Hypothesis

Age‑associated decline in the eukaryotic initiation factor 2B (eIF2B) complex, rather than PERK kinase activity, creates a translational bottleneck that prevents ATF4 protein accumulation despite phosphorylated eIF2α in senescent satellite cells. Restoring eIF2B activity will bypass this block, re‑establish the PERK‑ATF4‑autophagy axis, and rescue muscle regeneration.

Rationale

Recent work shows that PERK/eIF2α signaling is intact in aged muscle but ATF4 protein fails to accumulate【4】. This points to a defect downstream of eIF2α‑P, specifically in the ribosome reinitiation step that depends on eIF2B‑mediated GTP‑GDP exchange on eIF2. eIF2B activity declines with age in several tissues, and its inhibition reproduces the translational block seen in senescence【1】【2】. Moreover, the PERK arm is essential for satellite cell survival, differentiation and regeneration【3】, whereas IRE1 deletion does not affect regeneration, underscoring the non‑redundant role of PERK‑ATF4 output.

We hypothesize that age‑related oxidative stress or inflammatory signaling modifies eIF2B subunits (e.g., via phosphorylation of the ε subunit) reducing its guanine‑nucleotide exchange activity. Consequently, even when eIF2α is phosphorylated, ternary complex formation is insufficient to allow ribosomes to bypass the upstream ORFs of ATF4 mRNA, blocking its translation.

Predictions

1. In satellite cells isolated from old mice, eIF2B activity (measured by GDP‑GTP exchange assay) will be lower than in young cells, while PERK phosphorylation and eIF2α‑P levels remain comparable.
2. Pharmacological activation of eIF2B with ISRIB or a novel small‑molecule agonist will restore ATF4 protein levels in old satellite cells without altering upstream PERK signaling.
3. Rescue of ATF4 translation will reactivate autophagy genes (LC3‑II, p62 degradation) and reduce apoptosis markers (cleaved caspase‑3, CHOP) in aged satellite cells.
4. In vivo, periodic ISRIB treatment of aged mice undergoing muscle injury will improve satellite cell‑mediated regeneration (increased Pax7⁺MyoD⁺ cells, larger centrally nucleated fibers) and decrease fibrosis compared with vehicle.
5. Conversely, knock‑down of eIF2B ε subunit in young satellite cells will phenocopy the aged translational block, decreasing ATF4 protein despite normal PERK activation.

Experimental Approach

• Isolate primary satellite cells from young (3 mo) and old (24 mo) mice; assess PERK activation, eIF2α‑P, eIF2B activity (radiolabeled GTP exchange), and ATF4 protein by western blot.
• Treat cells with ISRIB (0.25 µM) or vehicle; measure ATF4 translation kinetics using puromycin‑based SUnSET assay and polysome profiling of ATF4 mRNA.
• Autophagy flux (LC3‑II turnover with bafilomycin) and apoptosis (caspase‑3 activity) will be quantified.
• For in vivo tests, aged mice receive tibialis anterior crush injury; ISRIB (2.5 mg/kg i.p.) or saline is administered daily for 5 days post‑injury. Regeneration is evaluated by histology (central nucleation, fibrosis scoring) and flow cytometry of satellite cells.
• Controls include PERK inhibitor (GSK2606414) to confirm dependence on PERK‑eIF2α signaling, and eIF2B ε siRNA to validate specificity.

If ISRIB restores ATF4 and regeneration in aged muscle, the hypothesis is supported; failure to rescue ATF4 despite restored eIF2B activity would falsify it.

[1] https://www.aging-us.com/article/102273/text [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC3429039/ [3] https://elifesciences.org/articles/22871 [4] https://doi.org/10.1101/2023.04.12.536613