The exosome pathway in K562 cells is regulated by Rab11

Hijacking multivesicular bodies enables long-term and exosome-mediated long-distance action of anthrax toxin

Anthrax lethal toxin is a classical AB toxin comprised of two components: protective antigen (PA) and lethal factor (LF). Here, we show that following assembly and endocytosis, PA forms a channel that translocates LF, not only into the cytosol, but also into the lumen of endosomal intraluminal vesicles (ILVs). These ILVs can fuse and release LF into the cytosol, where LF can proteolyze and disable host targets. We find that LF can persist in ILVs for days, fully sheltered from proteolytic degradation, both in vitro and in vivo. During this time, ILV-localized LF can be transmitted to daughter cells upon cell division. In addition, LF-containing ILVs can be delivered to the extracellular medium as exosomes. These can deliver LF to the cytosol of naive cells in a manner that is independent of the typical anthrax toxin receptor-mediated trafficking pathway, while being sheltered from neutralizing extracellular factors of the immune system.

Molecular lipidomics of exosomes released by PC-3 prostate cancer cells

The molecular lipid composition of exosomes is largely unknown. In this study, sophisticated shotgun and targeted molecular lipidomic assays were performed for in-depth analysis of the lipidomes of the metastatic prostate cancer cell line, PC-3, and their released exosomes. This study, based in the quantification of approximately 280 molecular lipid species, provides the most extensive lipid analysis of cells and exosomes to date. Interestingly, major differences were found in the lipid composition of exosomes compared to parent cells. Exosomes show a remarkable enrichment of distinct lipids, demonstrating an extraordinary discrimination of lipids sorted into these microvesicles. In particular, exosomes are highly enriched in glycosphingolipids, sphingomyelin, cholesterol, and phosphatidylserine (mol% of total lipids). Furthermore, lipid species, even of classes not enriched in exosomes, were selectively included in exosomes. Finally, it was found that there is an 8.4-fold enrichment of lipids per mg of protein in exosomes. The detailed lipid composition provided in this study may be useful to understand the mechanism of exosome formation, release and function. Several of the lipids enriched in exosomes could potentially be used as cancer biomarkers.

Tumor-derived exosomes are enriched in ΔNp73, which promotes oncogenic potential in acceptor cells and correlates with patient survival

Tumor-derived exosomes are emerging as local and systemic cell-to-cell mediators of oncogenic information through the horizontal transfer of mRNAs, microRNAs and proteins during tumorigenesis. The exosomal content has been described as biologically active when taken up by the recipient cell. Identifying the specific molecular cargo of exosomes will help to determine their function in specific steps of the tumorigenic process. Here we evaluate whether ΔNp73 is selectively packaged in tumor-derived exosomes, its function in the acceptor cells in vitro and in vivo and its prognosis potential in cancer. ΔNp73 messenger is enriched in tumor-derived exosomes, suggesting its active sorting in these microvesicles. We observed the transmission of this exosome cargo to different cell types and how it confers proliferation potential and chemoresistance to the acceptor cells in vitro and in animal models. Finally, our data support the potential prognostic value of exosomal ΔNp73 in colon cancer patients.

The unconventional secretion of stress-inducible protein 1 by a heterogeneous population of extracellular vesicles

The co-chaperone stress-inducible protein 1 (STI1) is released by astrocytes, and has important neurotrophic properties upon binding to prion protein (PrP(C)). However, STI1 lacks a signal peptide and pharmacological approaches pointed that it does not follow a classical secretion mechanism. Ultracentrifugation, size exclusion chromatography, electron microscopy, vesicle labeling, and particle tracking analysis were used to identify three major types of extracellular vesicles (EVs) released from astrocytes with sizes ranging from 20-50, 100-200, and 300-400 nm. These EVs carry STI1 and present many exosomal markers, even though only a subpopulation had the typical exosomal morphology. The only protein, from those evaluated here, present exclusively in vesicles that have exosomal morphology was PrP(C). STI1 partially co-localized with Rab5 and Rab7 in endosomal compartments, and a dominant-negative for vacuolar protein sorting 4A (VPS4A), required for formation of multivesicular bodies (MVBs), impaired EV and STI1 release. Flow cytometry and PK digestion demonstrated that STI1 localized to the outer leaflet of EVs, and its association with EVs greatly increased STI1 activity upon PrP(C)-dependent neuronal signaling. These results indicate that astrocytes secrete a diverse population of EVs derived from MVBs that contain STI1 and suggest that the interaction between EVs and neuronal surface components enhances STI1-PrP(C) signaling.

Exosomes function in cell-cell communication during brain circuit development

Exosomes are small extracellular vesicles that mediate intercellular signaling in the brain without requiring direct contact between cells. Although exosomes have been shown to play a role in neurological diseases and in response to nerve trauma, a role for exosome-mediated signaling in brain development and function has not yet been demonstrated. Here we review data building a case for exosome function in the brain.

Exosomes and communication between tumours and the immune system: are all exosomes equal?

Communication between cells is particularly important during tumour progression. Communication can take place through direct cell-cell interactions, but also through extracellular secretion of mediators acting at a distance. These mediators can be either soluble molecules or more complex structures called membrane vesicles, enclosing soluble factors within a lipid bilayer. A variety of extracellular membrane vesicles have been described, for instance microvesicles, ectosomes and a subtype called exosomes. The role of exosomes in tumour progression has been studied extensively in the last 10 years. In the present mini-review, we discuss our recent results, first showing the heterogeneity of the vesicles called exosomes and the probable existence of subpopulations of these exosomes, and secondly demonstrating that in vivo secretion of exosomes by some tumours can promote tumour progression, but that such a function cannot be generalized to all tumours and all exosomes.

Role of pancreatic cancer-derived exosomes in salivary biomarker development

Recent studies have demonstrated that discriminatory salivary biomarkers can be readily detected upon the development of systemic diseases such as pancreatic cancer, breast cancer, lung cancer, and ovarian cancer. However, the utility of salivary biomarkers for the detection of systemic diseases has been undermined due to the absence of the biological and mechanistic rationale as to why distal diseases from the oral cavity would lead to the development of discriminatory biomarkers in saliva. Here, we examine the hypothesis that pancreatic tumor-derived exosomes are mechanistically involved in the development of pancreatic cancer-discriminatory salivary transcriptomic biomarkers. We first developed a pancreatic cancer mouse model that yielded discriminatory salivary biomarkers by implanting the mouse pancreatic cancer cell line Panc02 into the pancreas of the syngeneic host C57BL/6. The role of pancreatic cancer-derived exosomes in the development of discriminatory salivary biomarkers was then tested by engineering a Panc02 cell line that is suppressed for exosome biogenesis, implanting into the C56BL/6 mouse, and examining whether the discriminatory salivary biomarker profile was ablated or disrupted. Suppression of exosome biogenesis results in the ablation of discriminatory salivary biomarker development. This study supports that tumor-derived exosomes provide a mechanism in the development of discriminatory biomarkers in saliva and distal systemic diseases.

Intercellular communication by exosome-derived microRNAs in cancer

The development of human cancers is a multistep process in which normal cells acquire characteristics that ultimately lead to their conversion into cancer cells. Many obstacles must be overcome for this process to occur; of these obstacles, is the ability to survive an inhospitable microenvironment. It is recognized that the intercommunication between tumor cells and their surrounding microenvironment is essential to overcoming this obstacle and for the tumor to progress, metastasize and establish itself at distant sites. Exosomes are membrane-derived vesicles that have recently been recognized as important mediators of intercellular communication, as they carry lipids, proteins, mRNAs and microRNAs that can be transferred to a recipient cell via fusion of the exosome with the target cell membrane. In the context of cancer cells, this process entails the transfer of cancer-promoting cellular contents to surrounding cells within the tumor microenvironment or into the circulation to act at distant sites, thereby enabling cancer progression. In this process, the transfer of exosomal microRNAs to a recipient cell where they can regulate target gene expression is of particular interest, both in understanding the basic biology of cancer progression and for the development of therapeutic approaches. This review discusses the exosome-mediated intercellular communication via microRNAs within the tumor microenvironment in human cancers, with a particular focus on breast cancer exosomes.

Transfer of extracellular vesicles during immune cell-cell interactions

The transfer of molecules between cells during cognate immune cell interactions has been reported, and recently a novel mechanism of transfer of proteins and genetic material such as small RNA between T cells and antigen-presenting cells (APCs) has been described, involving exchange of extracellular vesicles (EVs) during the formation of the immunological synapse (IS). EVs, a term that encompasses exosomes and microvesicles, has been implicated in cell-cell communication during immune responses associated with tumors, pathogens, allergies, and autoimmune diseases. This review focuses on EV transfer as a mechanism for the exchange of molecules during immune cell-cell interactions.

Exosomes: looking back three decades and into the future

Exosomes are extracellular membrane vesicles whose biogenesis by exocytosis of multivesicular endosomes was discovered in 1983. Since their discovery 30 years ago, it has become clear that exosomes contribute to many aspects of physiology and disease, including intercellular communication. We discuss the initial experiments that led to the discovery of exosomes and highlight some of the exciting current directions in the field.

Extracellular vesicles: exosomes, microvesicles, and friends

Cells release into the extracellular environment diverse types of membrane vesicles of endosomal and plasma membrane origin called exosomes and microvesicles, respectively. These extracellular vesicles (EVs) represent an important mode of intercellular communication by serving as vehicles for transfer between cells of membrane and cytosolic proteins, lipids, and RNA. Deficiencies in our knowledge of the molecular mechanisms for EV formation and lack of methods to interfere with the packaging of cargo or with vesicle release, however, still hamper identification of their physiological relevance in vivo. In this review, we focus on the characterization of EVs and on currently proposed mechanisms for their formation, targeting, and function.

Release of luminal exosomes contributes to TLR4-mediated epithelial antimicrobial defense

Exosomes are membranous nanovesicles released by most cell types from multi-vesicular endosomes. They are speculated to transfer molecules to neighboring or distant cells and modulate many physiological and pathological procedures. Exosomes released from the gastrointestinal epithelium to the basolateral side have been implicated in antigen presentation. Here, we report that luminal release of exosomes from the biliary and intestinal epithelium is increased following infection by the protozoan parasite Cryptosporidium parvum. Release of exosomes involves activation of TLR4/IKK2 signaling through promoting the SNAP23-associated vesicular exocytotic process. Downregulation of let-7 family miRNAs by activation of TLR4 signaling increases SNAP23 expression, coordinating exosome release in response to C. parvum infection. Intriguingly, exosomes carry antimicrobial peptides of epithelial cell origin, including cathelicidin-37 and beta-defensin 2. Activation of TLR4 signaling enhances exosomal shuttle of epithelial antimicrobial peptides. Exposure of C. parvum sporozoites to released exosomes decreases their viability and infectivity both in vitro and ex vivo. Direct binding to the C. parvum sporozoite surface is required for the anti-C. parvum activity of released exosomes. Biliary epithelial cells also increase exosomal release and display exosome-associated anti-C. parvum activity following LPS stimulation. Our data indicate that TLR4 signaling regulates luminal exosome release and shuttling of antimicrobial peptides from the gastrointestinal epithelium, revealing a new arm of mucosal immunity relevant to antimicrobial defense.

Exosomes are secreted membranous nanovesicles produced by a variety of cells. Exosomes shuttle various molecules to transfer them to neighboring or distant cells, and have been implicated as mediators in cell-cell communications to modulate physiological and pathological procedures. Here, we report that luminal release of exosomal vesicles is an important component of Toll-like receptor 4 (TLR4)-associated gastrointestinal epithelial defense against infection by Cryptosporidium parvum, an obligate intracellular protozoan that infects gastrointestinal epithelial cells. Activation of TLR4 signaling in host epithelial cells following C. parvum infection promotes luminal release of epithelial exosomes and exosomal shuttling of antimicrobial peptides from the epithelium. By direct binding to the C. parvum surface, exosomal vesicles reveal anti-C. parvum activity. Activation of TLR4 signaling in epithelial cells after LPS stimulation also increases exosomal release and exosome-associated anti-C. parvum activity. Therefore, we speculate that TLR4-mediated exosome release may be relevant to innate mucosal immunity in general, representing a new target for therapeutic intervention for infectious diseases at the mucosal surface.

Secretome analysis using a hollow fiber culture system for cancer biomarker discovery

Secreted proteins, collectively referred to as the secretome, were suggested as valuable biomarkers in disease diagnosis and prognosis. However, some secreted proteins from cell cultures are difficult to detect because of their intrinsically low abundance; they are frequently masked by the released proteins from lysed cells and the substantial amounts of serum proteins used in culture medium. The hollow fiber culture (HFC) system is a commercially available system composed of small fibers sealed in a cartridge shell; cells grow on the outside of the fiber. Recently, because this system can help cells grow at a high density, it has been developed and applied in a novel analytical platform for cell secretome collection in cancer biomarker discovery. This article focuses on the advantages of the HFC system, including the effectiveness of the system for collection of secretomes, and reviews the process of cell secretome collection by the HFC system and proteomic approaches to discover cancer biomarkers. The HFC system not only provides a high-density three-dimensional (3D) cell culture system to mimic tumor growth conditions in vivo but can also accommodate numerous cells in a small volume, allowing secreted proteins to be accumulated and concentrated. In addition, cell lysis rates can be greatly reduced, decreasing the amount of contamination by abundant cytosolic proteins from lysed cells. Therefore, the HFC system is useful for preparing a wide range of proteins from cell secretomes and provides an effective method for collecting higher amounts of secreted proteins from cancer cells. This article is part of a Special Issue entitled: An Updated Secretome.

The exosome secretory pathway transports amyloid precursor protein carboxyl-terminal fragments from the cell into the brain extracellular space

In vitro studies have shown that neuronal cell cultures secrete exosomes containing amyloid-β precursor protein (APP) and the APP-processing products, C-terminal fragments (CTFs) and amyloid-β (Aβ). We investigated the secretion of full-length APP (flAPP) and APP CTFs via the exosome secretory pathway in vivo. To this end, we developed a novel protocol designed to isolate exosomes secreted into mouse brain extracellular space. Exosomes with typical morphology were isolated from freshly removed mouse brains and from frozen mouse and human brain tissues, demonstrating that exosomes can be isolated from post-mortem tissue frozen for long periods of time. flAPP, APP CTFs, and enzymes that cleave both flAPP and APP CTFs were identified in brain exosomes. Although higher levels of both flAPP and APP CTFs were observed in exosomes isolated from the brains of transgenic mice overexpressing human APP (Tg2576) compared with wild-type control mice, there was no difference in the number of secreted brain exosomes. These data indicate that the levels of flAPP and APP CTFs associated with exosomes mirror the cellular levels of flAPP and APP CTFs. Interestingly, exosomes isolated from the brains of both Tg2576 and wild-type mice are enriched with APP CTFs relative to flAPP. Thus, we hypothesize that the exosome secretory pathway plays a pleiotropic role in the brain: exosome secretion is beneficial to the cell, acting as a specific releasing system of neurotoxic APP CTFs and Aβ, but the secretion of exosomes enriched with APP CTFs, neurotoxic proteins that are also a source of secreted Aβ, is harmful to the brain.

Mechanism of TNF-α autocrine effects in hypoxic cardiomyocytes: initiated by hypoxia inducible factor 1α, presented by exosomes

Excessive tumor necrosis factor-α (TNF-α) expression is increasingly thought to be detrimental to cardiomyocytes in acute myocardial infarction. During myocardial ischemia, TNF-α is mainly released from macrophages, but with persistent ischemia, it can originate from cardiomyocytes and contribute to cardiac remodeling. The initiating factor and exact molecular mechanism of TNF-α release from cardiomyocytes is presently unclear. In this study, we investigated direct effects of hypoxia on TNF-α expression of cardiomyocytes, the role of hypoxia inducible factor-1α (HIF-1α) in TNF-α regulation and potential secretory pathway of TNF-α. Elevated TNF-α expression and HIF-1α activation in primary cultured cardiomyocytes under hypoxia were detected by real-time PCR, Western blotting and immunofluorescence. TNF-α mRNA elevation and protein secretion were obviously inhibited by nucleofection of HIF-1α small interfering RNA (siRNA) and treatment with 2-methoxyestradiol (inhibitor of HIF-1α protein). Similar results were observed in HEK293 and HepG2 cells. Putative hypoxia response elements were identified in the human TNF-α gene promoter. Deletion analysis and site-directed mutagenesis demonstrated that HIF consensus binding sites spanning bp-1295 to bp-1292 relative to the transcription start site were functional for activation of the TNF-α promoter which was confirmed by electrophoretic mobility-shift assay (EMSA) and chromatin immunoprecipitation (ChIP) analysis. Exosomes (vesicles mediating a non-classical route of protein secretion) in supernatants from hypoxic cardiomyocytes were identified by an anti-CD63 antibody in Western blot and observed by electron microscopy. The presence of TNF-α within exosomes precipitated from supernatants of hypoxic cardiomyocytes was verified by immunoelectron microscopy and immunoblotting. Results of this study indicate that under hypoxia, HIF-1α initiates expression of TNF-α, mediated by exosomes in cardiomyocytes.

A subunit of eukaryotic translation initiation factor 2α-phosphatase (CreP/PPP1R15B) regulates membrane traffic

The constitutive reverter of eIF2α phosphorylation (CReP)/PPP1r15B targets the catalytic subunit of protein phosphatase 1 (PP1c) to phosphorylated eIF2α (p-eIF2α) to promote its dephosphorylation and translation initiation. Here, we report a novel role and mode of action of CReP. We found that CReP regulates uptake of the pore-forming Staphylococcus aureus α-toxin by epithelial cells. This function was independent of PP1c and translation, although p-eIF2α was involved. The latter accumulated at sites of toxin attack and appeared conjointly with α-toxin in early endosomes. CReP localized to membranes, interacted with phosphomimetic eIF2α, and, upon overexpression, induced and decorated a population of intracellular vesicles, characterized by accumulation of N-(lissamine rhodamine B sulfonyl)phosphatidylethanolamine (N-Rh-PE), a lipid marker of exosomes and intralumenal vesicles of multivesicular bodies. By truncation analysis, we delineated the CReP vesicle induction/association region, which comprises an amphipathic α-helix and is distinct from the PP1c interaction domain. CReP was also required for exocytosis from erythroleukemia cells and thus appears to play a broader role in membrane traffic. In summary, the mammalian traffic machinery co-opts p-eIF2α and CReP, regulators of translation initiation.

Exosomes as biomarker enriched microvesicles: characterization of exosomal proteins derived from a panel of prostate cell lines with distinct AR phenotypes

Prostate cancer is the leading type of cancer diagnosed in men. In 2010, ~217,730 new cases of prostate cancer were reported in the United States. Prompt diagnosis of the disease can substantially improve its clinical outcome. Improving capability for early detection, as well as developing new therapeutic targets in advanced disease are research priorities that will ultimately lead to better patient survival. Eukaryotic cells secrete proteins via distinct regulated mechanisms which are either ER/Golgi dependent or microvesicle mediated. The release of microvesicles has been shown to provide a novel mechanism for intercellular communication. Exosomes are nanometer sized cup-shaped membrane vesicles which are secreted from normal and cancerous cells. They are present in various biological fluids and are rich in characteristic proteins. Exosomes may thus have potential both in facilitating early diagnosis via less invasive procedures or be candidates for novel therapeutic approaches for castration resistance prostate cancer. Because exosomes have been shown previously to have a role in cell-cell communication in the local tumor microenvironment, conferring activation of numerous survival mechanisms, we characterized constitutive lipids, cholesterol and proteins from exosomes derived from six prostate cell lines and tracked their uptake in both cancerous and benign prostate cell lines respectively. Our comprehensive proteomic and lipidomic analysis of prostate derived exosomes could provide insight for future work on both biomarker and therapeutic targets for the treatment of prostate cancer.

Preliminary characterization of the murine membrane reticulocyte proteome

The maturation from reticulocyte (immature red blood cell) to erythrocyte (mature red blood cell) includes the loss or decreased expression of cell surface molecules through exosome formation and secretion. Identifying the molecules lost and the molecular events involved is important to our understanding of this final stage of erythropoiesis and of diseases where it is deranged. Also, the presence of certain cell surface molecules is likely responsible for the invasion of certain malaria parasites into reticulocytes. Using a global proteomics approach, we identified proteins potentially lost during and/or involved in the reticulocyte maturation process. The reticulocyte proteome has not yet been published, as previous such studies have focused on the mature erythrocyte. Membrane-rich fractions were fractionated by electrophoresis followed by analysis with tandem mass spectrometry. Seven hundred forty four proteins were identified in the reticulocyte-rich membrane fraction, 192 proteins in the erythrocyte-rich membrane fraction, with 157 common to both fractions. Many of the proteins found uniquely in the reticulocyte were associated with structures known to be in reticulocytes (mitochondria, Golgi). Additional proteins detected are or may be specifically involved in vesicle trafficking, a process important in the maturation process. A number of unique plasma membrane proteins were also identified. These results provide the groundwork for future targeted studies to improve our understanding of the mechanism of reticulocyte maturation and the role of reticulocytes in disease.

Exosomes: vesicular carriers for intercellular communication in neurodegenerative disorders

The intercellular transfer of misfolded proteins has received increasing attention in various neurodegenerative diseases characterized by the aggregation of specific proteins, as observed in Alzheimer's, Parkinson's and Huntington's disease. One hypothesis holds that intercellular dissemination of these aggregates within the central nervous system results in the seeded assembly of the cognate soluble protein in target cells, similar to that proposed for transmissible prion diseases. The molecular mechanisms underlying the intercellular transfer of these proteinaceous aggregates are poorly understood. Various transfer modes of misfolded proteins including continuous cell-cell contacts such as nanotubes, unconventional secretion or microvesicle/exosome-associated dissemination have been suggested. Cells can release proteins, lipids and nucleic acids by vesicular exocytosis pathways destined for horizontal transfer. Encapsulation into microvesicular/exosomal vehicles not only protects these molecules from degradation and dilution in the extracellular space but also facilitates delivery over large distances, e.g. within the blood flow or interstitial fluid. Specific surface ligands might allow the highly efficient and targeted uptake of these vesicles by recipient cells. In this review, we focus on the cell biology and function of neuronal microvesicles/exosomes and discuss the evidence for pathogenic intercellular protein transfer mediated by vesicular carriers.

Release of the glucose-regulated protein 94 by baby hamster kidney cells

Glucose-regulated protein 94 (grp94) is a major component of the endoplasmic reticulum (ER) lumen of eukaryotic cells. We showed that grp94 is released from baby hamster kidney (BHK-21) cells into a serum-free medium. The exit of grp94 into the medium was not related to the protein discharge due to cell death and was independent of de novo protein synthesis. The treatment of cells with brefeldin A and monensin, the inhibitors of the classical pathway of protein secretion, did not decrease the extracellular level of grp94, indicating that the discharge of grp94 from cells does not occur through the ER/Golgi-dependent pathway. Exosomes, membrane vesicles secreted by several cell types, were not involved in the release of grp94 from cells. Methyl-β-cyclodextrin, a substance that disrupts the lipid raft organization, considerably reduced the extracellular level of grp94, indicating that lipid rafts are involved in the liberation of grp94 from BHK-21 cells. The results suggest that BHK-21 cells release grp94 into the serum-free medium via the nonclassical secretory pathway in which lipid rafts play an important role.

Proteomic analysis of microvesicles released by the human prostate cancer cell line PC-3

Cancer biomarkers are invaluable tools for cancer detection, prognosis, and treatment. Recently, microvesicles have appeared as a novel source for cancer biomarkers. We present here the results from a proteomic analysis of microvesicles released to the extracellular environment by the metastatic prostate cancer cell line PC-3. Using nanocapillary liquid chromatography-tandem mass spectrometry 266 proteins were identified with two or more peptide sequences. Further analysis showed that 16% of the proteins were classified as extracellular and that intracellular proteins were annotated in a variety of locations. Concerning biological processes, the proteins found in PC-3 cell-released microvesicles are mainly involved in transport, cell organization and biogenesis, metabolic process, response to stimulus, and regulation of biological processes. Several of the proteins identified (tetraspanins, annexins, Rab proteins, integrins, heat shock proteins, cytoskeletal proteins, 14-3-3 proteins) have previously been found in microvesicles isolated from other sources. However, some of the proteins seem to be more specific to the vesicular population released by the metastatic prostate cancer PC-3 cell line. Among these proteins are the tetraspanin protein CD151 and the glycoprotein CUB domain-containing protein 1. Interestingly, our results show these proteins are promising biomarkers for prostate cancer and therefore candidates for clinical validation studies in biological fluids.

Hsp20 functions as a novel cardiokine in promoting angiogenesis via activation of VEGFR2

Heat shock proteins (Hsps) are well appreciated as intrinsic protectors of cardiomyocytes against numerous stresses. Recent studies have indicated that Hsp20 (HspB6), a small heat shock protein, was increased in blood from cardiomyopathic hamsters. However, the exact source of the increased circulating Hsp20 and its potential role remain obscure. In this study, we observed that the circulating Hsp20 was increased in a transgenic mouse model with cardiac-specific overexpression of Hsp20, compared with wild-type mice, suggesting its origin from cardiomyocytes. Consistently, culture media harvested from Hsp20-overexpressing cardiomyocytes by Ad.Hsp20 infection contained an increased amount of Hsp20, compared to control media. Furthermore, we identified that Hsp20 was secreted through exosomes, independent of the endoplasmic reticulum-Golgi pathway. To investigate whether extracellular Hsp20 promotes angiogenesis, we treated human umbilical vein endothelial cells (HUVECs) with recombinant human Hsp20 protein, and observed that Hsp20 dose-dependently promoted HUVEC proliferation, migration and tube formation. Moreover, a protein binding assay and immunostaining revealed an interaction between Hsp20 and VEGFR2. Accordingly, stimulatory effects of Hsp20 on HUVECs were blocked by a VEGFR2 neutralizing antibody and CBO-P11 (a VEGFR inhibitor). These in vitro data are consistent with the in vivo findings that capillary density was significantly enhanced in Hsp20-overexpressing hearts, compared to non-transgenic hearts. Collectively, our findings demonstrate that Hsp20 serves as a novel cardiokine in regulating myocardial angiogenesis through activation of the VEGFR signaling cascade.

Anticancer drugs cause release of exosomes with heat shock proteins from human hepatocellular carcinoma cells that elicit effective natural killer cell antitumor responses in vitro

Failure of immune surveillance related to inadequate host antitumor immune responses has been suggested as a possible cause of the high incidence of recurrence and poor overall survival outcome of hepatocellular carcinoma. The stress-induced heat shock proteins (HSPs) are known to act as endogenous "danger signals" that can improve tumor immunogenicity and induce natural killer (NK) cell responses. Exosome is a novel secretory pathway for HSPs. In our experiments, the immune regulatory effect of the HSP-bearing exosomes secreted by human hepatocellular carcinoma cells under stress conditions on NK cells was studied. ELISA results showed that the production of HSP60, HSP70, and HSP90 was up-regulated in both cell lines in a stress-specific manner. After exposure to hepatocellular carcinoma cell-resistant or sensitive anticancer drugs (hereafter referred to as "resistant" or "sensitive" anticancer drug), the membrane microvesicles were actively released by hepatocellular carcinoma cells, differing in their ability to present HSPs on the cell surface, which were characterized as exosomes. Acting as a decoy, the HSP-bearing exosomes efficiently stimulated NK cell cytotoxicity and granzyme B production, up-regulated the expression of inhibitory receptor CD94, and down-regulated the expression of activating receptors CD69, NKG2D, and NKp44. Notably, resistant anticancer drugs enhanced exosome release and generated more exosome-carried HSPs, which augmented the activation of the cytotoxic response. In summary, our findings demonstrated that exosomes derived from resistant anticancer drug-treated HepG2 cells conferred superior immunogenicity in inducing HSP-specific NK cell responses, which provided a clue for finding an efficient vaccine for hepatocellular carcinoma immunotherapy.

Comparison of ultracentrifugation, density gradient separation, and immunoaffinity capture methods for isolating human colon cancer cell line LIM1863-derived exosomes

Exosomes are 40-100nm extracellular vesicles that are released from a multitude of cell types, and perform diverse cellular functions including intercellular communication, antigen presentation, and transfer of oncogenic proteins as well as mRNA and miRNA. Exosomes have been purified from biological fluids and in vitro cell cultures using a variety of strategies and techniques. However, all preparations invariably contain varying proportions of other membranous vesicles that co-purify with exosomes such as shed microvesicles and apoptotic blebs. Using the colorectal cancer cell line LIM1863 as a cell model, in this study we performed a comprehensive evaluation of current methods used for exosome isolation including ultracentrifugation (UC-Exos), OptiPrep™ density-based separation (DG-Exos), and immunoaffinity capture using anti-EpCAM coated magnetic beads (IAC-Exos). Notably, all isolations contained 40-100nm vesicles, and were positive for exosome markers (Alix, TSG101, HSP70) based on electron microscopy and Western blotting. We employed a proteomic approach to profile the protein composition of exosomes, and label-free spectral counting to evaluate the effectiveness of each method. Based on the number of MS/MS spectra identified for exosome markers and proteins associated with their biogenesis, trafficking, and release, we found IAC-Exos to be the most effective method to isolate exosomes. For example, Alix, TSG101, CD9 and CD81 were significantly higher (at least 2-fold) in IAC-Exos, compared to UG-Exos and DG-Exos. Application of immunoaffinity capture has enabled the identification of proteins including the ESCRT-III component VPS32C/CHMP4C, and the SNARE synaptobrevin 2 (VAMP2) in exosomes for the first time. Additionally, several cancer-related proteins were identified in IAC-Exos including various ephrins (EFNB1, EFNB2) and Eph receptors (EPHA2-8, EPHB1-4), and components involved in Wnt (CTNNB1, TNIK) and Ras (CRK, GRB2) signalling.

The protein interaction network of extracellular vesicles derived from human colorectal cancer cells

Various mammalian cells including tumor cells secrete extracellular vesicles (EVs), otherwise known as exosomes and microvesicles. EVs are nanosized bilayered proteolipids and play multiple roles in intercellular communication. Although many vesicular proteins have been identified, their functional interrelationships and the mechanisms of EV biogenesis remain unknown. By interrogating proteomic data using systems approaches, we have created a protein interaction network of human colorectal cancer cell-derived EVs which comprises 1491 interactions between 957 vesicular proteins. We discovered that EVs have well-connected clusters with several hub proteins similar to other subcellular networks. We also experimentally validated that direct protein interactions between cellular proteins may be involved in protein sorting during EV formation. Moreover, physically and functionally interconnected protein complexes form functional modules involved in EV biogenesis and functions. Specifically, we discovered that SRC signaling plays a major role in EV biogenesis, and confirmed that inhibition of SRC kinase decreased the intracellular biogenesis and cell surface release of EVs. Our study provides global insights into the cargo-sorting, biogenesis, and pathophysiological roles of these complex extracellular organelles.

Competitive interactions of cancer cells and normal cells via secretory microRNAs

Normal epithelial cells regulate the secretion of autocrine and paracrine factors that prevent aberrant growth of neighboring cells, leading to healthy development and normal metabolism. One reason for tumor initiation is considered to be a failure of this homeostatic cell competitive system. Here we identify tumor-suppressive microRNAs (miRNAs) secreted by normal cells as anti-proliferative signal entities. Culture supernatant of normal epithelial prostate PNT-2 cells attenuated proliferation of PC-3M-luc cells, prostate cancer cells. Global analysis of miRNA expression signature revealed that a variety of tumor-suppressive miRNAs are released from PNT-2 cells. Of these miRNAs, secretory miR-143 could induce growth inhibition exclusively in cancer cells in vitro and in vivo. These results suggest that secretory tumor-suppressive miRNAs can act as a death signal in a cell competitive process. This study provides a novel insight into a tumor initiation mechanism.

Urinary exosomes and proteomics

A number of highly abundant proteins in urine have been identified through proteomics approaches, and some have been considered as disease-biomarker candidates. These molecules might be clinically useful in diagnosis of various diseases. However, none has proven to be specifically indicative of perturbations of cellular processes in cells associated with urogenital diseases. Exosomes could be released into urine which flows through the kidney, ureter, bladder and urethra, with a process of filtration and reabsorption. Urinary exosomes have been recently suggested as alternative materials that offer new opportunities to identify useful biomarkers, because these exosomes secreted from epithelial cells lining the urinary track might reflect the cellular processes associated with the pathogenesis of diseases in their donor cells. Proteomic analysis of such urinary exosomes assists the search of urinary biomarkers reflecting pathogenesis of various diseases and also helps understanding the function of urinary exosomes in urinary systems. Thus, it has been recently suggested that urinary exosomes are one of the most valuable targets for biomarker development and to understand pathophysiology of relevant diseases.

Erythrocyte membrane changes of chorea-acanthocytosis are the result of altered Lyn kinase activity

Acanthocytic RBCs are a peculiar diagnostic feature of chorea-acanthocytosis (ChAc), a rare autosomal recessive neurodegenerative disorder. Although recent years have witnessed some progress in the molecular characterization of ChAc, the mechanism(s) responsible for generation of acanthocytes in ChAc is largely unknown. As the membrane protein composition of ChAc RBCs is similar to that of normal RBCs, we evaluated the tyrosine (Tyr)-phosphorylation profile of RBCs using comparative proteomics. Increased Tyr phosphorylation state of several membrane proteins, including band 3, β-spectrin, and adducin, was noted in ChAc RBCs. In particular, band 3 was highly phosphorylated on the Tyr-904 residue, a functional target of Lyn, but not on Tyr-8, a functional target of Syk. In ChAc RBCs, band 3 Tyr phosphorylation by Lyn was independent of the canonical Syk-mediated pathway. The ChAc-associated alterations in RBC membrane protein organization appear to be the result of increased Tyr phosphorylation leading to altered linkage of band 3 to the junctional complexes involved in anchoring the membrane to the cytoskeleton as supported by coimmunoprecipitation of β-adducin with band 3 only in ChAc RBC-membrane treated with the Lyn-inhibitor PP2. We propose this altered association between membrane skeleton and membrane proteins as novel mechanism in the generation of acanthocytes in ChAc.

Hypoxia increases membranal and secreted HLA-DR in endothelial cells, rendering them T-cell activators

Transplantation involves preoperative ischemic periods that contribute to endothelial cell (EC) dysfunction and T-cell activation, leading to graft rejection. As hypoxia is a major constituent of ischemia, we evaluated its effect on the ability of ECs to express HLA-DR, which is required for presentation of antigens to T cells, and by itself serves as an important target for allogeneic T cells. Primary human umbilical vein ECs (HUVEC) and the human endothelial cell line EaHy926 were incubated in normoxia or hypoxia (PO(2) < 0.3%). Hypoxia increased the membranal expression (by 4-6 fold, P < 0.01) and secretion (by sixfold, P < 0.05) of HLA-DR protein, without influencing the accumulation of its mRNA. Alternative splicing, attenuated trafficking, or shedding from the plasma membrane were not observed, but the lysosomal inhibitor bafilomycin A1 reduced HLA-DR secretion. Hypoxia-induced endothelial HLA-DR elevated and diminished the secretion of IL-2 and IL-10, respectively, from co-cultured allogeneic CD4(+) T cells in a HLA-DR-dependent manner, as demonstrated by the use of monoclonal anti-HLA-DR. Our results indicate a yet not fully understood post-translational mechanism(s), which elevate both membranal and soluble HLA-DR expression. This elevation is involved in allogeneic T-cell activation, highlighting the pivotal role of ECs in ischemia/hypoxia-associated injury and graft rejection.

Lipid raft endocytosis and exosomal transport facilitate extracellular trafficking of annexin A2

Annexin A2 (AnxA2), a Ca(2+)-dependent phospholipid-binding protein, is known to associate with the plasma membrane and the endosomal system. Within the plasma membrane, AnxA2 associates in a Ca(2+) dependent manner with cholesterol-rich lipid raft microdomains. Here, we show that the association of AnxA2 with the lipid rafts is influenced not only by intracellular levels of Ca(2+) but also by N-terminal phosphorylation at tyrosine 23. Binding of AnxA2 to the lipid rafts is followed by the transport along the endocytic pathway to be associated with the intralumenal vesicles of the multivesicular endosomes. AnxA2-containing multivesicular endosomes fuse directly with the plasma membrane resulting in the release of the intralumenal vesicles into the extracellular environment, which facilitates the exogenous transfer of AnxA2 from one cell to another. Treatment with Ca(2+) ionophore triggers the association of AnxA2 with the specialized microdomains in the exosomal membrane that possess raft-like characteristics. Phosphorylation at Tyr-23 is also important for the localization of AnxA2 to the exosomal membranes. These results suggest that AnxA2 is trafficked from the plasma membrane rafts and is selectively incorporated into the lumenal membranes of the endosomes to escape the endosomal degradation pathway. The Ca(2+)-dependent exosomal transport constitutes a novel pathway of extracellular transport of AnxA2.

Extracellular heat shock proteins, cellular export vesicles, and the Stress Observation System: a form of communication during injury, infection, and cell damage. It is never known how far a controversial finding will go! Dedicated to Ferruccio Ritossa

Heat shock proteins (hsp) have been found to play a fundamental role in the recovery from multiple stress conditions and to offer protection from subsequent insults. The function of hsp during stress goes beyond their intracellular localization and chaperone role as they have been detected outside cells activating signaling pathways. Extracellular hsp are likely to act as indicators of the stress conditions, priming other cells, particularly of the immune system, to avoid the propagation of the insult. Some extracellular hsp, for instance Hsp70, are associated with export vesicles, displaying a robust activation of macrophages. We have coined the term Stress Observation System (SOS) for the mechanism for sensing extracellular hsp, which we propose is a form of cellular communication during stress conditions. An enigmatic and still poorly understood process is the mechanism for the release of hsp, which do not contain any consensus secretory signal. The export of hsp appears to be a very complex phenomenon encompassing different alternative pathways. Moreover, extracellular hsp may not come in a single flavor, but rather in a variety of physical conditions. This review addresses some of our current knowledge about the release and function of extracellular hsp, in particular those associated with vesicles.

Identification of an inhibitory budding signal that blocks the release of HIV particles and exosome/microvesicle proteins

We identify and characterize an inhibitory budding signal that acts dominantly to block the budding of otherwise budding-competent proteins, both viral and nonviral, and impairs the budding of several classic, budding-deficient HIV mutants. These findings expand our understanding of EMV biogenesis and resolve a number of previously paradoxical observations regarding the budding of HIV.

 Animal cells bud exosomes and microvesicles (EMVs) from endosome and plasma membranes. The combination of higher-order oligomerization and plasma membrane binding is a positive budding signal that targets diverse proteins into EMVs and retrovirus particles. Here we describe an inhibitory budding signal (IBS) from the human immunodeficiency virus (HIV) Gag protein. This IBS was identified in the spacer peptide 2 (SP2) domain of Gag, is activated by C-terminal exposure of SP2, and mediates the severe budding defect of p6-deficient and PTAP-deficient strains of HIV. This IBS also impairs the budding of CD63 and several other viral and nonviral EMV proteins. The IBS does not prevent cargo delivery to the plasma membrane, a major site of EMV and virus budding. However, the IBS does inhibit an interaction between EMV cargo proteins and VPS4B, a component of the endosomal sorting complexes required for transport (ESCRT) machinery. Taken together, these results demonstrate that inhibitory signals can block protein and virus budding, raise the possibility that the ESCRT machinery plays a role in EMV biogenesis, and shed new light on the role of the p6 domain and PTAP motif in the biogenesis of HIV particles.

Survivin is released from cancer cells via exosomes

Inhibitor of apoptosis (IAP) and Heat shock proteins (HSPs) provide assistance in protecting cells from stresses of hypoxia, imbalanced pH, and altered metabolic and redox states commonly found in the microenvironmental mixture of tumor and nontumor cells. HSPs are upregulated, cell-surface displayed and released extracellularly in some types of tumors, a finding that until now was not shared by members of the IAP family. The IAP Survivin has been implicated in apoptosis inhibition and the regulation of mitosis in cancer cells. Survivin exists in a number of subcellular locations such as the mitochondria, cytoplasm, nucleus, and most recently, the extracellular space. Our previous work showing that extracellular survivin was able to enhance cellular proliferation, survival and tumor cell invasion provides evidence that Survivin might be secreted via an unidentified exocytotic pathway. In the present study, we describe for the first time the exosome-release of Survivin to the extracellular space both basally and after proton irradiation-induced stress. To examine whether exosomes contributed to Survivin release from cancer cells, exosomes were purified from HeLa cervical carcinoma cells and exosome quantity and Survivin content were determined. We demonstrate that although proton irradiation does not influence the exosomal secretory rate, the Survivin content of exosomes isolated from HeLa cells treated with a sublethal dose of proton irradiation (3 Gy) is significantly higher than control. These data identify a novel secretory pathway by which Survivin can be actively released from cells in both the basal and stress-induced state.

Exosomes: immune properties and potential clinical implementations

To communicate, cells are known to release in their environment proteins which bind to receptors on surrounding cells. But cells also secrete more complex structures, called membrane vesicles, composed of a lipid bilayer with inserted transmembrane proteins, enclosing an internal content of hydrophilic components. Exosomes represent a specific subclass of such secreted membrane vesicles, which, despite having been described more than 20 years ago by two groups studying reticulocyte maturation, have only recently received attention from the scientific community. This renewed interest originated first from the description of exosome secretion by antigen-presenting cells, suggesting a potential role in immune responses, and very recently by the identification of the presence of RNA (both messenger and microRNA) in exosomes, suggesting a potential transfer of genetic information between cells. In this review, we will describe the conclusions of 20 years of studies on the immune properties of exosomes and the most recent advances on their roles and potential uses as markers or as therapeutic tools during pathologies, especially in cancer.

CD23 Sheddase A disintegrin and metalloproteinase 10 (ADAM10) is also required for CD23 sorting into B cell-derived exosomes

The low affinity receptor for IgE, CD23, is the natural regulator of IgE synthesis, and understanding both the synthesis and the catabolism of CD23 are, thus, important issues. Membrane CD23 is cleaved by a disintegrin and metalloproteinase 10 (ADAM10) and this cleavage influences the ability of CD23 to regulate IgE. In contrast to the belief that cleavage is a cell surface event, endosomal neutralization with NH(4)Cl was found to dramatically reduce CD23 cleavage, suggesting that the majority of CD23 cleavage occurred subsequent to internalization in the endosomal pathway and not at the cell surface. In line with this, full-length CD23 was shown to be sorted in an ADAM10-dependent manner into exosomes. Greatly increased ADAM10-mediated CD23 cleavage was seen at endosomal pH. Additionally, the stalk region of CD23 was found to interact with ADAM10 and ADAM10 binding of CD23 was found to be protease independent. SPR analysis of the interaction indicated about a 10-fold increase in the R(max) at endosomal pH (pH 5.8) compared with pH 7.4, whereas the affinity of the interaction was not significantly changed. The R(max) change, combined with the increased cleavage at endosomal pH, indicates greater accessibility of the CD23 stalk region for ADAM10 at the lower pH. These results indicate a model where CD23 internalization results in ADAM10-dependent incorporation into exosomes, followed by partial cleavage of CD23 by ADAM10 prior to being released from the cell. The increased cleavage at endosomal pH also has implications for other ADAM10 substrates.

Exosome secretion of dendritic cells is regulated by Hrs, an ESCRT-0 protein

Exosomes are nanovesicles derived from multivesicular bodies (MVBs) in antigen-presenting cells. The components of the ESCRT (endosomal sorting complex required for transport) pathway are critical for the formation of MVBs, however the relationship between the ESCRT pathway and the secretion of exosomes remains unclear. We here demonstrate that Hrs, an ESCRT-0 protein, is required for fascilitating the secretion of exosomes in dendritic cells (DCs). Ultrastructural analyses showed typical saucer-shaped exosomes in the culture supernatant from both the control and Hrs-depleted DCs. However, the amount of exosome secretion was significantly decreased in Hrs-depleted DCs following stimulations with ovalbumin (OVA) as well as calcium ionophore. Antigen-presentation activity was also suppressed in exsosomes purified from Hrs-depleted DCs, while no alteration in OVA degradation was seen in Hrs-depleted DCs. These data indicated that Hrs is involved in the regulation of antigen-presentation activity through the exosome secretion.

Exosomes in erythropoiesis

Multivesicular endosomes contain membrane vesicles which can be released into the extracellular environment as exosomes. This review describes the role of exosome secretion in the remodeling of the red cell plasma membrane during the last stage of erythropoietic differentiation. Herein, we propose that the exosome biogenesis involves several mechanisms of protein sorting and leads to partial or complete loss of membrane activities, in some cases in a regulated way.

An exosome-based secretion pathway is responsible for protein export from Leishmania and communication with macrophages

Specialized secretion systems are used by numerous bacterial pathogens to export virulence factors into host target cells. Leishmania and other eukaryotic intracellular pathogens also deliver effector proteins into host cells; however, the mechanisms involved have remained elusive. In this report, we identify exosome-based secretion as a general mechanism for protein secretion by Leishmania, and show that exosomes are involved in the delivery of proteins into host target cells. Comparative quantitative proteomics unambiguously identified 329 proteins in Leishmania exosomes, accounting for >52% of global protein secretion from these organisms. Our findings demonstrate that infection-like stressors (37 degrees C +/- pH 5.5) upregulated exosome release more than twofold and also modified exosome protein composition. Leishmania exosomes and exosomal proteins were detected in the cytosolic compartment of infected macrophages and incubation of macrophages with exosomes selectively induced secretion of IL-8, but not TNF-alpha. We thus provide evidence for an apparently broad-based mechanism of protein export by Leishmania. Moreover, we describe a mechanism for the direct delivery of Leishmania molecules into macrophages. These findings suggest that, like mammalian exosomes, Leishmania exosomes function in long-range communication and immune modulation.

Hsp60 is actively secreted by human tumor cells

BACKGROUND

Hsp60, a Group I mitochondrial chaperonin, is classically considered an intracellular chaperone with residence in the mitochondria; nonetheless, in the last few years it has been found extracellularly as well as in the cell membrane. Important questions remain pertaining to extracellular Hsp60 such as how generalized is its occurrence outside cells, what are its extracellular functions and the translocation mechanisms that transport the chaperone outside of the cell. These questions are particularly relevant for cancer biology since it is believed that extracellular chaperones, like Hsp70, may play an active role in tumor growth and dissemination.

METHODOLOGY/PRINCIPAL FINDINGS

Since cancer cells may undergo necrosis and apoptosis, it could be possible that extracellular Hsps are chiefly the result of cell destruction but not the product of an active, physiological process. In this work, we studied three tumor cells lines and found that they all release Hsp60 into the culture media by an active mechanism independently of cell death. Biochemical analyses of one of the cell lines revealed that Hsp60 secretion was significantly reduced, by inhibitors of exosomes and lipid rafts.

CONCLUSIONS/SIGNIFICANCE

Our data suggest that Hsp60 release is the result of an active secretion mechanism and, since extracellular release of the chaperone was demonstrated in all tumor cell lines investigated, our observations most likely reflect a general physiological phenomenon, occurring in many tumors.

Rab27a and Rab27b control different steps of the exosome secretion pathway

Exosomes are secreted membrane vesicles that share structural and biochemical characteristics with intraluminal vesicles of multivesicular endosomes (MVEs). Exosomes could be involved in intercellular communication and in the pathogenesis of infectious and degenerative diseases. The molecular mechanisms of exosome biogenesis and secretion are, however, poorly understood. Using an RNA interference (RNAi) screen, we identified five Rab GTPases that promote exosome secretion in HeLa cells. Among these, Rab27a and Rab27b were found to function in MVE docking at the plasma membrane. The size of MVEs was strongly increased by Rab27a silencing, whereas MVEs were redistributed towards the perinuclear region upon Rab27b silencing. Thus, the two Rab27 isoforms have different roles in the exosomal pathway. In addition, silencing two known Rab27 effectors, Slp4 (also known as SYTL4, synaptotagmin-like 4) and Slac2b (also known as EXPH5, exophilin 5), inhibited exosome secretion and phenocopied silencing of Rab27a and Rab27b, respectively. Our results therefore strengthen the link between MVEs and exosomes, and introduce ways of manipulating exosome secretion in vivo.

Massive secretion by T cells is caused by HIV Nef in infected cells and by Nef transfer to bystander cells

The HIV Nef protein mediates endocytosis of surface receptors that correlates with disease progression, but the link between this Nef function and HIV pathogenesis is not clear. Here, we report that Nef-mediated activation of membrane trafficking is bidirectional, connecting endocytosis with exocytosis as occurs in activated T cells. Nef expression induced an extensive secretory activity in infected and, surprisingly, also in noninfected T cells, leading to the massive release of microvesicle clusters, a phenotype observed in vitro and in 36%-87% of primary CD4 T cells from HIV-infected individuals. Consistent with exocytosis in noninfected cells, Nef is transferred to bystander cells upon cell-to-cell contact and subsequently induces secretion in an Erk1/2-dependent manner. Thus, HIV Nef alters membrane dynamics, mimicking those of activated T cells and causing a transfer of infected cell signaling (TOS) to bystander cells. This mechanism may help explain the detrimental effect on bystander cells seen in HIV infection.

Vesicular fractions of sunflower apoplastic fluids are associated with potential exosome marker proteins

Based on the presence of phospholipids in the extracellular fluids (EFs) of sunflower seeds, we have hypothesized on the existence of vesicles in the apoplastic compartment of plants. Ultracentrifugation of sunflower EF allowed the isolation of particles of 50-200 nm with apparent membrane organization. A small GTPase Rab was putatively identified in this vesicular fraction. Since Rab proteins are involved in vesicular traffic and their presence in exosomes from animal fluids has been demonstrated, evidence presented here supports the existence of exosome-like vesicles in apoplastic fluids of sunflower. Their putative contribution to intercellular communication in plants is discussed.

TI-VAMP/VAMP7 and VAMP3/cellubrevin: two v-SNARE proteins involved in specific steps of the autophagy/multivesicular body pathways

During reticulocyte maturation, some membrane proteins and organelles that are not required in the mature red cell are lost. Several of these proteins are released into the extracellular medium associated with the internal vesicles present in multivesicular bodies (MVBs). Likewise, organelles such as mitochondria and endoplasmic reticulum are wrapped into double membrane vacuoles (i.e., autophagosomes) and degraded via autophagy. Morphological, molecular, and biochemical studies have shown that autophagosomes fuse with MVBs forming the so-called amphisomes, a prelysosomal hybrid organelle. SNAREs are key molecules of the vesicle fusion machinery. TI-VAMP/VAMP7 and VAMP3/cellubrevin are two v-SNARE proteins involved in the endocytic and exocytic pathways. We have previously shown that in the human leukemic K562 cells, Rab11 decorates MVBs and it is necessary for fusion between autophagosomes with MVBs. In the present report, we present evidence indicating that VAMP3 is required for the fusion between MVBs with autophagosomes to generate the amphisome, allowing the maturation of the autophagosome, but it does not seem to be involved in the next step, i. e., fusion with the lysosome. On the other hand, we demonstrate that VAMP7 is necessary for this latter event, allowing the completion of the autophagic pathway. Furthermore, VAMP7 and ATPase NSF, a protein required for SNAREs disassembly, participate in the fusion between MVBs with the plasma membrane to release the internal vesicles (i.e., exosomes) into the extracellular medium.

Involvement of vps33a in the fusion of uroplakin-degrading multivesicular bodies with lysosomes

The apical surface of the terminally differentiated mouse bladder urothelium is largely covered by urothelial plaques, consisting of hexagonally packed 16-nm uroplakin particles. These plaques are delivered to the cell surface by fusiform vesicles (FVs) that are the most abundant cytoplasmic organelles. We have analyzed the functional involvement of several proteins in the apical delivery and endocytic degradation of uroplakin proteins. Although FVs have an acidified lumen and Rab27b, which localizes to these organelles, is known to be involved in the targeting of lysosome-related organelles (LROs), FVs are CD63 negative and are therefore not typical LROs. Vps33a is a Sec1-related protein that plays a role in vesicular transport to the lysosomal compartment. A point mutation in mouse Vps33a (Buff mouse) causes albinism and bleeding (Hermansky-Pudlak syndrome) because of abnormalities in the trafficking of melanosomes and platelets. These Buff mice showed a novel phenotype observed in urothelial umbrella cells, where the uroplakin-delivering FVs were almost completely replaced by Rab27b-negative multivesicular bodies (MVBs) involved in uroplakin degradation. MVB accumulation leads to an increase in the amounts of uroplakins, Lysosomal-associated membrane protein (LAMP)-1/2, and the activities of beta-hexosaminidase and beta-glucocerebrosidase. These results suggest that FVs can be regarded as specialized secretory granules that deliver crystalline arrays of uroplakins to the cell surface, and that the Vps33a mutation interferes with the fusion of MVBs with mature lysosomes thus blocking uroplakin degradation.

Proteomics of regulated secretory organelles

Regulated secretory organelles are important subcellular structures of living cells that allow the release in the extracellular space of crucial compounds, such as hormones and neurotransmitters. Therefore, the regulation of biogenesis, trafficking, and exocytosis of regulated secretory organelles has been intensively studied during the last 30 years. However, due to the large number of different regulated secretory organelles, only a few of them have been specifically characterized. New insights into regulated secretory organelles open crucial perspectives for a better comprehension of the mechanisms that govern cell secretion. The combination of subcellular fractionation, protein separation, and mass spectrometry is also possible to study regulated secretory organelles at the proteome level. In this review, we present different strategies used to isolate regulated secretory organelles, separate their protein content, and identify the proteins by mass spectrometry. The biological significance of regulated secretory organelles-proteomic analysis is discussed as well.

Heat shock protein 70 is secreted from endothelial cells by a non-classical pathway involving exosomes

Emerging evidence suggests that a high level of circulating heat shock protein 70 (HSP70) correlates with a lower risk of vascular disease; however, the biological significance of this inverse relationship has not been explored. Herein, we report that oxidative low density lipoprotein (Ox-LDL) and homocysteine (Hcy) induce HSP70 release from endothelial cells. In rat endothelial cells, Ox-LDL and Hcy induced robust release of HSP70, independent of the classical route of endoplasmic reticulum/Golgi protein trafficking or the formation of lipid rafts. In contrast, Ox-LDL and Hcy significantly enhanced the exosomal secretory rate and increased the HSP70 content of exosomes. Exogenous HSP70 had no impact on LPS-, Ox-LDL- and Hcy-induced activation of endothelial cells, whereas HSP70 did activate monocytes alone, resulting in monocyte adhesion to endothelial cells. These results indicate that exosome-dependent secretion of HSP70 from endothelial cells provides a novel paracrine mechanism to regulate vascular endothelial functional integrity.