Key Aging-Associated Alterations in Primary Microglia Response to Beta-Amyloid Stimulation

Abstract

Alzheimer’s disease (AD) is characterized by a progressive cognitive decline and believed to be driven by the self-aggregation of amyloid-β (Aβ) peptide into oligomers and fibrils that accumulate as senile plaques. It is widely accepted that microglia-mediated inflammation is a significant contributor to disease pathogenesis; however, different microglia phenotypes were identified along AD progression and excessive Aβ production was shown to dysregulate cell function. As so, the contribution of microglia to AD pathogenesis remains to be elucidated. In this study, we wondered if isolated microglia cultured for 16 days in vitro (DIV) would react differentially from the 2 DIV cells upon treatment with 1000 nM Aβ1–42 for 24 h. No changes in cell viability were observed and morphometric alterations associated to microglia activation, such as volume increase and process shortening, were obvious in 2 DIV microglia, but less evident in 16 DIV cells. These cells showed lower phagocytic, migration and autophagic properties after Aβ treatment than the 2 DIV cultured microglia. Reduced phagocytosis may derive from increased CD33 expression, reduced triggering receptor expressed on myeloid cells 2 (TREM2) and milk fat globule-EGF factor 8 protein (MFG-E8) levels, which were mainly observed in 16 DIV cells. Activation of inflammatory mediators, such as high mobility group box 1 (HMGB1) and pro-inflammatory cytokines, as well as increased expression of Toll-like receptor 2 (TLR2), TLR4 and fractalkine/CX3C chemokine receptor 1 (CX3CR1) cell surface receptors were prominent in 2 DIV microglia, while elevation of matrix metalloproteinase 9 (MMP9) was marked in 16 DIV cells. Increased senescence-associated β-galactosidase (SA-β-gal) and upregulated miR-146a expression that were observed in 16 DIV cells showed to increase by Aβ in 2 DIV microglia. Additionally, Aβ downregulated miR-155 and miR-124, and reduced the CD11b+ subpopulation in 2 DIV microglia, while increased the number of CD86+ cells in 16 DIV microglia. Simultaneous M1 and M2 markers were found after Aβ treatment, but at lower expression in the in vitro aged microglia. Data show key-aging associated responses by microglia when incubated with Aβ, with a loss of reactivity from the 2 DIV to the 16 DIV cells, which course with a reduced phagocytosis, migration and lower expression of inflammatory miRNAs. These findings help to improve our understanding on the heterogeneous responses that microglia can have along the progression of AD disease and imply that therapeutic approaches may differ from early to late stages.

CX3CR1 modulates cognitive dysfunction induced by sleep deprivation

“…We found a significant decrease in the level of TREM2 in the hippocampus after sleep deprivation in CX3CR1 −/− mice, which had a higher dendritic spine density. This result is consistent with a previous study that documents that the decreased expression of TREM2 is related to lower phagocytic properties using cell culture system [47]. Similar to TREM2, C3, another factor involved in synaptic pruning-complement proteins, was also reduced in CX3CR1 −/− mice after the stimuli of sleep deprivation.…”

Inflammation: the link between comorbidities, genetics, and Alzheimer’s disease

“…The AD-risk CD33 rs3865444 (C) allele is associated with increased TREM2 surface expression in monocytes, compared to rs3865444 (A), whereas blocking CD33 signaling leads to a reduction in TREM2 surface expression [ 200 ]. Measuring the levels of TREM2 and CD33 expressed by microglia at different time points following Aβ treatment revealed that TREM2 is expressed at a higher level initially, correlating with increased microglial phagocytosis [ 201 ]. Later time points in these experiments revealed that TREM2 expression subsequently decreases, whereas CD33 expression increases in association with diminished phagocytic capabilities.…”

Microglial phagocytosis in aging and Alzheimer's disease

“…Using this system, the authors found that cells kept longer in culture exhibited lower phagocytic properties, including that of amyloid beta. This change in phenotype could be possibly through decreased expression of TREM2 and MFG‐E8 or increased CD33 levels (Caldeira et al, ). Other studies have assessed the phagocytic capabilities of aged microglia to ingest myelin, which is targeted by the immune system in MS. For example, Hendrickx et al isolated myelin from a number of MS patients and assessed its uptake by human microglia and THP‐1 macrophages (Hendrickx, Schuurman, van Draanen, Hamann, & Huitinga, ).…”