Aβ42 Protofibrils Interact with and Are Trafficked through Microglial-Derived Microvesicles

Abstract

Microvesicles (MVs) and exosomes comprise a class of cell-secreted particles termed extracellular vesicles (EVs). These cargo-holding vesicles mediate cell-to-cell communication and have recently been implicated in neurodegenerative diseases such as Alzheimer's disease (AD). The two types of EVs are distinguished by the mechanism of cell release and their size, with the smaller exosomes and the larger MVs ranging from 30 to 100 nm and 100 nm to 1 μm in diameter, respectively. MV numbers are increased in AD and appear to interact with amyloid-β peptide (Aβ), the primary protein component of the neuritic plaques in the AD brain. Because microglial cells play such an important role in AD-linked neuroinflammation, we sought to characterize MVs shed from microglial cells, better understand MV interactions with Aβ, and determine whether internalized Aβ may be incorporated into secreted MVs. Multiple strategies were used to characterize MVs shed from BV-2 microglia after ATP stimulation. Confocal images of isolated MVs bound to fluorescently labeled annexin-V via externalized phosphatidylserine revealed a polydisperse population of small spherical structures. Dynamic light scattering measurements yielded MV diameters ranging from 150 to 600 nm. Electron microscopy of resin-embedded MVs cut into thin slices showed well-defined uranyl acetate-stained ring-like structures in a similar diameter range. The use of a fluorescently labeled membrane insertion probe, NBD C-HPC, effectively tracked MVs in binding experiments, and an Aβ ELISA confirmed a strong interaction between MVs and Aβ protofibrils but not Aβ monomers. Despite the lesser monomer interaction, MVs had an inhibitory effect on monomer aggregation. Primary microglia rapidly internalized Aβ protofibrils, and subsequent stimulation of the microglia with ATP resulted in the release of MVs containing the internalized Aβ protofibrils. The role of MVs in neurodegeneration and inflammation is an emerging area, and further knowledge of MV interaction with Aβ may shed light on extracellular spread and influence on neurotoxicity and neuroinflammation.

Inflammatory mechanisms in neurodegeneration

“…; Gouwens et al . ). This process appears to be sensitive to Aβ conformation and has implications for spreading of Aβ and neuroinflammation.…”

Neuroimmune Crosstalk through Extracellular Vesicles in Health and Disease

“…Similarly, in vivo inhibition of exosome synthesis by neutral syphingomyelinase-2 inhibitor also reduced tau propagation in adeno-associated virus (AAV)-based and P301S tauopathy mouse models [61]. The cellular origin of exosomes carrying abnormal proteins (Aβ and tau) is still unclear as neurons, astrocytes, and reactive microglia have all been reported to secrete them [5,[62][63][64][65]. Following engulfment of abnormally accumulated neuronal tau, microglia release tau-enriched exosomes, which are then taken up by neurons, further contributing to disease spread in vivo [61] (Figure 2B).…”

Increased Microglial Exosomal miR-124-3p Alleviates Neurodegeneration and Improves Cognitive Outcome after rmTBI

“…16 On the other hand, these EVs may also exert detrimental effects by promoting the spread of Ab to surrounding cells, such as neurons, when the microglia are overloaded with Ab. [17][18][19] In addition, the expression of Ab clearance genes could be downregulated with the progression of AD in response to pro-inflammatory cytokines produced by activated microglia. 20,21 These studies suggest that the beneficial/detrimental effects of microglial exosomes are determined by the progressing stages of AD and are regulated by various factors, such as inflammatory cytokines in the microenvironment of injured brain.…”