Prion strains are differentially released through the exosomal pathway

The structural line between prion and "prion-like": Insights from prion protein and tau

The concept of 'prion-like' behavior has emerged in the study of diseases involving protein misfolding where fibrillar structures, called amyloids, self-propagate and induce disease in a fashion similar to prions. From a biological standpoint, in order to be considered 'prion-like,' a protein must traverse cells and tissues and further propagate via a templated conformational change. Since 2017, cryo-electron microscopy structures from patient-derived 'prion-like' amyloids, in particular tau, have been presented and revealed structural similarities shared across amyloids. Since 2021, cryo-EM structures from prions of known infectivity have been added to the ex vivo amyloid structure family. In this review, we discuss current proposals for the 'prion-like' mechanisms of spread for tau and prion protein as well as discuss different influencers on structures of aggregates from tauopathies and prion diseases. Lastly, we discuss some of the current hypotheses for what may distinguish structures that are 'prion-like' from transmissible prion structures.

Large extracellular vesicles transfer more prions and infect cell culture better than small extracellular vesicles

Prions are responsible for a number of lethal neurodegenerative and transmissible diseases in humans and animals. Extracellular vesicles, especially small exosomes, have been extensively studied in connection with various diseases. In contrast, larger microvesicles are often overlooked. In this work, we compared the ability of large extracellular vesicles (lEVs) and small extracellular vesicles (sEVs) to spread prions in cell culture. We utilized CAD5 cell culture model of prion infection and isolated lEVs by 20,000×g force and sEVs by 110,000×g force. The lEV fraction was enriched in β-1 integrin with a vesicle size starting at 100 nm. The fraction of sEVs was partially depleted of β-1 integrin with a mean size of 79 nm. Both fractions were enriched in prion protein, but the lEVs contained a higher prion-converting activity. In addition, lEV infection led to stronger prion signals in both cell cultures, as detected by cell and western blotting. These results were verified on N2a-PK1 cell culture. Our data suggest the importance of lEVs in the trafficking and spread of prions over extensively studied small EVs.

Contribution of Extracellular Vesicles and Molecular Chaperones in Age-Related Neurodegenerative Disorders of the CNS

Many neurodegenerative disorders are characterized by the abnormal aggregation of misfolded proteins that form amyloid deposits which possess prion-like behavior such as self-replication, intercellular transmission, and consequent induction of native forms of the same protein in surrounding cells. The distribution of the accumulated proteins and their correlated toxicity seem to be involved in the progression of nervous system degeneration. Molecular chaperones are known to maintain proteostasis, contribute to protein refolding to protect their function, and eliminate fatally misfolded proteins, prohibiting harmful effects. However, chaperone network efficiency declines during aging, prompting the onset and the development of neurological disorders. Extracellular vesicles (EVs) are tiny membranous structures produced by a wide range of cells under physiological and pathological conditions, suggesting their significant role in fundamental processes particularly in cellular communication. They modulate the behavior of nearby and distant cells through their biological cargo. In the pathological context, EVs transport disease-causing entities, including prions, α-syn, and tau, helping to spread damage to non-affected areas and accelerating the progression of neurodegeneration. However, EVs are considered effective for delivering therapeutic factors to the nervous system, since they are capable of crossing the blood-brain barrier (BBB) and are involved in the transportation of a variety of cellular entities. Here, we review the neurodegeneration process caused mainly by the inefficiency of chaperone systems as well as EV performance in neuropathies, their potential as diagnostic biomarkers and a promising EV-based therapeutic approach.

Mesenchymal Stem Cell-Derived Extracellular Vesicles Therapy for Pulmonary Hypertension: A Comprehensive Review of Preclinical Studies

Pulmonary hypertension (PH) is a type of clinical pathophysiological syndrome characterized by a progressive increase in pulmonary vascular resistance and subsequent progressive failure of the right heart function, and is a common complication of many diseases. Mesenchymal stem cells (MSCs) autonomously home to sites damaged by disease, repair damaged tissues, and participate in the regulation of systemic inflammation and immune responses, which have good clinical application prospects. Extracellular vesicles (EVs), such as exosomes and microvesicles, participate in various biological activities by regulating intercellular communication. Exosomes secreted into the extracellular environment also affect the host immune system. MSC-derived extracellular vesicles (MSC-EVs), as a mediator in the paracrine processes of MSCs, carry biologically active substances such as proteins, lipids, mRNA, and micro-RNA. MSC-EVs therapies, safer than cell-based treatments, have been shown to be effective in modulating macrophages to support anti-inflammatory phenotypes, which are strongly related to histological and functional benefits in preclinical models of pulmonary hypertension. The main effects of active substances and their potential medical value have attracted wide attention from researchers. This article reviews the role and relevant mechanisms of MSC-EVs in the treatment of pulmonary hypertension in recent studies and provides a basis for their future clinical applications.

Extracellular vesicles with diagnostic and therapeutic potential for prion diseases

Prion diseases (PrD) or transmissible spongiform encephalopathies (TSE) are invariably fatal and pathogenic neurodegenerative disorders caused by the self-propagated misfolding of cellular prion protein (PrP^C) to the neurotoxic pathogenic form (PrP^TSE) via a yet undefined but profoundly complex mechanism. Despite several decades of research on PrD, the basic understanding of where and how PrP^C is transformed to the misfolded, aggregation-prone and pathogenic PrP^TSE remains elusive. The primary clinical hallmarks of PrD include vacuolation-associated spongiform changes and PrP^TSE accumulation in neural tissue together with astrogliosis. The difficulty in unravelling the disease mechanisms has been related to the rare occurrence and long incubation period (over decades) followed by a very short clinical phase (few months). Additional challenge in unravelling the disease is implicated to the unique nature of the agent, its complexity and strain diversity, resulting in the heterogeneity of the clinical manifestations and potentially diverse disease mechanisms. Recent advances in tissue isolation and processing techniques have identified novel means of intercellular communication through extracellular vesicles (EVs) that contribute to PrP^TSE transmission in PrD. This review will comprehensively discuss PrP^TSE transmission and neurotoxicity, focusing on the role of EVs in disease progression, biomarker discovery and potential therapeutic agents for the treatment of PrD.

Propagation and Dissemination Strategies of Transmissible Spongiform Encephalopathy Agents in Mammalian Cells

Transmissible spongiform encephalopathies or prion disorders are fatal infectious diseases that cause characteristic spongiform degeneration in the central nervous system. The causative agent, the so-called prion, is an unconventional infectious agent that propagates by converting the host-encoded cellular prion protein PrP into ordered protein aggregates with infectious properties. Prions are devoid of coding nucleic acid and thus rely on the host cell machinery for propagation. While it is now established that, in addition to PrP, other cellular factors or processes determine the susceptibility of cell lines to prion infection, exact factors and cellular processes remain broadly obscure. Still, cellular models have uncovered important aspects of prion propagation and revealed intercellular dissemination strategies shared with other intracellular pathogens. Here, we summarize what we learned about the processes of prion invasion, intracellular replication and subsequent dissemination from ex vivo cell models.

The Role of Extracellular Vesicles in Systemic Lupus Erythematosus

Extracellular Vesicles (EVs) are small vesicles that can be actively secreted by most cell types into the extracellular environment. Evidence indicates that EVs can carry microRNAs (miRNAs), long non-coding RNAs (lncRNAs), tRNA-derived small RNAs (tsRNAs), proteins, and lipids to target cells or tissue organizations. Latest studies show that EVs play a vital role in the immune modulation and may contribute to the pathogenesis of autoimmune diseases. Systemic lupus erythematosus (SLE) is a common autoimmune disease characterized by abnormal T cell activation and sustained production of autoantibodies against self-antigens, resulting in inflammation and damage to multiple systems. Pathogenic mechanisms of SLE, however, are still not well understood. In this review, we summarize the latest research advances on the functions and mechanisms of EVs, and its role in the pathogenesis, diagnosis, and treatment of SLE.

Cerebrospinal Fluid and Plasma Small Extracellular Vesicles and miRNAs as Biomarkers for Prion Diseases

Diagnosis of transmissible spongiform encephalopathies (TSEs), or prion diseases, is based on the detection of proteinase K (PK)-resistant PrP^Sc in post-mortem tissues as indication of infection and disease. Since PrP^Sc detection is not considered a reliable method for in vivo diagnosis in most TSEs, it is of crucial importance to identify an alternative source of biomarkers to provide useful alternatives for current diagnostic methodology. Ovine scrapie is the prototype of TSEs and has been known for a long time. Using this natural model of TSE, we investigated the presence of PrP^Sc in exosomes derived from plasma and cerebrospinal fluid (CSF) by protein misfolding cyclic amplification (PMCA) and the levels of candidate microRNAs (miRNAs) by quantitative PCR (qPCR). Significant scrapie-associated increase was found for miR-21-5p in plasma-derived but not in CSF-derived exosomes. However, miR-342-3p, miR-146a-5p, miR-128-3p and miR-21-5p displayed higher levels in total CSF from scrapie-infected sheep. The analysis of overexpressed miRNAs in this biofluid, together with plasma exosomal miR-21-5p, could help in scrapie diagnosis once the presence of the disease is suspected. In addition, we found the presence of PrP^Sc in most CSF-derived exosomes from clinically affected sheep, which may facilitate in vivo diagnosis of prion diseases, at least during the clinical stage.

From Cell Culture to Organoids-Model Systems for Investigating Prion Strain Characteristics

Prion diseases are the hallmark protein folding neurodegenerative disease. Their transmissible nature has allowed for the development of many different cellular models of disease where prion propagation and sometimes pathology can be induced. This review examines the range of simple cell cultures to more complex neurospheres, organoid, and organotypic slice cultures that have been used to study prion disease pathogenesis and to test therapeutics. We highlight the advantages and disadvantages of each system, giving special consideration to the importance of strains when choosing a model and when interpreting results, as not all systems propagate all strains, and in some cases, the technique used, or treatment applied, can alter the very strain properties being studied.

Circulating Exosomal miRNA as Diagnostic Biomarkers of Neurodegenerative Diseases

Neurodegenerative diseases (NDDs) are a group of diseases caused by chronic and progressive degeneration of neural tissue. The main pathological manifestations are neuronal degeneration and loss in the brain and/or spinal cord. Common NDDs include Alzheimer disease (AD), Parkinson disease (PD), Huntington disease (HD), and amyotrophic lateral sclerosis (ALS). The complicated pathological characteristics and different clinical manifestations of NDDs result in a lack of sensitive and efficient diagnostic methods. In addition, no sensitive biomarkers are available to monitor the course of NDDs, predict their prognosis, and monitor the therapeutic response. Despite extensive research in recent years, analysis of amyloid β (Aβ) and α-synuclein has failed to effectively improve NDD diagnosis. Although recent studies have indicated circulating miRNAs as promising diagnostic biomarkers of NDDs, the miRNA in the peripheral circulation is susceptible to interference by other components, making circulating miRNA results less consistent. Exosomes are small membrane vesicles with a diameter of approximately 30-100 nm that transport proteins, lipids, mRNA, and miRNA. Because recent studies have shown that exosomes have a double-membrane structure that can resist ribonuclease in the blood, giving exosomal miRNA high stability and making them resistant to degradation, they may become an ideal biomarker of circulating fluids. In this review, we discuss the applicability of circulating exosomal miRNAs as biomarkers, highlight the technical aspects of exosomal miRNA analysis, and review studies that have used circulating exosomal miRNAs as candidate diagnostic biomarkers of NDDs.

CNS-Derived Blood Exosomes as a Promising Source of Biomarkers: Opportunities and Challenges

Eukaryotic cells release different types of extracellular vesicles (EVs) including exosomes, ectosomes, and microvesicles. Exosomes are nanovesicles, 30–200 nm in diameter, that carry cell- and cell-state-specific cargo of proteins, lipids, and nucleic acids, including mRNA and miRNA. Recent studies have shown that central nervous system (CNS)-derived exosomes may carry amyloidogenic proteins and facilitate their cell-to-cell transfer, thus playing a critical role in the progression of neurodegenerative diseases, such as tauopathies and synucleinopathies. CNS-derived exosomes also have been shown to cross the blood-brain-barrier into the bloodstream and therefore have drawn substantial attention as a source of biomarkers for various neurodegenerative diseases as they can be isolated via a minimally invasive blood draw and report on the biochemical status of the CNS. However, although isolating specific brain-cell-derived exosomes from the blood is theoretically simple and the approach has great promise, practical details are of crucial importance and may compromise the reproducibility and utility of this approach, especially when different laboratories use different protocols. In this review we discuss the role of exosomes in neurodegenerative diseases, the usefulness of CNS-derived blood exosomes as a source of biomarkers for these diseases, and practical challenges associated with the methodology of CNS-derived blood exosomes and subsequent biomarker analysis.

Targetting Exosomes as a New Biomarker and Therapeutic Approach for Alzheimer's Disease

Alzheimer’s disease (AD) is a neurodegenerative disease that mainly occurs in old age and involves progressive cognitive impairment. AD has become a major global issue for public health, with approximately 24 million people currently affected by the disease. Estimates indicted that this number will quadruple by 2050. Because of the high incidence of AD, there is an urgent need to develop new strategies to diagnose and treat AD. Many recent studies have indicated the multiple, yet somewhat controversial, roles of exosomes in AD. Although the underlying mechanisms by which exosomes play a role in AD are still unknown, current evidence suggests that exosomes can carry and spread toxic amyloid-beta, and hyperphosphorylated tau, between cells, and then induce apoptosis, thus contributing to the loss of neurons. In addition, exosomes appear to possess the ability to reduce brain amyloid-beta, and tau hyperphosphorylation, and transfer neuroprotective substances between neural cells. The accumulating data brings hope that the application of exosomes may be helpful for early diagnostics and the identification of new therapeutic targets for AD. Here, we summarized the various roles of exosomes, and how they might relate to the pathogenesis of AD. We also highlight the potential application of exosomes as a therapeutic option in AD therapy.

Sporadic Creutzfeldt-Jakob disease prion infection of human cerebral organoids

For the transmissible, neurogenerative family of prion diseases, few human models of infection exist and none represent structured neuronal tissue. Human cerebral organoids are self-organizing, three-dimensional brain tissues that can be grown from induced pluripotent stem cells. Organoids can model aspects of neurodegeneration in Alzheimer's Disease and Down's Syndrome, reproducing tau hyperphosphorylation and amyloid plaque pathology. To determine whether organoids could be used to reproduce human prion infection and pathogenesis, we inoculated organoids with two sporadic Creutzfeldt-Jakob Disease prion subtypes. Organoids showed uptake, followed by clearance, of the infectious inoculum. Subsequent re-emergence of prion self-seeding activity indicated de novo propagation. Organoid health assays, prion titer, prion protein electrophoretic mobility and immunohistochemistry demonstrated inoculum-specific differences. Our study shows, for the first time, that cerebral organoids can model aspects of human prion disease and thus offer a powerful system for investigating different human prion subtype pathologies and testing putative therapeutics.

Environmental neurotoxicant-induced dopaminergic neurodegeneration: a potential link to impaired neuroinflammatory mechanisms

With the increased incidence of neurodegenerative diseases worldwide, Parkinson's disease (PD) represents the second-most common neurodegenerative disease. PD is a progressive multisystem neurodegenerative disorder characterized by a marked loss of nigrostriatal dopaminergic neurons and the formation of Lewy pathology in diverse brain regions. Although the mechanisms underlying dopaminergic neurodegeneration remain poorly characterized, data from animal models and postmortem studies have revealed that heightened inflammatory responses mediated via microglial and astroglial activation and the resultant release of proinflammatory factors may act as silent drivers of neurodegeneration. In recent years, numerous studies have demonstrated a positive association between the exposure to environmental neurotoxicants and the etiology of PD. Although it is unclear whether neuroinflammation drives pesticide-induced neurodegeneration, emerging evidence suggests that the failure to dampen neuroinflammatory mechanisms may account for the increased vulnerability to pesticide neurotoxicity. Furthermore, recent studies provide additional evidence that shifts the focus from a neuron-centric view to glial-associated neurodegeneration following pesticide exposure. In this review, we propose to summarize briefly the possible factors that regulate neuroinflammatory processes during environmental neurotoxicant exposure with a focus on the potential roles of mitochondria-driven redox mechanisms. In this context, a critical discussion of the data obtained from experimental research and possible epidemiological studies is included. Finally, we hope to provide insights on the pivotal role of exosome-mediated intercellular transmission of aggregated proteins in microglial activation response and the resultant dopaminergic neurodegeneration after exposure to pesticides. Collectively, an improved understanding of glia-mediated neuroinflammatory signaling might provide novel insights into the mechanisms that contribute to neurodegeneration induced by environmental neurotoxicant exposure.

Cellular and Molecular Mechanisms of Prion Disease

Prion diseases are rapidly progressive, incurable neurodegenerative disorders caused by misfolded, aggregated proteins known as prions, which are uniquely infectious. Remarkably, these infectious proteins have been responsible for widespread disease epidemics, including kuru in humans, bovine spongiform encephalopathy in cattle, and chronic wasting disease in cervids, the latter of which has spread across North America and recently appeared in Norway and Finland. The hallmark histopathological features include widespread spongiform encephalopathy, neuronal loss, gliosis, and deposits of variably sized aggregated prion protein, ranging from small, soluble oligomers to long, thin, unbranched fibrils, depending on the disease. Here, we explore recent advances in prion disease research, from the function of the cellular prion protein to the dysfunction triggering neurotoxicity, as well as mechanisms underlying prion spread between cells. We also highlight key findings that have revealed new therapeutic targets and consider unanswered questions for future research.

Cell biology of prion infection

The development of multiple cell culture models of prion infection over the last two decades has led to a significant increase in our understanding of how prions infect cells. In particular, new techniques to distinguish exogenous from endogenous prions have allowed us for the first time to look in depth at the earliest stages of prion infection through to the establishment of persistent infection. These studies have shown that prions can infect multiple cell types, both neuronal and nonneuronal. Once in contact with the cell, they are rapidly taken up via multiple endocytic pathways. After uptake, the initial replication of prions occurs almost immediately on the plasma membrane and within multiple endocytic compartments. Following this acute stage of prion replication, persistent prion infection may or may not be established. Establishment of a persistent prion infection in cells appears to depend upon the achievement of a delicate balance between the rate of prion replication and degradation, the rate of cell division, and the efficiency of prion spread from cell to cell. Overall, cell culture models have shown that prion infection of the cell is a complex and variable process which can involve multiple cellular pathways and compartments even within a single cell.

Specialized Intercellular Communications via Cytonemes and Nanotubes

In recent years, thin membrane protrusions such as cytonemes and tunneling nanotubes have emerged as a novel mechanism of intercellular communication. Protrusion-based cellular interactions allow for specific communication between participating cells and have a distinct spectrum of advantages compared to secretion- and diffusion-based intercellular communication. Identification of protrusion-based signaling in diverse systems suggests that this mechanism is a ubiquitous and prevailing means of communication employed by many cell types. Moreover, accumulating evidence indicates that protrusion-based intercellular communication is often involved in pathogenesis, including cancers and infections. Here we review our current understanding of protrusion-based intercellular communication.

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.

Extracellular Vesicles: Multimodal Envoys in Neural Maintenance and Repair

The physiology of the central nervous system (CNS) is built on a foundation of connection, integration, and the exchange of complex information among brain cells. Emerging evidence indicates that extracellular vesicles (EVs) are key players in the intercellular communication that underlies physiological processes such as synaptic plasticity and the maintenance of myelination. Furthermore, upon injury to the CNS, EVs may propagate inflammation across the blood-brain barrier and beyond, and also appear to mediate neuroprotection and modulate regenerative processes. In neurodegenerative diseases, EVs may play roles in the formation, spreading, and clearance of toxic protein aggregates. Here, we discuss the physiological roles of EVs in the healthy and the diseased CNS, with a focus on recent findings and emerging concepts.

Exosomes and their role in the intercellular trafficking of normal and disease associated prion proteins

Over the past decade, small extracellular vesicles called exosomes have been observed to harbour protein and genetic cargo that can assist in health and also cause disease. Many groups are extensively investigating the mechanisms involved that regulate the trafficking and packaging of exosomal contents and how these processes may be deregulated in disease. Prion diseases are transmissible neurodegenerative disorders and are characterized by the presence of detectable misfolded prion proteins. The disease associated form of the prion protein can be found in exosomes and its transmissible properties have provided a reliable experimental read out that can be used to understand how exosomes and their cargo are involved in cell-cell communication and in the spread of prion diseases. This review reports on the current understanding of how exosomes are involved in the intercellular spread of infectious prions. Furthermore, we discuss how these principles are leading future investigations in developing new exosome based diagnostic tools and therapeutic drugs that could be applied to other neurodegenerative diseases.

How do PrPSc Prions Spread between Host Species, and within Hosts?

Prion diseases are sub-acute neurodegenerative diseases that affect humans and some domestic and free-ranging animals. Infectious prion agents are considered to comprise solely of abnormally folded isoforms of the cellular prion protein known as PrP^Sc. Pathology during prion disease is restricted to the central nervous system where it causes extensive neurodegeneration and ultimately leads to the death of the host. The first half of this review provides a thorough account of our understanding of the various ways in which PrP^Sc prions may spread between individuals within a population, both horizontally and vertically. Many natural prion diseases are acquired peripherally, such as by oral exposure, lesions to skin or mucous membranes, and possibly also via the nasal cavity. Following peripheral exposure, some prions accumulate to high levels within the secondary lymphoid organs as they make their journey from the site of infection to the brain, a process termed neuroinvasion. The replication of PrP^Sc prions within secondary lymphoid organs is important for their efficient spread to the brain. The second half of this review describes the key tissues, cells and molecules which are involved in the propagation of PrP^Sc prions from peripheral sites of exposure (such as the lumen of the intestine) to the brain. This section also considers how additional factors such as inflammation and aging might influence prion disease susceptibility.

Generation of Infectious Prions and Detection with the Prion-Infected Cell Assay

Cell lines propagating prions are an efficient and useful means for studying the cellular and molecular mechanisms implicated in prion disease. Utilization of cell-based models has led to the finding that PrP^C and PrP^Sc are released from cells in association with extracellular vesicles known as exosomes. Exosomes have been shown to act as vehicles for infectivity, transferring infectivity between cell lines and providing a mechanism for prion spread between tissues. Here, we describe the methods for generating a prion-propagating cell line with prion-infected brain homogenate, cell lysate, conditioned media, and exosomes and also detection of protease-resistant PrP with the prion-infected cell assay.

The spread of prion-like proteins by lysosomes and tunneling nanotubes: Implications for neurodegenerative diseases

Victoria and Zurzolo discuss current evidence for the emerging role of lysosomal damage and tunneling nanotubes in the intercellular propagation of prion and prion-like proteins in neurodegenerative disease.

Progression of pathology in neurodegenerative diseases is hypothesized to be a non–cell-autonomous process that may be mediated by the productive spreading of prion-like protein aggregates from a “donor cell” that is the source of misfolded aggregates to an “acceptor cell” in which misfolding is propagated by conversion of the normal protein. Although the proteins involved in the various diseases are unrelated, common pathways appear to be used for their intercellular propagation and spreading. Here, we summarize recent evidence of the molecular mechanisms relevant for the intercellular trafficking of protein aggregates involved in prion, Alzheimer’s, Huntington’s, and Parkinson’s diseases. We focus in particular on the common roles that lysosomes and tunneling nanotubes play in the formation and spreading of prion-like assemblies.

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 and the Prion Protein: More than One Truth

Exosomes are involved in the progression of neurodegenerative diseases. The cellular prion protein (PrP^C) is highly expressed on exosomes. In neurodegenerative diseases, PrP^C has at least two functions: It is the substrate for the generation of pathological prion protein (PrP^Sc), a key player in the pathophysiology of prion diseases. On the other hand, it binds neurotoxic amyloid-beta (Aß) oligomers, which are associated with initiation and progression of Alzheimer's disease (AD). This has direct consequences for the role of exosomal expressed PrP^C. In prion diseases, exosomal PrP leads to efficient dissemination of pathological prion protein, thus promoting spreading and transmission of the disease. In AD, exosomal PrP^C can bind and detoxify Aß oligomers thus acting protective. In both scenarios, assessment of the state of PrP^C on exosomes derived from blood or cerebrospinal fluid (CSF) may be useful for diagnostic workup of these diseases. This review sums up current knowledge of the role of exosomal PrP^C on different aspects of Alzheimer's and prion disease.

The role of exosomes in the pathogenesis of Alzheimer' disease

Exosomes are small vesicles secreted by most cell types including neurons that function in intercellular communication through transfer of their cargo or encapsulate and eliminate unnecessary cellular components and therefore have a broad impact on nerve development, activation and regeneration. In addition, exosomes have been observed to be involved in spreading pathological misfolded proteins, thereby leading to the onset and propagation of disease. Alzheimer disease (AD) is the most common form of dementia and characterized by two types of lesions: amyloid plaques and neurofibrillary tangles. Accumulating evidence has demonstrated that exosomes are associated with amyloid precursor (APP) and Tau proteins and play a controversial role in Alzheimer’s disease process. In this review, we will discuss the role of exosomes in the metabolism and secretion of APP and Tau proteins and their subsequent impact on AD pathogenesis.

Isolation of Exosomes and Microvesicles from Cell Culture Systems to Study Prion Transmission

Extracellular vesicles (EVs) are composed of microvesicles and exosomes. Exosomes are small membrane vesicles (40-120 nm sized) of endosomal origin released in the extracellular medium from cells when multivesicular bodies fuse with the plasma membrane, whereas microvesicles (i.e., shedding vesicles, 100 nm to 1 μm sized) bud from the plasma membrane. Exosomes and microvesicles carry functional proteins and nucleic acids (especially mRNAs and microRNAs) that can be transferred to surrounding cells and tissues and can impact multiple dimensions of the cellular life. Most of the cells, if not all, from neuronal to immune cells, release exosomes and microvesicles in the extracellular medium, and all biological fluids including blood (serum/plasma), urine, cerebrospinal fluid, and saliva contain EVs.Prion-infected cultured cells are known to secrete infectivity into their environment. We characterized this cell-free form of prions and showed that infectivity was associated with exosomes. Since exosomes are produced by a variety of cells, including cells that actively accumulate prions, they could be a vehicle for infectivity in body fluids and could participate to the dissemination of prions in the organism. In addition, such infectious exosomes also represent a natural, simple, biological material to get key information on the abnormal PrP forms associated with infectivity.In this chapter, we describe first a method that allows exosomes and microvesicles isolation from prion-infected cell cultures and in a second time the strategies to characterize the prions containing exosomes and their ability to disseminate the prion agent.

Emerging roles of extracellular vesicles in neurodegenerative disorders: focus on HIV-associated neurological complications

Exosomes are membrane-enriched extracellular vesicles with a proposed diameter in the range of 30-100 nm. They are released during both normal homeostasis as well as under pathological conditions by most cell types. In recent years, there has been robust interest in the study of these vesicles as conduits for the delivery of information between cells in both analogous as well as disparate tissues. Their ability to transport specialized cargo including signaling mediators, proteins, messenger RNA and miRNAs characterizes these vesicles as primary facilitators of cell-to-cell communication and regulation. Exosomes have also been demonstrated to have important roles in the field of cancer biology and metastasis. More recently, their role in several neurodegenerative disorders has been gaining increased momentum as these particles have been shown to promote the spread of toxic factors such as amyloid beta and prions, adding further validity to their role as important regulators of disease pathogenesis. This review briefly summarizes current findings and thoughts on exosome biology in the context of neurodegenerative disorders and the manipulation of these particles for the development of potential therapeutic strategies.

Exosomes in the Pathology of Neurodegenerative Diseases

More than 30 years ago, two unexpected findings were discovered that challenged conventional thinking in biology. The first was the identification of a misfolded protein with transmissible properties associated with a group of neurodegenerative diseases known as transmissible spongiform encephalopathies. The second was the discovery of a new pathway used for the extracellular release of biomolecules, including extracellular vesicles called exosomes. Two decades later, the convergence of these pathways was shown when exosomes were found to play a significant role in both the transmission and propagation of protein aggregates in disease. Recent research has now revealed that the majority of proteins involved in neurodegenerative diseases are transported in exosomes, and that external stresses due to age-related impairment of protein quality control mechanisms can promote the transcellular flux of these proteins in exosomes. Significantly, exosomes provide an environment that can induce the conformational conversion of native proteins into aggregates that can be transmitted to otherwise aggregate-free cells in the brain. Here we review the current roles of exosomes in the pathology of neurodegenerative diseases.

Oligodendroglial deletion of ESCRT-I component TSG101 causes spongiform encephalopathy

BACKGROUND INFORMATION

Vacuolation of the central nervous system (CNS) is observed in patients with transmissible spongiform encephalopathy, HIV-related encephalopathy and some inherited diseases, but the underlying cellular mechanisms remain poorly understood. Mice lacking the mahogunin ring finger-1 (MGRN1) E3 ubiquitin ligase develop progressive, widespread spongiform degeneration of the CNS. MGRN1 ubiquitinates and regulates tumour susceptibility gene 101 (TSG101), a central component of the endosomal trafficking machinery. As loss of MGRN1 is predicted to cause partial TSG101 loss-of-function, we hypothesised that CNS vacuolation in Mgrn1 null mice may be caused by the accumulation of multi-cisternal endosome-like 'class E' vacuolar protein sorting (vps) compartments similar to those observed in Tsg101-depleted cells in culture.

RESULTS

To test this hypothesis, Tsg101 was deleted from mature oligodendroglia in vivo. This resulted in severe spongiform encephalopathy, histopathologically similar to that observed in Mgrn1 null mutant mice but with a more rapid onset. Vacuoles in the brains of Tsg101-deleted and Mgrn1 mutant mice labelled with endosomal markers, consistent with an endosomal origin. Vacuoles in the brains of mice inoculated with Rocky Mountain Laboratory (RML) prions did not label with these markers, indicating a different origin, consistent with previously published studies that indicate RML prions have a primary effect on neurons and cause vacuolation in an MGRN1-independent manner. Oligodendroglial deletion of Rab7, which mediates late endosome-to-lysosome trafficking and autophagosome-lysosome fusion, did not cause spongiform change.

CONCLUSIONS

Our data suggest that the formation of multi-cisternal 'class E' vps endosomal structures in oligodendroglia leads to vacuolation.

SIGNIFICANCE

This work provides the first evidence that disrupting multi-vesicular body formation in oligodendroglia can cause white matter vacuolation and demyelination. HIV is known to hijack the endosomal sorting machinery, suggesting that HIV infection of the CNS may also act through this pathway to cause encephalopathy.

Are prions transported by plasma exosomes?

Blood has been shown to contain disease-associated misfolded prion protein (PrP(TSE)) in animals naturally and experimentally infected with various transmissible spongiform encephalopathy (TSE) agents, and in humans infected with variant Creutzfeldt-Jakob disease (vCJD). Recently, we have demonstrated PrP(TSE) in extracellular vesicle preparations (EVs) containing exosomes from plasma of mice infected with mouse-adapted vCJD by Protein Misfolding Cyclic Amplification (PMCA). Here we report the detection of PrP(TSE) by PMCA in EVs from plasma of mice infected with Fukuoka-1 (FU), an isolate from a Gerstmann-Sträussler-Scheinker disease patient. We used Tga20 transgenic mice that over-express mouse cellular prion protein, to assay by intracranial injections the level of infectivity in a FU-infected brain homogenate from wild-type mice (FU-BH), and in blood cellular components (BCC), consisting of red blood cells, white blood cells and platelets, plasma EVs, and plasma EVs subjected to multiple rounds of PMCA. Only FU-BH and plasma EVs from FU-infected mice subjected to PMCA that contained PrP(TSE) transmitted disease to Tga20 mice. Plasma EVs not subjected to PMCA and BCC from FU-infected mice failed to transmit disease. These findings confirm the high sensitivity of PMCA for PrP(TSE) detection in plasma EVs and the efficiency of this in vitro method to produce highly infectious prions. The results of our study encourage further research to define the role of EVs and, more specifically exosomes, as blood-borne carriers of PrP(TSE).

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 in neurodegenerative disease - pathogenesis to biomarkers

To develop effective disease-modifying therapies for neurodegenerative diseases, reliable markers of diagnosis, disease activity and progression are a research priority. The fact that neurodegenerative pathology is primarily associated with distinct subsets of cells in discrete areas of the CNS makes the identification of relevant biomarker molecules a challenge. The trafficking of macromolecules from the CNS to the cerebrospinal fluid and blood, mediated by extracellular vesicles (EVs), presents a promising source of CNS-specific biomarkers. EVs are released by almost all cell types and carry a cargo of protein and nucleic acid that varies according to the cell of origin. EV output changes with cell status and reflects intracellular events, so surface marker expression can be used to identify the cell type from which EVs originate. EVs could, therefore, provide an enriched pool of information about core neuropathogenic, cell-specific processes. This Review examines the current knowledge of the biology and function of EVs, discusses the evidence for their involvement in the pathogenesis of neurodegenerative diseases, and considers their potential as biomarkers of disease.

Axonal maintenance, glia, exosomes, and heat shock proteins

Of all cellular specializations, the axon is especially distinctive because it is a narrow cylinder of specialized cytoplasm called axoplasm with a length that may be orders of magnitude greater than the diameter of the cell body from which it originates. Thus, the volume of axoplasm can be much greater than the cytoplasm in the cell body. This fact raises a logistical problem with regard to axonal maintenance. Many of the components of axoplasm, such as soluble proteins and cytoskeleton, are slowly transported, taking weeks to months to travel the length of axons longer than a few millimeters after being synthesized in the cell body. Furthermore, this slow rate of supply suggests that the axon itself might not have the capacity to respond fast enough to compensate for damage to transported macromolecules. Such damage is likely in view of the mechanical fragility of an axon, especially those innervating the limbs, as rapid limb motion with high impact, like running, subjects the axons in the limbs to considerable mechanical force. Some researchers have suggested that local, intra-axonal protein synthesis is the answer to this problem. However, the translational state of axonal RNAs remains controversial. We suggest that glial cells, which envelop all axons, whether myelinated or not, are the local sources of replacement and repair macromolecules for long axons. The plausibility of this hypothesis is reinforced by reviewing several decades of work on glia-axon macromolecular transfer, together with recent investigations of exosomes and other extracellular vesicles, as vehicles for the transmission of membrane and cytoplasmic components from one cell to another.

Astrocyte-to-neuron intercellular prion transfer is mediated by cell-cell contact

Prion diseases are caused by misfolding of the cellular protein PrP(C) to an infectious conformer, PrP(Sc). Intercellular PrP(Sc) transfer propagates conversion and allows infectivity to move from the periphery to the brain. However, how prions spread between cells of the central nervous system is unclear. Astrocytes are specialized non-neuronal cells within the brain that have a number of functions indispensable for brain homeostasis. Interestingly, they are one of the earliest sites of prion accumulation in the brain. A fundamental question arising from this observation is whether these cells are involved in intercellular prion transfer and thereby disease propagation. Using co-culture systems between primary infected astrocytes and granule neurons or neuronal cell lines, we provide direct evidence that prion-infected astrocytes can disseminate prion to neurons. Though astrocytes are capable of secreting PrP, this is an inefficient method of transferring prion infectivity. Efficient transfer required co-culturing and direct cell contact. Astrocytes form numerous intercellular connections including tunneling nanotubes, containing PrP(Sc), often colocalized with endolysosomal vesicles, which may constitute the major mechanism of transfer. Because of their role in intercellular transfer of prions astrocytes may influence progression of the disease.

Depletion of microglia and inhibition of exosome synthesis halt tau propagation

Accumulation of pathological tau protein is a major hallmark of Alzheimer's disease. Tau protein spreads from the entorhinal cortex to the hippocampal region early in the disease. Microglia, the primary phagocytes in the brain, are positively correlated with tau pathology, but their involvement in tau propagation is unknown. We developed an adeno-associated virus-based model exhibiting rapid tau propagation from the entorhinal cortex to the dentate gyrus in 4 weeks. We found that depleting microglia dramatically suppressed the propagation of tau and reduced excitability in the dentate gyrus in this mouse model. Moreover, we demonstrate that microglia spread tau via exosome secretion, and inhibiting exosome synthesis significantly reduced tau propagation in vitro and in vivo. These data suggest that microglia and exosomes contribute to the progression of tauopathy and that the exosome secretion pathway may be a therapeutic target.

Emerging roles of exosomes in normal and pathological conditions: new insights for diagnosis and therapeutic applications

From the time when they were first described in the 1970s by the group of Johnstone and Stahl, exosomes are a target of constant research. Exosomes belong to the family of nanovesicles which are of great interest for their many functions and potential for diagnosis and therapy in multiples diseases. Exosomes originate from the intraluminal vesicles of late endosomal compartments named multivesicular bodies and the fusion of these late endosomes with the cell membrane result in the release of the vesicles into the extracellular compartment. Moreover, their generation can be induced by many factors including extracellular stimuli, such as microbial attack and other stress conditions. The primary role attributed to exosomes was the removal of unnecessary proteins from the cells. Now, several studies have demonstrated that exosomes are involved in cell–cell communication, even though their biological function is not completely clear. The participation of exosomes in cancer is the field of microvesicle research that has expanded more over the last years. Evidence proving that exosomes derived from tumor-pulsed dendritic cells, neoplastic cells, and malignant effusions are able to present antigens to T-cells, has led to numerous studies using them as cell-free cancer vaccines. Because exosomes derive from all cell types, they contain proteins, lipids, and micro RNA capable of regulating a variety of target genes. Much research is being conducted, which focuses on the employment of these vesicles as biomarkers in the diagnosis of cancer in addition to innovative biomarkers for diagnosis, prognosis, and management of cardiovascular diseases. Interesting findings indicating the role of exosomes in the pathogenesis of several diseases have encouraged researchers to consider their therapeutic potential not only in oncology but also in the treatment of autoimmune syndromes and neurodegenerative disorders such as Alzheimer’s and Parkinson’s disease, in addition to infectious diseases such as tuberculosis, diphtheria, and toxoplasmosis as well as infections caused by prions or viruses such as HIV. The aim of this review is to disclose the emerging roles of exosomes in normal and pathological conditions and to discuss their potential therapeutic applications.

Prion aggregates transfer through tunneling nanotubes in endocytic vesicles

Transmissible spongiform encephalopathies (TSEs) are a group of neurodegenerative diseases caused by the misfolding of the cellular prion protein to an infectious form PrP(Sc). The intercellular transfer of PrP(Sc) is a question of immediate interest as the cell-to-cell movement of the infectious particle causes the inexorable propagation of disease. We have previously identified tunneling nanotubes (TNTs) as one mechanism by which PrP(Sc) can move between cells. Here we investigate further the details of this mechanism and show that PrP(Sc) travels within TNTs in endolysosomal vesicles. Additionally we show that prion infection of CAD cells increases both the number of TNTs and intercellular transfer of membranous vesicles, thereby possibly playing an active role in its own intercellular transfer via TNTs.