Hypothesis

Chronic leakage of lysosomal cathepsin B into the cytosol and extracellular space sensitizes nociceptive neurons through direct cleavage of TRPV1 and TRPA1 channels, while concurrent decline of cystatin B removes endogenous inhibition, amplifying protease activity. This protease‑mediated sensitization lowers pain thresholds and simultaneously accelerates epigenetic aging via inflammasome‑driven NF‑κB signaling and histone cleavage. Consequently, individual pain sensitivity predicts biological age more accurately than conventional clocks because it integrates lysosomal dysfunction, inflammatory burden, and neuronal excitability in a single physiologic readout.

Mechanistic Rationale

• Cathepsin B as a nociceptive modulator: Cytosolic cathepsin B can cleave the N‑terminal domain of TRPV1, increasing its open probability and reducing activation threshold (source). Extracellular cathepsin B, packaged in exosomes, can bind neuronal surface receptors and trigger intracellular calcium fluxes that potentiate spike firing in C‑fibers.
• Cystatin B loss removes the brake: Age‑related decline of cystatin B (source) shifts the protease/antiprotease equilibrium toward uncontrolled cathepsin B and cathepsin L activity. Unopposed cathepsin L accesses the nucleus, cleaving histone H3 tails and establishing a senescent chromatin state (source).
• Inflammasome coupling: Leaked cathepsin B activates the NLRP3 inflammasome, driving caspase‑1‑mediated IL‑1β maturation and NF‑κB transcription (source). Persistent NF‑κB signaling sustains expression of cyclooxygenase‑2 and prostaglandin E2, further sensitizing nociceptors.
• Exosomal spread: Aged cells export cathepsin B‑rich exosomes that fuse with sensory neurons, delivering protease activity directly to pain‑processing circuits. This creates a feed‑forward loop where lysosomal aging in peripheral tissues amplifies central pain perception.

Testable Predictions

1. Cross‑sectional correlation: In adults aged 30‑80, exosomal cathepsin B activity (measured by fluorogenic assay) will inversely correlate with pressure‑pain threshold (PPT) and heat‑pain threshold (HPT) after adjusting for sex, BMI, and comorbid pain conditions (p < 0.01).
2. Longitudinal acceleration: Individuals whose baseline exosomal cathepsin B activity lies in the top quartile will show a 0.5‑year‑per‑year faster increase in GrimAge epigenetic clock over a 3‑year follow‑up, independent of baseline CRP or IL‑6 levels.
3. Causal intervention: Pharmacologic inhibition of cathepsin B (using a selective, blood‑brain‑barrier‑permeable inhibitor) in a randomized, double‑blind trial of older adults with elevated baseline protease activity will raise PPT/HPT by ≥15 % and attenuate GrimAge acceleration relative to placebo after 12 months.
4. Cystatin B rescue: Intranasal delivery of recombinant cystatin B will reduce exosomal cathepsin B activity and improve pain thresholds in a murine model of lysosomal aging, accompanied by decreased histone H3 cleavage in dorsal root ganglia.

Experimental Design

• Cohort: Recruit 500 community‑dwelling participants stratified by decade of life. Collect fasting blood for exosome isolation (ultracentrifugation), plasma cytokine panel, and DNA methylation array (Illumina EPIC). Quantify exosomal cathepsin B activity using a Z‑FR‑AMC substrate normalized to exosomal protein (BCA). Measure PPT with a computerized algometer and HPT with a thermode (baseline 32 °C, ramp 1 °C/s).
• Analysis: Use linear mixed‑effects models to assess cross‑sectional associations; apply joint longitudinal‑survival modeling to test whether baseline protease activity predicts epigenetic clock slope. Adjust for confounders via propensity scoring.
• Intervention sub‑study: 60 participants with high cathepsin B activity receive either cathepsin B inhibitor (e.g., Ca‑074Me analog) or placebo for 12 months, with primary outcomes of pain threshold change and secondary outcome of ΔGrimAge.

Falsifiability

If exosomal cathepsin B activity shows no correlation with pain thresholds after rigorous covariate adjustment, or if modulating cathepsin B/cystatin B fails to alter pain sensitivity or epigenetic aging rates, the hypothesis would be refuted. Conversely, confirming the predicted links would establish pain threshold as a functional, integrative biomarker of lysosomal aging and biological age.