Basic Science and Pathogenesis.

Background: Host commensal gut microbes are shown to be crucial for microglial maturation, and functions that involve innate immune responses to maintain brain homeostasis. Sex has a crucial role in the incidence of neurological diseases with females showing higher progression of AD compared with males. Transcriptomics has been a powerful tool for the characterization of microglial phenotypes however, there is a large gap in relating to their functional protein abundances. Here, we generated 'APPPS1-21-CD11br' a translating ribosome affinity purification (TRAP) mice model and assessed the influence of gut microbiome on microglial protein networks during their phenotypic transition in a sex-specific manner.

Methods: APPPS1-21 mice were crossed with WT-CD11br mice wherein a CD11b-promoter drives expression of ribosomal protein L10a (RpL10) that is fused to FLAG/EGFP at the amino-terminus to generate APPPS1-21-CD11br transgenic mice. Six groups of mice that included WT-CD11br, antibiotic (ABX) or vehicle-treated APPPS1-21-CD11br male and female were sacrificed at 7-weeks of age (n = 15/group) and used for immunoprecipitation of microglial polysomes from cortical homogenates using anti-FLAG antibody. Liquid chromatography coupled to tandem-mass-spectrometry (LC-MSMS) and label-free quantification was used to identify newly synthesized peptides extracted from polysomes.

Results: Proteomic analysis reveal that Aβ-induced microglial activation resulted in increased FcgR-mediated-phagocytosis and actin organization in male and female AD mice respectively. ABX-treatment resulted in substantial remodeling of epigenetic landscape, leading to a metabolic shift that accommodates the increased bioenergetic and biosynthetic demands associated with microglial polarization in a sex-specific manner. While microglia in ABX-treated male mice exhibited a metabolic shift towards ketogenesis to promote neuroprotective phenotype mediating lysosomal Aβ clearance, microglia in ABX-treated female mice exhibited persistent mitochondrial dysfunction and impaired clearance associated with inflammatory phenotypes. Targeted metabolomics show sex-specific changes in immunomodulatory gut metabolites mediated by ABX-treatment.

Conclusions: Our studies provide first snapshot of dynamic translational state of microglia in cerebral Aβ-amyloidosis models with an altered gut-microbiome. Gut metabolites can support microglial metabolic plasticity to modulate immune responses and amyloid clearance in a sex-specific manner. Reprogramming of microglial metabolism could be a promising strategy for restoring their physiological functions in a manner that might halt the progression of AD.