NFATc4-dependent myofiber metabolic state directs senescent cell SASP phenotype in skeletal muscle

Hypothesis

Restoring NFATc4 activity in aged skeletal muscle myofibers shifts the metabolic milieu that senescent cells sense, converting their SASP from a proinflammatory, stem‑cell‑inhibitory state to a regenerative, pro‑myogenic profile.

Mechanistic Rationale

NFATc4 drives expression of miR‑23a, which represses atrogenes and sustains oxidative fiber identity [3] In young muscle, NFATc4‑dependent miR‑23a elevates intracellular glutamate and glutamine export via SLC1A5, increasing extracellular glutamine that senescent cells take up through ASCT2. Glutamine fuels the hexosamine biosynthesis pathway in senescent cells, promoting O‑GlcNAcylation of NF‑κB p65 and biasing SASP toward IL‑10 and TGF‑β1 secretion [4] When NFATc4 activity declines with age, glutamine efflux drops, senescent cells experience reduced O‑GlcNAc signaling, leading to heightened NF‑κB activity and a SASP rich in IL‑6, IL‑8 and MMP‑9 that impairs MuSC proliferation [2] Thus, the senescent cell's "negotiation" output is dictated by the myofiber‑derived metabolic cue set by NFATc4.

Testable Predictions

1. Pharmacological activation of NFATc4 (e.g., with low‑dose cyclosporine A‑free calcineurin agonist) in aged mouse muscle will increase extracellular glutamine levels by ~30 % within 48 h [5]
2. Senescent cells isolated from treated muscles will show a shift in SASP composition: IL‑6 and IL‑8 decrease >50 %, while TGF‑β1 and VEGF‑A increase >2‑fold, measured by multiplex ELISA.
3. Co‑culture of these senescent cells with MuSCs will rescue proliferation rates to ≥80 % of young‑control levels, whereas senescent cells from untreated aged muscle will suppress proliferation ≤40 % of control.
4. Genetic ablation of ASCT2 in senescent cells will block the glutamine‑dependent SASP switch, confirming metabolite sensing as the intermediary step.

Experimental Approach

Use 24‑month‑old C57BL/6 mice; treat one tibialis anterior with AAV9‑mediated constitutively active NFATc4, contralateral with AAV9‑GFP control. After 7 days, measure interstitial glutamine via microdialysis and LC‑MS. Fluorescence‑activated cell sorting (FACS) for p16^Ink4a^+ CD45^‑ senescent fibroblasts; collect conditioned medium for SASP profiling. Perform MuSC proliferation assays (EdU incorporation) in vitro with conditioned medium. Validate metabolite dependence by adding glutamine‑free medium or ASCT2 inhibitor (V‑9302) to senescent cultures.

Falsifiability

If NFATc4 activation fails to raise extracellular glutamine, or if senescent SASP does not shift toward TGF‑β1/VEGF despite increased glutamine, the hypothesis is refuted. Likewise, if MuSC proliferation is not rescued despite the SASP shift, the proposed mechanistic link between myofiber metabolism and senescent cell negotiation is invalid.

Broader Implication

This reframes senolytics not as indiscriminate clearance but as a potential adjuvant to metabolic rejuvenation: restoring myofiber NFATc4 activity could re-educate senescent cells, allowing them to resume their hostage‑negotiator role without triggering the secondary crisis of stem‑cell exhaustion.