IF subretinal or intravitreal delivery of a dual-component gene therapy vector — encoding (1) ATP6V0D1 (V-ATPase V0 domain subunit D1) to restore lysosomal acidification, AND (2) a codon-optimized, lysosome-targeted carotenoid cleavage dioxygenase (CCD) derived from soil-bacterial retinoid-catabolizing organisms (e.g., Brevundimonas or Sphingomonas genus homologs), engineered with an N-terminal LAMP1-signal peptide for lysosomal compartment retention and acid-range (pH 4.5–5.5) activity optimization — is administered via AAV2/5 vector to aged (18–22 month), female C57BL/6J mice with documented RPE lipofuscin accumulation,

THEN quantified autofluorescent lipofuscin burden in RPE flat mounts will decrease by ≥40% from baseline at 12 weeks post-injection, lysosomal pH measured by ratiometric LysoSensor Yellow/Blue will fall by ≥0.7 pH units from the pathological range (~5.8–6.0) toward physiological range (~4.5–5.0), and cathepsin D activity (fluorometric Z-Arg-Arg-AMC cleavage assay at pH 4.6) will increase ≥50% above vehicle controls,

BECAUSE the causal chain proceeds through two independent, sequentially enabling repair mechanisms:

1. A2E/bisretinoid accumulation inhibits V-ATPase assembly, raising lysosomal pH to 5.4–6.0 in RPE cells, which directly suppresses cathepsin D autocatalytic maturation and enzymatic activity below functional thresholds (impaired lysosomal degradation, not just pH, is required for lipofuscin-like autofluorescent granule formation) (ATP6V0D1 restores lysosomal acidification and autophagic flux in A2E-loaded RPE).

2. ATP6V0D1 overexpression promotes V-ATPase V1–V0 domain reassembly, restoring electrochemical proton gradient, lowering lysosomal pH to physiological 4.5–5.0, and recovering cathepsin D proteolytic activity against soluble/proteinaceous substrates (ATP6V0D1 overexpression restores lysosomal acidification and autophagic flux in A2E-loaded RPE) (DOI).

3. However, re-acidification alone is insufficient to clear pre-existing lipofuscin, because advanced bisretinoid species (oxidized A2E, A2E photo-oxidation products, cross-linked bis-retinoid conjugates) possess steric and chemical structures resistant to cathepsin D hydrolysis even at optimal pH — the cross-linking and pyridinium ring geometry of mature lipofuscin prevents enzyme active site access (advanced cross-linked lipofuscin species resist degradation even after pH normalization) (DOI). [SPECULATIVE: this resistance is well-supported mechanistically but the specific chemistry of bisretinoid cross-links susceptible to CCD enzymes has not been validated in RPE context.]

4. Bacterial carotenoid cleavage dioxygenases (CCDs) cleave polyene double-bond systems via oxidative scission at specific positions in the retinoid backbone — the same conjugated polyene system present in the bis-retinoid core of A2E and related...

SENS category: GlycoSENS

Key references: • doi.org/10.1167/iovs.62.9.39]. • doi.org/10.1167/iovs.62.9.39