Hypothesis\n\nCore proposition: In aging skeletal muscle, chronic low-grade calcium dysregulation activates calpain, which cleaves calcineurin into a constitutively active C-terminal fragment (Cn‑C). This fragment retains phosphatase activity toward NFAT but exhibits altered substrate specificity, preferentially dephosphorylating NFATc4 at sites that antagonize RSK2‑mediated phosphorylation. The resulting shift in the NFATc4 phosphorylation balance promotes a conformation that exposes a cryptic DNA‑binding domain, enabling direct NFATc4 occupancy of the MuRF1 and MAFbx/Atrogin-1 promoters. Consequently, what is normally a protective, constitutively nuclear NFATc4 program becomes a driver of atrophy gene transcription.\n\n## Mechanistic rationale\n- Calcineurin is known to be converted into a proteolytically active fragment by calpain under severe calcium overload ([3]), a condition that becomes more prevalent with age due to leaky RyR channels and mitochondrial dysfunction.\n- NFATc4’s nuclear import and stability are uniquely co‑regulated by the kinase RSK2 ([1]); RSK2 phosphorylates NFATc4 on serine residues that favor a repressive chromatin state.\n- We propose that Cn‑C preferentially dephosphorylates the same RSK2 target sites, tipping the kinase/phosphatase balance toward dephosphorylation and unveiling a DNA‑binding interface that is otherwise masked.\n- This model explains the lack of documented direct NFAT binding to atrophy gene promoters: the interaction only occurs when calcineurin is cleaved, a state not captured in basal condition studies.\n\n## Testable predictions\n1. Biochemical: Aged mouse tibialis anterior will show increased levels of the C‑terminal calcineurin fragment (detectable by an antibody against the cleavage site) correlating with elevated calpain activity.\n2. Phosphoproteomics: NFATc4 from aged muscle will display reduced phosphorylation at RSK2‑targeted serine residues (e.g., Ser^172) compared with young controls.\n3. Chromatin: Chromatin immunoprecipitation followed by qPCR (ChIP‑qPCR) will reveal enriched NFATc4 occupancy at the MuRF1 and MAFbx promoters in aged muscle, which diminishes after treatment with a calpain inhibitor (e.g., MDL-28170) or an RSK2 activator.\n4. Functional: Overexpression of a cleavage‑resistant calcineurin mutant (lacking the calpain target site) in aged muscle will attenuate atrophy gene expression and preserve fiber cross‑sectional area, whereas expression of the Cn‑C fragment in young muscle will be sufficient to induce atrophy.\n\n## Experimental approach\n- Use Western blot with cleavage‑specific calcineurin antibodies and activity assays to quantify Cn‑C in young (3 mo) vs aged (24 mo) mouse gastrocnemius.\n- Perform phospho‑specific NFATc4 Westerns or mass‑spec phosphoproteimetry after immunoprecipitation of NFATc4.\n- Conduct ChIP‑qPCR for NFATc4 at MuRF1 and MAFbx promoters under basal conditions and after pharmacological modulation (calpain inhibitor, RSK2 inhibitor/activator).\n- Generate AAV vectors expressing either wild‑type calcineurin, cleavage‑resistant mutant (D→A at calpain site), or the isolated Cn‑C fragment; inject into tibialis anterior and assess atrophy markers (MuRF1, MAFbx mRNA) and muscle morphology after 2 weeks.\n- Include appropriate controls (GFP, empty vector) and replicate in both sexes.\n\n## Potential implications\nIf validated, this hypothesis would redefine the calcineurin/NFAT axis from a indirectly protective pathway to a direct effector of atrophy when hijacked by age‑dependent proteolysis. It identifies a druggable node—the calpain‑calcineurin cleavage step—or the RSK2‑NFATc4 phosphorylation axis for interventions aimed at preventing sarcopenia.\n\nReferences\n[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC2726382/\n[2] https://pmc.ncbi.nlm.nih.gov/articles/PMC317085/\n[3] https://www.frontiersin.org/journals/aging-neuroscience/articles/10.3389/fnagi.2018.00199/full\n[4] https://pubmed.ncbi.nlm.nih.gov/24336651/\n[5] https://www.pnas.org/doi/10.1073/pnas.0401835101