Hypothesis

Oxidative modification of cystatin B reduces its ability to inhibit cytosolic cathepsin D, thereby amplifying cathepsin D‑mediated apoptosis and senescence during aging.

Mechanistic Rationale

Lysosomal destabilization releases cathepsin D into the cytosol (1). Cystatin B, the chief endogenous inhibitor of cysteine cathepsins, normally binds cathepsin D with high affinity and limits its proteolytic activity. Age‑associated oxidative stress promotes S‑glutathionylation or S‑nitrosylation of the reactive cysteine residues in cystatin B’s inhibitor domain. This post‑translational change lowers cystatin B’s binding constant for cathepsin D and can even convert cystatin B into a low‑efficiency substrate, facilitating cathepsin D’s proteolytic cycle. Unchecked cathepsin D then accelerates Bax translocation to mitochondria and caspase‑3 activation, driving the ROS‑Nrf2‑senescence loop described in cathepsin D‑deficient models (2). In parallel, the observed rise in cathepsin D levels and activity in aged brain (3) and the fall in cathepsin L activity suggest a shift toward cysteine‑cathepsin dependence that cystatin B normally buffers. Loss of cystatin B inhibition therefore provides a mechanistic link between lysosomal leak, oxidative stress, and the tissue‑specific cathepsin dysregulation seen in neurodegeneration and photoaged skin (4).

Testable Predictions

1. Aged tissues will show increased S‑glutathionylated cystatin B concurrent with decreased cathepsin B‑cystatin B complex formation.
2. Preventing cystatin B oxidation (e.g., via overexpression of a cysteine‑to‑serine mutant or treatment with glutaredoxin‑1) will rescue cathepsin D activity, lower ROS accumulation, and delay senescence in cultured neurons subjected to lysosomal destabilization.
3. Neuron‑specific cystatin B knockout will exacerbate cathepsin D‑dependent apoptosis and accelerate behavioral decline in mouse models of aging, whereas cystatin B overexpression will be protective.

Experimental Approach

• Biochemical assay: Immunoprecipitate cystatin B from young vs. old mouse brain, probe for S‑glutathionylation using anti‑GSH antibody, and measure cathepsin D activity in the immunoprecipitate.
• Cell model: Treat SH‑SY5Y cells with Leu‑Leu‑OMe to induce lysosomal permeabilization, then transfect with wild‑type or oxidation‑resistant cystatin B (Cys→Ser). Quantify cathepsin D‑mediated Bax mitochondrial translocation (Western blot), caspase‑3 cleavage, ROS (DCFDA), and senescence (SA‑β‑gal).
• In vivo: Use AAV‑mediated neuron‑specific cystatin B overexpression or CRISPR knockout in aged mice; assess lysosomal integrity (LAMP1 staining), cathepsin D cytosolic fraction, neurodegeneration markers (NeuN loss, phosphorylated tau), and cognitive performance (Morris water maze).

If oxidative inactivation of cystatin B is a key amplifier of cytosolic cathepsin D pathology, restoring cystatin B function should break the leak‑ROS‑senescence axis and mitigate age‑related neurodegeneration.