Hypothesis

Age-associated up-regulation of CD38 not only consumes intracellular NAD+ but also releases NAD+ metabolites into the extracellular space, where they act as competitive inhibitors of SIRT2 deacetylase activity on cytosolic tubulin. Reduced SIRT2-mediated deacetylation stabilizes acetyl-tubulin, making microtubules more rigid and impairing mechanotransduction pathways that normally restrain NF-kB signaling. Consequently, cells experience a feed-forward loop: inflammation drives CD38 expression, CD38-derived NAD+ loss suppresses SIRT2, stiffened cytoskeleton amplifies inflammatory signaling, and further CD38 induction ensues.

Mechanistic Basis

CD38 is a bifunctional enzyme that hydrolyzes NAD+ to ADP-ribose and cyclic ADP-ribose, both of which can be exported via connexin hemichannels or ATP-binding cassette transporters [2]. Extracellular ADP-ribose has been shown to inhibit sirtuins by occupying the NAD+-binding pocket without supporting catalysis [7]. SIRT2 preferentially deacetylates alpha-tubulin at Lys40, a modification that promotes microtubule flexibility and facilitates the turnover of focal adhesions [5]. When SIRT2 activity falls, acetyl-tubulin accumulates, microtubules become hyper-stable, and cells display reduced tensile strain sensing. Mechanical strain normally activates the tumor suppressor LATS1/2 of the Hippo pathway, which phosphorylates and retains YAP in the cytoplasm; loss of strain leads to nuclear YAP, which cooperates with NF-kB to boost transcription of pro-inflammatory cytokines [4]. Thus, CD38-driven NAD+ loss can mechanistically link metabolic decline to a cytoskeletal-mediated amplification of inflammation.

Testable Predictions

1. In aged tissues, extracellular ADP-ribose concentrations will correlate positively with CD38 expression and inversely with SIRT2 activity measured by acetyl-tubulin levels.
2. Pharmacological blockade of CD38 ecto-enzymatic activity (using antibodies or small-molecule inhibitors) will lower extracellular ADP-ribose, restore SIRT2-dependent tubulin deacetylation, and reduce SASP secretion from fibroblasts independent of changes in immune-cell infiltration.
3. Genetic overexpression of a NAD+-insensitive SIRT2 mutant (e.g., SIRT2-H187Y) in murine muscle will rescue microtubule dynamicity, diminish YAP nuclear localization, and extend healthspan even when CD38 remains elevated.

Experimental Approach

• Measure extracellular ADP-ribose in plasma and interstitial fluid of young vs. old mice using LC-MS/MS; correlate with tissue CD38 mRNA (qPCR) and SIRT2 activity (immunoblot for acetyl-tubulin).
• Treat aged mice with a CD38-specific inhibitor (e.g., 78c) or control for 8 weeks; assess SASP cytokines (IL-6, TNF-alpha) in serum, microtubule acetylation (immunofluorescence), and YAP localization.
• Generate a muscle-specific SIRT2-H187Y knock-in line; cross with aged wild-type and CD38-overexpressing mice; evaluate grip strength, treadmill endurance, and histology for fibrosis.
• Use traction force microscopy on primary fibroblasts exposed to recombinant ADP-ribose to quantify changes in substrate stiffness sensing and NF-kB reporter activity.

Potential Implications

If validated, this hypothesis reframes NAD+ decline as not merely an energy shortage but as a signaling imbalance that remodels the cytoskeleton to amplify inflammatory programs. It suggests that targeting the extracellular NAD+ metabolome—or bolstering SIRT2-dependent microtubule dynamics—could break the inflammation-CD38 loop and improve tissue function without requiring global NAD+ supplementation, thereby avoiding the risks of masking underlying DNA damage observed in high-dose NR/NMN regimens [6,7]