Hypothesis

Protein aggregates in aging cells act as a redox‑regulated filtration system that selectively sequesters oxidatively modified proteins into liquid‑liquid phase separated (LLPS) condensates. Sulfenylation of cysteine residues promotes LLPS by increasing hydrophobicity and promoting multivalent interactions, while persulfidation maintains the condensates in a reversible, chaperone‑rich state. When persulfidation declines with age, sulfenylated clients become trapped, leading to irreversible amyloid‑like aggregation.

Key predictions

1. In young cells, oxidative stress triggers transient sulfenylation of a subset of cytosolic proteins, which drives their partitioning into chaperone‑enriched LLPS droplets that reduce soluble toxic species [1][2].
2. Persulfidation, mediated by hydrogen sulfide (H2S) signaling, coats the same cysteine residues, preventing over‑cross‑linking and allowing rapid disassembly of droplets upon stress relief [5].
3. With aging, mitochondrial H2S production falls, decreasing persulfidation; sulfenylated proteins accumulate in droplets, undergo further oxidation to sulfonylation, and the condensates mature into stable, insoluble aggregates [3][4].
4. Artificially boosting persulfidation (e.g., via NaHS donation) in aged models should dissolve existing aggregates without increasing soluble oxidative damage, whereas inhibiting sulfenylation (e.g., with dimedone) should block aggregate formation even under oxidative stress.

Experimental approach

• Use quantitative cysteine‑oxidation proteomics (OxICAT) to map sulfenylation vs persulfidation sites on proteins isolated from LLPS fractions of young vs old human fibroblasts [6].
• Perform FRAP (fluorescence recovery after photobleaching) on GFP‑tagged candidate clients to assess droplet reversibility under manipulations of H2S donors or sulfenylation inhibitors.
• Treat aged mouse muscle with NaHS or a persulfidation‑enhancing transgene and measure insoluble protein load, chaperone co‑localization, and functional readouts (grip strength, treadmill endurance) [4].
• Parallelly, administer dimedone to young mice exposed to paraquat and quantify whether sulfenylation blockade prevents LLPS formation and subsequent aggregation [2].

Potential outcomes and falsification

• If boosting persulfidation fails to reduce insoluble protein or does not increase chaperone occupancy, the hypothesis that persulfidation maintains LLPS reversibility is falsified.
• If sulfenylation inhibition does not decrease LLPS formation under oxidative stress, the role of sulfenylation as a LLPS trigger is questionable.
• Conversely, observing a shift from soluble sulfenylated species to insoluble sulfonylated species concomitant with loss of persulfidation would support the model.

By framing aggregation as a redox‑controlled filtration step, this hypothesis links the protective LLPS paradigm directly to the metabolic decline of sulfur motility, offering a testable route to distinguish beneficial sequestration from pathological deposition and suggesting that therapeutic persulfidation may restore proteome order without globally suppressing oxidative signaling.