Peter Attia MD395 – Brain lipidology: understanding APOE, cholesterol homeostasis, Alzheimer’s disease, & more
CHAPTERS
- 0:01 – 13:23
Peripheral cholesterol transport: apoA and apoB lipoproteins
Peter Attia sets up the episode and asks for a short primer on how cells make and move cholesterol outside the brain. Tom Dayspring explains how cells synthesize and efflux cholesterol, how HDL and apoB particles carry it, and how reverse cholesterol transport works directly and indirectly through LDL.
- •Every cell can make its own cholesterol, and excess cholesterol can crystallize and kill a cell
- •Lipoproteins are needed because lipids can't travel in watery plasma; HDLs use apoA-I and the liver and intestine make apoB particles
- •Most HDL cholesterol is handed to LDL, whose main job is returning cholesterol to the liver
- •'Peripheral' means anything outside the brain
- •Dayspring flags the belief that very low LDL will deprive the brain as something he will address
- 13:23 – 18:23
Why lowering plasma LDL doesn't deplete body cholesterol
Attia and Dayspring address what happens to reverse cholesterol transport when LDL falls, and compare plasma cholesterol with total body stores. They note that red blood cells carry more plasma cholesterol than lipoproteins do and that the brain holds about 20 times more cholesterol than the liver.
- •When LDL falls, cells are in balance and there is less need to return cholesterol to the liver
- •Red blood cell membranes are the biggest carrier of cholesterol in the bloodstream
- •The brain holds roughly 20 to 25 grams of the body's ~140 grams, versus 3 to 5 grams in the liver
- •Halving plasma cholesterol changes total body cholesterol by only a couple of percent
- 18:23 – 27:27
How apoB particles drive atherosclerosis and why risk varies
Dayspring explains that cholesterol entering the artery wall via apoB particles is the defining feature of atherosclerosis, and that particle concentration and defective clearance drive entry. Responding to Attia's question about people with similar lipids but different outcomes, he lists metabolic health, inflammation, blood pressure and smoking as modifiers, along with unexplained protective factors.
- •Atherosclerosis requires cholesterol buildup in the artery wall, delivered by apoB particles
- •Entry is by diffusion once apoB concentration exceeds a threshold, and the process takes decades
- •Defective clearance by LDL receptors is the main driver of high apoB
- •Insulin resistance, chronic inflammation, blood pressure, smoking and autoimmune disease affect risk
- •Dayspring advises against assuming you're one of the genetically protected people
- 27:27 – 37:44
Brain cholesterol synthesis and the apoE lipoprotein system
Attia asks whether the brain relies on peripheral cholesterol. Dayspring explains that apoB particles can't cross the blood-brain barrier, that the brain makes its own cholesterol from fetal life, and that neurons stop synthesizing it around age 10, leaving astrocytes to supply them via apoE-containing HDL-like particles.
- •Brain cholesterol has essentially no relationship to plasma cholesterol; young children with very low LDL still grow brains
- •Oligodendrocytes produce about 70% of brain cholesterol, largely for myelin
- •Neurons stop making cholesterol at about age 10 because synthesis costs over 30 ATP per molecule
- •Astrocytes package cholesterol into apoE lipoproteins that travel through the matrisome
- •Neurons take these up with LDL receptors, LRP1 and scavenger receptor B1
- 37:44 – 43:54
How the LDL receptor recognizes apoB and apoE
Attia asks how the LDL receptor binds apoB at the liver and how neurons can pull in tiny apoE particles. Dayspring describes the receptor binding domains and conformation effects on LDL clearance, then explains that LRP1, which recognizes only apoE, does most of the work in the brain.
- •A small binding domain on apoB must be properly exposed for LDL receptor recognition
- •Small or large LDLs clear less well, which can raise LDL particle counts
- •LDL receptors work as dimers, a recent NIH finding
- •LRP1 only recognizes apoE and clears chylomicrons and VLDLs in the liver
- •Neurons express LRP1 and scavenger receptor B1, so they are largely looking for apoE
- 43:54 – 47:38
Desmosterol versus lathosterol as brain synthesis markers
Attia asks about the balance between the two cholesterol synthesis pathways in the brain. Dayspring explains that peripheral cells mainly use the lathosterol pathway, while after age 10 brain cholesterol synthesis goes through desmosterol, making plasma desmosterol a marker of brain synthesis.
- •Peripheral cells mostly use the lathosterol pathway, except steroidogenic tissues
- •Mature neurons don't synthesize cholesterol, so brain synthesis runs through desmosterol
- •Plasma desmosterol correlates highly with cerebrospinal fluid and brain desmosterol
- •Plasma lathosterol mostly reflects other cells, so it isn't a brain marker
- •Dayspring says he doesn't have an answer for why evolution set it up this way
- 47:38 – 52:48
APOE genotypes and Alzheimer's risk
Attia clarifies the difference between the APOE gene and the apoE protein, and Dayspring explains how isoforms arise from single amino acid differences. Attia gives approximate population frequencies of the genotypes and the Alzheimer's risk associated with E3-E4 and E4-E4.
- •APOE in capitals is the gene; apoE is the protein
- •One amino acid difference changes the protein's shape and what it can bind
- •About 55% of people are E3-E3, 20 to 25% E3-E4, and 1 to 2% E4-E4
- •E3-E4 carries roughly two to three times the risk of E3-E3; E4-E4 is now estimated at about eight to twelve times
- •Risk rises non-linearly with E4 copies
- 52:48 – 1:00:04
HDL function beyond cholesterol and its link to the brain
Dayspring explains that HDL's importance lies in its protein cargo and roles in innate immunity rather than its cholesterol content. Very small HDL particles can cross the blood-brain barrier and join apoE brain HDLs, and he explains how apoE4 makes brain HDL dysfunctional.
- •HDL cholesterol tells little about HDL function; over 200 proteins have been found on HDL
- •The tiniest HDLs carry protective proteins and may cross the blood-brain barrier, possibly via scavenger receptor B1
- •The brain can't make apoA-I, so any apoA-I there comes from plasma
- •ApoE4 binds neuronal receptors poorly, disrupting cholesterol delivery to neurons
- 1:00:04 – 1:09:10
Cholesterol, amyloid and 24S-hydroxycholesterol clearance in Alzheimer's
Attia asks how brain cholesterol relates to amyloid and tau. Dayspring links excess neuronal membrane cholesterol to amyloid-beta 42 production and describes how neurons convert cholesterol to 24S-hydroxycholesterol for export to the liver, making it a biomarker, alongside desmosterol.
- •Autopsies of Alzheimer's brains show cholesterol-overloaded neurons
- •Too much membrane cholesterol favors secretase cleavage of amyloid precursor protein into the more toxic amyloid 42
- •Brain cholesterol has a half-life of about five years, versus days in the periphery
- •Neurons turn cholesterol into water-soluble 24S-hydroxycholesterol, which crosses the blood-brain barrier to the liver
- •Plasma 24S-hydroxycholesterol marks brain cholesterol stress but isn't commercially available
- 1:09:10 – 1:19:45
Statins and the brain: evidence and hypotheses
Attia asks what statins do in the brain. Dayspring says statins are the only apoB-lowering class that enters the brain and that all of them reduce brain cholesterol synthesis at steady state. They discuss neutral-or-beneficial trial data, hypothetical over-suppression causing brain fog, and monitoring desmosterol and 24S-hydroxycholesterol.
- •Only statins among the apoB-lowering drugs penetrate the blood-brain barrier; lipophilicity matters less at steady state
- •Meta-analyses show neutrality or benefit for dementia, though it was a secondary outcome
- •Dayspring hypothesizes brain fog reflects over-suppression of brain cholesterol synthesis
- •A study found low plasma desmosterol associated with more cognitive impairment
- •24S-hydroxycholesterol falls on statins, so desmosterol could signal over-suppression
- 1:19:45 – 1:23:58
Ezetimibe and possible cognitive benefit
Attia raises ezetimibe, which works in the gut yet may help the brain. Dayspring cites neurologists' anecdotal belief in cognitive benefit and a metabolite, ezetimibe glucuronide, that may cross the blood-brain barrier, and they discuss how it could be studied.
- •Ezetimibe itself can't cross the blood-brain barrier, but ezetimibe glucuronide can in small amounts
- •Animal studies suggest it interferes with hexokinase and brain protein glycosylation, reducing inflammation
- •Neurologists Richard Isaacson and Kelly Andiotis anecdotally see cognitive benefit
- •Dayspring says a combination of low-dose statin and ezetimibe could suit E4 carriers with apoB issues
- •Attia suggests banked serum from existing trials could be used to check p-tau and amyloid ratios
- 1:23:58 – 1:32:11
Omega-3 EPA and DHA in brain health
Attia asks about EPA and DHA for the brain. Dayspring describes how omega-3s are absorbed and delivered to the brain via phospholipid transfer protein, and both speakers discuss that evidence is mainly observational, with an omega-3 index of 8 to 9% as a target.
- •Omega-3s from triglyceride or phospholipid carriers are absorbed as free fatty acids or lysophospholipids
- •A phospholipid transfer protein delivers the lysophospholipid form to a specific receptor at the blood-brain barrier
- •Both DHA and EPA are important in the brain
- •Evidence is observational, with no level-one randomized trials for brain outcomes
- •Bill Harris's work suggests an omega-3 index of 8 to 9% is adequate; Dayspring sees little downside to trying for more
- 1:32:11 – 1:44:09
Obicetrapib, CETP inhibition and Alzheimer's biomarkers
Attia closes with obicetrapib and the BROADWAY trial. Dayspring explains that genetic CETP loss of function is linked to less cognitive impairment, that Alzheimer's biomarkers moved favorably in the trial, and that raised apoA-I might help rescue E4 brain HDL, and both discuss the need for well-designed future trials.
- •CETP loss of function genetically is associated with less Alzheimer's or cognitive impairment
- •BROADWAY biomarkers such as p-tau and amyloid 40/42 ratio moved in the right direction
- •Obicetrapib raises apoA-I and may bring protective proteins into the brain and help E4 brain HDL
- •Further cognitive and imaging studies are planned; the drug is not yet FDA approved
- •Attia stresses patient selection and duration, citing PREDIMED as an example of choosing the right population
