A mitochondrial-immune axis drives the transcriptomic transition from brain aging to Alzheimer's disease

Aging is the primary risk factor for Alzheimer's disease (AD), yet the molecular transitions linking normal brain aging to neurodegeneration remain poorly defined. Here, we performed integrative bulk transcriptomic analyses across a multi-region mouse aging atlas, a human aging-to-AD cohort, and an independent human AD validation dataset. Aging is associated with a progressive, region-specific increase in transcriptional perturbation, with the entorhinal cortex and choroid plexus showing the most pronounced age-associated remodeling. Females develop more extensive late-stage remodeling than males, characterized by stronger immune activation and greater suppression of mitochondrial metabolic pathways. Across cohorts, aging drives a coordinated shift toward immune activation and suppression of oxidative phosphorylation and respiratory-chain programs that is amplified in AD. Aged brains occupy an intermediate molecular state between young and AD conditions, supporting a continuum model. Together, our findings define a sex-modulated mitochondrial-immune axis linking normal aging to AD and highlight early immune-metabolic changes as potential intervention targets.