Hypothesis

Aging reprograms the transcriptional output of constitutively nuclear NFATc4 by altering its cofactor repertoire, shifting activity from slow‑fiber‑promoting genes to atrophy‑inducing genes such as atrogin-1 and MuRF1.

Rationale

NFATc4 shows persistent nuclear localization in both slow and fast fibers regardless of contractile activity[1], yet its target genes are undefined. In young muscle, NFATc4 cooperates with MEF2 and HDAC5 to drive oxidative fiber genes[4]. Aging elevates cytosolic HDAC4 and reduces MEF2 activity[5], favoring NFATc4‑HDAC4 complexes that repress MEF2 targets and permit recruitment of AP‑1 components that activate atrogene promoters[2]. This model explains how a constantly present transcription factor can switch from protective to deleterious signaling without changes in its localization.

Predictions

1. NFATc4 chromatin binding in old muscle will be enriched at atrogene promoters and depleted at slow‑fiber loci compared with young muscle.
2. Co‑immunoprecipitation will show increased HDAC4‑NFATc4 and decreased MEF2‑NFATc4 interactions in aged muscle.
3. Genetic disruption of HDAC4‑NFATc4 interaction (e.g., HDAC4‑binding‑deficient NFATc4 knock‑in) will attenuate age‑dependent atrogene expression and preserve muscle mass, while overexpression of MEF2 will rescue the young‑like transcriptional program.

Experimental Approach

• ChIP‑seq for NFATc4 in isolated EDL and soleus from 3‑month and 24‑month mice; quantify peaks at Foxo3, Murf1, Atrogin1, Myh7, Myh2 loci.
• Co‑IP / mass spec of NFATc4 complexes from the same samples to assess HDAC4, MEF2, and AP‑1 subunits.
• In vivo validation: AAV‑mediated expression of NFATc4 mutant lacking the HDAC4‑binding domain in aged mice; measure muscle fiber CSA, grip strength, and atrogene mRNA.
• Rescue test: AAV‑MEF2 overexpression in aged mice to see if it restores slow‑fiber gene expression despite NFATc4 nuclear presence.

Potential Outcomes and Falsification

• If NFATc4 binding shifts toward atrogenes and HDAC4 interaction rises with age, the hypothesis is supported.
• If NFATc4 binding pattern and cofactor composition remain unchanged between young and old muscle, or HDAC4 disruption fails to affect atrogene expression, the hypothesis is falsified.

This framework directly links age‑dependent calcium‑dependent calcineurin activity to a cofactor‑driven transcriptional switch, offering a testable mechanism for sarcopenia that preserves the ability to distinguish beneficial NFATc4 signaling in exercise from maladaptive signaling in aging.