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Extracellular Vesicle Biology in Alzheimer's Disease and Related Tauopathy - PubMed

Review

Extracellular Vesicle Biology in Alzheimer's Disease and Related Tauopathy

Annina M DeLeo et al. J Neuroimmune Pharmacol. 2018 Sep.

Abstract

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

Keywords: Alzheimer’s disease; Amyloid-β peptide; Chronic traumatic encephalopathy; Exosomes; Extracellular vesicles; Microglia; Microtubule-associated protein tau; Microvesicles; Tauopathy.

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Conflict of interest statement

Disclaimers

The authors have no conflict of interest relevant to this article.

Figures

Fig 1
Fig 1. Schema of exosome and microvesicle synthesis and cargo molecules

Left Schematic of exosome biogenesis. First, the plasma membrane invaginates creating the early endosome. The early endosome then buds inwardly for a second time creating intraluminal vesicles (ILVs) and becomes the late endosome or multivesicular body (MVB). The MVB can then fuse with the lysosome and ILVs along with their contents will be degraded (bottom middle). Alternately, the MVB can fuse with the plasma membrane, releasing ILVs into the extracellular space, at which point they are considered exosomes. Ectosomes or microvesicles are created by the outward budding of the plasma membrane (bottom right). Right Schema of a single exosome. Exosomes are recognized by their expression of tetraspanins (blue) such as CD9, CD81, and CD63 as well as lipid rafts and Flotilin 1 protein (green). Depending on their cell of origin, exosomes may possess cell-specific receptors (red). Internally, exosomes contain DNA (grey), RNA (grey strand), and proteins (purple) specially packaged during their assembly.

Fig. 2
Fig. 2. Schema for the staging of Alzheimer’s disease and Chronic Traumatic Encephalopathy based on tauopathy

Left Schema of tau deposition (red) in Alzheimer’s disease as based on the 6 Braak & Braak stages. Top Left In stages I-II of AD, tau deposits are seen in the transentorhinal and entorhinal cortices (EC), as well as in the locus coeruleus (LC). Middle Left In AD stages III-IV, tau is additionally found in the CA1 region, subiculum, amygdala, and putamen, and stains more intensely in the EC. Bottom Left In stages V-VI, tau deposits are found throughout the hippocampus and cortex. The brain has also undergone shrinkage at this point. Right The four stages of Chronic Traumatic Encephalopathy in coronal view. Top Right CTE stage I is characterized by tau deposits in the depths cortical sulci. Middle Top Right In CTE stage II, tau has spread from the depths of the sulci to areas closer to the brain surface as well as other sulcal depths, and have begun to appear in the LC. Middle Bottom Right In CTE stage III, the brain begins to exhibit atrophy and ventricular enlargement. NFTs are now present in the hippocampus, EC, and amygdala, as well as being more widespread in the cortex. Bottom Right Stage VI of CTE is characterized by gross brain weight loss and septal defects, and widespread presence of pathological tau.

Fig. 3
Fig. 3. Schema of mechanisms for the spread of pathological protein seeding in the brain via direct and indirect routes

1 An event occurs that alters normal protein monomers, such as tau (black) into pathological misfolded proteins (red). In the case of tau, this event may be hyperphosphorylation. 2 The misfolded protein templates other monomers to misfold. Together, these misfolded proteins aggregate, eventually leading to cell death. 3 The misfolded protein escapes the dying cell and 4 is taken up by a nearby recipient cell. 5 In the recipient cell, the misfolded protein again templates other proteins to misfold and aggregate. 6 Alternatively, microglia phagocytose the cytopathic cells, thereby taking up the misfolded protein. 7 Misfolded proteins may be shuttled into endosomes and processed through endolysosomal degradation or incorporated into the multivesicular bodies (MVBs), where intraluminal vesicles package misfolded proteins and then 8 are released into the extracellular space as exosomes. 9 The exosomes may then be taken up by recipient cells via membrane fusion, directly releasing their cargo, or may be taken up by endocytosis. 10 The misfolded protein in the endocytosed exosome reaches the cytoplasm and templates and aggregates in the recipient cell. While in neuro-degenerative diseases such as Alzheimer’s and CTE this type of seeding is thought to occur across synapses, exosomes can also travel laterally or across long distances to be taken up by distal neurons or even other cell types, and cause pathological aggregation.

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