Hypothesis

Chronic activation of the hexosamine biosynthetic pathway (HBP) generates excess UDP‑GlcNAc, driving O‑GlcNAcylation of nuclear lamina proteins that mimics a perpetual nutrient‑abundance signal. This signal sustains mTORC1 activity and suppresses autophagy, accelerating aging. Inhibiting mTORC1 with rapamycin or lowering HBP flux (e.g., with GFAT inhibitors) both reduce this aberrant O‑GlcNAcylation, thereby extending lifespan not by repairing damage but by correcting a misinterpreted metabolic cue.

Mechanistic Basis

• HBP flux supplies UDP‑GlcNAc, the substrate for O‑GlcNAc transferase (OGT). OGT modifies lamin A/C and associated chromatin regulators, altering lamina stiffness and heterochromatin organization OGT deletion reduces mTORC1; OGA knockout enhances mTORC1.
• Persistent O‑GlcNAcylation of lamin A/C impairs its phosphorylation cycle, leading to nuclear envelope blebbing, DNA‑damage signaling, and inflammaging—phenotypes also seen with RagC‑mediated mTORC1 activation activating mTORC1 via RagC mutants shortens mouse lifespan ~30% via inflammaging/senescence.
• mTORC1 inhibition lowers O‑GlcNAc levels indirectly by decreasing HBP flux through reduced glycolysis and glutamine uptake, while direct HBP inhibition (e.g., azaserine) lowers UDP‑GlcNAc independent of mTORC1 high glucose impairs β‑cell calcium responses via HBP metabolism; azaserine prevents this; glucosamine recapitulates it. Thus, both interventions converge on a common downstream effector: lamina‑associated O‑GlcNAcylation.

Testable Predictions

1. In wild‑type mice, chronic low‑dose azaserine (GFAT inhibitor) will reduce O‑GlcNAcylation of lamin A/C in heart and liver to the same extent as rapamycin treatment, without altering phospho‑S6K (a mTORC1 read‑out).
2. Mice lacking OGT specifically in cardiomyocytes will show extended lifespan comparable to rapamycin‑treated wild‑type mice, and combining cardiomyocyte‑specific OGT knockout with rapamycin will not produce additive lifespan extension.
3. Expressing a non‑O‑GlcNAcylatable lamin A/C mutant (serine‑to‑alanine at major O‑GlcNAc sites) in a progeroid model will rescue nuclear shape abnormalities and extend lifespan, mimicking the effect of HBP inhibition.
4. Metabolomic profiling will reveal that azaserine lowers UDP‑GlcNAc levels in tissues while leaving intracellular ATP and amino‑acid pools unchanged, confirming that the lifespan effect stems from altered protein modification rather than energy stress.

Experimental Design

• Treatment groups: (i) vehicle, (ii) rapamycin (14 ppm diet), (iii) azaserine (50 mg/kg i.p. twice weekly), (iv) rapamycin + azaserine, (v) cardiomyocyte‑specific OGT knockout, (vi) OGT knockout + rapamycin.
• Readouts: lifespan monitoring, O‑GlcNAc immunoblotting of lamin A/C, immunofluorescence for nuclear envelope morphology, phospho‑S6K western blot, circulating cytokines (IL‑6, TNF‑α) as inflammaging markers, and UDP‑GlcNAc quantification via LC‑MS.
• Statistical analysis: Kaplan‑Meier survival curves with log‑rank test; two‑way ANOVA for biochemical endpoints; significance set at p<0.05.

Potential Outcomes

If the hypothesis is correct, azaserine will phenocopy rapamycin’s reduction of lamin O‑GlcNAcylation and extension of lifespan, and the combination will not be additive, indicating a shared downstream mechanism. Conversely, if azaserine fails to affect lamin O‑GlcNAcylation or lifespan, or if combining it with rapamycin yields additive benefits, the hypothesis would be falsified, suggesting that mTOR inhibition acts through distinct or additional pathways beyond HBP‑mediated O‑GlcNAcylation.

This framework directly links nutrient‑sensing deception to a concrete structural nuclear defect, offering a falsifiable route to test whether longevity interventions are merely imposing a physiological ‘hardship’ signal on aging cells.