Age-linked Xi escape in microglia drives NAD+ biosynthesis, enhancing glial metabolic support for neurons and tempering inflammasome activation, thereby contributing to the female longevity advantage.

Mechanistic Basis

Aging promotes epigenetic remodeling of the inactive X chromosome (Xi) in female glial cells, increasing escape of specific loci {1}. Among these escapees are genes governing NAD+ salvage, notably Nampt and Napsa, which are enriched for DNA repair and immune regulation functions {2}. Chromatin accessibility gains at these loci are mediated by the histone demethylase KDM6B, whose expression rises with age in female microglia but not in male microglia due to the absence of a second X allele to buffer dosage fluctuations. Elevated Nampt activity raises intracellular NAD+, fueling SIRT1-dependent deacetylation of NF‑κB and NLRP3 components, which dampens pro-inflammatory cytokine release and sustains microglial phagocytic efficiency. The resulting NAD+ surplus also shuttles to neighboring neurons via astrocytic lactate‑NAD+ coupling, bolstering neuronal SIRT3 activity and mitochondrial resilience against oxidative stress.

Testable Predictions

1. In aged mouse hippocampus, female microglia will show ~2-fold higher Nampt protein and NAD+ levels compared with male microglia; this difference will be abolished in female mice carrying a conditional knockout of Kdm6b in Cx3cr1-positive cells.
2. Pharmacological inhibition of Nampt (using FK866) in aged female mice will reduce microglial NAD+ to male-like levels and recapitulate male-typical increases in hippocampal IL-1β and cognitive decline in Morris water maze performance.
3. Overexpression of Nampt specifically in male microglia will elevate NAD+, suppress inflammasome signaling, and extend male median lifespan by at least 10 % without altering gonadal hormone levels.
4. Single-cell RNA-seq of aging human prefrontal cortex will reveal a sex-biased increase in NAMPT transcript escaping XI in microglial clusters, correlating with lower expression of NLRP3 and IL1B in females.

Potential Falsification

If aged female microglia do not exhibit higher Nampt or NAD+ relative to males, or if Kdm6b loss does not diminish the sex difference, the epigenetic‑driven Xi escape hypothesis for NAD+ biosynthesis loses support. Likewise, if Nampt manipulation fails to alter inflammasome activity or cognitive outcomes in either sex, the proposed mechanistic link between X‑chromosome dosage, NAD+ metabolism, and neuroimmune aging would be refuted.