SummaryEpidemiological and clinical studies during the last several years have suggested a link between cholesterol metabolism and Alzheimer’s disease (AD) pathogenesis. Despite the therapeutic potential of this link, the mechanisms by which cholesterol metabolism influence AD pathogenesis remain uncertain. Drugs that ameliorate AD phenotypes by interfering cholesterol metabolism have been suggested, but the rationale for using those is poorly defined and in all of the cases the molecular mechanisms which account for their beneficial effects are virtually unknown. As a result it is difficult, if possible at all, to outline drug development and discovery strategies based on molecular, cellular and pharmacological principles.For several years now we have been investigating the role of nuclear Liver X Receptors (LXR), and metabolic pathways controlled by their major responsive genes, in AD pathogenesis. LXRa and LXRb, are transcription factors that control the expression of genes involved in cholesterol metabolism. We have found that activated LXR regulate metabolic pathways of brain cholesterol intra- and extracellular transport that influence Amyloid Precursor Protein (APP) proteolytic processing, b-amyloid deposition and its clearance from brain. The results from our studies have been confirmed by other groups using different approaches and different animal models. We and others have shown for example, that treatments of neuronal cell lines and primary neurons with natural or synthetic LXR ligands decrease Ab secretion. The effect was attributed primarily to the transcriptional upregulation of ABCA1 cholesterol transporter and other LXR responsive genes, cholesterol and phospholipids efflux and their membrane redistribution with changes in activities of proteolytic enzymes responsible for Ab generation. The result is a shift of APP processing towards the nonamyloidogenic pathway. Moreover, we have demonstrated that in vivo treatment of young AD model mice with a synthetic LXR ligand, increased the expression of LXR responsive genes in CNS and decreased soluble b-amyloid levels in their brains. Our studies, confirmed by two other groups using different APP transgenic mice, show that ABCA1 deficiency in APP expressing mice leads to a dramatic decrease of soluble apoE and an increased deposition of Ab in the brain. More recently we have expanded the experimental approaches and addressed questions related to gene expression levels. We have found that the application of LXR ligands influenced secretion of inflammatory cytokines and increased neuronal survival following Ab or LPS treatment. Finally, our preliminary data show that 2 years old APP expressing, ABCA1 heterozygous (APP23/ABCA1+/-) mice, together with unique phenotype have memory deficits. The result from these behavioral experiments are particularly important in the context of recently published studies on the effect of T0901317 which completely reversed the memory deficits in 20 weeks old AD model mice. LXR ligands are considered one of the most promising targets for prevention and treatment of atherosclerosis and most recently together with ABCA1 have emerged as such for Alzheimer disease. Until now, however, LXR ligands for clinical use are not available on the market.Altogether the data as outlined above, as well as the preliminary unpublished results generated in our laboratory substantiate the design of a drug discovery study for identification of LXR ligands and their systematic evaluation in well established in vitro and in vivo model systems for APP processing, Ab secretion/aggregation, amyloid deposition and clearance. We believe the screening and validation methodologies as outlined in our proposal meet the criteria to screen natural and synthetic libraries and to identify compounds with inhibitory effect on Ab formation and deposition, at the same time facilitating clearance of amyloid deposits in the brain.