Elevated Expression of MiR-17 in Microglia of Alzheimer’s Disease Patients Abrogates Autophagy-Mediated Amyloid-β Degradation

Abstract

Autophagy is a proposed route of amyloid-β (Aβ) clearance by microglia that is halted in Alzheimer’s Disease (AD), though mechanisms underlying this dysfunction remain elusive. Here, primary microglia from adult AD (5xFAD) mice were utilized to demonstrate that 5xFAD microglia fail to degrade Aβ and express low levels of autophagy cargo receptor NBR1. In 5xFAD mouse brains, we show for the first time that AD microglia express elevated levels of microRNA cluster Mirc1/Mir17-92a, which is known to downregulate autophagy proteins. By in situ hybridization in post-mortem AD human tissue sections, we observed that the Mirc1/Mir17-92a cluster member miR-17 is also elevated in human AD microglia, specifically in the vicinity of Aβ deposits, compared to non-disease controls. We show that NBR1 expression is negatively correlated with expression of miR-17 in human AD microglia via immunohistopathologic staining in human AD brain tissue sections. We demonstrate in healthy microglia that autophagy cargo receptor NBR1 is required for Aβ degradation. Inhibiting elevated miR-17 in 5xFAD mouse microglia improves Aβ degradation, autophagy, and NBR1 puncta formation in vitro and improves NBR1 expression in vivo. These findings offer a mechanism behind dysfunctional autophagy in AD microglia which may be useful for therapeutic interventions aiming to improve autophagy function in AD.

Key Genes and Biochemical Networks in Various Brain Regions Affected in Alzheimer’s Disease

“…Among the identified miRNAs, hsa-mir-17-5p, hsa-mir-106a-5p and hsa-mir-373-3p were found to interact with robust DEGs of both pathways. Recently, the elevated levels of hsa-mir-17-5p have been reported in the human AD brain and the microglia of AD mice models, coupled with the down-regulation of autophagy markers, that consequently impaired Aβ clearance [59]. In contrast to our results, decreased serum level of hsa-mir-106a-5p has previously been observed in AD patients [60].…”

Caspase-4/11 exacerbates disease severity in SARS–CoV-2 infection by promoting inflammation and immunothrombosis

“…Corresponding secondary antibodies conjugated with horseradish peroxidase in combination with enhanced chemiluminescence reagent (Amersham, RPN2209) were used to visualize protein bands. Densitometry analyses were performed by normalizing target protein bands to their respective loading control (β-Actin) using ImageJ software as previously described ( 24 , 51 ).…”

“…Lungs were removed from infected mice and fixed in 10% formalin at RT. Sample preparation, processing, H&E staining, and semiquantitative slide evaluation using ordinal grading scales were performed as previously described ( 51 ). Lungs used for immunofluorescence (IF) staining and RNAscope ISH technique were embedded in optimal cutting temperature compound (OCT) and flash frozen, while lung tissue used for immunohistochemistry (IHC) was embedded in paraffin blocks.…”

“…IF staining of mouse lung sections was performed as previously described ( 51 ). Slides were washed three times for 15 min with PBS to remove residual OCT.…”

The Potential Role of miRNA-Regulated Autophagy in Alzheimer’s Disease

“…In the PC12 cell model of AD, the overexpression of let-7a can exacerbate the toxicity of Aβ1-40 to neurons by impaired autophagy through the inhibition of the PI3K/Akt/mTOR signaling pathway [ 80 ]. In addition, inhibiting the elevation of miR-17 in AD microglia can activate autophagy, accelerate the clearance of Aβ, and mitigate the progression of AD [ 81 ]. Mitochondrial function is also considered to be an important factor in the occurrence and development of AD.…”