Mechanistic description
The strongest synthesis is an indirect mechanism in glia: APOE4 promotes cholesterol sequestration in late endosome/lysosome compartments, lowering the ER-accessible cholesterol pool sensed by SCAP despite normal or elevated total cellular cholesterol. This weakens SCAP-INSIG retention, increases SREBP2 processing, and may explain the paradox of cholesterol accumulation alongside increased cholesterol biosynthesis.
Evidence for (4)
APOE4 glia show lysosomal cholesterol accumulation and altered cholesterol homeostasis consistent with compartmental misrouting rather than simple cholesterol deficiency.
Recent data support APOE4-associated cholesterol sequestration and glial lipid trafficking defects relevant to ER sterol sensing.
ER-accessible cholesterol is the key regulatory pool controlling SCAP-SREBP retention and SREBP activation.
NPC-like lysosomal export failure is known to impair ER feedback and sustain SREBP activation.
Evidence against (3)
No study directly demonstrates the full APOE4-to-NPC1/ER-contact-to-SCAP causal chain in human glia.
Lysosomal cholesterol accumulation could be secondary to broader autophagy or stress defects rather than the primary driver of SREBP2 activation.
Therapeutic cholesterol mobilizers such as cyclodextrins are pleiotropic and carry significant safety liabilities including ototoxicity risk.