes appear to be different in size and composition from ectosomes. Exosome-associated MAC has also been described. Although research on ectosomes and exosomes is still limited, physiological roles in coagulation, vascular functions, angiogenesis, wound healing and development have been attributed to these shed membrane vesicles. On the other hand, there are indications that elevated levels of ectosomes and exosomes may predispose to morbidity. Membrane vesicles released by cells exposed to complement MAC may play roles in health and disease beyond protection from cell death.

The Platelet Microparticle Proteome

Platelet-derived microparticles are the most abundant type of microparticle in human blood and contribute to many biologically significant processes. Here, we report the first proteomic analysis of microparticles generated from activated platelets. Using 1D SDS-PAGE and liquid chromatography coupled to a linear ion trap mass spectrometer, the identification of 578 proteins was accomplished using a minimum of 5 MS/MS detections of at least two different peptides for each protein. These microparticles displayed many proteins intrinsic to and well-characterized on platelets. For example, microparticles in these experiments were found to contain membrane surface proteins including GPIIIa, GPIIb, and P-selectin, as well other platelet proteins such as the chemokines CXCL4, CXCL7, and CCL5. In addition, approximately 380 of the proteins identified were not found in two previous studies of the platelet proteome. Since several of the proteins detected here have been previously implicated in microparticle formation and/or pathological function, it is hoped that this study will help fuel future work concerning the possible role of microparticles in various disease states.

Microparticles released by human neutrophils adhere to erythrocytes in the presence of complement

The release of cell surface-derived microparticles, or ectosomes, has now been described for many different cell types. In various diseases characterized by systemic inflammation, the numbers of ectosomes released from specific cell-types are found increased manifold in the circulation. Their pro-inflammatory and pro-coagulant functions make them potentially important actors in disease establishment and/or progression. Until now, ectosomes have been believed to be free in the circulation. Herein, we provide evidence for sequestration of ectosomes derived from human polymorphonuclear neutrophils to erythrocytes, similarly to immune complexes. We show that ectosomes activate and bind complement in vitro. In whole blood, opsonization of ectosomes by complement mediated their immune adherence to erythrocytes through complement receptor 1. Taken together, our data suggest an important role for complement and erythrocytes in the sequestration, and possibly clearance, of blood-borne ectosomes stemming from neutrophils. The immune adherence described here may modify the biological activity and function of ectosomes.

Distinct Signaling Pathways Are Involved in Leukosialin (CD43) Down-regulation, Membrane Blebbing, and Phospholipid Scrambling during Neutrophil Apoptosis

Although leukosialin (CD43) membrane expression decreases during neutrophil apoptosis, the CD43 molecule, unexpectedly, is neither proteolyzed nor internalized. We thus wondered whether it could be shed on bleb-derived membrane vesicles. Membrane blebbing is a transient event, hardly appreciated during the asynchronous, spontaneous apoptosis of neutrophils. Cell pre-synchronization at 15 degrees C made it possible to observe numerous blebbing neutrophils for a short 1-h period at 37 degrees C. CD43 down-regulation co-occurred with the blebbing stage and phosphatidylserine externalization, shortly after mitochondria depolarization and before nuclear condensation. Blebs detaching from the cell body were observed by time lapse fluorescence microscopy, and the release of bleb-derived vesicles was followed by flow cytometry. Phosphatidylserine externalization required caspases and protein kinase C (PKC) but not the myosin light chain kinase (MLCK). By contrast, bleb formation and release was caspase- and PKC-independent but required an active MLCK, whereas CD43 down-regulation involved caspases but neither PKC nor MLCK. Furthermore, CD43 appeared mostly excluded from membrane blebs by electron microscopy. Thus, CD43 down-regulation does not result from the release of bleb-derived vesicles. Ultracentrifugation of apoptotic cell supernatants made it possible to recover <1 microM microparticles, which contained the entire CD43 molecule. These microparticles expressed neutrophil membrane markers such as CD11b, CD66b, and CD63, together with CD43. In conclusion, we show that the three early membrane events of apoptosis, namely blebbing, phosphatidylserine externalization, and CD43 down-regulation, result from different signaling pathways and can occur independently from one another. CD43 down-regulation results from the shedding of microparticles released during apoptosis but unrelated to the blebbing.

Proteomic analysis of melanoma-derived exosomes by two-dimensional polyacrylamide gel electrophoresis and mass spectrometry

Exosomes are 40-100 nm vesicles released by numerous cell types and are thought to have a variety of roles depending on their origin. Exosomes derived from antigen presenting cells have been shown to be capable of initiating immune responses in vivo and eradicating established tumours in murine models. Tumour-derived exosomes can be utilised as a source of tumour antigen for cross-priming to T-cells and are thus of interest for use in anti-tumour immunotherapy. Further exploration into the protein composition of exosomes may increase our understanding of their potential roles in vivo and this study has examined the proteome of exosomes purified from cell supernatants of the melanoma cell lines MeWo and SK-MEL-28. The vesicular nature and size (30-100 nm) of the purified exosomes was confirmed by electron microscopy and sucrose density gradient centrifugation. Western blotting demonstrated the absence of calnexin and cytochrome c, verifying the purity of the exosome preparations, as well as enrichment of MHC class I and the tumour-associated antigens Mart-1 and Mel-CAM. The two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) protein profiles of exosomes from the two cell lines were highly comparable and strikingly different from the profiles of the total cell lysates. Mass spectrometric sequencing identified proteins present in 49 protein spots in the exosome lysates. Several of these have been identified previously in exosomes but some are novel, including p120 catenin, radixin, and immunoglobulin superfamily member 8 (PGRL). Proteins present in whole-cell lysates that were significantly reduced or excluded from exosomes were also identified and included several mitochondrial and lysosomal proteins, again confirming the proposed endosomal origin of exosomes. This study presents a starting point for future more in-depth protein studies of tumour-derived exosomes which will aid the understanding of their biogenesis and targeting for use in anti-tumour immunotherapy protocols.

Ceramides and glycosphingolipids in maturation process: leukemic cells as an experimental model

Leukemic cells were used as experimental material to demonstrate changes in the content of GSLs during the development and maturation of neutrophils. The most abundant cellular GSL is LacCer. An elevation in the LacCer level occurs twice during the maturation process: initially, on formation of azurophil granules, and subsequently, (a more significant rise) on formation of specific granules. The formation of the latter is accompanied by an increase in the level of GalGalCer. During the maturation of myeloblasts, there is a simultaneous growth in the content of LacCer and GM3 as well as that of their common precursors, that is, free ceramides. Like other tumor cells, GM3 rich myeloblasts in the peripheral blood from patients with AML are characterized by shedding of gangliosides. The quantitative Cer/GlcCer ratio in these cells seems to be advantageous for the efficacy of chemotherapy in the induction of apoptosis. Myelo- and metamyelocytes achieve the highest level of GSLs. Their entry into the full maturity stage is accompanied by a decrease in the level of GSLs. Patterns of GSLs expression change greatly during development and maturation. However, with respect to the composition and content of GSLs, there are no significant differences between normal and leukemic mature neutrophils. At each stage of the development and maturation of myelogenous leukemic cells, as well as in normal mature neutrophils, there occurs the synthesis of the same molecular species both free ceramides and ceramide portions of LacCer, precursor of more complex GSLs.

Activated polymorphonuclear neutrophils disseminate anti-inflammatory microparticles by ectocytosis

On activation, human neutrophils release microparticles, called ectosomes, directly from the cell surface membrane. Microparticles from platelets, endothelial cells, and monocytes were reported to support coagulation or to modulate vascular homeostasis by activating monocytes as well as endothelial cells. We find that neutrophil ectosomes have no proinflammatory activity on human macrophages as assessed by the release of interleukin 8 (IL-8) and tumor necrosis factor alpha (TNFalpha). On the contrary, ectosomes increase the release of transforming growth factor beta1 (TGFbeta1), suggesting that ectosomes down-modulate cellular activation in macrophages. Polymorphonuclear neutrophil (PMN) ectosomes are able to block inflammatory response of macrophages to zymosan and lipopolysaccharide (LPS). We show that an early-phase TGFbeta1 secretion and the exposure of phosphatidylserine on the surface of ectosomes independently contribute to this effect. Ectosome-cell contact was sufficient for their immunomodulatory function as shown by blocking phagocytosis with cytochalasin D. Thus, neutrophils release potent anti-inflammatory effectors, in the form of ectosomes, at the earliest stage of inflammation, already providing a drive to its resolution.

Endothelium-derived microparticles impair endothelial function in vitro

Endothelial cell dysfunction (ECD) is emerging as the common denominator for diverse and highly prevalent cardiovascular diseases. Recently, an increased number of procoagulant circulating endothelial microparticles (EMPs) has been identified in patients with acute myocardial ischemia, preeclampsia, and diabetes, which suggests that these particles represent a surrogate marker of ECD. Our previous studies showed procoagulant potential of endothelial microparticles and mobilization of microparticles by PAI-1. The aim of this study was to test the effects of isolated EMPs on the vascular endothelium. EMPs impaired ACh-induced vasorelaxation and nitric oxide production by aortic rings obtained from Sprague-Dawley rats in a concentration-dependent manner. This effect was accompanied by increased superoxide production by aortic rings and cultured endothelial cells that were coincubated with EMPs and was inhibited by a SOD mimetic and blunted by an endothelial nitric oxide synthase inhibitor. Superoxide was also produced by isolated EMP. In addition, p22(phox) subunit of NADPH-oxidase was detected in EMP. Our data strongly suggest that circulating EMPs directly affect the endothelium and thus not only act as a marker for ECD but also aggravate preexisting ECD.

Toward oxidative lipidomics of cell signaling

Involvement and role of oxidatively modified lipids become increasingly evident in different signaling pathways, although specific mechanisms remain to be explored. The current forum focuses on several aspects of signaling by oxidatively modified lipid molecules during apoptosis and implications of different oxidized phospholipids in elimination of apoptotic cells and regulation of inflammatory response. Studies of signaling mechanisms by oxidatively modified lipids represent a rapidly expanding field of oxidative lipidomics, a new and exciting research focus at the interface of traditional lipid/membrane biochemistry and biophysics with free radical chemistry and cell biology.