Microglial autophagy‐mediated clearance of amyloid‐beta plaques is dysfunctional in Alzheimer’s disease mice

Abstract Background Autophagy is a conserved homeostatic cellular process for clearing dysfunctional organelles, pathogens, and protein aggregates. Neurons in patients with Alzheimer’s disease (AD) exhibit accumulation of immature autophagic vacuoles, which is thought to contribute to amyloid‐beta (Aβ) plaque build‐up and neurotoxicity. However, it is still unclear what causes autophagic dysfunction in neurons and how it contributes to disease progression. Moreover, the role of autophagy in microglia, the innate immune cell and phagocyte of the CNS, has not been explored. We hypothesized that autophagy in microglia in AD is dysfunctional, leading to the inability of microglia to effectively clear Aβ plaque protein aggregates. Method We purify microglia from adult mice of an AD mouse model (5xFAD – 5 familial mutations of AD). We are utilizing a novel method developed in our lab to quantify Aβ internalization and degradation which does not require radioactive or fluorophore labeling. Additionally, we are able to visualize Aβ autophagic trafficking utilizing microglia from a mouse with GFP+ LC3, a key autophagy protein involved in the elongation of the autophagosome. Lastly, to understand the components at play in autophagy processing of Aβ in microglia, we are able to reduce expression of various autophagy proteins and microRNAs which target autophagy utilizing a magnetic nanoparticle transfection reagent. Result Microglia from AD mice and age‐matched wild‐type littermates internalize the same amount of Aβ, but AD microglia do not efficiently degrade Aβ in vitro . When stimulating autophagy using rapamycin, degradation of Aβ in AD microglia improves. We are able to see that microglia traffic Aβ to an LC3+ autophagosome, and that this trafficking and subsequent Aβ‐degradation is dependent on the expression of various autophagy molecules. Furthermore, we have reduced expression of microRNAs which downregulate autophagy and are overexpressed in AD, and recovered the degradative capacity of AD microglia in vitro . Conclusion Together these results indicate that Aβ is degraded via autophagy and that autophagy is defective in AD microglia. Our projects in the lab are seeking to establish additional consequences beyond failure of Aβ‐clearance, such as inability to regulate expression of inflammatory cytokines, and mechanisms behind dysregulated autophagy, such as epigenetic changes.