IF a single intravenous dose of AAV-PHP.eB carrying a bicistronic construct under the microglial Tmem119/F4/80 promoter — co-expressing human CYP27A1 and human ABCA1 (the reverse-cholesterol lipid transporter) — is delivered at 2×10¹¹ vg/mouse to 20–22-month-old male and female C57BL/6J mice,

THEN brain 7-ketocholesterol (7KC) levels measured by isotope-dilution LC-MS/MS will be reduced by ≥50% relative to AAV-PHP.eB-CYP27A1-alone controls at 12 weeks post-injection, with concurrent normalization of microglial morphology (Iba1/CD68 immunofluorescence), reduction in hippocampal TNF-α and IL-1β protein, and ≥20% improvement in Morris Water Maze latency — WITHOUT elevation of plasma 27-OHC above the age-matched untreated control range,

BECAUSE the following step-by-step causal chain is supported:

1. 7KC accumulates within aged microglial lysosomes and mitochondria, driving lysosomal permeabilization, NF-κB-mediated neuroinflammation, and oxiapoptophagy — a damage state that is already-present in aged CNS and cannot be reversed without active enzymatic clearance. (7KC drives microglial activation, migration, and angiogenicity)[https://doi.org/10.1038/srep09144]

2. CYP27A1 (mitochondrial sterol 27-hydroxylase) catalyzes the direct hydroxylation of 7KC at the C-27 position to form 7KCh-27OH and subsequently to the carboxylated acid 7KCh-27COOH, both of which are more hydrophilic than 7KC and capable of cellular export — as directly demonstrated in RPE cells expressing endogenous CYP27A1. (CYP27A1 converts 7KC to 7KCh-27OH and 7KCh-27COOH in RPE)[https://doi.org/10.1194/jlr.m014217]

3. [CRITICAL MECHANISTIC GAP — ADDRESSED BY THIS HYPOTHESIS]: A directly contradicting dataset reveals that in extra-hepatic cells (ARPE19 cells and RPE/choroid explants), comprehensive HPLC-UV and LC-MS searches detect NO hydroxylated or sulfated 7KC derivatives, and instead show that extra-hepatic 7KC metabolism proceeds predominantly via SOAT1/cPLA2α-mediated esterification followed by selective efflux to HDL acceptors. (Absence of hydroxylated 7KC derivatives in extra-hepatic cells; dominant pathway is SOAT1-esterification → HDL efflux)[https://doi.org/10.1016/j.bbalip.2015.01.007] This implies that even when CYP27A1 is overexpressed, if ABCA1-mediated efflux capacity is insufficient, the hydroxylated products 7KCh-27OH and 7KCh-27COOH may fail to exit the cell — potentially re-accumulating or generating secondary oxidative species. [SPECULATIVE] The efflux bottleneck, not enzymatic conversion alone, may be rate-limiting in aged microglia.

4. ABCA1 co-expression directly addresses this efflux bottleneck: ABCA1 mediates active transport of hydroxylated and esterified oxysterols (including 7KC metabolites) across the plasma membrane to apolipoprotein acceptors in the extracellular space, completing the "convert → export" pipeline and preventing intracellular re-accumulation of 7KCh-27OH. [SPECULATIVE — direct evidence for ABCA1-mediated 7KCh-27OH efflux i...

SENS category: LysoSENS

Key references: • doi.org/10.1038/srep09144] • doi.org/10.1194/jlr.m014217] • doi.org/10.1016/j.bbalip.2015.01.007] • doi.org/10.1101/2025.06.06.658349] • doi.org/10.1016/j.bcp.2013.02.002]