Hypothesis

We propose that BAG3-dependent chaperone‑assisted selective autophagy (CASA) directly degrades NAD+-consuming enzymes PARP1 and CD38, thereby modulating cellular NAD+ levels. In aging tissues, increased BAG3 expression attempts to limit NAD+ loss by removing these enzymes; when BAG3 activity fails, NAD+ depletion accelerates, linking proteostasis to redox metabolism.

Mechanistic Rationale

• NAD+ decline with age is driven by upregulated PARP1 (DNA damage response) and CD38 (NAD+ glycohydrolase) activity [1]
• BAG3 expression rises with age and stress via the BAG1‑BAG3 switch and Nrf2 signaling, promoting CASA-mediated autophagy of damaged proteins [2,3,4]
• Both PARP1 and CD38 are cytosolic/nuclear proteins that can be recognized by HSP70‑HSPB8‑BAG3 complexes and targeted for p62‑dependent autophagic clearance, a principle demonstrated for other HSP70 clients [5]
• Nrf2, which drives BAG3 transcription, is activated by oxidative stress and DNA damage—both consequences of NAD+ depletion—creating a potential feedback loop where NAD+ loss triggers BAG3 up‑regulation, which then attempts to restore NAD+ by degrading PARP1/CD38.

Testable Predictions

1. In aged mouse muscle or heart, immunoprecipitation of BAG3 complexes will show increased association with PARP1 and CD38 compared with young tissue.
2. Genetic or pharmacological inhibition of BAG3 in aged animals will lead to higher PARP1/CD38 protein levels, lower NAD+ concentrations, and exacerbated DNA damage markers.
3. Overexpression of BAG3 in aged tissue will reduce PARP1/CD38 abundance, elevate NAD+ levels, and improve functional outcomes (e.g., grip strength, ejection fraction).
4. Inhibition of autophagy (e.g., with chloroquine) will block the BAG3‑mediated decline of PARP1/CD38, confirming lysosomal dependence.

Experimental Approach

• Use co‑immunoprecipitation and proximity ligation assays to detect BAG3‑PARP1/CD38 interactions in young vs. old murine tissues.
• Measure PARP1 and CD38 protein turnover using cycloheximide chase assays in primary cardiomyocytes with BAG3 siRNA or CRISPR activation.
• Quantify NAD+ levels via enzymatic cycling assay and assess DNA damage (γH2AX) under BAG3 manipulation.
• Perform functional assays: treadmill endurance for muscle, echocardiography for heart.
• Include controls: Nrf2 knockout to test dependence of BAG3 induction on oxidative stress signaling.

Potential Implications

If validated, this hypothesis reframes NAD+ decline not as a passive consequence of wear‑and‑tear but as a node in a proteostatic‑metabolic feedback circuit. Enhancing BAG3‑mediated turnover of NAD+‑consuming enzymes could represent a strategy to preserve NAD+ without supplementation, linking chaperone‑autophagy pathways to metabolic health in aging.

References

[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC7442590/ [2] https://pubmed.ncbi.nlm.nih.gov/21681022/ [3] https://www.frontiersin.org/journals/molecular-neuroscience/articles/10.3389/fnmol.2017.00177/full [4] https://pmc.ncbi.nlm.nih.gov/articles/PMC5801049/ [5] https://www.ahajournals.org/doi/full/10.1161/CIRCULATIONAHA.121.056589