Hypothesis

We hypothesize that aging reduces the effective concentration of cytosolic cystatin B, the chief endogenous inhibitor of cathepsins, thereby unmasking the toxic potential of leaked cathepsin D and B. While cystatin B is known to inhibit cathepsins in lysosomes, its cytosolic pool is poorly characterized. We propose that two age‑dependent processes diminish free cystatin B: (1) transcriptional downregulation of the CSTB gene in neurons and microglia, and (2) oxidative modification of cystatin B that impairs its inhibitory affinity and promotes its sequestration into lipofuscin‑laden lysosomes. Consequently, cathepsins that escape through LMP encounter insufficient inhibition, leading to amplified oxidative stress, mitochondrial dysfunction, and cell death.

Mechanistic Reasoning

Cystatin B contains a reactive site that forms a reversible covalent bond with the active‑site cysteine of cathepsins. Oxidation of its conserved methionine or cysteine residues can alter this site, decreasing binding affinity. In aged brain, lipid peroxidation products such as 4‑hydroxynonenal adduct cystatin B, as shown in other contexts. Moreover, lipofuscin granules, which accumulate with age, have a high affinity for basic proteins and may trap cystatin B, further lowering its cytosolic availability. This creates a feed‑forward loop: more cathepsin leakage → more oxidative stress → more cystatin B inactivation → more leakage.

Predictions

1. In aged rat cortex and hippocampus, cytosolic cystatin B levels (measured by subcellular fractionation and western blot) will be significantly lower than in young adults, despite unchanged total CSTB mRNA.
2. Oxidized cystatin B (detected by biotin‑switch assay or mass spec) will correlate positively with cytosolic cathepsin D activity across individuals.
3. Genetic overexpression of cystatin B specifically in the cytosol (using a N‑terminal myristoylation signal) in aged mice will reduce cytosolic cathepsin D/B activity, lower ROS and Nrf2 suppression, decrease senescence markers (p16^INK4a, SA‑β‑gal), and improve neuronal survival.
4. Acute knockdown of cystatin B in young primary microglia will phenocopy aged lysosomal leakage: increased cathepsin B release, mitochondrial depolarization, and heightened pyroptosis upon myelin debris exposure.
5. Small‑molecule stabilizers of cystatin B (e.g., NSC‑632839 analogs) will rescue the proteolytic imbalance without altering lysosomal cathepsin content or activity.

Experimental Approach

• Fractionate young (3 mo) and aged (24 mo) rat brains to isolate cytosol and lysosomes; quantify cystatin B and cathepsins by ELISA and activity assays.
• Perform immunoblotting for oxidized cystatin B using anti‑methionine‑sulfoxide antibodies.
• Use AAV‑PHP.eB vectors delivering cystatin B‑myristoylated construct to aged mice; assess behavior (rotarod, novel object recognition) and histology after 8 weeks.
• CRISPRi knockdown of CSTB in BV2 microglia; expose to aggregated myelin; measure cathepsin B release (fluorogenic substrate), mitochondrial membrane potential (JC‑1), and caspase‑1/pyroptosis (LDH release, IL‑1β).
• Treat cultures with cystatin B‑stabilizing compound; repeat leakage assays to test rescue.

If these predictions hold, restoring cytosolic cystatin B would break the vicious cycle linking LMP, cathepsin toxicity, and neurodegeneration, offering a therapeutic strategy that preserves lysosomal cathepsin function while neutralizing their cytoplasmic menace.