Aggregates as Redox Buffers: Sequestration of Oxidatively Damaged Proteins Limits Free‑Ion Mediated Toxicity in Aging Neurons
Hypothesis
Aging neurons convert oxidatively damaged, soluble proteins into amyloid‑like deposits not merely to hide toxic species but to sequester redox‑active metal ions (Fe²⁺/Cu⁺) that catalyze further oxidative damage, thereby acting as a proteostatic sink that lowers cellular labile iron and copper pools.
Mechanistic Rationale
Oxidative modification of cysteine and methionine residues promotes liquid‑liquid phase separation, nucleating chaperone‑enriched deposits such as Q‑bodies and PACs [https://pubmed.ncbi.nlm.nih.gov/33386485/]. These sites preferentially capture proteins bearing carbonyl or dityrosine cross‑links, which often retain bound transition metals. By incorporating these metallated clients into β‑sheet‑rich aggregates, the cell reduces the concentration of free Fe²⁺ and Cu⁺ that drive Fenton chemistry and lipid peroxidation [https://doi.org/10.1101/2023.11.07.566021]. The protective effect of aggregates therefore depends on their metal‑binding capacity rather than inertness alone.
Testable Predictions
- Metal content – Isolated detergent‑resistant aggregates from aged mouse brain will show higher Fe and Cu ICP‑MS signals per unit protein than soluble fractions, and this enrichment will correlate with decreased labile iron pools measured by calcein‑AM fluorescence in situ.
- Genetic manipulation – Neuronal over‑expression of the yeast sequestrase Btn2/Hsp42 ortholog (e.g., human HSPB8) in C. elegans will increase aggregate formation, lower cytosolic labile Fe²⁺ (detected with FerroOrange), and extend lifespan; conversely, CRISPR‑mediated knock‑down of Btn2 will raise free metal levels and accelerate age‑dependent motility decline.
- Pharmacological challenge – Treating aggregate‑rich cultures with a membrane‑impermeable chelator (e.g., deferoxamine‑conjugated to dextran) will not rescue toxicity if aggregates are intact, but will exacerbate oxidative damage when aggregates are disrupted by low‑dose SDS or Hsp70 inhibitor VER‑155008, indicating that the aggregate‑bound metal pool is sequestered and not readily exchangeable.
- Rescue experiment – Expressing a mutant HSPB8 unable to bind metal‑oxidized clients (Cys→Ser) will fail to reduce labile iron despite forming visible deposits, linking metal sequestration directly to the protective phenotype.
Potential Implications
If aggregates serve as redox buffers, therapeutic strategies that indiscriminately dissolve amyloid plaques may release sequestered metals, worsening oxidative stress. Instead, enhancing the capacity of chaperone‑mediated sequestration (e.g., via HSPB8 activators) or promoting safe metal export (e.g., ferroportin up‑regulation) could preserve the protective function of aggregates while lowering overall oxidative burden. This reframes anti‑aggregation approaches: the goal may be to modulate aggregate composition rather than eliminate them outright.