Tangles, Toxicity, and Tau Secretion in AD - New Approaches to a Vexing Problem

Regulation of neuroinflammation in Alzheimer's disease via nanoparticle-loaded phytocompounds with anti-inflammatory and autophagy-inducing properties

BACKGROUND

Alzheimer's disease (AD) is characterized by neuroinflammation linked to amyloid β (Aβ) aggregation and phosphorylated tau (τ) protein in neurofibrillary tangles (NFTs). Key elements in Aβ production and NFT assembly, like γ-secretase and p38 mitogen-activated protein kinase (p38MAPK), contribute to neuroinflammation. In addition, impaired proteosomal and autophagic pathways increase Aβ and τ aggregation, leading to neuronal damage. Conventional neuroinflammation drugs have limitations due to unidirectional therapeutic approaches and challenges in crossing the Blood-Brain Barrier (BBB). Clinical trials for non-steroidal anti-inflammatory drugs (NSAIDs) and other therapeutics remain uncertain. Novel strategies addressing the complex pathogenesis and BBB translocation are needed to effectively tackle AD-related neuroinflammation.

PURPOSE

The current scenario demands for a much-sophisticated theranostic measures which could be achieved via customized engineering and designing of novel nanotherapeutics. As, these therapeutics functions as a double edge sword, having the efficiency of unambiguous targeting, multiple drug delivery and ability to cross BBB proficiently.

METHODS

Inclusion criteria involve selecting recent, English-language studies from the past decade (2013-2023) that explore the regulation of neuroinflammation in neuroinflammation, Alzheimer's disease, amyloid β, tau protein, nanoparticles, autophagy, and phytocompounds. Various study types, including clinical trials, experiments, and reviews, were considered. Exclusion criteria comprised non-relevant publication types, studies unrelated to Alzheimer's disease or phytocompounds, those with methodological flaws, duplicates, and studies with inaccessible data.

RESULTS

In this study, polymeric nanoparticles loaded with specific phytocompounds and coated with an antibody targeting the transferrin receptor (anti-TfR) present on BBB. Thereafter, the engineered nanoparticles with the ability to efficiently traverse the BBB and interact with target molecules within the brain, could induce autophagy, a cellular process crucial for neuronal health, and exhibit potent anti-inflammatory effects. Henceforth, the proposed combination of desired phytocompounds, polymeric nanoparticles, and anti-TfR coating presents a promising approach for targeted drug delivery to the brain, with potential implications in neuroinflammatory conditions such as Alzheimer's disease.

Dual role of brain-derived extracellular vesicles in dementia-related neurodegenerative disorders: cargo of disease spreading signals and diagnostic-therapeutic molecules

Neurodegenerative disorders are one of the most common causes of disability and represent 6.3% of the global burden of disease. Among them, Alzheimer's, Parkinson's, and Huntington's diseases cause cognitive decline, representing the most disabling symptom on both personal and social levels. The molecular mechanisms underlying the onset and progression of dementia are still poorly understood, and include secretory factors potentially affecting differentiated neurons, glial cells and neural stem cell niche. In the last decade, much attention has been devoted to exosomes as novel carriers of information exchanged among both neighbouring and distant cells. These vesicles can be generated and internalized by different brain cells including neurons, neural stem cells, astrocytes, and microglia, thereby affecting neural plasticity and cognitive functions in physiological and pathological conditions. Here, we review data on the roles of exosomes as carriers of bioactive molecules potentially involved in the pathogenesis of neurodegenerative disorders and detectable in biological fluids as biomarkers of dementia. We also discuss the experimental evidence of the therapeutic potential of stem cell-derived vesicles in experimental models of neurodegeneration-dependent cognitive decline.

Tau kinetics in Alzheimer's disease

The cytoskeletal protein tau is implicated in the pathogenesis of Alzheimer's disease which is characterized by intra-neuronal neurofibrillary tangles containing abnormally phosphorylated insoluble tau. Levels of soluble tau are elevated in the brain, the CSF, and the plasma of patients with Alzheimer's disease. To better understand the causes of these elevated levels of tau, we propose a three-compartment kinetic model (brain, CSF, and plasma). The model assumes that the synthesis of tau follows zero-order kinetics (uncorrelated with compartmental tau levels) and that the release, absorption, and clearance of tau is governed by first-order kinetics (linearly related to compartmental tau levels). Tau that is synthesized in the brain compartment can be released into the interstitial fluid, catabolized, or retained in neurofibrillary tangles. Tau released into the interstitial fluid can mix with the CSF and eventually drain to the plasma compartment. However, losses of tau in the drainage pathways may be significant. The kinetic model estimates half-life of tau in each compartment (552 h in the brain, 9.9 h in the CSF, and 10 h in the plasma). The kinetic model predicts that an increase in the neuronal tau synthesis rate or a decrease in tau catabolism rate best accounts for observed increases in tau levels in the brain, CSF, and plasma found in Alzheimer's disease. Furthermore, the model predicts that increases in brain half-life of tau in Alzheimer's disease should be attributed to decreased tau catabolism and not to increased tau synthesis. Most clearance of tau in the neuron occurs through catabolism rather than release to the CSF compartment. Additional experimental data would make ascertainment of the model parameters more precise.

The Immune System as a Therapeutic Target for Alzheimer’s Disease

Alzheimer’s disease (AD) is a heterogeneous neurodegenerative disorder and is the most common cause of dementia. Furthermore, aging is considered the most critical risk factor for AD. However, despite the vast amount of research and resources allocated to the understanding and development of AD treatments, setbacks have been more prominent than successes. Recent studies have shown that there is an intricate connection between the immune and central nervous systems, which can be imbalanced and thereby mediate neuroinflammation and AD. Thus, this review examines this connection and how it can be altered with AD. Recent developments in active and passive immunotherapy for AD are also discussed as well as suggestions for improving these therapies moving forward.

Endoplasmic reticulum stress induces Alzheimer's disease-like phenotypes in the neuron derived from the induced pluripotent stem cell with D678H mutation on amyloid precursor protein

Alzheimer's disease (AD), a progressive neurodegenerative disorder, is mainly caused by the interaction of genetic and environmental factors. The impact of environmental factors on the genetic mutation in the amyloid precursor protein (APP) is not well characterized. We hypothesized that Endoplasmic Reticulum (ER) stress would promote disease for the patient carrying the APP D678H mutation. Therefore, we analyzed the impact of a familial AD mutation on amyloid precursor protein (APP D678H) under ER stress. Induced pluripotent stem cell (iPSC) from APP D678H mutant carrier was differentiated into neurons, which were then analyzed for AD-like changes. Immunocytochemistry and whole-cell patch-clamp recording revealed that the derived neurons on day 28 after differentiation showed neuronal markers and electrophysiological properties similar to those of mature neurons. However, the APP D678H mutant neurons did not have significant alterations in the levels of amyloid-β (Aβ) and phosphorylated tau (pTau) compared to its isogenic wild-type neuron. Only under ER stress, the neurons with the APP D678H mutation had more Aβ and pTau via immune detection assays. The higher level of Aβ in the APP D678H mutant neurons was probably due to the increased level of β-site APP cleaving enzyme (BACE1) and decreased level of Aβ degrading enzymes under ER stress. Increased Aβ and pTau under ER stress reduced the N-methyl-D-aspartate receptor (NMDAR) in Western blot analysis and altered electrophysiological properties in the mutant neurons. Our study provides evidence that the interaction between genetic mutation and ER stress would induce AD-like changes.

Brain Exosomes: Friend or Foe in Alzheimer's Disease?

Alzheimer's disease (AD) is the most common neurodegenerative disease. It is known to be a multifactorial disease and several causes are associated with its occurrence as well as progression. However, the accumulation of amyloid beta (Aβ) is widely considered its major pathogenic hallmark. Additionally, neurofibrillary tangles (NFT), mitochondrial dysfunction, oxidative stress, and aging (cellular senescence) are considered as additional hits affecting the disease pathology. Several studies are now suggesting important role of inflammation in AD, which shifts our thought towards the brain's resident immune cells, microglia, and astrocytes; how they interact with neurons; and how these interactions are affected by intra and extracellular stressful factors. These interactions can be modulated by different mechanisms and pathways, in which exosomes could play an important role. Exosomes are multivesicular bodies secreted by nearly all types of cells. The exosomes secreted by glial cells or neurons affect the interactions and thus the physiology of these cells by transmitting miRNAs, proteins, and lipids. Exosomes can serve as a friend or foe to the neuron function, depending upon the carried signals. Exosomes, from the healthy microenvironment, may assist neuron function and health, whereas, from the stressed microenvironment, they carry oxidative and inflammatory signals to the neurons and thus prove detrimental to the neuronal function. Furthermore, exosomes can cross the blood-brain barrier (BBB), and from the blood plasma they can enter the brain cells and activate microglia and astrocytes. Exosomes can transport Aβ or Tau, cytokines, miRNAs between the cells, and alter the physiology of recipient cells. They can also assist in Aβ clearance and regulation of synaptic activity. The exosomes derived from different cells play different roles, and this field is still in its infancy stage. This review advocates exosomes' role as a friend or foe in neurodegenerative diseases, especially in the case of Alzheimer's disease.

Glial cells and adaptive immunity in frontotemporal dementia with tau pathology

Neuroinflammation is involved in the aetiology of many neurodegenerative disorders including Alzheimer's disease, Parkinson's disease and motor neuron disease. Whether neuroinflammation also plays an important role in the pathophysiology of frontotemporal dementia is less well known. Frontotemporal dementia is a heterogeneous classification that covers many subtypes, with the main pathology known as frontotemporal lobar degeneration. The disease can be categorized with respect to the identity of the protein that causes the frontotemporal lobar degeneration in the brain. The most common subgroup describes diseases caused by frontotemporal lobar degeneration associated with tau aggregation, also known as primary tauopathies. Evidence suggests that neuroinflammation may play a role in primary tauopathies with genome-wide association studies finding enrichment of genetic variants associated with specific inflammation-related gene loci. These loci are related to both the innate immune system, including brain resident microglia, and the adaptive immune system through possible peripheral T-cell involvement. This review discusses the genetic evidence and relates it to findings in animal models expressing pathogenic tau as well as to post-mortem and PET studies in human disease. Across experimental paradigms, there seems to be a consensus regarding the involvement of innate immunity in primary tauopathies, with increased microglia and astrocyte density and/or activation, as well as increases in pro-inflammatory markers. Whilst it is less clear as to whether inflammation precedes tau aggregation or vice versa; there is strong evidence to support a microglial contribution to the propagation of hyperphosphorylated in tau frontotemporal lobar degeneration associated with tau aggregation. Experimental evidence-albeit limited-also corroborates genetic data pointing to the involvement of cellular adaptive immunity in primary tauopathies. However, it is still unclear whether brain recruitment of peripheral immune cells is an aberrant result of pathological changes or a physiological aspect of the neuroinflammatory response to the tau pathology.

Mechanistic roles for altered O-GlcNAcylation in neurodegenerative disorders

Neurodegenerative diseases such as Alzheimer's and Parkinson's remain highly prevalent and incurable disorders. A major challenge in fully understanding and combating the progression of these diseases is the complexity of the network of processes that lead to progressive neuronal dysfunction and death. An ideal therapeutic avenue is conceivably one that could address many if not all of these multiple misregulated mechanisms. Over the years, chemical intervention for the up-regulation of the endogenous posttranslational modification (PTM) O-GlcNAc has been proposed as a potential strategy to slow down the progression of neurodegeneration. Through the development and application of tools that allow dissection of the mechanistic roles of this PTM, there is now a growing body of evidence that O-GlcNAc influences a variety of important neurodegeneration-pertinent mechanisms, with an overall protective effect. As a PTM that is appended onto numerous proteins that participate in protein quality control and homeostasis, metabolism, bioenergetics, neuronal communication, inflammation, and programmed death, O-GlcNAc has demonstrated beneficence in animal models of neurodegenerative diseases, and its up-regulation is now being pursued in multiple clinical studies.

Stress Response Is the Main Trigger of Sporadic Amyloidoses

Amyloidoses are a group of diseases associated with the formation of pathological protein fibrils with cross-β structures. Approximately 5-10% of the cases of these diseases are determined by amyloidogenic mutations, as well as by transmission of infectious amyloids (prions) between organisms. The most common group of so-called sporadic amyloidoses is associated with abnormal aggregation of wild-type proteins. Some sporadic amyloidoses are known to be induced only against the background of certain pathologies, but in some cases the cause of amyloidosis is unclear. It is assumed that these diseases often occur by accident. Here we present facts and hypotheses about the association of sporadic amyloidoses with vascular pathologies, trauma, oxidative stress, cancer, metabolic diseases, chronic infections and COVID-19. Generalization of current data shows that all sporadic amyloidoses can be regarded as a secondary event occurring against the background of diseases provoking a cellular stress response. Various factors causing the stress response provoke protein overproduction, a local increase in the concentration or modifications, which contributes to amyloidogenesis. Progress in the treatment of vascular, metabolic and infectious diseases, as well as cancers, should lead to a significant reduction in the risk of sporadic amyloidoses.

Tau is not necessary for amyloid-β-induced synaptic and memory impairments

The amyloid hypothesis posits that the amyloid-beta (Aβ) protein precedes and requires microtubule-associated protein tau in a sort of trigger-bullet mechanism leading to Alzheimer's disease (AD) pathology. This sequence of events has become dogmatic in the AD field and is used to explain clinical trial failures due to a late start of the intervention when Aβ already activated tau. Here, using a multidisciplinary approach combining molecular biological, biochemical, histopathological, electrophysiological, and behavioral methods, we demonstrated that tau suppression did not protect against Aβ-induced damage of long-term synaptic plasticity and memory, or from amyloid deposition. Tau suppression could even unravel a defect in basal synaptic transmission in a mouse model of amyloid deposition. Similarly, tau suppression did not protect against exogenous oligomeric tau-induced impairment of long-term synaptic plasticity and memory. The protective effect of tau suppression was, in turn, confined to short-term plasticity and memory. Taken together, our data suggest that therapies downstream of Aβ and tau together are more suitable to combat AD than therapies against one or the other alone.

Paired Helical Filament-Forming Region of Tau (297-391) Influences Endogenous Tau Protein and Accumulates in Acidic Compartments in Human Neuronal Cells

Assembly of tau protein into paired helical filaments and straight filaments is a key feature of Alzheimer's disease. Aggregation of tau has been implicated in neurodegeneration, cellular toxicity and the propagation, which accompanies disease progression. We have reported previously that a region of tau (297–391), referred to as dGAE, assembles spontaneously in physiological conditions to form paired helical filament-like fibres in vitro in the absence of additives such as heparin. This provides a valuable tool with which to explore the effects of tau in cell culture. Here we have studied the cellular uptake of soluble oligomeric and fibrillar forms of dGAE and examined the downstream consequences of tau internalisation into differentiated SH-SY5Y neuroblastoma cells using fluorescence and electron microscopy alongside structural and biochemical analyses. The assembled dGAE shows more acute cytotoxicity than the soluble, non-aggregated form. Conversely, the soluble form is much more readily internalised and, once within the cell, is able to associate with endogenous tau resulting in increased phosphorylation and aggregation of endogenous tau, which accumulates in lysosomal/endosomal compartments. It appears that soluble oligomeric forms are able to propagate tau pathology without being acutely toxic. The model system we have developed now permits the molecular mechanisms of propagation of tau pathology to be studied in vitro in a more physiological manner with a view to development of novel therapeutic approaches.

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Tau297–391 aggregates to form filaments toxic to cells in the absence of additives.

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Internalisation of soluble tau297–391 into cells leads to increased insoluble Ptau.

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Soluble tau297–391 accumulates with endogenous tau in endo-lysosomal compartments.

Cellular polyamines condense hyperphosphorylated Tau, triggering Alzheimer's disease

Many gaps in our understanding of Alzheimer's disease remain despite intense research efforts. One such prominent gap is the mechanism of Tau condensation and fibrillization. One viewpoint is that positively charged Tau is condensed by cytosolic polyanions. However, this hypothesis is likely based on an overestimation of the abundance and stability of cytosolic polyanions and an underestimation of crucial intracellular constituents - the cationic polyamines. Here, we propose an alternative mechanism grounded in cellular biology. We describe extensive molecular dynamics simulations and analysis on physiologically relevant model systems, which suggest that it is not positively charged, unmodified Tau that is condensed by cytosolic polyanions but negatively charged, hyperphosphorylated Tau that is condensed by cytosolic polycations. Our work has broad implications for anti-Alzheimer's research and drug development and the broader field of tauopathies in general, potentially paving the way to future etiologic therapies.

The Role of Protein Misfolding and Tau Oligomers (TauOs) in Alzheimer's Disease (AD)

Although the causative role of the accumulation of amyloid β 1-42 (Aβ42) deposits in the pathogenesis of Alzheimer's disease (AD) has been under debate for many years, it is supposed that the toxicity soluble oligomers of Tau protein (TauOs) might be also the pathogenic factor acting on the initial stages of this disease. Therefore, we performed a thorough search for literature pertaining to our investigation via the MEDLINE/PubMed database. It was shown that soluble TauOs, especially granular forms, may be the most toxic form of this protein. Hyperphosphorylated TauOs can reduce the number of synapses by missorting into axonal compartments of neurons other than axon. Furthermore, soluble TauOs may be also responsible for seeding Tau pathology within AD brains, with probable link to AβOs toxicity. Additionally, the concentrations of TauOs in the cerebrospinal fluid (CSF) and plasma of AD patients were higher than in non-demented controls, and revealed a negative correlation with mini-mental state examination (MMSE) scores. It was postulated that adding the measurements of TauOs to the panel of CSF biomarkers could improve the diagnosis of AD.

Improving Mouse Models for Dementia. Are All the Effects in Tau Mouse Models Due to Overexpression?

Mouse models of Alzheimer's disease have commonly used transgenic overexpression of genes involved in production of amyloid β (APP and/or PSEN1/2) or Tau (MAPT) with mutations that result in familial forms of dementia. We discuss possible improvements that may create full models while avoiding the problems of overexpression and report synaptic results in APPKI models. We stress use of inappropriate controls without overexpression of the normal human protein and the mismatch between the learning deficits reported in mice with plaques but no tangles and the human condition. We focus on Tau overexpression, including new data that support previous reports of the grossly nonlinear relationship between Tau overexpression and neurofibrillary tangle load, with a twofold increase in Tau protein, resulting in a 100-fold increase in tangle density. These data also support the hypothesis that a high concentration of soluble Tau, in overexpression models, plays an important direct role in neurodegeneration, rather than only via aggregation. Finally, we hypothesize that there is an optimal concentration range over which Tau can bind to microtubules and a threshold beyond which much of the overexpressed protein is unable to bind. The excess thus causes toxicity in ways not necessarily related to the process in human dementias.

Mapping interactions with the chaperone network reveals factors that protect against tau aggregation

A network of molecular chaperones is known to bind proteins ('clients') and balance their folding, function and turnover. However, it is often unclear which chaperones are critical for selective recognition of individual clients. It is also not clear why these key chaperones might fail in protein-aggregation diseases. Here, we utilized human microtubule-associated protein tau (MAPT or tau) as a model client to survey interactions between ~30 purified chaperones and ~20 disease-associated tau variants (~600 combinations). From this large-scale analysis, we identified human DnaJA2 as an unexpected, but potent, inhibitor of tau aggregation. DnaJA2 levels were correlated with tau pathology in human brains, supporting the idea that it is an important regulator of tau homeostasis. Of note, we found that some disease-associated tau variants were relatively immune to interactions with chaperones, suggesting a model in which avoiding physical recognition by chaperone networks may contribute to disease.

Extracellular Vesicles Containing P301L Mutant Tau Accelerate Pathological Tau Phosphorylation and Oligomer Formation but Do Not Seed Mature Neurofibrillary Tangles in ALZ17 Mice

In Alzheimer's disease, the distribution of neurofibrillary tangles, a histological hallmark comprised of phosphorylated forms of the protein tau, follows a distinct pattern through anatomically connected brain regions. The well-documented correlation between the severity of tau pathology and disease progression implies a prion-like seeding and spreading mechanism for tau. Experimentally, this has been addressed in transgenic mice by the injection of protein lysates isolated from brains of transgenic mice or patients with tauopathies, including AD, that were shown to behave like seeds, accelerating tau pathology and tangle formation in predisposed mice. More specifically, in vivo data suggest that brain lysates from mice harboring the P301S mutation of tau can seed protein aggregation when injected into the hippocampi of human wild-type tau transgenic ALZ17 mice. Here, we compared the seeding potential of lysates and extracellular vesicles enriched for exosomes (EVs) from wild-type and human P301L tau transgenic rTg4510 mouse brains. We show that transgenic EVs cause increased tau phosphorylation and soluble oligomer formation in a manner comparable to that of freely available proteins in brain lysates, a prerequisite for the formation of mature protein aggregates.

Exosomes: a novel therapeutic target for Alzheimer's disease?

Extracellular exosomes are formed inside the cytoplasm of cells in compartments known as multivesicular bodies. Thus, exosomes contain cytoplasmic content. Multivesicular bodies fuse with the plasma membrane and release exosomes into the extracellular environment. Comprehensive research suggests that exosomes act as both inflammatory intermediaries and critical inducers of oxidative stress to drive progression of Alzheimer's disease. An important role of exosomes in Alzheimer's disease includes the formation of neurofibrillary tangles and beta-amyloid production, clearance, and accumulation. In addition, exosomes are involved in neuroinflammation and oxidative stress, which both act as triggers for beta-amyloid pathogenesis and tau hyperphosphorylation. Further, it has been shown that exosomes are strongly associated with beta-amyloid clearance. Thus, effective measures for regulating exosome metabolism may be novel drug targets for Alzheimer's disease.

Tau accumulation in the retina promotes early neuronal dysfunction and precedes brain pathology in a mouse model of Alzheimer's disease

Background

Tau is an axon-enriched protein that binds to and stabilizes microtubules, and hence plays a crucial role in neuronal function. In Alzheimer’s disease (AD), pathological tau accumulation correlates with cognitive decline. Substantial visual deficits are found in individuals affected by AD including a preferential loss of retinal ganglion cells (RGCs), the neurons that convey visual information from the retina to the brain. At present, however, the mechanisms that underlie vision changes in these patients are poorly understood. Here, we asked whether tau plays a role in early retinal pathology and neuronal dysfunction in AD.

Methods

Alterations in tau protein and gene expression, phosphorylation, and localization were investigated by western blots, qPCR, and immunohistochemistry in the retina and visual pathways of triple transgenic mice (3xTg) harboring mutations in the genes encoding presenilin 1 (PS1M146 V), amyloid precursor protein (APPSwe), and tau (MAPTP301L). Anterograde axonal transport was assessed by intraocular injection of the cholera toxin beta subunit followed by quantification of tracer accumulation in the contralateral superior colliculus. RGC survival was analyzed on whole-mounted retinas using cell-specific markers. Reduction of tau expression was achieved following intravitreal injection of targeted siRNA.

Results

Our data demonstrate an age-related increase in endogenous retinal tau characterized by epitope-specific hypo- and hyper-phosphorylation in 3xTg mice. Retinal tau accumulation was observed as early as three months of age, prior to the reported onset of behavioral deficits, and preceded tau aggregation in the brain. Intriguingly, tau build up occurred in RGC soma and dendrites, while tau in RGC axons in the optic nerve was depleted. Tau phosphorylation changes and missorting correlated with substantial defects in anterograde axonal transport that preceded RGC death. Importantly, targeted siRNA-mediated knockdown of endogenous tau improved anterograde transport along RGC axons.

Conclusions

Our study reveals profound tau pathology in the visual system leading to early retinal neuron damage in a mouse model of AD. Importantly, we show that tau accumulation promotes anterograde axonal transport impairment in vivo, and identify this response as an early feature of neuronal dysfunction that precedes cell death in the AD retina. These findings provide the first proof-of-concept that a global strategy to reduce tau accumulation is beneficial to improve axonal transport and mitigate functional deficits in AD and tauopathies.

LTP and memory impairment caused by extracellular Aβ and Tau oligomers is APP-dependent

The concurrent application of subtoxic doses of soluble oligomeric forms of human amyloid-beta (oAβ) and Tau (oTau) proteins impairs memory and its electrophysiological surrogate long-term potentiation (LTP), effects that may be mediated by intra-neuronal oligomers uptake. Intrigued by these findings, we investigated whether oAβ and oTau share a common mechanism when they impair memory and LTP in mice. We found that as already shown for oAβ, also oTau can bind to amyloid precursor protein (APP). Moreover, efficient intra-neuronal uptake of oAβ and oTau requires expression of APP. Finally, the toxic effect of both extracellular oAβ and oTau on memory and LTP is dependent upon APP since APP-KO mice were resistant to oAβ- and oTau-induced defects in spatial/associative memory and LTP. Thus, APP might serve as a common therapeutic target against Alzheimer's Disease (AD) and a host of other neurodegenerative diseases characterized by abnormal levels of Aβ and/or Tau.

DOI:http://dx.doi.org/10.7554/eLife.26991.001

Tau Oligomers: Cytotoxicity, Propagation, and Mitochondrial Damage

Aging has long been considered as the main risk factor for several neurodegenerative disorders including a large group of diseases known as tauopathies. Even though neurofibrillary tangles (NFTs) have been examined as the main histopathological hallmark, they do not seem to play a role as the toxic entities leading to disease. Recent studies suggest that an intermediate form of tau, prior to NFT formation, the tau oligomer, is the true toxic species. However, the mechanisms by which tau oligomers trigger neurodegeneration remain unknown. This review summarizes recent findings regarding the role of tau oligomers in disease, including release from cells, propagation from affected to unaffected brain regions, uptake into cells, and toxicity via mitochondrial dysfunction. A greater understanding of tauopathies may lead to future advancements in regards to prevention and treatment.

The Role of Stress as a Risk Factor for Progressive Supranuclear Palsy

BACKGROUND

PSP, like Alzheimer's disease (AD), is a tauopathy. The etiopathogenesis of PSP is not well known and the role of stress has not yet been examined. Recent studies have shown that stress increases the risk for developing AD. This study investigates the role of stress as a risk factor for PSP.

OBJECTIVE

B To examine the association between the development of progressive supranuclear palsy (PSP) and self-reported life stressors.

METHODS

76 patients diagnosed with PSP according to the NINDS-SPSP criteria and 68 age-matched unrelated controls were administered a life stressor questionnaire. Stress was quantified as total number of events, number of life changing events, and number of events characterized by self-rated severity. Conditional odds ratio (OR) was calculated for each measure, with participants in the highest quartile of each measure being defined as high-exposure in relation to all other participants.

RESULTS

There were no significant differences between the reported number of total events or life-changing events in cases and controls. However, we found 24.4% of cases (N = 11) and 9.1% of controls (N = 5) had a higher exposure to high severity events, yielding an OR of 3.2 (p = 0.04).

CONCLUSIONS

We found that cases have over a three times greater odds of high exposure to high-severity events than controls prior to the clinical development of PSP, while there were no differences in overall number of reported events. Our findings suggest that high exposure to highly stressful events may be associated with the development of PSP.

Extrasynaptic NMDA receptor-induced tau overexpression mediates neuronal death through suppressing survival signaling ERK phosphorylation

Intracellular accumulation of the hyperphosphorylated tau is a pathological hallmark in the brain of Alzheimer disease. Activation of extrasynaptic NMDA receptors (E-NMDARs) induces excitatory toxicity that is involved in Alzheimer's neurodegeneration. However, the intrinsic link between E-NMDARs and the tau-induced neuronal damage remains elusive. In the present study, we showed in cultured primary cortical neurons that activation of E-NMDA receptors but not synaptic NMDA receptors dramatically increased tau mRNA and protein levels, with a simultaneous neuronal degeneration and decreased neuronal survival. Memantine, a selective antagonist of E-NMDARs, reversed E-NMDARs-induced tau overexpression. Activation of E-NMDARs in wild-type mouse brains resulted in neuron loss in hippocampus, whereas tau deletion in neuronal cultures and in the mouse brains rescued the E-NMDARs-induced neuronal death and degeneration. The E-NMDARs-induced tau overexpression was correlated with a reduced ERK phosphorylation, whereas the increased MEK activity, decreased binding and activity of ERK phosphatase to ERK, and increased ERK phosphorylation were observed in tau knockout mice. On the contrary, addition of tau proteins promoted ERK dephosphorylation in vitro. Taking together, these results indicate that tau overexpression mediates the excitatory toxicity induced by E-NMDAR activation through inhibiting ERK phosphorylation.

CSF Biomarkers and Its Associations with 18F-AV133 Cerebral VMAT2 Binding in Parkinson's Disease-A Preliminary Report

OBJECTIVE

Cerebrospinal fluid (CSF) biomarkers, such as α-synuclein (α-syn), amyloid beta peptide 1-42 (Aβ1-42), phosphorylated tau (181P) (p-tau), and total tau (t-tau), have long been associated with the development of Parkinson disease (PD) and other neurodegenerative diseases. In this investigation, we reported the assessment of CSF biomarkers and their correlations with vesicular monoamine transporter 2 (VMAT2) bindings measured with 18F-9-fluoropropyl-(+)-dihydrotetrabenazine (18F-AV133) that is being developed as a biomarker for PD. We test the hypothesis that monoaminergic degeneration was correlated with CSF biomarker levels in untreated PD patients.

METHODS

The available online data from the Parkinson's Progression Markers Initiative study (PPMI) project were collected and analyzed, which include demographic information, clinical evaluations, CSF biomarkers (α-syn, Aβ1-42, p-tau, and t-tau), 18F-AV133 brain PET, and T1 weighted MRIs. Region of interest (ROI) and voxel-wise Pearson correlation between standardized uptake value ratio (SUVR) and CSF biomarkers were calculated.

RESULTS

Our major findings are: 1) Compared with controls, CSF α-syn and tau levels decreased significantly in PD; 2) α-syn was closely correlated with Aβ1-42 and tau in PD, especially in early-onset patients; and 3) hypothesis-driven ROI analysis found a significant negative correlation between CSF Aβ1-42 levels and VMAT2 densities in post cingulate, left caudate, left anterior putamen, and left ventral striatum in PDs. CSF t-tau and p-tau levels were significantly negatively related to VMAT2 SUVRs in substantia nigra and left ventral striatum, respectively. Voxel-wise analysis showed that left caudate, parahippocampal gyrus, insula and temporal lobe were negatively correlated with Aβ1-42. In addition, superior frontal gyrus and transverse temporal gyrus were negatively correlated with CSF p-tau levels.

CONCLUSION

These results suggest that monoaminergic degeneration in PD is correlated with CSF biomarkers associated with cognitive impairment in neurodegenerative diseases including Alzheimer's disease. The association between loss of dopamine synaptic function and pathologic protein accumulations in PD indicates an important role of CSF biomarkers in PD development.

FRMD4A-cytohesin signaling modulates the cellular release of tau

One of the defining pathological features of Alzheimer's disease is the intraneuronal accumulation of tau (also known as MAPT) protein. Tau is also secreted from neurons in response to various stimuli and accumulates in the cerebrospinal fluid of Alzheimer's disease patients. Tau pathology might spread from cell to cell through a mechanism involving secretion and uptake. Here, we developed an assay to follow cellular release and uptake of tau dimers. Individual silencing of ten common late-onset Alzheimer's disease risk genes in HEK293T cells expressing the tau reporters suggested that FRMD4A is functionally linked to tau secretion. FRMD4A depletion by using RNA interference (RNAi) reduced and overexpression increased tau secretion. The activity of cytohesins, interactors of FRMD4A and guanine-nucleotide-exchange factors of Arf6, was necessary for FRMD4A-induced tau secretion. Increased Arf6 and cell polarity signaling through Par6 and atypical protein kinase Cζ (aPKCζ) stimulated tau secretion. In mature cortical neurons, FRMD4A RNAi or inhibition of cytohesins strongly upregulated secretion of endogenous tau. These results suggest that FRMD4A, a genetic risk factor for late-onset Alzheimer's disease, regulates tau secretion by activating cytohesin-Arf6 signaling. We conclude that genetic risk factors of Alzheimer's disease might modulate disease progression by altering tau secretion.

Cellular and molecular modifier pathways in tauopathies: the big picture from screening invertebrate models

Abnormal tau accumulations were observed and documented in post-mortem brains of patients affected by Alzheimer's disease (AD) long before the identification of mutations in the Microtubule-associated protein tau (MAPT) gene, encoding the tau protein, in a different neurodegenerative disease called Frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17). The discovery of mutations in the MAPT gene associated with FTDP-17 highlighted that dysfunctions in tau alone are sufficient to cause neurodegeneration. Invertebrate models have been diligently utilized in investigating tauopathies, contributing to the understanding of cellular and molecular pathways involved in disease etiology. An important discovery came with the demonstration that over-expression of human tau in Drosophila leads to premature mortality and neuronal dysfunction including neurodegeneration, recapitulating some key neuropathological features of the human disease. The simplicity of handling invertebrate models combined with the availability of a diverse range of experimental resources make these models, in particular Drosophila a powerful invertebrate screening tool. Consequently, several large-scale screens have been performed using Drosophila, to identify modifiers of tau toxicity. The screens have revealed not only common cellular and molecular pathways, but in some instances the same modifier has been independently identified in two or more screens suggesting a possible role for these modifiers in regulating tau toxicity. The purpose of this review is to discuss the genetic modifier screens on tauopathies performed in Drosophila and C. elegans models, and to highlight the common cellular and molecular pathways that have emerged from these studies. Here, we summarize results of tau toxicity screens providing mechanistic insights into pathological alterations in tauopathies. Key pathways or modifiers that have been identified are associated with a broad range of processes including, but not limited to, phosphorylation, cytoskeleton organization, axonal transport, regulation of cellular proteostasis, transcription, RNA metabolism, cell cycle regulation, and apoptosis. We discuss the utility and application of invertebrate models in elucidating the cellular and molecular functions of novel and uncharacterized disease modifiers identified in large-scale screens as well as for investigating the function of genes identified as risk factors in genome-wide association studies from human patients in the post-genomic era. In this review, we combined and summarized several large-scale modifier screens performed in invertebrate models to identify modifiers of tau toxicity. A summary of the screens show that diverse cellular processes are implicated in the modification of tau toxicity. Kinases and phosphatases are the most predominant class of modifiers followed by components required for cellular proteostasis and axonal transport and cytoskeleton elements.

Extracellular Tau Oligomers Produce An Immediate Impairment of LTP and Memory

Non-fibrillar soluble oligomeric forms of amyloid-β peptide (oAβ) and tau proteins are likely to play a major role in Alzheimer's disease (AD). The prevailing hypothesis on the disease etiopathogenesis is that oAβ initiates tau pathology that slowly spreads throughout the medial temporal cortex and neocortices independently of Aβ, eventually leading to memory loss. Here we show that a brief exposure to extracellular recombinant human tau oligomers (oTau), but not monomers, produces an impairment of long-term potentiation (LTP) and memory, independent of the presence of high oAβ levels. The impairment is immediate as it raises as soon as 20 min after exposure to the oligomers. These effects are reproduced either by oTau extracted from AD human specimens, or naturally produced in mice overexpressing human tau. Finally, we found that oTau could also act in combination with oAβ to produce these effects, as sub-toxic doses of the two peptides combined lead to LTP and memory impairment. These findings provide a novel view of the effects of tau and Aβ on memory loss, offering new therapeutic opportunities in the therapy of AD and other neurodegenerative diseases associated with Aβ and tau pathology.

Extracellular Membrane Vesicles as Vehicles for Brain Cell-to-Cell Interactions in Physiological as well as Pathological Conditions

Extracellular vesicles are involved in a great variety of physiological events occurring in the nervous system, such as cross talk among neurons and glial cells in synapse development and function, integrated neuronal plasticity, neuronal-glial metabolic exchanges, and synthesis and dynamic renewal of myelin. Many of these EV-mediated processes depend on the exchange of proteins, mRNAs, and noncoding RNAs, including miRNAs, which occurs among glial and neuronal cells. In addition, production and exchange of EVs can be modified under pathological conditions, such as brain cancer and neurodegeneration. Like other cancer cells, brain tumours can use EVs to secrete factors, which allow escaping from immune surveillance, and to transfer molecules into the surrounding cells, thus transforming their phenotype. Moreover, EVs can function as a way to discard material dangerous to cancer cells, such as differentiation-inducing proteins, and even drugs. Intriguingly, EVs seem to be also involved in spreading through the brain of aggregated proteins, such as prions and aggregated tau protein. Finally, EVs can carry useful biomarkers for the early diagnosis of diseases. Herein we summarize possible roles of EVs in brain physiological functions and discuss their involvement in the horizontal spreading, from cell to cell, of both cancer and neurodegenerative pathologies.

Cdk5 at crossroads of protein oligomerization in neurodegenerative diseases: facts and hypotheses

Cyclin-dependent kinase 5 (Cdk5) is involved in proper neurodevelopment and brain function and serves as a switch between neuronal survival and death. Overactivation of Cdk5 is associated with many neurodegenerative disorders such as Alzheimer's or Parkinson's diseases. It is believed that in those diseases Cdk5 may be an important link between disease-initiating factors and cell death effectors. A common hallmark of neurodegenerative disorders is incorrect folding of specific proteins, thus leading to their intra- and extracellular accumulation in the nervous system. Abnormal Cdk5 signaling contributes to dysfunction of individual proteins and has a substantial role in either direct or indirect interactions of proteins common to, and critical in, different neurodegenerative diseases. While the roles of Cdk5 in α-synuclein (ASN) - tau or β-amyloid peptide (Aβ) - tau interactions are well documented, its contribution to many other pertinent interactions, such as that of ASN with Aβ, or interactions of the Aβ - ASN - tau triad with prion proteins, did not get beyond plausible hypotheses and remains to be proven. Understanding of the exact position of Cdk5 in the deleterious feed-forward loop critical for development and progression of neurodegenerative diseases may help designing successful therapeutic strategies of several fatal neurodegenerative diseases. Cyclin-dependent kinase 5 (Cdk5) is associated with many neurodegenerative disorders such as Alzheimer's or Parkinson's diseases. It is believed that in those diseases Cdk5 may be an important factor involved in protein misfolding, toxicity and interaction. We suggest that Cdk5 may contribute to the vicious circle of neurotoxic events involved in the pathogenesis of different neurodegenerative diseases.

Analysis of tau post-translational modifications in rTg4510 mice, a model of tau pathology

BACKGROUND

Microtubule associated protein tau is the major component of the neurofibrillary tangles (NFTs) found in the brains of patients with Alzheimer's disease and several other neurodegenerative diseases. Tau mutations are associated with frontotemperal dementia with parkinsonism on chromosome 17 (FTDP-17). rTg4510 mice overexpress human tau carrying the P301L FTDP-17 mutation and develop robust NFT-like pathology at 4-5 months of age. The current study is aimed at characterizing the rTg4510 mice to better understand the genesis of tau pathology and to better enable the use of this model in drug discovery efforts targeting tau pathology.

RESULTS

Using a panel of immunoassays, we analyzed the age-dependent formation of pathological tau in rTg4510 mice and our data revealed a steady age-dependent accumulation of pathological tau in the insoluble fraction of brain homogenates. The pathological tau was associated with multiple post-translational modifications including aggregation, phosphorylation at a wide variety of sites, acetylation, ubiquitination and nitration. The change of most tau species reached statistical significance at the age of 16 weeks. There was a strong correlation between the different post-translationally modified tau species in this heterogeneous pool of pathological tau. Total tau in the cerebrospinal fluid (CSF) displayed a multiphasic temporal profile distinct from the steady accumulation of pathological tau in the brain. Female rTg4510 mice displayed significantly more aggressive accumulation of pathological tau in the brain and elevation of total tau in CSF than their male littermates.

CONCLUSION

The immunoassays described here were used to generate the most comprehensive description of the changes in various tau species across the lifespan of the rTg4510 mouse model. The data indicate that development of tauopathy in rTg4510 mice involves the accumulation of a pool of pathological tau that carries multiple post-translational modifications, a process that can be detected well before the histological detection of NFTs. Therapeutic treatment targeting tau should therefore aim to reduce all tau species associated with the pathological tau pool rather than reduce specific post-translational modifications. There is still much to learn about CSF tau in physiological and pathological processes in order to use it as a translational biomarker in drug discovery.

Tau proteins in the temporal and frontal cortices in patients with vascular dementia

We previously reported that, in the brains of older patients with vascular dementia (VaD), there is a distinctive accumulation of detergent-extractable soluble amyloid-β, with a predominance of Aβ42 species. It is unclear, however, if tau proteins also accumulate in the brains of older VaD subjects. Using antibody-specific immunoassays, we assessed concentrations of total tau (t-tau) and phosphorylated tau protein, measured at 3 phosphorylated sites (i.e. Thr181, Ser202/Thr205, and Ser262), as well as synaptophysin in the temporal and frontal cortices of 18 VaD, 16 Alzheimer disease (AD), and 16 normal age-matched control subjects. There was selective loss of t-tau protein in VaD compared with controls and AD subjects (p < 0.021 and p < 0.001, respectively). In contrast, phosphorylated tau levels were similar to controls in VaD in both regions, but they were increased in the temporal lobes of patients with AD (p < 0.01 and p < 0.0001 for Ser202/Thr205 and Ser262 phosphorylated sites, respectively). The reduced t-tau in the VaD group was unrelated to any low-level neurofibrillary or amyloid pathology or age at death. These findings suggest that breaches of microvascular or microstructural tissue integrity subsequent to ischemic injury in older age may modify tau protein metabolism or phosphorylation and have effects on the burden of neurofibrillary pathology characteristic of AD.

Genetic markers in biological fluids for aging-related major neurocognitive disorder

Aging-related major neurocognitive disorder (NCD), formerly named dementia, comprises of the different acquired diseases whose primary deficit is impairment in cognitive functions such as complex attention, executive function, learning and memory, language, perceptual/motor skills, and social cognition, and that are related to specific brain regions and/or networks. According to its etiology, the most common subtypes of major NCDs are due to Alzheimer' s disease (AD), vascular disease (VaD), Lewy body disease (LBD), and frontotemporal lobar degeneration (FTLD). These pathologies are frequently present in mixed forms, i.e., AD plus VaD or AD plus LBD, thus diagnosed as due to multiple etiologies. In this paper, the definitions, criteria, pathologies, subtypes and genetic markers for the most common age-related major NCD subtypes are summarized. The current diagnostic criteria consider cognitive decline leading to major NCD or dementia as a progressive degenerative process with an underlying neuropathology that begins before the manifestation of symptoms. Biomarkers associated with this asymptomatic phase are being developed as accurate risk factor and biomarker assessments are fundamental to provide timely treatment since no treatments to prevent or cure NCD yet exist. Biological fluid assessment represents a safer, cheaper and less invasive method compared to contrast imaging studies to predict NCD appearance. Genetic factors particularly have a key role not only in predicting development of the disease but also the age of onset as well as the presentation of comorbidities that may contribute to the disease pathology and trigger synergistic mechanisms which may, in turn, accelerate the neurodegenerative process and its resultant behavioral and functional disorders.

Tau monoclonal antibody generation based on humanized yeast models: impact on Tau oligomerization and diagnostics

A link between Tau phosphorylation and aggregation has been shown in different models for Alzheimer disease, including yeast. We used human Tau purified from yeast models to generate new monoclonal antibodies, of which three were further characterized. The first antibody, ADx201, binds the Tau proline-rich region independently of the phosphorylation status, whereas the second, ADx215, detects an epitope formed by the Tau N terminus when Tau is not phosphorylated at Tyr(18). For the third antibody, ADx210, the binding site could not be determined because its epitope is probably conformational. All three antibodies stained tangle-like structures in different brain sections of THY-Tau22 transgenic mice and Alzheimer patients, and ADx201 and ADx210 also detected neuritic plaques in the cortex of the patient brains. In hippocampal homogenates from THY-Tau22 mice and cortex homogenates obtained from Alzheimer patients, ADx215 consistently stained specific low order Tau oligomers in diseased brain, which in size correspond to Tau dimers. ADx201 and ADx210 additionally reacted to higher order Tau oligomers and presumed prefibrillar structures in the patient samples. Our data further suggest that formation of the low order Tau oligomers marks an early disease stage that is initiated by Tau phosphorylation at N-terminal sites. Formation of higher order oligomers appears to require additional phosphorylation in the C terminus of Tau. When used to assess Tau levels in human cerebrospinal fluid, the antibodies permitted us to discriminate patients with Alzheimer disease or other dementia like vascular dementia, indicative that these antibodies hold promising diagnostic potential.

Autophagy-related protein 7 deficiency in amyloid β (Aβ) precursor protein transgenic mice decreases Aβ in the multivesicular bodies and induces Aβ accumulation in the Golgi

Alzheimer disease (AD) is biochemically characterized by increased levels of amyloid β (Aβ) peptide, which aggregates into extracellular Aβ plaques in AD brains. Before plaque formation, Aβ accumulates intracellularly in both AD brains and in the brains of AD model mice, which may contribute to disease progression. Autophagy, which is impaired in AD, clears cellular protein aggregates and participates in Aβ metabolism. In addition to a degradative role of autophagy in Aβ metabolism we recently showed that Aβ secretion is inhibited in mice lacking autophagy-related gene 7 (Atg7) in excitatory neurons in the mouse forebrain. This inhibition of Aβ secretion leads to intracellular accumulation of Aβ. Here, we used fluorescence and immunoelectron microscopy to elucidate the subcellular localization of the intracellular Aβ accumulation which accumulates in Aβ precursor protein mice lacking Atg7. Autophagy deficiency causes accumulation of p62(+) aggregates, but these aggregates do not contain Aβ. However, knockdown of Atg7 induced Aβ accumulation in the Golgi and a concomitant reduction of Aβ in the multivesicular bodies. This indicates that Atg7 influences the transport of Aβ possibly derived from Golgi to multivesicular bodies.

Frequent and symmetric deposition of misfolded tau oligomers within presynaptic and postsynaptic terminals in Alzheimer's disease

The accumulation of neurofibrillary tangles in Alzheimer’s disease (AD) propagates with characteristic spatiotemporal patterns which follow brain network connections, implying trans-synaptic transmission of tauopathy. Since misfolded tau has been shown to transmit across synapses in AD animal models, we hypothesized that synapses in AD patients may contain misfolded tau. By immunofluorescence imaging of bipartite synapses from AD subjects, we detected tau protein in 38.4% of presynaptic and 50.9% of postsynaptic terminals. The pre/post distribution for hyperphosphorylated tau was 26.9%/30.7%, and for misfolded tau 18.3%/19.3%. In the temporal cortex, microscopic aggregates of tau, containing ultra-stable oligomers, were estimated to accumulate within trillions of synapses, outnumbering macroscopic tau aggregates such as tangles by 10000 fold. Non-demented elderly also showed considerable synaptic tau hyperphosphorylation and some misfolding, implicating the synapse as one of the first subcellular compartments affected by tauopathy. Misfolding of tau protein appeared to occur in situ inside synaptic terminals, without mislocalizing or mistrafficking. Misfolded tau at synapses may represent early signs of neuronal degeneration, mediators of synaptotoxicity, and anatomical substrates for transmitting tauopathy, but its actual role in these processes remain to be elucidated.

Environmental factors in the development and progression of late-onset Alzheimer's disease

Late-onset Alzheimer's disease (LOAD) is an age-related neurodegenerative disorder characterized by gradual loss of synapses and neurons, but its pathogenesis remains to be clarified. Neurons live in an environment constituted by neurons themselves and glial cells. In this review, we propose that the neuronal degeneration in the AD brain is partially caused by diverse environmental factors. We first discuss various environmental stresses and the corresponding responses at different levels. Then we propose some mechanisms underlying the specific pathological changes, in particular, hypothalamic-pituitary adrenal axis dysfunction at the systemic level; cerebrovascular dysfunction, metal toxicity, glial activation, and Aβ toxicity at the intercellular level; and kinase-phosphatase imbalance and epigenetic modification at the intracellular level. Finally, we discuss the possibility of developing new strategies for the prevention and treatment of LOAD from the perspective of environmental stress. We conclude that environmental factors play a significant role in the development of LOAD through multiple pathological mechanisms.

In vivo tracking of tau pathology using positron emission tomography (PET) molecular imaging in small animals

Hyperphosphorylation of the tau protein leading to the formation of neurofibrillary tangles (NFTs) is a common feature in a wide range of neurodegenerative diseases known as tauopathies, which include Alzheimer's disease (AD) and the frontotemporal dementias (FTDs). Although heavily investigated, the mechanisms underlying the pathogenesis and progression of tauopathies have yet to be fully understood. In this context, several rodent models have been developed that successfully recapitulate the behavioral and neurochemical features of tau pathology, aiming to achieve a better understanding of the link between tau and neurodegeneration. To date, behavioral and biochemical parameters assessed using these models have been conducted using a combination of memory tasks and invasive methods such as cerebrospinal fluid (CSF) sampling or post-mortem analysis. Recently, several novel positron emission tomography (PET) radiopharmaceuticals targeting tau tangles have been developed, allowing for non-invasive in vivo quantification of tau pathology. Combined with tau transgenic models and microPET, these tracers hold the promise of advancing the development of theoretical models and advancing our understanding of the natural history of AD and non-AD tauopathies. In this review, we briefly describe some of the most important insights for understanding the biological basis of tau pathology, and shed light on the opportunity for improved modeling of tau pathology using a combination of tau-radiopharmaceuticals and animal models.

Exosomes and their role in CNS viral infections

Exosomes are small membrane-bound vesicles that carry biological macromolecules from the site of production to target sites either in the microenvironment or at distant sites away from the origin. Exosomal content of cells varies with the cell type that produces them as well as environmental factors that alter the normal state of the cell such as viral infection. Human DNA and RNA viruses alter the composition of host proteins as well as incorporate their own viral proteins and other cargo into the secreted exosomes. While numerous viruses can infect various cell types of the CNS and elicit damaging neuropathologies, few have been studied for their exosomal composition, content, and function on recipient cells. Therefore, there is a pressing need to understand how DNA and RNA viral infections in CNS control exosomal release. Some of the more recent studies including HIV-1, HTLV-1, and EBV-infected B cells indicate that exosomes from these infections contain viral miRNAs, viral transactivators, and a host of cytokines that can control the course of infection. Finally, because exosomes can serve as vehicles for the cellular delivery of proteins and RNA and given that the blood-brain barrier is a formidable challenge in delivering therapeutics to the brain, exosomes may be able to serve as ideal vehicles to deliver protein or RNA-based therapeutics to the brain.