Hypothesis

Programmed NAD+ decline during aging is not a passive byproduct of metabolic wear but an active downstream effect of reduced GH/IGF-1 signaling. Specifically, low IGF-1 activity increases hepatic FOXO1-driven transcription of CD38, raising NAD+ consumption and enforcing a hypometabolic, longevity‑promoting state.

Mechanistic Rationale

• Reduced GH/IGF-1 signaling activates FOXO transcription factors, which have been shown to bind the Cd38 promoter in liver cells1.
• Elevated CD38 accelerates NAD+ breakdown, lowering substrate availability for sirtuins and PARPs, thereby dampening anabolic processes and reinforcing stress‑resistance pathways2.
• This creates a feedback loop: low NAD+ further stabilizes FOXO activity by inhibiting AKT signaling, maintaining high CD38 expression3.
• The loop aligns with the observed parallel decline of GH/IGF-1 and NAD+ in natural aging, whereas caloric restriction uncouples them by sustaining NAD+ via NAMPT upregulation despite low IGF-12.

Testable Predictions

1. Liver‑specific Cd38 knockdown in IGF-1R‑deficient mice will restore NAD+ levels to youth‑like concentrations without altering circulating IGF-1.
2. Restored NAD+ in these mice will prematurely reactivate mTORC1 signaling, decrease stress‑resistance markers, and shorten lifespan relative to IGF-1R‑deficient controls.
3. Conversely, hepatic overexpression of CD38 in wild‑type mice will mimic the NAD+ low, IGF‑1 low phenotype, extending healthspan even when GH/IGF-1 signaling is intact.

Experimental Approach

• Generate three mouse cohorts: (a) liver‑specific IGF-1R knockout (IGF-1R^LKO), (b) IGF-1R^LKO with AAV‑mediated shRNA against Cd38, (c) wild‑type with AAV‑mediated CD38 overexpression.
• Measure hepatic and serum NAD+, NAD+/NADH ratio, sirtuin activity, and PARP activity at 3, 12, and 24 months.
• Assess phenotypic readouts: body composition, insulin tolerance, senescence-associated β‑galactosidase, cytokine profile, tumor incidence, and survival.
• Perform RNA‑seq to verify FOXO1 binding to Cd38 promoter and downstream metabolic gene expression.

Potential Outcomes and Falsifiability

• If CD38 knockdown fails to raise NAD+ or does not alter lifespan in IGF-1R^LKO mice, the hypothesis that CD38 mediates the NAD+ decline is falsified.
• If CD38 overexpression does not reproduce the low‑NAD+/low‑IGF‑1 phenotype or extend healthspan, the proposed directionality of the loop is invalid.
• Successful validation would position CD38 as a programmable ‘metabolic brake’ that translates attenuated somatotropic signaling into a deliberate NAD+ budget cut, supporting the idea that aging reflects a strategic downgrade of cellular ambitions rather than mere damage accumulation.