Cholesterol Metabolism
A defect in cholesterol metabolism is an appealing hypothesis because it ties together the apolipoprotein E (APOE) genetic risk, amyloid production and aggregation, and vasculopathy of Alzheimer's disease. However, proof is also lacking for this hypothesis. Cholesterol is an essential component of neuronal membranes and is concentrated in sphingolipid islands termed “lipid rafts.” Rafts are ordered platforms for the assembly of β-secretases and γ-secretases and processing of amyloid precursor protein into Aβ168 (Figure 1 and Figure 2). Aβ generation and aggregation are promoted and clearance from the brain is reduced when an overabundance of esterified cholesterol decreases membrane lipid turnover. Glial-derived APOE is the primary cholesterol transporter in the brain. A major determinant of the risk of late-onset Alzheimer's disease is the APOE isoform inheritance pattern (APOE2, APOE3, or APOE4)169; a single E4 allele increases the risk by a factor of 4, and two E4 alleles increase the risk by a factor of 19.170 APOE4 is not only a pathological chaperone, promoting Aβ deposition171 and tau phosphorylation,172 but it is also the least effective of the three in promoting healthy membrane lipid turnover and the uptake of lipoprotein particles.
High serum cholesterol levels in midlife increase the risk of Alzheimer's disease.173 In observational studies, use of statins was shown to be associated with a reduced risk. Statins appear to reduce the membrane pool of free cholesterol.174 Other actions of statins that are not dependent on cholesterol include reductions in inflammation175 and isoprenoids and up-regulation of both α-secretase176 and vascular function. One prospective trial of statins showed cognitive improvements in patients with mild Alzheimer's disease,177 but a recent multicenter trial did not.178 Thus, the benefit of statins remains controversial. An alternative pharmacologic approach is to limit the esterification of cholesterol.179 Improvement of membrane biophysics and function through ingestion of n–3 fatty acid supplements has also been studied (NCT00440050).180