Pharmacological Ablation of Astrocytes Reduces Aβ Degradation and Synaptic Connectivity in an Ex Vivo Model of Alzheimer’s Disease

BackgroundAstrocytes provide a vital support to neurons in normal and pathological conditions. In Alzheimer's disease (AD) brains, reactive astrocytes have been found surrounding amyloid plaques, forming an astrocytic scar. However, their role and potential mechanisms whereby they affect neuroinflammation, amyloid pathology, and synaptic density in AD remain unclear.MethodsTo explore the role of astrocytes on A beta pathology and neuroinflammatory markers, we pharmacologically ablated them in organotypic brain culture slices (OBCSs) from 5XFAD mouse model of AD and wild-type (WT) littermates with the selective astrocytic toxin L-alpha-aminoadipate (L-AAA). To examine the effects on synaptic circuitry, we measured dendritic spine number and size in OBCSs from Thy-1-GFP transgenic mice incubated with synthetic A beta 42 or double transgenics Thy-1-GFP/5XFAD mice treated with LAAA or vehicle for 24 h.ResultsTreatment of OBCSs with L-AAA resulted in an increased expression of pro-inflammatory cytokine IL-6 in conditioned media of WTs and 5XFAD slices, associated with changes in microglia morphology but not in density. The profile of inflammatory markers following astrocytic loss was different in WT and transgenic cultures, showing reductions in inflammatory mediators produced in astrocytes only in WT sections. In addition, pharmacological ablation of astrocytes led to an increase in A beta levels in homogenates of OBCS from 5XFAD mice compared with vehicle controls, with reduced enzymatic degradation of A beta due to lower neprilysin and insulin-degrading enzyme (IDE) expression. Furthermore, OBSCs from wild-type mice treated with L-AAA and synthetic amyloid presented 56% higher levels of A beta in culture media compared to sections treated with A beta alone, concomitant with reduced expression of IDE in culture medium, suggesting that astrocytes contribute to A beta clearance and degradation. Quantification of hippocampal dendritic spines revealed a reduction in their density following L-AAA treatment in all groups analyzed. In addition, pharmacological ablation of astrocytes resulted in a decrease in spine size in 5XFAD OBCSs but not in OBCSs from WT treated with synthetic A beta compared to vehicle control.ConclusionsAstrocytes play a protective role in AD by aiding A beta clearance and supporting synaptic plasticityWe would like to thank Maria Tortora for her advice on the GFP-Thy1 mice. We thank Imperial College London for the Presidential Fellowship to ND and the Dunhill Foundation Medical Trust (grant reference R591/0717) for funding Dr Bibiana Mota’s salar