Cholesterol Transport

Neurons make very little of their own cholesterol. They import it, largely from astrocytes, carried on ApoE particles — which is why the strongest genetic risk factor for late-onset Alzheimer's disease is a lipid transporter, and why this corpus treats cholesterol logistics as a candidate for the disease's central lesion rather than as a metabolic sideline.

The recurring observation across several independent lines here is that the field's frameworks disagree about the direction of the traffic while agreeing on the address.

The deficiency account

Rappoport (submission 57) argues the failure is one of supply. Astrocyte-to-neuron cholesterol transport via ApoE particles fails; without delivered cholesterol the neuron cannot form the lipid rafts that synaptic stabilisation requires; and the neuron is left trapped in chronic positive plasticity, where Aβ and hyperphosphorylated tau act as persistent retraction signals. On this reading APOE4's contribution is a transport defect, not a toxicity.

The signal-never-cleared account

The MAM/lipid programme arrives at the same molecule from the opposite side. There, C99 is a cholesterol sensor — a signal raised when the cell needs cholesterol moved — and the lesion is that the signal is never switched off, so contact-site function is upregulated rather than lost. Same molecule, same compartment, opposite sign.

The corpus's master reading of its scientist revisitations found this pattern is not confined to cholesterol: eight of nine independent programmes locate the lesion on the return leg of a cycle whose forward leg is compensating. Cholesterol transport is the clearest single instance.

Where the traffic is druggable

Van der Kant and colleagues showed in patient-derived iPSC neurons that cholesterol metabolism regulates tau and amyloid-β independently; that promoting cholesterol efflux by non-esterification routes reduced tau phosphorylation; and that the protective effect of statins on tau depended on cholesterol esterification. Esterification is an ACAT1 reaction, and ACAT1 is a contact-site enzyme — so the druggable step sits at the same structure the MAM account implicates.

A separate arm runs through oxysterols: glial CYP27A1 activation and increased influx of peripheral 27-hydroxycholesterol aggravating oxidative stress, with ApoE4 acting via its cholesterol-transporter function though a route through brain glucose dysmetabolism may also exist.

Why it keeps returning

Cholesterol transport is where the corpus's membrane, energy and genetics stories meet. The membrane needs it to organise receptors; the ApoE genotype determines how well it moves; and the corpus's caveolin work shows cognition can be preserved by restoring raft scaffolding without touching amyloid — which is the strongest available argument that the lipid axis is causal rather than correlated.

Related

Lipid Transport · Lipid Dysregulation · Membrane Biophysics · ER Stress · Synaptic Plasticity · APOE4 Hub · Endosomal Nexus