Hypothesis

We propose that age‑dependent loss of the ER chaperones Calr and ERp57 shifts the ER luminal redox potential toward a more oxidizing environment, which directly biases IRE1 toward its active RNase conformation while simultaneously impairing PERK signaling through sequestration of eIF2α phosphatases by oxidized ERp57 fragments. This redox‑driven arm imbalance explains why some tissues show PERK‑dependent adaptive responses (e.g., skeletal muscle satellite cells) whereas others exhibit IRE1‑mediated pro‑apoptotic signaling (e.g., aged heart) and why the brain displays a globally attenuated UPR.

Mechanistic Basis

Calr and ERp57 are not only chaperones but also major contributors to ER redox buffering via their thiol‑disulfide isomerase activity. Their decline reduces the pool of reduced cysteine residues available to neutralize oxidizing equivalents generated by protein folding. Consequently, misfolded proteins accumulate less because the oxidative stress itself modifies IRE1’s luminal domain, promoting dimerization and RNase activation independent of canonical load sensing (see redox‑sensing model of IRE1 IRE1 redox activation). Oxidized ERp57 fragments, generated during chaperone depletion, bind and inhibit the phosphatase complex that dephosphorylates p‑eIF2α, thereby prolonging PERK‑mediated translational attenuation beyond the adaptive window (PERK‑eIF2α phosphatase regulation Phosphatase study). In tissues with high secretory demand (skeletal muscle), transient PERK activity remains beneficial for satellite cell proliferation, but chronic oxidation tips the balance toward IRE1‑JNK‑NF‑κB signaling, driving inflammation and apoptosis (IRE1‑JNK link IRE1‑JNK). In neurons, reduced ER lumen oxidation fails to trigger either arm sufficiently, resulting in a blunted UPR that cannot clear aggregates (neuronal UPR attenuation Neuronal UPR).

Testable Predictions

1. Restoring ER luminal reducing capacity (e.g., ER‑targeted overexpression of thioredoxin or treatment with ER‑permeable antioxidants) will normalize IRE1 RNase activity and reduce CHOP expression in aged mouse heart without affecting PERK signaling in skeletal muscle.
2. Mutating the cysteine residues of ERp57 that become oxidized upon aging will prevent its inhibitory binding to eIF2α phosphatases, preserving PERK‑dependent translational recovery after stress.
3. Tissue‑specific redox sensors (roGFP2‑ER) will show a more oxidizing ER lumen in aged heart and liver, but a relatively reduced lumen in aged brain, correlating with the observed UPR phenotypes.
4. Pharmacological inhibition of IRE1 RNase (e.g., MKC‑3943) will rescue senescence markers in aged cardiovascular tissue, whereas PERK activation (e.g., Sephin1) will enhance satellite cell regeneration in aged muscle.

Experimental Approach

• Measure ER redox state using roGFP2‑ER in isolated tissues from young (3 mo) and aged (24 mo) mice; correlate with Calr/ERp57 levels by immunoblot.
• Assess IRE1 RNase activity (XBP1 splicing) and PERK signaling (p‑eIF2α, ATF4) under basal conditions and after tunicamycin challenge.
• Apply ER‑targeted antioxidant (Mito‑ER‑TXN) or overexpress redox‑dead ERp57(CxxS) mutant via AAV; evaluate CHOP, caspase‑12, senescence (p16^INK4a), and functional outcomes (echocardiography, grip strength, gait analysis).
• Use tissue‑specific CRISPR knockout of the redox‑sensing cysteines in IRE1 to test whether blocking its oxidation abrogates the age‑dependent hyperactivation.
• Analyze outcomes with two‑way ANOVA (age × treatment) and post‑hoc tests; a significant interaction supporting redox‑dependent arm bias would validate the hypothesis, whereas lack of redox change or unchanged UPR patterning would falsify it.