Microglial senescence contributes to female-biased neuroinflammation in the aging mouse hippocampus: implications for Alzheimer’s disease

Abstract

Background Microglia, the brain’s principal immune cells, have been implicated in the pathogenesis of Alzheimer’s disease (AD), a condition shown to affect more females than males. Although sex differences in microglial function and transcriptomic programming have been described across development and in disease models of AD, no studies have comprehensively identified the sex divergences that emerge in the aging mouse hippocampus. Further, existing models of AD generally develop pathology (amyloid plaques and tau tangles) early in life and fail to recapitulate the aged brain environment associated with late-onset AD. Here, we examined and compared transcriptomic and translatomic sex effects in young and old murine hippocampal microglia. Methods Hippocampal tissue from C57BL6/N and microglial NuTRAP mice of both sexes were collected at young (5–6 month-old [mo]) and old (22–25 mo) ages. Cell sorting and affinity purification techniques were used to isolate the microglial transcriptome and translatome for RNA-sequencing and differential expression analyses. Flow cytometry, qPCR, and imaging approaches were used to confirm the transcriptomic and translatomic findings. Results There were marginal sex differences identified in the young hippocampal microglia, with most differentially expressed genes (DEGs) restricted to the sex chromosomes. Both sex chromosomally and autosomally encoded sex differences emerged with aging. These sex DEGs identified at old age were primarily female-biased and enriched in senescent and disease-associated microglial signatures. Normalized gene expression values can be accessed through a searchable web interface (https://neuroepigenomics.omrf.org/). Pathway analyses identified upstream regulators induced to a greater extent in females than in males, including inflammatory mediators IFNG, TNF, and IL1B, as well as AD-risk genes TREM2 and APP. Conclusions These data suggest that female microglia adopt disease-associated and senescent phenotypes in the aging mouse hippocampus, even in the absence of disease pathology, to a greater extent than males. This sexually divergent microglial phenotype may explain the difference in susceptibility and disease progression in the case of AD pathology. Future studies will need to explore sex differences in microglial heterogeneity in response to AD pathology and determine how sex-specific regulators (i.e., sex chromosomal or hormonal) elicit these sex effects.

Consistent specificity and efficiency of tamoxifen-mediated cre induction across ages

“…Comparing old mice to young mice, with both ages of induction, we see an enrichment for reactivated microglial subtypes, such as DAMs, ARMs, MgNDs, and WAMs, while also seeing a depletion of homeostatic microglia. This is consistent with previous findings by our labs and others 40,63,64 , and there is no differential effect due to age of induction.…”

“…In addition to microglial phenotypic changes, microglial senescence is emerging as a contributing factor to inflammaging 65,66 . Evidence demonstrates that senescent microglia contribute to cognitive decline and white matter degradation 67 , and this pattern is observed in multiple different microglial subtypes and disease progressions 40,63,68 . We used similar GSEA approaches with senescent cell marker lists ( Supplemental Table 3 ) to detect enrichment of senescent cells in our aged mice.…”

“…have an impact on cre efficiency and specificity, and ability to detect age-related translatome changes in Cx3cr1-cre;NuTRAP mice 32 . We have used this experimental design of TRAP and INTACT and a floxed NuTRAP mouse model to better understand the transcriptional and epigenetic changes with age in various cell types 38–40 . The key findings from this study are that age of Tam induction had no significant effect on the efficiency or specificity of cre recombinase induction, isolation of microglial specific translatome profiles, or the ability to identify age-related microglial translatomic alterations.…”

Selective agonism of GPR34 stimulates microglial uptake and clearance of amyloid β fibrils

“…In contrast, multiple mouse studies consistently revealed a significant reduction in GPR34 levels in aged microglia compared to young adults (SI Appendix, Fig. S6) (35)(36)(37), suggesting that an age-related decline in microglial GPR34 expression may be a conserved phenomenon across species.…”

Altered Metabolism and DAM-signatures in Female Brains and Microglia with Aging

“…In rodent models of AD, female microglia are more glycolytic, and less phagocytic than male mice (Guillot-Sestier et al, 2021). In addition, recent studies have shown sex-specific changes in senescence and disease associated microglia (DAM) signatures, which become more pronounced with aging (Ocanas et al, 2023). Therefore, microglia in aged females exhibit exacerbated metabolic reprogramming associated with AD pathology.…”