Loss of apolipoprotein E exacerbates microglial and astrocyte reactivity to sustained systemic glucocerebrosidase inhibition

Abstract

Background: Increased lipid storage is observed within the brains of patients with Alzheimer's disease (AD). Genetic risk factors for AD involved in lipid processing include APOE, ABCA7 and TREM2. Lipid transport between neurons and glia is induced by mitochondrial stress and NMDA-induced excitotoxicity in vitro and is dependent on glial apolipoprotein E (ApoE) expression. Therefore, this suggests that lipid transport between neurons and glia of the brain by ApoE and intracellular lipid processing are critical cellular adaptations to stress. This in turn suggests that insufficient ApoE activity may underpin the elevated lipid storage associated with neurodegenerative diseases. Method: To determine whether ApoE function is required for cellular adaptation to increased lipid load relevant to dementia, 3-month-old WT and ApoE-KO mice received daily intraperitoneal injections of the glucocerebrosidase inhibitor, CBE, for 18 d. Animals were sacrificed 1 d after the final dose and perfused with paraformaldehyde for immunohistochemistry, or were perfused with saline and dissected to isolate the cortex, hippocampus and substantia nigra for biochemical analysis by Western blot. Microglial and astrocyte reactivity to CBE were assessed by Iba1 and GFAP immunohistochemistry respectively. Result: Microglial reactivity was greater in the dentate gyrus, substantia nigra and cortical layer V in ApoE-KO mice after 18 d CBE, relative to WT. Loss of ApoE also elevated astrocyte reactivity in the dentate gyrus, substantia nigra and cortical layer V after 18 d CBE. Ongoing analyses into the molecular and cellular consequences of prolonged CBE include changes in lipid transport, tau phosphorylation, lysosome biogenesis, autophagy and proteostasis, to determine how loss of ApoE function may impact cellular adaptation to intracellular lipid formation resulting from sustained glucocerebrosidase inhibition. Conclusion:The exacerbated glial activation in ApoE-KO animals receiving CBE suggests that ApoE expression and function may, in part, determine the capacity of neuron-glia networks to overcome damaging stimuli. When ApoE function is insufficient, failed resolution of lipids may then lead to persistent damage and degeneration.Therefore, characterizing the consequences of limited lipid clearance on the adapta-