Hypothesis

NAD+ decline forms a spatially organized gradient emanating from senescent cell foci, and this gradient functions as a signaling cue that reprograms chromatin in neighboring cells via ADP‑ribosylation of lamina‑associated proteins, thereby locking those cells into a low‑energy, low‑identity state that reinforces tissue aging.

Mechanistic Basis

Senescent cells secrete SASP factors that up‑regulate CD38 in adjacent cells, increasing NAD+ consumption and creating a local NAD+ sink. Falling NAD+ reduces SIRT1 activity, but more importantly, it limits the substrate for nuclear ARTs (ADP‑ribosyltransferases) that modify lamin A/C and lamin B1. When NAD+ is scarce, these lamins become hypo‑ADP‑ribosylated, leading to tighter lamina‑chromatin interactions, reduced accessibility of regeneration‑promoting enhancers, and a shift toward a heterochromatic state. This chromatin change lowers expression of NAD+ biosynthetic genes (NAMPT, NMNAT) and reinforces the sink—a positive feedback loop that spreads the NAD+ low zone outward.

Testable Predictions

1. In aged mouse liver or kidney, high‑resolution spatial transcriptomics will reveal a radial decrease in NAMPT/NMNAT transcripts and a reciprocal increase in CD38/PARP transcripts centered on p16^Ink4a^‑positive senescent cells.
2. Artificially elevating NAD+ locally (e.g., with NMN‑loaded liposomes targeted to senescent cell neighborhoods) will restore lamin ADP‑ribosylation, open chromatin at regenerative loci, and increase proliferative markers in the surrounding tissue.
3. Genetic ablation of CD38 specifically in senescent cells will flatten the NAD+ gradient, prevent lamina hypo‑ADP‑ribosylation, and attenuate age‑related functional decline without altering global NAD+ levels.

Potential Experiments

• Perform Xenium spatial transcriptomics on 24‑month‑old mouse tissue, staining for p16^Ink4a^, NAMPT, NMNAT1‑3, CD38, PARP1, and lamin A/C; quantify expression gradients using radial binning.
• Use a genetically encoded NAD+ sensor (SoNar) injected into the same tissue to correlate metabolic readouts with transcriptomic maps.
• Apply senescent‑cell‑specific CD38 knockout (p16‑Cre;Cd38^fl/fl^) mice and compare lamina ADP‑ribosylation (via ADP‑ribose immunoblotting of nuclear extracts) and regeneration after partial hepatectomy to wild‑type controls.
• Rescue experiments: inject NMN‑PEG‑liposomes functionalized with a senescent‑cell‑homing peptide (e.g., targeting uPAR) and measure changes in chromatin accessibility (ATAC‑seq) and functional outcomes.

If the predicted NAD+ gradients and their chromatin effects are absent, or if manipulating CD38/NAD+ does not alter lamina ADP‑ribosylation and regeneration, the hypothesis would be falsified.