Stress Granule Sequestration of ATG Proteins Drives ERK-Dependent Autophagy Suppression in Aging

Hypothesis

Aged cells actively suppress autophagy by trapping ATG5/7 and LC3 proteins in stress granules, which removes these proteins from their role as scaffolds for ERK phosphorylation. Loss of scaffolded ERK shifts signaling toward a persistently nuclear pool that represses autophagy genes, creating a self‑reinforcing loop that maintains the senescent state.

Mechanistic Basis

• In young cells, ATG proteins serve as platforms that tether ERK to autophagosomes, favoring cytoplasmic ERK activity that supports protective autophagy via moderate Beclin 1 upregulation [1].
• With age, oxidative stress and RAS/MAPK hyperactivity increase G3BP1‑dependent stress granule formation [4]. ATG5/7 and LC3 are recruited into these granules, reducing their availability for ERK scaffolding [3].
• Cytoplasmic ERK activity falls, while nuclear ERK accumulates due to impaired phosphatase recruitment (PP2A/PP2C inhibition by ROS‑induced caveolin‑1) [4]. Nuclear ERK phosphorylates transcriptional repressors (e.g., FOXO3) that silence autophagy promoters, overriding any residual Beclin 1 induction.
• The resulting autophagy deficit further impairs clearance of damaged mitochondria, amplifying ROS and sustaining stress granule assembly—a positive feedback loop.

Testable Predictions

1. Inhibiting stress granule nucleation (e.g., G3BP1 knock‑down or 1,6‑hexanediol treatment) will increase cytoplasmic ERK phosphorylation and restore LC3‑II flux in senescent fibroblasts, even when ERK remains constitutively active.
2. Forced cytoplasmic retention of ERK (via an ERK‑AKAP fusion) will rescue autophagy and reduce SA‑β‑gal positivity in aged cells independent of stress granule status.
3. Conversely, enhancing stress granule formation in young cells will mimic the aged phenotype: decreased autophagy, increased nuclear ERK, and premature senescence markers.

Experimental Design

• Use human IMR‑90 fibroblasts passaged to replicative senescence and young controls.
• Measure stress granule formation (G3BP1 immunofluorescence), ERK subcellular fractionation (Western blot), autophagic flux (LC3‑II turnover with bafilomycin A1), and senescence (SA‑β‑gal, p16).
• Apply interventions: siRNA against G3BP1, 1,6‑hexanediol pulse, ERK‑AKAP construct overexpression, and MEK inhibitor (U0126) as control.
• Perform rescue experiments: overexpress LC3‑B mutant unable to bind G3BP1 to test scaffold specificity.

Potential Outcomes

• If stress granule disruption restores cytoplasmic ERK signaling and autophagy while reducing senescence markers, the hypothesis is supported.
• If autophagy remains suppressed despite normalized ERK localization or if stress granule manipulation fails to affect senescence, the hypothesis would be falsified, indicating that alternative mechanisms dominate autophagy shutdown in aging.

This framework links two observed age‑altered systems—stress granule dynamics and ERK compartmentalization—to provide a concrete, falsifiable explanation for why autophagy is actively turned off rather than merely worn out.