The simple assumption would be that, in the same way as a high blood level of cholesterol damages the vascular endothelium in the periphery, it also damages the brain vasculature, hence increasing the risk for VD. However, it is also LY2157299 possible that abnormal cholesterol metabolism has a direct effect on the brain not mediated by its effect

on the cerebral vasculature. This is supported by data showing that brain cholesterol metabolism and transfer is at least partly independent of systemic cholesterol metabolism. The main source of brain cholesterol is de novo synthesis in the brain itself, rather than transport from plasma,53,54 which possesses a distinct Inhibitors,research,lifescience,medical set of lipoproteins.55,56 Furthermore, it is assumed that the major role of the apolipoproteins implicated in AD in the brain is redistribution Inhibitors,research,lifescience,medical of cholesterol between different brain compartments rather than transfer to and from the plasma.57 There exist a number of hypotheses explaining

the direct effect of cholesterol on the brain and on brain pathological processes. The degree of activity of the different amyloid precursor proteins (APPs) cleaving enzymes varies according Inhibitors,research,lifescience,medical to the surrounding lipid moiety: environments richer in cholesterol promote β- and γ-secretase, which produce insoluble amyloid plaques. Furthermore, β-amyloid also acts as a seed for the amyloid plaque in a lipidrich membrane.58 On the other hand, conditions poorer in cholesterol promote α-secretase activity, which docs not create plaques.59-61 Hence brain cholesterol metabolism has an independent, effect on amyloid plaque Inhibitors,research,lifescience,medical formation, not mediated by vascular pathology, thus potentially directly contributing to AD pathology. Interestingly, recent, studies have shown a decrease in AD prevalence among

patients treated with cholesterollowering Inhibitors,research,lifescience,medical drugs from the statin group.62,63 Statins are compounds that inhibit HMG CoA (3-hydroxy-3-methyl-glutaryl coenzyme A) reductase, a enzyme central to the process of de novo cholesterol synthesis. Studies have shown that statins lower the risk of developing dementia most independently of their effect, on plasma lipid levels63,64 or exposure to other lipid-lowering drugs.63 These results suggest that statins have properties additional to their systemic lipid-lowering effect, some of which are probably associated with central nervous system (CNS) protection.65 Taken together, these lines of evidence suggest that, apart from its atherogenic effect, cholesterol is involved in several metabolic pathways in the brain, some of which may be relevant to the pathological process associated with plaque and tangle formation. The relationship between AD and apolipoprotein E (ApoE) also indicates a direct role for abnormal lipoprotein metabolism on AD pathology that, is not mediated by vascular lesions. ApoE’ is a protein involved in lipid transport and has three isoforms: ApoE2, ApoE3, and ApoE4.