‘Chicken and Egg’
“There is a strong link between the brain and the heart, and we know that cardiovascular risk factors have an overlap with dementia risk factors — especially vascular dementia,” said Dr. Mills.
She explained that an atherogenic lipid profile results in narrowing of the arteries, with less blood reaching the brain. “This can lead to stress in the brain, which drives inflammation and pathology.”
But cholesterol itself plays an important role in inflammation, Dr. Hansen said. In the periphery, it is “part of an integral response to tissue damage and infection.”
In the brain, once cholesterol is synthesized by the astrocytes, it is transported to neurons via the apolipoprotein E (APOE) protein, which plays a role in brain cholesterol homeostasis, Dr. Mills explained. Those with the ε4 allele of APOE (APOE4) tend to have faultier transport and storage of lipids in the brain, relative to the other APOE variants.
It’s known that individuals with APOE4 are particularly vulnerable to late-onset AD, Dr. Hansen observed. By contrast, APOE2 has a more protective effect. “Most people have APOE3, which is ‘in between,’ ” he said.
When there is neuronal uptake of “invading cholesterol,” not only is amyloid produced but also neuroinflammatory cytokines, further driving inflammation. A vicious cycle ensues: Cholesterol induces cytokine release; and cytokine release, in turn, induces cholesterol synthesis — which “suggests an autocatalytic function of cholesterol in the escalation of inflammation,” Dr. Hansen suggested. He noted that permeability of the BBB also allows inflammatory cytokines from elsewhere in the body to invade the brain, further driving inflammation.
Dr. Mills elaborated: “We know that generally, in dementia, there appear to be some changes in cholesterol metabolism in the brain, but it’s a chicken-and-egg question. We know that as the disease progresses, neurons are dying and getting remodeled. Do these changes have to do with the degenerative process, or are the changes in the cholesterol metabolism actually driving the degenerative disease process? It’s probably a combination, but it’s unclear at this point.”
Lipids in Plasma vs CSF
Dr. Mills explained that HDL particles in the brain differ from those in the periphery. “In the CNS, you have ‘HDL-like particles,’ which are similar in size and composition [to HDL in the periphery] but aren’t the same particles.” The brain itself generates HDL-like lipoproteins, which are produced by astrocytes and other glial cells and found in cerebrospinal fluid (CSF).
Dyslipidemia in the periphery can be a marker for cardiovascular pathology. In the brain, “it can be an indication that there is active damage going on, depending on which compartment you’re looking at.”
She noted that plasma lipid levels and brain CSF lipid levels are “very different.” Research suggests that HDL in the CSF exhibits similar heterogeneity to plasma HDL, but these CSF lipoproteins present at 100-fold lower concentrations, compared to plasma HDL and have unique combinations of protein subpopulations. Lipidomics analysis studies show that these compartments “get very different readings, in terms of the predominant lipid disease state, and they are regulated differently from the way lipids in the periphery are regulated.”
In the brain, the cholesterol “needs to get shuttled from glial cells to neurons,” so defects in the transport process can disrupt overall brain homeostasis, said Dr. Mills. But since the brain system is separate from the peripheral system, measuring plasma lipids is more likely to point to cardiovascular risks, while changes reflected in CSF lipids are “more indicative of alteration in lipid homeostasis in the brain.”