This review discusses the profound connection between microglia, neuroinflammation, and Alzheimer's disease (AD).Theories have been postulated, tested, and modified over several decades. The findings have further bolstered the belief that microglia-mediated inflammation is both a product and contributor to AD pathology and progression. Distinct microglia phenotypes and their function, microglial recognition and response to protein aggregates in AD, and the overall role of microglia in AD are areas that have received considerable research attention and yielded significant results. The following article provides a historical perspective of microglia, a detailed discussion of multiple microglia phenotypes including dark microglia, and a review of a number of areas where microglia intersect with AD and other pathological neurological processes. The overall breadth of important discoveries achieved in these areas significantly strengthens the hypothesis that neuroinflammation plays a key role in AD. Future determination of the exact mechanisms by which microglia respond to, and attempt to mitigate, protein aggregation in AD may lead to new therapeutic strategies.
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.
Nine bis(tryptophan) derivatives (BTs) and two control compounds were synthesized and tested for antimicrobial activity against two Escherichia coli strains and a Staphylococcus aureus strain. The effects of linker type, shape, and conformational rigidity were manifested in dramatic differences in altering tetracycline potency when coadministered with that antibiotic. A reversal of resistance was observed for an E. coli strain having a TetA efflux pump. Survival of mammalian cells was assayed with good result.
Immunothrombosis—immune-mediated activation of coagulation—is protective against pathogens, but excessive immunothrombosis can result in pathological thrombosis and multiorgan damage, as in severe coronavirus disease 2019 (COVID-19). The NACHT-, LRR-, and pyrin domain-containing protein 3 (NLRP3) inflammasome produces major proinflammatory cytokines of the interleukin (IL)-1 family, IL-1β and IL-18, and induces pyroptotic cell death. Activation of the NLRP3 inflammasome pathway also promotes immunothrombotic programs including release of neutrophil extracellular traps and tissue factor by leukocytes, and prothrombotic responses by platelets and the vascular endothelium. NLRP3 inflammasome activation occurs in patients with COVID-19 pneumonia. In preclinical models, NLRP3 inflammasome pathway blockade restrains COVID-19-like hyperinflammation and pathology. Anakinra, recombinant human IL-1 receptor antagonist, showed safety and efficacy and is approved for the treatment of hypoxaemic COVID-19 patients with early signs of hyperinflammation. The non-selective NLRP3 inhibitor colchicine reduced hospitalization and death in a subgroup of COVID-19 outpatients but is not approved for the treatment of COVID-19. Additional COVID-19 trials testing NLRP3 inflammasome pathway blockers are inconclusive or ongoing. We herein outline the contribution of immunothrombosis to COVID-19-associated coagulopathy, and review preclinical and clinical evidence suggesting an engagement of the NLRP3 inflammasome pathway in the immunothrombotic pathogenesis of COVID-19. We also summarize current efforts to target the NLRP3 inflammasome pathway in COVID-19, and discuss challenges, unmet gaps, and the therapeutic potential that inflammasome-targeted strategies may provide for inflammation-driven thrombotic disorders including COVID-19.
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