Higher-order oligomerization targets plasma membrane proteins and HIV gag to exosomes

The Arc gene: Retroviral heritage in cognitive functions

Stabilization of neuronal plastic changes is mediated by transient gene expression, including transcription of the activity-regulated cytoskeleton-associated gene (Arc), also known as Arg 3.1. Arc is implicated in several types of synaptic plasticity, including synaptic scaling, long-term potentiation, and long-term depression. However, the precise mechanisms by which Arc mediates these forms of long-term plasticity are unclear. It was recently found that Arc protein is capable of forming capsid-like structures and of transferring its own mRNA to neighboring cells. Moreover, Arc mRNA undergoes activity-dependent translation in these "transfected" cells. These new data raise unexpected possibilities for the mechanisms of the Arc action, and many intriguing questions concerning the role of Arc transcellular traffic in neuronal plasticity. In this mini-review, we discuss a possible link between the role of Arc in learning and memory and the virus-like properties of this protein. Additionally, we highlight some of the emerging questions for future neurobiological studies and translational applications of Arc transsynaptic effects.

Vehicles of intercellular communication: exosomes and HIV-1

The terms extracellular vesicles, microvesicles, oncosomes, or exosomes are often used interchangeably as descriptors of particles that are released from cells and comprise a lipid membrane that encapsulates nucleic acids and proteins. Although these entities are defined based on a specific size range and/or mechanism of release, the terminology is often ambiguous. Nevertheless, these vesicles are increasingly recognized as important modulators of intercellular communication. The generic characterization of extracellular vesicles could also be used as a descriptor of enveloped viruses, highlighting the fact that extracellular vesicles and enveloped viruses are similar in both composition and function. Their high degree of similarity makes differentiating between vesicles and enveloped viruses in biological specimens particularly difficult. Because viral particles and extracellular vesicles are produced simultaneously in infected cells, it is necessary to separate these populations to understand their independent functions. We summarize current understanding of the similarities and differences of extracellular vesicles, which henceforth we will refer to as exosomes, and the enveloped retrovirus, HIV-1. Here, we focus on the presence of these particles in semen, as these are of particular importance during HIV-1 sexual transmission. While there is overlap in the terminology and physical qualities between HIV-1 virions and exosomes, these two types of intercellular vehicles may differ depending on the bio-fluid source. Recent data have demonstrated that exosomes from human semen serve as regulators of HIV-1 infection that may contribute to the remarkably low risk of infection per sexual exposure.

Extracellular Vesicles as Conduits of Non-Coding RNA Emission and Intercellular Transfer in Brain Tumors

Non-coding RNA (ncRNA) species have emerged in as molecular fingerprints and regulators of brain tumor pathogenesis and progression. While changes in ncRNA levels have been traditionally regarded as cell intrinsic there is mounting evidence for their extracellular and paracrine function. One of the key mechanisms that enables ncRNA to exit from cells is their selective packaging into extracellular vesicles (EVs), and trafficking in the extracellular space and biofluids. Vesicular export processes reduce intracellular levels of specific ncRNA in EV donor cells while creating a pool of EV-associated ncRNA in the extracellular space and biofluids that enables their uptake by other recipient cells; both aspects have functional consequences. Cancer cells produce several EV subtypes (exosomes, ectosomes), which differ in their ncRNA composition, properties and function. Several RNA biotypes have been identified in the cargo of brain tumor EVs, of which microRNAs are the most studied, but other species (snRNA, YRNA, tRNA, and lncRNA) are often more abundant. Of particular interest is the link between transforming oncogenes and the biogenesis, cargo, uptake and function of tumor-derived EV, including EV content of oncogenic RNA. The ncRNA repertoire of EVs isolated from cerebrospinal fluid and serum is being developed as a liquid biopsy platform in brain tumors.

The role of exosomal transport of viral agents in persistent HIV pathogenesis

Human immunodeficiency virus (HIV) infection, despite great advances in antiretroviral therapy, remains a lifelong affliction. Though current treatment regimens can effectively suppress viral load to undetectable levels and preserve healthy immune function, they cannot fully alleviate all symptoms caused by the presence of the virus, such as HIV-associated neurocognitive disorders. Exosomes are small vesicles that transport cellular proteins, RNA, and small molecules between cells as a mechanism of intercellular communication. Recent research has shown that HIV proteins and RNA can be packaged into exosomes and transported between cells, to pathogenic effect. This review summarizes the current knowledge on the diverse mechanisms involved in the sorting of viral elements into exosomes and the damage those exosomal agents can inflict. In addition, potential therapeutic options to counteract exosome-mediated HIV pathogenesis are reviewed and considered.

Exosome as a Novel Shuttle for Delivery of Therapeutics across Biological Barriers

The effective delivery of target-specific siRNA to the brain by exploiting the exosomes derived from dendritic cells renders the paradigm shift for the prospective use of nanosized exosomes as a delivery system. Although the in vivo targeting strategies by other nanovesicles like liposomes exist, still this novel exosome-based delivery approach holds an inclusive dominance of in vivo security and reduced immunogenicity. Achieving promising exosome-based delivery strategies warrants more desirable exploration of their biology. Over the years, the invention of novel production, characterization, targeting strategies, and cargo loading techniques of exosome improved its ability to reach clinics. Essentially, exosome-based delivery of therapeutics assures to conquer the major hurdles, like delivery of cargos across impermeable biological barriers, like the blood-brain barrier, biocompatibility, increased solubility, metabolic stability, improved circulation time, target specific delivery, and pharmacokinetics, and thereby enhanced the efficacy of loaded therapeutic agents. In this article, we cover the current status of exosome as a delivery vehicle for therapeutics and the challenges that need to be overcome, and we also discuss future perspectives of this exciting field of research to transform it from bench to clinical reality.

Post-translational and transcriptional dynamics - regulating extracellular vesicle biology

Introduction: Extracellular vesicles (EVs) are secreted into their extracellular environment, contain a specific repertoire of cellular cargo, and represent a novel vehicle for cell-cell communication. Protein post-translational modifications (PTMs) are emerging as major effectors of EV biology and function, and in turn, regulate cellular signaling. Areas covered: Discovery and investigation of PTMs such as methylation, glycosylation, acetylation, phosphorylation, sumoylation, and many others has established fundamental roles for PTMs within EVs and associated EV function. The application of enrichment strategies for modifications, high-resolution quantitative mass spectrometry-based proteomics, and improved technological approaches have provided key insights into identification and characterization of EV-based PTMs. Recently, an overwhelming appreciation for the diversity of modifications, including post-transcriptional modifications, dynamic roles of these modifications, and their emerging interplay, including protein-protein, protein-lipid, protein-RNA, and variable RNA modifications, is emerging. At a cellular level, such interplay is essential for gene expression/genome organization, protein function and localization, RNA metabolism, cell division, and cell signaling. Expert commentary: The understanding of these modifications and interactions will provide strategies toward how distinct cargo is localized, sorted, and delivered through EVs to mediate intercellular function, with further understanding of such modifications and intermolecular interactions will provide advances in EV-based therapeutic strategies.

Inside job: how the ESCRTs release HIV-1 from infected cells

Human immunodeficiency virus type 1 (HIV-1) hijacks the host endosomal sorting complex required for transport (ESCRT) proteins in order to release infectious viral particles from the cell. ESCRT recruitment is virtually essential for the production of infectious virus, despite that the main structural protein of HIV-1, Gag, is capable of self-assembling and eventually budding from membranes on its own. Recent data have reinforced the paradigm of ESCRT-dependent particle release while clarifying why this rapid release is so critical. The ESCRTs were originally discovered as integral players in endosome maturation and are now implicated in many important cellular processes beyond viral and endosomal budding. Nearly all of these roles have in common that membrane scission occurs from the inward face of the membrane neck, which we refer to as 'reverse topology' scission. A satisfactory mechanistic description of reverse-topology membrane scission by ESCRTs remains a major challenge both in general and in the context of HIV-1 release. New observations concerning the fundamental scission mechanism for ESCRTs in general, and the process of HIV-1 release specifically, have generated new insights in both directions, bringing us closer to a mechanistic understanding.

Autophagy, EVs, and Infections: A Perfect Question for a Perfect Time

Autophagy, a highly conserved process, serves to maintain cellular homeostasis in response to an extensive variety of internal and external stimuli. The classic, or canonical, pathway of autophagy involves the coordinated degradation and recycling of intracellular components and pathogenic material. Proper regulation of autophagy is critical to maintain cellular health, as alterations in the autophagy pathway have been linked to the progression of a variety of physiological and pathological conditions in humans, namely in aging and in viral infection. In addition to its canonical role as a degradative pathway, a more unconventional and non-degradative role for autophagy has emerged as an area of increasing interest. This process, known as secretory autophagy, is gaining widespread attention as many viruses are believed to use this pathway as a means to release and spread viral particles. Moreover, secretory autophagy has been found to intersect with other intracellular pathways, such as the biogenesis and secretion of extracellular vesicles (EVs). Here, we provide a review of the current landscape surrounding both degradative autophagy and secretory autophagy in relation to both aging and viral infection. We discuss their key features, while describing their interplay with numerous different viruses (i.e. hepatitis B and C viruses, Epstein-Barr virus, SV40, herpesviruses, HIV, chikungunya virus, dengue virus, Zika virus, Ebola virus, HTLV, Rift Valley fever virus, poliovirus, and influenza A virus), and compare secretory autophagy to other pathways of extracellular vesicle release. Lastly, we highlight the need for, and emphasize the importance of, more thorough methods to study the underlying mechanisms of these pathways to better advance our understanding of disease progression.

The Ambiguous Roles of Extracellular Vesicles in HIV Replication and Pathogenesis

Cells from all kingdoms of life can release membrane-enclosed vesicles to the extracellular milieu. These extracellular vesicles (EVs) may function as mediators of intercellular communication, allowing the transfer of biologically active molecules between cells and organisms. It has become clear that HIV particles and certain types of EVs, such as exosomes, share many similarities regarding morphology, composition, and biogenesis. This review presents a summary of the literature describing the intricate relationship between HIV and EVs biogenesis. Also, we discuss the latest progress toward understanding the mechanisms by which EVs influence HIV pathogenesis, as well as, how HIV modulates EVs composition in infected cells to facilitate viral spread.

Extracellular Vesicle Biology in Alzheimer's Disease and Related Tauopathy

Extracellular vesicles (EVs) are physiological vesicles secreted from most eukaryotes and contain cargos of their cell of origin. EVs, and particularly a subset of EV known as exosomes, are emerging as key mediators of cell to cell communication and waste management for cells both during normal organismal function and in disease. In this review, we investigate the rapidly growing field of exosome biology, their biogenesis, cargo loading, and uptake by other cells. We particularly consider the role of exosomes in Alzheimer's disease, both as a pathogenic agent and as a disease biomarker. We also explore the emerging role of exosomes in chronic traumatic encephalopathy. Finally, we highlight open questions in these fields and the possible use of exosomes as therapeutic targets and agents.

Transmembrane Domains Mediate Intra- and Extracellular Trafficking of Epstein-Barr Virus Latent Membrane Protein 1

EBV latent membrane protein 1 (LMP1) is released from latently infected tumor cells in small membrane-enclosed extracellular vesicles (EVs). Accumulating evidence suggests that LMP1 is a major driver of EV content and functions. LMP1-modified EVs have been shown to influence recipient cell growth, migration, differentiation, and regulation of immune cell function. Despite the significance of LMP1-modified exosomes, very little is known about how this viral protein enters or manipulates the host EV pathway. In this study, LMP1 deletion mutants were generated to assess protein regions required for EV trafficking. Following transfection of LMP1 or mutant plasmids, EVs were collected by differential centrifugation, and the levels of specific cargo were evaluated by immunoblot analysis. The results demonstrate that, together, the N terminus and transmembrane region 1 of LMP1 are sufficient for efficient sorting into EVs. Consistent with these findings, a mutant lacking the N terminus and transmembrane domains 1 through 4 (TM5-6) failed to be packaged into EVs, and exhibited higher colocalization with endoplasmic reticulum and early endosome markers than the wild-type protein. Surprisingly, TM5-6 maintained the ability to colocalize and form a complex with CD63, an abundant exosome protein that is important for the incorporation of LMP1 into EVs. Other mutations within LMP1 resulted in enhanced levels of secretion, pointing to potential positive and negative regulatory mechanisms for extracellular vesicle sorting of LMP1. These data suggest new functions of the N terminus and transmembrane domains in LMP1 intra- and extracellular trafficking that are likely downstream of an interaction with CD63.IMPORTANCE EBV infection contributes to the development of cancers, such as nasopharyngeal carcinoma, Burkitt lymphoma, Hodgkin's disease, and posttransplant lymphomas, in immunocompromised or genetically susceptible individuals. LMP1 is an important viral protein expressed by EBV in these cancers. LMP1 is secreted in extracellular vesicles (EVs), and the transfer of LMP1-modified EVs to uninfected cells can alter their physiology. Understanding the cellular machinery responsible for sorting LMP1 into EVs is limited, despite the importance of LMP1-modified EVs. Here, we illustrate the roles of different regions of LMP1 in EV packaging. Our results show that the N terminus and TM1 are sufficient to drive LMP1 EV trafficking. We further show the existence of potential positive and negative regulatory mechanisms for LMP1 vesicle sorting. These findings provide a better basis for future investigations to identify the mechanisms of LMP1 targeting to EVs, which could have broad implications in understanding EV cargo sorting.

Cellular mechanisms responsible for cell-to-cell spreading of prions

Prions are infectious agents that cause fatal neurodegenerative diseases. Current evidence indicates that they are essentially composed of an abnormally folded protein (PrPSc). These abnormal aggregated PrPSc species multiply in infected cells by recruiting and converting the host PrPC protein into new PrPSc. How prions move from cell to cell and progressively spread across the infected tissue is of crucial importance and may provide experimental opportunity to delay the progression of the disease. In infected cells, different mechanisms have been identified, including release of infectious extracellular vesicles and intercellular transfer of PrPSc-containing organelles through tunneling nanotubes. These findings should allow manipulation of the intracellular trafficking events targeting PrPSc in these particular subcellular compartments to experimentally address the relative contribution of these mechanisms to in vivo prion pathogenesis. In addition, such information may prompt further experimental strategies to decipher the causal roles of protein misfolding and aggregation in other human neurodegenerative diseases.

Proteomic Analysis of Exosomes and Its Application in HIV-1 Infection

Exosomes are 30-100 nm extracellular vesicles secreted from late endosomes by various types of cells. Numerous studies have suggested that exosomes play significant roles in human immunodeficiency virus 1 (HIV-1) biogenesis. Proteomics coupled with exosome fractionation has been successfully used to identify various exosomal proteins and helped to uncover the interactions between exosomes and HIV-1. To inform the current progress in the intersection of exosome, proteomics, and HIV-1, this review is focused on: i) analyzing different exosome isolation, purification methods, and their implications in HIV-1 studies; ii) evaluating the roles of various proteomic techniques in defining exosomal contents; iii) discussing the research and clinical applications of proteomics and exosome in HIV-1 biology.

Exosome markers associated with immune activation and oxidative stress in HIV patients on antiretroviral therapy

Exosomes are nanovesicles released from most cell types including immune cells. Prior studies suggest exosomes play a role in HIV pathogenesis, but little is known about exosome cargo in relation to immune responses and oxidative stress. Here, we characterize plasma exosomes in HIV patients and their relationship to immunological and oxidative stress markers. Plasma exosome fractions were isolated from HIV-positive subjects on ART with suppressed viral load and HIV-negative controls. Exosomes were characterized by electron microscopy, nanoparticle tracking, immunoblotting, and LC-MS/MS proteomics. Plasma exosomes were increased in HIV-positive subjects compared to controls, and correlated with increased oxidative stress markers (cystine, oxidized cys-gly) and decreased PUFA (DHA, EPA, DPA). Untargeted proteomics detected markers of exosomes (CD9, CD63, CD81), immune activation (CD14, CRP, HLA-A, HLA-B), oxidative stress (CAT, PRDX1, PRDX2, TXN), and Notch4 in plasma exosomes. Exosomal Notch4 was increased in HIV-positive subjects versus controls and correlated with immune activation markers. Treatment of THP-1 monocytic cells with patient-derived exosomes induced expression of genes related to interferon responses and immune activation. These results suggest that exosomes in ART-treated HIV patients carry proteins related to immune activation and oxidative stress, have immunomodulatory effects on myeloid cells, and may have pro-inflammatory and redox effects during pathogenesis.

Adiponectin/T-cadherin system enhances exosome biogenesis and decreases cellular ceramides by exosomal release

Adiponectin, an adipocyte-derived circulating protein, accumulates in vasculature, heart, and skeletal muscles through interaction with a unique glycosylphosphatidylinositol-anchored cadherin, T-cadherin. Recent studies have demonstrated that such accumulation is essential for adiponectin-mediated cardiovascular protection. Here, we demonstrate that the adiponectin/T-cadherin system enhances exosome biogenesis and secretion, leading to the decrease of cellular ceramides. Adiponectin accumulated inside multivesicular bodies, the site of exosome generation, in cultured cells and in vivo aorta, and also in exosomes in conditioned media and in blood, together with T-cadherin. The systemic level of exosomes in blood was significantly affected by adiponectin or T-cadherin in vivo. Adiponectin increased exosome biogenesis from the cells, dependently on T-cadherin, but not on AdipoR1 or AdipoR2. Such enhancement of exosome release accompanied the reduction of cellular ceramides through ceramide efflux in exosomes. Consistently, the ceramide reduction by adiponectin was found in aortas of WT mice treated with angiotensin II, but not in T-cadherin-knockout mice. Our findings provide insights into adiponectin/T-cadherin-mediated organ protection through exosome biogenesis and secretion.

Role of Exosomes in Human Retroviral Mediated Disorders

Retroviruses comprise an ancient and varied group of viruses with the unique ability to integrate DNA from an RNA transcript into the genome, a subset of which are able to integrate in humans. The timing of these integrations during human history has dictated whether these viruses have remained exogenous and given rise to various human diseases or have become inseparable from the host genome (endogenous retroviruses). Given the ability of retroviruses to integrate into the host and subsequently co-opt host cellular process for viral propagation, retroviruses have been shown to be closely associated with several cellular processes including exosome formation. Exosomes are 30-150 nm unilamellar extracellular vesicles that originate from intraluminal vesicles (ILVs) that form in the endosomal compartment. Exosomes have been shown to be important in intercellular communication and immune cell function. Almost every cell type studied has been shown to produce these types of vesicles, with the cell type dictating the contents, which include proteins, mRNA, and miRNAs. Importantly, recent evidence has shown that infection by viruses, including retroviruses, alter the contents and subsequent function of produced exosomes. In this review, we will discuss the important retroviruses associated with human health and disease. Furthermore, we will delve into the impact of exosome formation and manipulation by integrated retroviruses on human health, survival, and human retroviral disease pathogenesis.

Extracellular vesicles as potential biomarkers for alcohol- and drug-induced liver injury and their therapeutic applications

Extracellular vesicles (EVs) are small membranous vesicles originating from various cells and tissues, including the liver parenchymal hepatocytes and nonparenchymal cells such as Kupffer and stellate cells. Recently, the pathophysiological role of EVs, such as exosomes and microvesicles, has been increasingly recognized based on their properties of intercellular communications. These EVs travel through the circulating blood and interact with specific cells and then deliver their cargos such as nucleic acids and proteins into recipient cells. In addition, based on their stabilities, circulating EVs from body fluids such as blood, cerebrospinal fluid, urine, saliva, semen, breast milk and amniotic fluids are being studied as a valuable source of potential biomarkers for providing information about the physiological status of original cells or tissues. In addition, EVs are considered potential therapeutic agents due to their ability for intercellular communications between different cell types within the liver and between various organs through transfer of their cargos. In this review, we have briefly described recent advances in the characteristics and pathophysiological roles of EVs in alcoholic liver disease (ALD) or drug-induced liver injury (DILI) and discuss their advantages in the discovery of potential biomarkers and therapeutic agents.

Exosomal miR-9 Released from HIV Tat Stimulated Astrocytes Mediates Microglial Migration

Chronic neuroinflammation still remains a common underlying feature of HIV-infected patients on combined anti-retroviral therapy (cART). Previous studies have reported that despite near complete suppression of virus replication by cART, cytotoxic viral proteins such as HIV trans-activating regulatory protein (Tat) continue to persist in tissues such as the brain and the lymph nodes, thereby contributing, in part, to chronic glial activation observed in HIV-associated neurological disorders (HAND). Understanding how the glial cells cross talk to mediate neuropathology is thus of paramount importance. MicroRNAs (miR) also known as regulators of gene expression, have emerged as key paracrine signaling mediators that regulate disease pathogenesis and cellular crosstalk, through their transfer via the extracellular vesicles (EV). In the current study we have identified a novel function of miR-9, that of mediating microglial migration. We demonstrate that miR-9 released from Tat-stimulated astrocytes can be taken up by microglia resulting in their migratory phenotype. Exposure of human astrocytoma (A172) cells to HIV Tat resulted in induction and release of miR-9 in the EVs, which, was taken up by microglia, leading in turn, increased migration of the latter cells, a process that could be blocked by both an exosome inhibitor GW4869 or a specific target protector of miR-9. Furthermore, it was also demonstrated that EV miR-9 mediated inhibition of the expression of target PTEN, via its binding to the 3'UTR seed sequence of the PTEN mRNA, was critical for microglial migration. To validate the role of miR-9 in this process, microglial cells were treated with EVs loaded with miR-9, which resulted in significant downregulation of PTEN expression with a concomitant increase in microglial migration. These findings were corroborated by transfecting microglia with a specific target protector of PTEN, that blocked miR-9-mediated downregulation of PTEN as well as microglial migration. In vivo studies wherein the miR-9 precursor-transduced microglia were transplanted into the striatum of mice, followed by assessing their migration in response to a stimulus administered distally, further validated the role of miR-9 in mediating microglial migration. Collectively, our findings provide evidence that glial crosstalk via miRs released from EVs play a vital role in mediating disease pathogenesis and could provide new avenues for development of novel therapeutic strategies aimed at dampening neuropathogenesis.

Characterization of Trichuris muris secreted proteins and extracellular vesicles provides new insights into host-parasite communication

Whipworms are parasitic nematodes that live in the gut of more than 500 million people worldwide. Owing to the difficulty in obtaining parasite material, the mouse whipworm Trichuris muris has been extensively used as a model to study human whipworm infections. These nematodes secrete a multitude of compounds that interact with host tissues where they orchestrate a parasitic existence. Herein we provide the first comprehensive characterization of the excretory/secretory products of T. muris. We identify 148 proteins secreted by T. muris and show for the first time that the mouse whipworm secretes exosome-like extracellular vesicles (EVs) that can interact with host cells. We use an Optiprep® gradient to purify the EVs, highlighting the suitability of this method for purifying EVs secreted by a parasitic nematode. We also characterize the proteomic and genomic content of the EVs, identifying >350 proteins, 56 miRNAs (22 novel) and 475 full-length mRNA transcripts mapping to T. muris gene models. Many of the miRNAs putatively mapped to mouse genes are involved in regulation of inflammation, implying a role in parasite-driven immunomodulation. In addition, for the first time to our knowledge, colonic organoids have been used to demonstrate the internalization of parasite EVs by host cells. Understanding how parasites interact with their host is crucial to develop new control measures. This first characterization of the proteins and EVs secreted by T. muris provides important information on whipworm-host communication and forms the basis for future studies.

Extracellular vesicle-mediated bone metabolism in the bone microenvironment

Extracellular vesicles (EVs) are phospholipid membrane-enclosed entities containing specific proteins, RNA, miRNA, and lncRNA. EVs are released by various cells and play a vital role in cell communication by transferring their contents from the host cells to the recipient cells. The role of EVs has been characterized in a wide range of physiological and pathophysiological processes. In this context, we highlight recent advances in our understanding of the regulatory effects of EVs, with a focus on bone metabolism and the bone microenvironment. The roles of EVs in cell communication among bone-related cells, stem cells, tumor cells, and other cells under physiological or pathological conditions are also discussed. In addition, promising applications for EVs in treating bone-related diseases are proposed.

Post-translational add-ons mark the path in exosomal protein sorting

Extracellular vesicles (EVs) are released by cells to the extracellular environment to mediate inter-cellular communication. Proteins, lipids, nucleic acids and metabolites shuttled in these vesicles modulate specific functions in recipient cells. The enrichment of selected sets of proteins in EVs compared with global cellular levels suggests the existence of specific sorting mechanisms to specify EV loading. Diverse post-translational modifications (PTMs) of proteins participate in the loading of specific elements into EVs. In this review, we offer a perspective on PTMs found in EVs and discuss the specific role of some PTMs, specifically Ubiquitin and Ubiquitin-like modifiers, in exosomal sorting of protein components. The understanding of these mechanisms will provide new strategies for biomedical applications. Examples include the presence of defined PTM marks on EVs as novel biomarkers for the diagnosis and prognosis of certain diseases, or the specific import of immunogenic components into EVs for vaccine generation.

Port-to-port delivery: Mobilization of toxic sphingolipids via extracellular vesicles

The discovery that most cells produce extracellular vesicles (EVs) and release them in the extracellular milieu has spurred the idea that these membranous cargoes spread pathogenic mechanisms. In the brain, EVs may have multifold and important physiological functions, from deregulating synaptic activity to promoting demyelination to changes in microglial activity. The finding that small EVs (exosomes) contain α-synuclein and β-amyloid, among other pathogenic proteins, is an example of this notion, underscoring their potential role in the brains of patients with Parkinson's and Alzheimer's diseases. Given that they are membranous vesicles, we speculate that EVs also have an intrinsic capacity to incorporate sphingolipids. In conditions under which these lipids are elevated to toxic levels, such as in Krabbe's disease and metachromatic leukodystrophy, EVs may contribute to spread disease from sick to healthy cells. In this essay, we discuss a working hypothesis that brain cells in sphingolipidoses clear some of the accumulated lipid material to attempt restoring cell homeostasis via EV secretion. We hypothesize that secreted sphingolipid-loaded EVs shuttle pathogenic lipids to cells that are not intrinsically affected, contributing to establishing non-cell-autonomous defects. © 2016 Wiley Periodicals, Inc.

Biogenesis and function of ESCRT-dependent extracellular vesicles

From bacteria to humans, cells secrete a large variety of membrane-bound extracellular vesicles. Only relatively recently has it however started to become clear that the exovesicular transport of proteins and RNAs is important for normal physiology and numerous pathological conditions. Extracellular vesicles can be formed through the release of the intralumenal vesicles of multivesicular endosomes as so-called exosomes, or through direct, ectosomal, budding from the cell surface. Through their ability to promote the bending of membranes away from the cytoplasm, the components of the Endosomal Sorting Complex Required for Transport (ESCRT) have been implicated in both exo- and ectosomal biogenesis. Studies of the ESCRT machinery may therefore provide important insights into the formation and function of extracellular vesicles. In the present review, we first describe the cell biological mechanisms through which ESCRT components contribute to the biogenesis of different types of extracellular vesicles. We then discuss how recent functional studies have started to uncover important roles of ESCRT-dependent extracellular vesicles in a wide variety of processes, including the transport of developmental signaling molecules and embryonic morphogenesis, the regulation of social behavior and host-pathogen interactions, as well as the etiology and progression of neurodegenerative pathologies and cancer.

Mechanisms of HIV Neuropathogenesis: Role of Cellular Communication Systems

BACKGROUND

One of the major complications of Human Immunodeficiency Virus (HIV) infection is the development of HIV-Associated Neurocognitive Disorders (HANDs) in approximately 50-60% of HIV infected individuals. Despite undetectable viral loads in the periphery owing to anti-retroviral therapy, neuroinflammation and neurocognitive impairment are still prevalent in HIV infected individuals. Several studies indicate that the central nervous system (CNS) abnormalities observed in HIV infected individuals are not a direct effect of viral replication in the CNS, rather these neurological abnormalities are associated with amplification of HIV specific signals by unknown mechanisms. We propose that some of these mechanisms of damage amplification are mediated by gap junction channels, pannexin and connexin hemichannels, tunneling nanotubes and microvesicles/exosomes.

OBJECTIVE

Our laboratory and others have demonstrated that HIV infection targets cell to cell communication by altering all these communication systems resulting in enhanced bystander apoptosis of uninfected cells, inflammation and viral infection. Here we discuss the role of these communication systems in HIV neuropathogenesis.

CONCLUSION

In the current manuscript, we have described the mechanisms by which HIV "hijacks" these host cellular communication systems, leading to exacerbation of HIV neuropathogenesis, and to simultaneously promote the survival of HIV infected cells, resulting in the establishment of viral reservoirs.

Exosomal tetraspanins mediate cancer metastasis by altering host microenvironment

The metastases of malignant tumors develop through a cascade of events. The establishment of a pre-metastatic micro-environment is initiated by communication between tumors and host. Exosomes come into focus as the most potent intercellular communicators playing a pivotal role in this process. Cancer cells release exosomes into the extracellular environment prior to metastasis. Tetraspanin is a type of 4 times transmembrane proteins. It may be involved in cell motility, adhesion, morphogenesis, as well as cell and vesicular membrane fusion. The exosomal tetraspanin network is a molecular scaffold connecting various proteins for signaling transduction. The complex of tetraspanin-integrin determines the recruiting cancer exosomes to pre-metastatic sites. Tetraspanin is a key element for the target cell selection of exosomes uptake that may lead to the reprogramming of target cells. Reprogrammed target cells assist pre-metastatic niche formation. Previous reviews have described the biogenesis, secretion and intercellular interaction of exosomes in various tumors. However, there is a lack of reviews on the topic of exosomal tetraspanin in the context of cancer. In this review, we will describe the main characteristics of exosomal tetraspanin in cancer cells. We will also discuss how the cancer exosomal tetraspanin alters extracellular environment and regulates cancer metastasis.

Role of TIM-4 in exosome-dependent entry of HIV-1 into human immune cells

Exosomes, 30–200 nm nanostructures secreted from donor cells and internalized by recipient cells, can play an important role in the cellular entry of some viruses. These microvesicles are actively secreted into various body fluids, including blood, urine, saliva, cerebrospinal fluid, and breast milk. We successfully isolated exosomes from human breast milk and plasma. The size and concentration of purified exosomes were measured by nanoparticle tracking, while Western blotting confirmed the presence of the exosomal-associated proteins CD9 and CD63, clathrin, and T cell immunoglobulin and mucin proteins (TIMs). Through viral infection assays, we determined that HIV-1 utilizes an exosome-dependent mechanism for entry into human immune cells. The virus contains high amounts of phosphatidylserine (PtdSer) and may bind PtdSer receptors, such as TIMs. This mechanism is supported by our findings that exosomes from multiple sources increased HIV-1 entry into T cells and macrophages, and viral entry was potently blocked with anti-TIM-4 antibodies.

Interplay between Autophagy, Exosomes and HIV-1 Associated Neurological Disorders: New Insights for Diagnosis and Therapeutic Applications

The autophagy–lysosomal pathway mediates a degradative process critical in the maintenance of cellular homeostasis as well as the preservation of proper organelle function by selective removal of damaged proteins and organelles. In some situations, cells remove unwanted or damaged proteins and RNAs through the release to the extracellular environment of exosomes. Since exosomes can be transferred from one cell to another, secretion of unwanted material to the extracellular environment in exosomes may have an impact, which can be beneficial or detrimental, in neighboring cells. Exosome secretion is under the influence of the autophagic system, and stimulation of autophagy can inhibit exosomal release and vice versa. Neurons are particularly vulnerable to degeneration, especially as the brain ages, and studies indicate that imbalances in genes regulating autophagy are a common feature of many neurodegenerative diseases. Cognitive and motor disease associated with severe dementia and neuronal damage is well-documented in the brains of HIV-infected individuals. Neurodegeneration seen in the brain in HIV-1 infection is associated with dysregulation of neuronal autophagy. In this paradigm, we herein provide an overview on the role of autophagy in HIV-associated neurodegenerative disease, focusing particularly on the effect of autophagy modulation on exosomal release of HIV particles and how this interplay impacts HIV infection in the brain. Specific autophagy–regulating agents are being considered for therapeutic treatment and prevention of a broad range of human diseases. Various therapeutic strategies for modulating specific stages of autophagy and the current state of drug development for this purpose are also evaluated.

Viral effects on the content and function of extracellular vesicles

The release of membrane-bound vesicles from cells is being increasingly recognized as a mechanism of intercellular communication. Extracellular vesicles (EVs) or exosomes are produced by virus-infected cells and are thought to be involved in intercellular communication between infected and uninfected cells. Viruses, in particular oncogenic viruses and viruses that establish chronic infections, have been shown to modulate the production and content of EVs. Viral microRNAs, proteins and even entire virions can be incorporated into EVs, which can affect the immune recognition of viruses or modulate neighbouring cells. In this Review, we discuss the roles that EVs have during viral infection to either promote or restrict viral replication in target cells. We will also discuss our current understanding of the molecular mechanisms that underlie these roles, the potential consequences for the infected host and possible future diagnostic applications.

Serum extracellular vesicle depletion processes affect release and infectivity of HIV-1 in culture

Extracellular vesicles (EVs) are involved in intercellular communication and affect processes including immune and antiviral responses. Blood serum, a common cell culture medium component, is replete with EVs and must be depleted prior to EV-related experiments. The extent to which depletion processes deplete non-EV particles is incompletely understood, but depleted serum is associated with reduced viability and growth in cell culture. Here, we examined whether serum depleted by two methods affected HIV-1 replication. In cell lines, including HIV-1 latency models, increased HIV-1 production was observed, along with changes in cell behavior and viability. Add-back of ultracentrifuge pellets (enriched in EVs but possibly other particles) rescued baseline HIV-1 production. Primary cells were less sensitive to serum depletion processes. Virus produced under processed serum conditions was more infectious. Finally, changes in cellular metabolism, surface markers, and gene expression, but not miRNA profiles, were associated with depleted serum culture. In conclusion, depleted serum conditions have a substantial effect on HIV-1 production and infectivity. Dependence of cell cultures on “whole serum” must be examined carefully along with other experimental variables, keeping in mind that the effects of EVs may be accompanied by or confused with those of closely associated or physically similar particles.

Extracellular Vesicles Carry HIV Env and Facilitate Hiv Infection of Human Lymphoid Tissue

Cells productively infected with HIV-1 release virions along with extracellular vesicles (EVs) whose biogenesis, size, and physical properties resemble those of retroviruses. Here, we found that a significant number of EVs (exosomes) released by HIV-1 infected cells carry gp120 (Env), a viral protein that mediates virus attachment and fusion to target cells, and also facilitates HIV infection in various indirect ways. Depletion of viral preparations of EVs, in particular of those that carry gp120, decreases viral infection of human lymphoid tissue ex vivo. Thus, EVs that carry Env identified in our work seem to facilitate HIV infection and therefore may constitute a new therapeutic target for antiviral strategy.

Prions on the run: How extracellular vesicles serve as delivery vehicles for self-templating protein aggregates

Extracellular vesicles (EVs) are actively secreted, membrane-bound communication vehicles that exchange biomolecules between cells. EVs also serve as dissemination vehicles for pathogens, including prions, proteinaceous infectious agents that cause transmissible spongiform encephalopathies (TSEs) in mammals. Increasing evidence accumulates that diverse protein aggregates associated with common neurodegenerative diseases are packaged into EVs as well. Vesicle-mediated intercellular transmission of protein aggregates can induce aggregation of homotypic proteins in acceptor cells and might thereby contribute to disease progression. Our knowledge of how protein aggregates are sorted into EVs and how these vesicles adhere to and fuse with target cells is limited. Here we review how TSE prions exploit EVs for intercellular transmission and compare this to the transmission behavior of self-templating cytosolic protein aggregates derived from the yeast prion domain Sup 35 NM. Artificial NM prions are non-toxic to mammalian cell cultures and do not cause loss-of-function phenotypes. Importantly, NM particles are also secreted in association with exosomes that horizontally transmit the prion phenotype to naive bystander cells, a process that can be monitored with high accuracy by automated high throughput confocal microscopy. The high abundance of mammalian proteins with amino acid stretches compositionally similar to yeast prion domains makes the NM cell model an attractive model to study self-templating and dissemination properties of proteins with prion-like domains in the mammalian context.

Exosomes: mobile platforms for targeted and synergistic signaling across cell boundaries

Intercellular communications play a vital role during tissue patterning, tissue repair, and immune reactions, in homeostasis as well as in disease. Exosomes are cell-derived secreted vesicles, extensively studied for their role in intercellular communication. Exosomes have the intrinsic ability to package multiple classes of proteins and nucleic acids within their lumens and on their membranes. Here, we explore the hypothesis that exosomal targeting may represent a cellular strategy that has evolved to deliver specific combinations of signals to specific target cells and influence normal or pathological processes. This review aims to evaluate the available evidence for this hypothesis and to identify open questions whose answers will illuminate our understanding and applications of exosome biology.

Exosomes in immunoregulation of chronic lung diseases

Exosomes are nano-sized, membrane-bound vesicles released from cells that transport cargo including DNA, RNA, and proteins, between cells as a form of intercellular communication. In addition to their role in intercellular communication, exosomes are beginning to be appreciated as agents of immunoregulation that can modulate antigen presentation, immune activation, suppression, and surveillance. This article summarizes how these multifaceted functions of exosomes may promote development and/or progression of chronic inflammatory lung diseases including asthma, chronic obstructive pulmonary disease, and pulmonary fibrosis. The potential of exosomes as a novel therapeutic is also discussed.

Label-free Proteomic Analysis of Exosomes Derived from Inducible Hepatitis B Virus-Replicating HepAD38 Cell Line

Hepatitis B virus (HBV) infection is a major health problem worldwide. Recent evidence suggests that some viruses can manipulate the infection process by packing specific viral and cellular components into exosomes, small nanometer-sized (30-150 nm) vesicles secreted from various cells. However, the impact of HBV replication on the content of exosomes produced by hepatocytes has not been fully delineated. In this work, an HBV-inducible cell line HepAD38 was used to directly compare changes in the protein content of exosomes secreted from HepAD38 cells with or without HBV replication. Exosomes were isolated from supernantants of HepAD38 cells cultured with or without doxycycline (dox) and their purity was confirmed by transmission electron microscopy (TEM) and Western immunoblotting assays. Ion-intensity based label-free LC-MS/MS quantitation technologies were applied to analyze protein content of exosomes from HBV replicating cells [referred as HepAD38 (dox^-)-exo] and from HBV nonreplicating cells [referred as HepAD38 (dox⁺)-exo]. A total of 1412 exosomal protein groups were identified, among which the abundance of 35 proteins was significantly changed following HBV replication. Strikingly, 5 subunit proteins from the 26S proteasome complex, including PSMC1, PSMC2, PSMD1, PSMD7 and PSMD14 were consistently enhanced in HepAD38 (dox^-)-exo. Bioinformatic analysis of differential exosomal proteins confirmed the significant enrichment of components involved in the proteasomal catabolic process. Proteasome activity assays further suggested that HepAD38 (dox^-)-exo had enhanced proteolytic activity compared with HepAD38 (dox⁺)-exo. Furthermore, human peripheral monocytes incubated with HepAD38 (dox^-)-exo induced a significantly lower level of IL-6 secretion compared with IL-6 levels from HepAD38 (dox⁺)-exo. Irreversible inhibition of proteasomal activity within exosomes restored higher production of IL-6 by monocytes, suggesting that transmission of proteasome subunit proteins by HepAD38 (dox^-)-exo might modulate the production of pro-inflammatory molecules in the recipient monocytes. These results revealed the composition and potential function of exosomes produced during HBV replication, thus providing a new perspective on the role of exosomes in HBV-host interaction.

Prelysosomal Compartments in the Unconventional Secretion of Amyloidogenic Seeds

A mechanistic link between neuron-to-neuron transmission of secreted amyloid and propagation of protein malconformation cytopathology and disease has recently been uncovered in animal models. An enormous interest in the unconventional secretion of amyloids from neurons has followed. Amphisomes and late endosomes are the penultimate maturation products of the autophagosomal and endosomal pathways, respectively, and normally fuse with lysosomes for degradation. However, under conditions of perturbed membrane trafficking and/or lysosomal deficiency, prelysosomal compartments may instead fuse with the plasma membrane to release any contained amyloid. After a brief introduction to the endosomal and autophagosomal pathways, we discuss the evidence for autophagosomal secretion (exophagy) of amyloids, with a comparative emphasis on Aβ_1-42 and α-synuclein, as luminal and cytosolic amyloids, respectively. The ESCRT-mediated import of cytosolic amyloid into late endosomal exosomes, a known vehicle of transmission of macromolecules between cells, is also reviewed. Finally, mechanisms of lysosomal dysfunction, deficiency, and exocytosis are exemplified in the context of genetically identified risk factors, mainly for Parkinson's disease. Exocytosis of prelysosomal or lysosomal organelles is a last resort for clearance of cytotoxic material and alleviates cytopathy. However, they also represent a vehicle for the concentration, posttranslational modification, and secretion of amyloid seeds.

The Multifaceted Functions of Exosomes in Health and Disease: An Overview

Exosomes are extracellular vesicles of 50-150 nm in diameter secreted by basically all cell types. They mediate micro-communication among cells, tissues, and organs under both healthy and disease conditions by virtue of their ability to deliver macromolecules to target cells. Research on exosomes is a rapidly growing field, however many aspects of their biogenesis and functions still await a complete clarification. In our review we summarize most recent findings regarding biogenesis, structure, and functions of exosomes. In addition, an overview regarding the role of exosomes in both infectious and non-infectious diseases is provided. Finally, the use of exosomes as biomarkers and delivery tools for therapeutic molecules is addressed. Considering the body of literature data, exosomes have to be considered key components of the intercellular communication in both health and disease.

Surface Profiling of Extracellular Vesicles from Plasma or Ascites Fluid Using DotScan Antibody Microarrays

DotScan antibody microarrays were initially developed for the extensive surface profiling of live leukemia and lymphoma cells. DotScan's diagnostic capability was validated with an extensive clinical trial using mononuclear cells from the blood or bone marrow of leukemia or lymphoma patients. DotScan has also been used for the profiling of surface proteins on peripheral blood mononuclear cells (PBMC) from patients with HIV, liver disease, and stable and progressive B-cell chronic lymphocytic leukemia (CLL). Fluorescence multiplexing allowed the simultaneous profiling of cancer cells and leukocytes from disaggregated colorectal and melanoma tumor biopsies after capture on DotScan. In this chapter, we have used DotScan for the surface profiling of extracellular vesicles (EV) recovered from conditioned growth medium of cancer cell lines and the blood of patients with CLL. The detection of captured EV was performed by enhanced chemiluminescence (ECL) using biotinylated antibodies that recognized antigens expressed on the surface of the EV subset of interest. DotScan was also used to profile EV from the blood of healthy individuals and the ascites fluid of ovarian cancer patients. DotScan binding patterns of EV from human plasma and other body fluids may yield diagnostic or prognostic signatures for monitoring the incidence, treatment, and progression of cancers.

Imaging of Isolated Extracellular Vesicles Using Fluorescence Microscopy

High-resolution fluorescence microscopy approaches enable the study of single objects or biological complexes. Single object studies have the general advantage of uncovering heterogeneity that may be hidden during the ensemble averaging which is common in any bulk conventional biochemical analysis. The implementation of single object analysis in the study of extracellular vesicles (EVs) may therefore be used to characterize specific properties of vesicle subsets which would be otherwise undetectable. We present a protocol for staining isolated EVs with a fluorescent lipid dye and attaching them onto a glass slide in preparation for imaging with total internal reflection fluorescence microscopy (TIRF-M) or other high-resolution microscopy techniques.

Extracellular vesicles in liver pathobiology: Small particles with big impact

Extracellular vesicles (EVs) are nanometer-sized, membrane-bound vesicles released by cells into the extracellular milieu. EVs are now recognized to play a critical role in cell-to-cell communication. EVs contain important cargo in the form of proteins, lipids, and nucleic acids and serve as vectors for delivering this cargo from donor to acceptor or target cell. EVs are released under both physiologic and pathologic conditions, including liver diseases, and exert a wide range of effects on target cells. This review provides an overview on EV biogenesis, secretion, cargo, and target cell interactions in the context of select liver diseases. Specifically, the diverse roles of EVs in nonalcoholic steatohepatitis, alcoholic liver disease, viral hepatitis, cholangiopathies, and hepatobiliary malignancies are emphasized. Liver diseases often result in an increased release of EVs and/or in different cargo sorting into these EVs. Either of these alterations can drive disease pathogenesis. Given this fact, EVs represent a potential target for therapeutic intervention in liver disorders. Because altered EV composition may reflect the underlying disease condition, circulating EVs can be exploited for diagnostic and prognostic purposes as a liquid biopsy. Furthermore, ex vivo modified or synthesized EVs can be engineered as therapeutic nano-shuttles. Finally, we highlight areas that merit further investigation relevant to understanding how EVs regulate liver disease pathogenesis. (Hepatology 2016;64:2219-2233).

Physiological and pathological roles of exosomes in the nervous system

Exosomes represent a subtype of extracellular nanovesicles that are generated from the luminal budding of limiting endosomal membranes and subsequent exocytosis. They encapsulate or associate with obsolete molecules to eliminate or to transfer their cargos in intercellular communication. The exosomes are also released and transported between neurons and glia in the nervous system, having a broad impact on nerve development, activation and regeneration. Accumulating evidence suggests that the exosomes are attributed to the pathogenesis of several neurodegenerative diseases such as prion disease, Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, as well as aging, in which the exosomes lack the capacity for cellular self-repair and spread their enclosed pathological agents among neurons. In this article, we review the current proposed functions of exosomes in physiological and pathological processes in the nervous system.

Exosomes in Cancer Disease

Cancer diagnosis and therapy is steadily improving. Still, diagnosis is frequently late and diagnosis and follow-up procedures mostly are time-consuming and expensive. Searching for tumor-derived exosomes (TEX) in body fluids may provide an alternative, minimally invasive, yet highly reliable diagnostic tool. Beyond this, there is strong evidence that TEX could become a potent therapeutics. Exosomes, small vesicles delivered by many cells of the organism, are found in all body fluids. Exosomes are characterized by lipid composition, common and donor cell specific proteins, mRNA, small non-coding RNA including miRNA and DNA. Particularly the protein and miRNA markers received much attention as they may allow for highly specific diagnosis and can provide hints toward tumor aggressiveness and progression, where exosome-based diagnosis and follow-up is greatly facilitated by the recovery of exosomes in body fluids, particularly the peripheral blood. Beyond this, exosomes are the most important intercellular communicators that modulate, instruct, and reprogram their surrounding as well as distant organs. In concern about TEX this includes message transfer from tumor cells toward the tumor stroma, the premetastatic niche, the hematopoietic system and, last but not least, the instruction of non-cancer stem cells by cancer-initiating cells (CIC). Taking this into account, it becomes obvious that "tailored" exosomes offer themselves as potent therapeutic delivery system. In brief, during the last 4-5 years there is an ever-increasing, overwhelming interest in exosome research. This boom appears fully justified provided the content of the exosomes becomes most thoroughly analyzed and their mode of intercellular interaction can be unraveled in detail as this knowledge will open new doors toward cancer diagnosis and therapy including immunotherapy and CIC reprogramming.

Toll-like receptor 3-activated macrophages confer anti-HCV activity to hepatocytes through exosomes

Exosomes are a class of cell-released small vesicles that mediate intercellular communication by delivering functional factors to recipient cells. During hepatitis C virus (HCV) infection, the interaction between liver resident macrophages and hepatocytes is a key component in liver innate immunity. In this study, we explored the role of exosomes in the delivery of innate anti-HCV factors to hepatocytes from macrophages. We showed that supernatant from TLR3-activated macrophage cultures could efficiently inhibit HCV replication in Huh7 cells. This macrophage-mediated anti-HCV activity was through exosomes because inhibiting exosomes could abrogate the action of macrophages. Further analyses demonstrated that TLR3-activated macrophages release exosomes that contain anti-HCV microRNA (miRNA)-29 family members. Inhibiting miRNA29 could restore HCV replication. These findings suggest a novel antiviral mechanism in liver innate immunity against HCV infection and provide insights to support further studies on developing exosome-based delivery system for disease treatment.-Zhou, Y., Wang, X., Sun, L., Zhou, L., Ma, T.-C., Song, L., Wu, J.-G., Li, J.-L., Ho, W.-Z. Toll-like receptor 3-activated macrophages confer anti-HCV activity to hepatocytes through exosomes.

Horizontal Transmission of Cytosolic Sup35 Prions by Extracellular Vesicles

Prions are infectious protein particles that replicate by templating their aggregated state onto soluble protein of the same type. Originally identified as the causative agent of transmissible spongiform encephalopathies, prions in yeast (Saccharomyces cerevisiae) are epigenetic elements of inheritance that induce phenotypic changes of their host cells. The prototype yeast prion is the translation termination factor Sup35. Prions composed of Sup35 or its modular prion domain NM are heritable and are transmitted vertically to progeny or horizontally during mating. Interestingly, in mammalian cells, protein aggregates derived from yeast Sup35 NM behave as true infectious entities that employ dissemination strategies similar to those of mammalian prions. While transmission is most efficient when cells are in direct contact, we demonstrate here that cytosolic Sup35 NM prions are also released into the extracellular space in association with nanometer-sized membrane vesicles. Importantly, extracellular vesicles are biologically active and are taken up by recipient cells, where they induce self-sustained Sup35 NM protein aggregation. Thus, in mammalian cells, extracellular vesicles can serve as dissemination vehicles for protein-based epigenetic information transfer.

IMPORTANCE

Prions are proteinaceous infectious particles that propagate by templating their quaternary structure onto nascent proteins of the same kind. Prions in yeast act as heritable epigenetic elements that can alter the phenotype when transmitted to daughter cells or during mating. Prion activity is conferred by so-called prion domains often enriched in glutamine and asparagine residues. Interestingly, many mammalian proteins also contain domains with compositional similarity to yeast prion domains. We have recently provided a proof-of-principle demonstration that a yeast prion domain also retains its prion activity in mammalian cells. We demonstrate here that cytosolic prions composed of a yeast prion domain are also packaged into extracellular vesicles that transmit the prion phenotype to bystander cells. Thus, proteins with prion-like domains can behave as proteinaceous information molecules that exploit the cellular vesicle trafficking machinery for intercellular long-distance dissemination.

Extracellular Vesicles Exploit Viral Entry Routes for Cargo Delivery

Extracellular vesicles (EVs) have emerged as crucial mediators of intercellular communication, being involved in a wide array of key biological processes. Eukaryotic cells, and also bacteria, actively release heterogeneous subtypes of EVs into the extracellular space, where their contents reflect their (sub)cellular origin and the physiologic state of the parent cell. Within the past 20 years, presumed subtypes of EVs have been given a rather confusing diversity of names, including exosomes, microvesicles, ectosomes, microparticles, virosomes, virus-like particles, and oncosomes, and these names are variously defined by biogenesis, physical characteristics, or function. The latter category, functions, in particular the transmission of biological signals between cells in vivo and how EVs control biological processes, has garnered much interest. EVs have pathophysiological properties in cancer, neurodegenerative disorders, infectious disease, and cardiovascular disease, highlighting possibilities not only for minimally invasive diagnostic applications but also for therapeutic interventions, like macromolecular drug delivery. Yet, in order to pursue therapies involving EVs and delivering their cargo, a better grasp of EV targeting is needed. Here, we review recent progress in understanding the molecular mechanisms underpinning EV uptake by receptor-ligand interactions with recipient cells, highlighting once again the overlap of EVs and viruses. Despite their highly heterogeneous nature, EVs require common viral entry pathways, and an unanticipated specificity for cargo delivery is being revealed. We discuss the challenges ahead in delineating specific roles for EV-associated ligands and cellular receptors.

Exosome-associated release, uptake, and neurotoxicity of HIV-1 Tat protein

HIV-1 Tat is an indispensible transactivator for HIV gene transcription and replication. It has been shown to exit cells as a free protein and enter neighboring cells or interact with surface receptors of neighboring cells to regulate gene expression and cell function. In this study, we report, for the first time, exosome-associated Tat release and uptake. Using a HIV-1 LTR-driven luciferase reporter-based cell assay and Western blotting or in combination with exosome inhibitor, OptiPrep gradient fractionation, and exosome depletion, we demonstrated significant presence of HIV-1 Tat in exosomes derived from Tat-expressing primary astrocytes, Tat-transfected U373.MG and 293T, and HIV-infected MT4. We further showed that exosome-associated Tat from Tat-expressing astrocytes was capable of causing neurite shortening and neuron death, further supporting that this new form of extracellular Tat is biologically active. Lastly, we constructed a Tat mutant deleted of its basic domain and determined the role of the basic domain in Tat trafficking into exosomes. Basic domain-deleted Tat exhibited no apparent effects on Tat trafficking into exosomes, while maintained its dominant-negative function in Tat-mediated LTR transactivation. Taken together, these results show a significant fraction of Tat is secreted and present in the form of exosomes and may contribute to the stability of extracellular Tat and broaden the spectrum of its target cells.

Extracellular Vesicles and a Novel Form of Communication in the Brain

In numerous neurodegenerative diseases, the interplay between neurons and glia modulates the outcome and progression of pathology. One particularly intriguing mode of interaction between neurons, astrocytes, microglia, and oligodendrocytes is characterized by the release of extracellular vesicles that transport proteins, lipids, and nucleotides from one cell to another. Notably, several proteins that cause disease, including the prion protein and mutant SOD1, have been detected in glia-derived extracellular vesicles and observed to fuse with neurons and trigger pathology in vitro. Here we review the structural and functional characterization of such extracellular vesicles in neuron-glia interactions. Furthermore, we discuss possible mechanisms of extracellular vesicle biogenesis and release from activated glia and microglia, and their effects on neurons. Given that exosomes, the smallest type of extracellular vesicles, have been reported to recognize specific cellular populations and act as carriers of very specialized cargo, a thorough analysis of these vesicles may aid in their engineering in vitro and targeted delivery in vivo, opening opportunities for therapeutics.