ABSTRACT
Repeated exposure to toll-like receptor 4 (TLR4) ligands, such as lipopolysaccharide (LPS), reduces responses of monocytes/macrophages to LPS (LPS/endotoxin tolerance). Microglial exposure to Aß deposits, a TLR4 ligand, may cause "Aß/LPS tolerance," leading to decreased Aß clearance. We demonstrated that microglial activation by LPS is diminished in Aß deposit-bearing 12-month-old model mice of Alzheimer's disease (AD), compared with non-AD mice and Aß deposit-free 2-month-old AD mice. Because miR-146a plays a predominant role in inducing TLR tolerance in macrophages and because miR-146a in extracellular vesicles (EVs) shed by inflammatory macrophages increases in circulation, we investigated potential roles of miR-146a and inflammatory EVs in inducing TLR tolerance in microglia and in altering expression of inflammatory AD risk genes. We found that miR-146a upregulation induces TLR tolerance and alters expression of inflammatory AD risk genes in response to LPS treatment in BV2 microglia. LPS brain injection altered expression of the AD risk genes in 12-month-old AD mice but not in non-AD littermates. EVs from inflammatory macrophages polarize BV2 microglia to M1 phenotype and induce TLR tolerance. Microglia exposed to Aß in the brain show reduced cytokine responses to systemic inflammation due to peripheral LPS injection, indicating TLR/Aß tolerance in microglia. Our results suggest that increased miR-146a induces microglial Aß/LPS tolerance and that circulating EVs shed by inflammatory macrophages contribute to microglial Aß/LPS tolerance, leading to reduced Aß clearance. Our study also suggests that altered expression of inflammatory AD risk genes may contribute to AD development via the same molecular mechanism underlying LPS tolerance.
Subject(s)
Alzheimer Disease/genetics , Exosomes/genetics , MicroRNAs/metabolism , Toll-Like Receptor 4/metabolism , Alzheimer Disease/metabolism , Amyloid beta-Peptides/genetics , Amyloid beta-Peptides/toxicity , Animals , Brain/cytology , Brain/drug effects , Brain/metabolism , Exosomes/metabolism , Lipopolysaccharides/toxicity , Mice , Mice, Inbred C57BL , MicroRNAs/genetics , Microglia/drug effects , Microglia/metabolism , Presenilin-1/genetics , RAW 264.7 Cells , Signal TransductionABSTRACT
Parenteral immunization of transgenic mouse models of Alzheimer disease (AD) with synthetic amyloid beta-peptide (Abeta) prevented or reduced Abeta deposits and attenuated their memory and learning deficits. A clinical trial of immunization with synthetic Abeta, however, was halted due to brain inflammation, presumably induced by a toxic Abeta, T-cell- and/or Fc-mediated immune response. Another issue relating to such immunizations is that some AD patients may not be able to raise an adequate immune response to Abeta vaccination due to immunological tolerance or age-associated decline. Because peripheral administration of antibodies against Abeta also induced clearance of amyloid plaques in the model mice, injection of humanized Abeta antibodies has been proposed as a possible therapy for AD. By screening a human single-chain antibody (scFv) library for Abeta immunoreactivity, we have isolated a scFv that specifically reacts with oligomeric Abeta as well as amyloid plaques in the brain. The scFv inhibited Abeta amyloid fibril formation and Abeta-mediated cytotoxicity in vitro. We have tested the efficacy of the human scFv in a mouse model of AD (Tg2576 mice). Relative to control mice, injections of the scFv into the brain of Tg2576 mice reduced Abeta deposits. Because scFvs lack the Fc portion of the immunoglobulin molecule, human scFvs against Abeta may be useful to treat AD patients without eliciting brain inflammation.
