Hypothesis

Age‑dependent depletion of ER luminal chaperones (e.g., calreticulin, ERp57) does not merely raise basal ER stress; it actively remodels the ER membrane through selective ER‑phagy that preferentially removes PERK‑favoring ER subdomains. This remodeling shifts the UPR arm balance toward chronic IRE1α hyperactivation and XBP1 splicing, while diminishing PERK‑eIF2α‑ATF4 signaling that is essential for satellite cell survival and myogenic differentiation. Restoring chaperone levels therefore rescues muscle regeneration not only by suppressing maladaptive IRE1 signaling but also by reversing the ER‑phagy‑driven loss of PERK‑competent ER.

Mechanistic Rationale

1. Chaperone scarcity exposes hydrophobic peptide segments on nascent proteins, increasing the likelihood of misfolded protein accumulation in specific ER subdomains that are enriched for PERK‑activating complexes (PERK‑GRP78 interactions).
2. Misfolded protein‑rich ER patches become substrates for ER‑phagy receptors such as FAM134B and SEC62, which recognize exposed luminal domains and recruit the autophagy machinery (LC3, ATG proteins).
3. Selective removal of PERK‑rich ER sheets reduces the local concentration of PERK activators, lowering PERK autophosphorylation and downstream eIF2α‑ATF4‑CHOP signaling despite overall ER stress.
4. Concurrently, IRE1α‑containing ER tubules, which are less dependent on luminal chaperone occupancy for activation, persist or expand. Their sustained oligomerization drives elevated XBP1 splicing and downstream pro‑survival or, when chronic, pro‑apoptotic outputs.
5. This creates a tissue‑specific hierarchy: in skeletal muscle, PERK signaling is required for satellite cell proliferation and differentiation, so its selective loss contributes to age‑related regenerative decline; in other tissues (e.g., liver, brain) where IRE1α outputs dominate stress adaptation, the same remodeling exacerbates maladaptive pathways.

Testable Predictions

• Prediction 1: Aged mouse skeletal muscle will show increased colocalization of ER‑phagy markers (LC3‑II, FAM134B) with PERK‑containing ER fractions, but not with IRE1α fractions, compared with young muscle.
• Prediction 2: Genetic or pharmacological inhibition of ER‑phagy (e.g., FAM134B knockout, Sec62 siRNA) in aged mice will restore PERK‑eIF2α‑ATF4 signaling, reduce XBP1 splicing, and improve muscle regeneration after injury without exacerbating CHOP‑mediated apoptosis.
• Prediction 3: Overexpression of calreticulin in aged muscle will decrease ER‑phagy flux (measured by mCherry‑GFP‑LC3 ER‑phagy reporter) and re‑balance PERK/IRE1 activity, mirroring the effects seen with chaperone restoration in [1].
• Prediction 4: In vitro, tunicamycin‑treated C2C12 myotubes subjected to ER‑phagy inducers (e.g., starvation) will exhibit decreased PERK phosphorylation and increased IRE1α‑XBP1 splicing, an effect reversible by chaperone co‑overexpression.

Potential Experiments

• Perform subcellular fractionation of ER from young vs. old mouse gastrocnemius, followed by immunoblotting for PERK, IRE1α, LC3, and chaperones to quantify ER‑phagy selectivity.
• Use AAV‑mediated FAM134B shRNA delivery to aged mouse muscle prior to cardiotoxin injury; assess Pax7+ satellite cell proliferation, MyoD expression, and force recovery.
• Monitor live‑cell ER‑phagy using the mCherry‑GFP‑LC3 reporter in primary myoblasts cultured from young and old donors, with and without calreticulin overexpression.
• Measure downstream outputs: p‑eIF2α, ATF4, CHOP, XBP1s splicing, and protein synthesis rates (SUnSET assay) to link ER‑phagy manipulation to functional UPR arm activity.

If these experiments confirm that ER‑phagy selectively erodes PERK‑competent ER in aging muscle, it will reframe chaperone loss as an initiator of organelle remodeling that directly dictates UPR arm specificity, offering a dual‑target strategy: boost chaperones and modulate ER‑phagy to restore regenerative PERK signaling while curbing detrimental IRE1α activity.