IF a directed-evolution-optimized, M6P-glycopeptide-conjugated horseradish peroxidase variant (M6P-HRP*; estimated ~50–200 nM intravitreal dose; prepared by chemical conjugation of yeast-derived M6P glycopeptides to evolved HRP with counter-selected phospholipid oxidation activity) is administered by intravitreal injection every 3 months to 4-month-old male and female Abca4-Rdh8 double-knockout (DKO) mice, a validated model of accelerated bisretinoid accumulation,

THEN total RPE bisretinoid burden — quantified as the combined HPLC-MS signal for A2E and lysoA2PE (the alkyl-ether lysoA2PE species recently identified as a major human RPE bisretinoid fraction) — will be reduced by ≥50% relative to vehicle-injected controls at the 12-month endpoint, accompanied by ≥25% recovery of RPE lysosomal chymotrypsin-like proteasomal activity and preservation of scotopic ERG b-wave amplitude,

BECAUSE the following mechanistic chain is supported by converging evidence:

1. HRP cleaves A2E at lysosomal pH. Wild-type HRP reduces A2E by ~40% in ARPE-19 cells via C=C epoxidation of the polyene side arms, and HRP activity peaks at pH 5.5, exactly matching lysosomal lumen pH — establishing enzymatic feasibility of the damage-removal step. (Wu et al. JACS 2011, cited in research context; mechanism confirmed in bisretinoid degradation review) (Bisretinoid degradation and the ubiquitin-proteasome system)[https://doi.org/10.1007/978-1-4614-3209-8_75]

2. The M6P receptor pathway delivers active peroxidase to RPE lysosomes. Native myeloperoxidase (MPO), which carries endogenous M6P glycans, is internalized by RPE cells through the cation-independent M6P receptor (CI-M6PR), trafficked intact to lysosomes, retains peroxidase catalytic activity in that compartment, and eliminates lysosomal A2E — directly demonstrating that the M6P → CI-M6PR → lysosome → active peroxidase chain operates in the precise target cell type. (Extracellular MPO internalized by RPE via M6P receptor)[https://doi.org/10.1074/jbc.m116.739441]

3. Yeast-derived M6P glycopeptide conjugation provides a validated chemical M6P-tagging strategy for non-mammalian proteins. M6P-containing glycopeptides prepared from glyco-engineered Saccharomyces cerevisiae, after mild acid uncapping to expose terminal M6P ligands and conjugated to heterologous proteins via two-step chemistry, efficiently target those proteins to lysosomes in human fibroblasts — providing a practical route to M6P-tag HRP, which is a plant glycoprotein incapable of acquiring endogenous mammalian M6P marks. (M6P glycopeptides from yeast direct lysosomal targeting)[https://doi.org/10.1038/s41598-018-26913-4]

4. A2E lysosomal accumulation causally impairs proteasomal function, creating a self-amplifying damage loop that enzymatic degradation can break. Concurrent with HRP-mediated A2E degradation, chymotrypsin-like proteasome activity recovered by ~36% and trypsin-like activity by ~18% — demonstrating that bisretinoid ...

SENS category: LysoSENS

Key references: • doi.org/10.1007/978-1-4614-3209-8_75] • doi.org/10.1074/jbc.m116.739441] • doi.org/10.1038/s41598-018-26913-4] • doi.org/10.1194/jlr.m084459] • doi.org/10.1016/j.fob.2014.11.003]