The ERAD-Retrotranslocation Signal: How Misfolded Protein Fragments Actively Silence Autophagy via H3K9 Deacetylation in Aging

Autophagy isn't passively failing in aging—it's being actively suppressed by a coordinated, compartment-spanning signaling axis. The research context reveals a metabolic trap: high ER acetyl-CoA jams autophagosome formation (Atg9a-Fam134b step), while low nuclear acetyl-CoA deacetylates histones to silence TFEB/ATG genes (PMC8068152). But what coordinates this dual suppression? The missing link is likely a retrograde signal from the ERAD-proteasome system to the nucleus.

The Core Hypothesis

I propose that age-related proteostatic stress generates specific peptide fragments from ERAD (ER-associated degradation) of misfolded proteins. These fragments, translocated to the cytosol, act as signaling molecules that systemically inhibit nuclear ACSS2 activity and promote SIRT1-mediated deacetylation of H3K9 at autophagy gene promoters. This creates a self-reinforcing loop: proteotoxicity → ERAD → signal → epigenetic silencing → reduced autophagy → increased proteotoxicity.

Mechanistic Rationale

1. Source of Signal: Aged cells accumulate misfolded proteins, overwhelming the ERAD system (PMC4893201). Incomplete proteasomal degradation of retrotranslocated ERAD substrates could generate small, stable peptide fragments.
2. Transduction: These fragments could act as endogenous inhibitors of nuclear-cytoplasmic acetyl-CoA shuttles. Specifically, they might bind and inhibit the nuclear translocation or activity of ACSS2, which is known to boost histone acetylation on autophagy genes in neurons (PMC8068152).
3. Epigenetic Execution: Reduced nuclear acetyl-CoA flux, as documented in aged liver and neurons (PMC8068152, Science Advances), favors SIRT1-mediated deacetylation. The signal would enhance SIRT1 activity or recruitment to H3K9 at autophagy promoters, mimicking the SIRT1 occupancy increase seen in aged T cells (JCI).
4. Conservation & Paradox Resolution: This explains the compartmental paradox. The same proteotoxic stress that elevates ER acetyl-CoA (blocking autophagosomes) also generates a signal that depletes nuclear acetyl-CoA (silencing genes). The process is reversible—clearing misfolded proteins or supplementing young plasma factors (Aging Cell) would remove the signal, allowing nuclear acetyl-CoA recovery and TFEB-driven gene re-expression.

Falsifiable Predictions

• Prediction 1: In vitro, introducing synthetic peptide fragments derived from common misfolded ERAD substrates (e.g., from mutant progerin or APOE4) to young cells should specifically inhibit nuclear ACSS2 localization and reduce H3K9 acetylation at TFEB and ATG gene promoters within hours, preceding autophagy inhibition.
• Prediction 2: Genetic knockdown of key ERAD components (e.g., HRD1, SEL1L) in aged cells should paradoxically improve autophagy gene transcription and nuclear H3K9 acetylation, despite increasing total misfolded protein load, by blocking the generation of the retrograde signal.
• Prediction 3: The peptide signal should be detectable in the plasma of aged but not young organisms. Plasma exchange from young to old animals should rapidly reduce its levels, correlating with restored nuclear ACSS2 function and autophagy gene expression (Aging Cell).

This model frames autophagy suppression as an active, adaptive (though maladaptive) response to proteotoxic stress, mediated by a novel ERAD-to-chromatin signaling axis. It shifts the focus from metabolic wear-and-tear to regulated signaling from the protein misfolding crisis.