Hypothesis

Female advantage in lifespan stems from biallelic expression of the X‑linked Klotho gene that escapes X‑inactivation, producing a 1.3‑1.5‑fold increase in soluble Klotho (sKl). This excess sKl does more than blunt circulating FGF23; it reshapes FGF23 receptor signaling bias in kidney distal tubules and brain microglia, favoring FGFR1c‑mediated MAPK pathways over the calcineurin‑NFAT axis that drives hypertrophy and neuroinflammation. Consequently, XX individuals maintain lower tissue FGF23 activity and preserve Wnt/β‑catenin‑dependent anti‑inflammatory programs across the kidney‑brain‑bone axis, while XY individuals, with a single Klotho allele, are biased toward pathogenic FGF23/FGFR4 signaling, accelerating fibrosis, cardiac remodeling, and cognitive decline.

Testable predictions

1. Allelic expression analysis of Klotho in male vs female mouse kidney distal tubules and cortical microglia will show ~2‑fold higher total Klotho mRNA in females, with allele‑specific RNA‑seq detecting expression from both X chromosomes in females but only the paternal X in males (due to imprinting escape).
2. Circulating sKl levels will correlate with the ratio of phospho‑ERK1/2 to phospho‑NFATc3 in isolated kidney tubules and microglia, a ratio that is significantly higher in females.
3. CRISPR‑mediated deletion of one Klotho allele in female mice will reduce sKl to male‑like levels, shift the ERK/NFAT ratio toward NFAT dominance, and recapitulate male‑typical aging phenotypes (increased cardiac fibrosis, cortical microglial IL‑1β elevation, shortened lifespan) without altering sex hormone levels.
4. Conversely, transgenic overexpression of Klotho in male mice to achieve female‑level sKl will restore the ERK/NFAT bias, suppress FGF23‑driven cardiac hypertrophy, and improve performance in hippocampal‑dependent memory tasks.

Mechanistic insight Klotho functions as a co‑factor that sterically hinders FGFR4 recruitment while promoting FGFR1c‑FGFR2c heterodimerization. In the presence of high sKl, FGF23 preferentially activates FGFR1c‑mediated MAPK signaling, which phosphorylates and inhibits GSK‑3β, thereby stabilizing β‑catenin and transcription of anti‑inflammatory genes (e.g., IL‑10, Nrf2 targets). Low sKl shifts the balance toward FGFR4, which couples to calcineurin‑NFAT, driving pro‑fibrotic and pro‑inflammatory transcription. This dual‑receptor bias provides a mechanistic link between X‑dosage of Klotho and sex‑dimorphic trajectories of kidney fibrosis, cardiac remodeling, and neuroinflammation.

Falsifiability If allele‑specific expression shows no biallelic Klotho expression in female tissues, or if manipulating Klotho dosage does not shift the ERK/NFAT ratio or affect aging phenotypes independently of sex hormones, the hypothesis would be refuted.