Hypothesis

Protein aggregates in aging cells act as redox buffers that temporarily lock away hyperoxidized cysteine residues, preventing aberrant signaling. We've observed that peroxiredoxin enzymes, by reducing sulfenic/sulfinic acids on client proteins, keep biomolecular condensates in a liquid‑like state; when peroxiredoxin activity drops below a threshold, thiol hyperoxidation drives LLPS toward irreversible amyloid‑like solids. It's known that peroxiredoxin levels decline with age, shifting the balance toward solidification.

Mechanistic Basis

• Aggregates enrich for proteins containing redox‑sensitive cysteines (e.g., peroxiredoxin targets) that become sulfenylated under oxidative stress.
• Peroxiredoxins bind these sulfenylated motifs via their cysteine‑containing active site, preventing cross‑β stacking and maintaining dynamic LLPS (see 4).
• As peroxiredoxin levels decline with age, sulfenylated proteins accumulate, promoting thiol hyperoxidation and solid transition (3).
• The resulting solid aggregates sequester chaperones (e.g., Hsp40) and UPS components, creating a vicious cycle of proteostasis collapse (5,6).

Testable Predictions

1. Overexpressing peroxiredoxin in aged C. elegans will increase the fraction of liquid‑like condensates (measured by FRAP) and delay the appearance of Thioflavin‑positive aggregates.
2. Pharmacological inhibition of peroxiredoxin will accelerate solid‑aggregate formation even under mild oxidative stress (e.g., low paraquat).
3. Mutating the redox‑sensitive cysteines of a model client protein (e.g., GFP‑Cys) will abolish its incorporation into protective condensates and render it prone to pathological aggregation independent of peroxiredoxin levels.

Potential Experiments

• Generate transgenic worms expressing peroxiredoxin‑GFP and a liquid‑condensate reporter (e.g., Ddx‑4::mFRAP). Quantify recovery rates in young vs. old worms with/without peroxiredoxin overexpression.
• Use thiol‑specific biotin‑switch assay to track sulfenylation of aggregate‑associated proteins under varying peroxiredoxin activity.
• Perform lipid‑dye staining (e.g., Nile Red) to differentiate liquid vs. solid aggregate states in vivo.

Implications

If peroxiredoxin maintains the liquid state of protective aggregates, therapeutic strategies that boost peroxiredoxin activity—or mimic its chaperone‑like function—could preserve the adaptive role of aggregation while blocking its transition to toxicity. This reframes anti‑aggregation therapies from global inhibition to selective stabilization of condensates.

References

[1] https://www.fightaging.org/archives/2015/05/protein-aggregation-as-a-protective-mechanism/ [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC11897469/ [3] https://doi.org/10.1101/2023.11.07.566021 [4] https://doi.org/10.1101/2024.06.13.598862 [5] https://pmc.ncbi.nlm.nih.gov/articles/PMC4539002/ [6] https://elifesciences.org/articles/48240 [7] https://doi.org/10.1126/science.aag3048