Nuclear receptors as lipid sensors at the blood-brain barrier

Nuclear receptors represent a large super family of ligand-dependent transcription factors that regulate the expression of target genes to affect diverse metabolic processes. Two families, peroxisome-proliferator activated receptors (PPARs) and liver X receptors (LXRs), control the transcription of key-genes involved in lipid metabolism, transport, and elimination. PPARs and LXRs are nuclear receptors that control central pathways in (chole)sterol and fatty acid metabolism. The goal of this proposal is to characterize ligand-activated regulation of these nuclear receptors and to define their role as lipid-sensors and regulators of cholesterol and fatty acid metabolism at the blood-brain barrier (BBB). The proposed role of PPARs and LXRs at the BBB is based on the following background: (i) Several neurodegenerative disorders are tightly coupled to functional lipid and lipoprotein metabolism; (ii) Brain function and development rely on a supply with essential polyunsaturated fatty acids from the circulation; (iii) Excess brain cholesterol is eliminated after oxidative conversion to 24(S)OH- cholesterol; (iv) Brain endothelial cells express target genes for nuclear receptors that display gate-keeping functions in peripheral reverse cholesterol transport and/or in fatty acid uptake. The central hypothesis underlying the present application suggests that LXR and PPAR activation regulates reverse cholesterol transport and fatty acid supply at the BBB. Therefore, these nuclear receptors may provide promising drug targets for the treatment of lipid-related neurodegenerative disorders in the near future. In order to validate this hypothesis the following AIMS will be explored: 1. Identification of the most potent endogenous ligands and therapeutic drugs for activation of the nuclear receptors PPARa, b, g and LXRa, b in porcine brain capillary endothelial cells. 2. Evaluation of nuclear receptor-mediated regulation of the expression of specific target genes taking central roles during lipid turnover at the BBB. 3. Effects of (synthetic) agonists for PPARs and LXRs on elimination of cholesterol and 24(S)OH-cholesterol (i.e., on reverse cholesterol transport) across an in vitro model of the BBB. 4. Effects of nuclear receptor ligands on the supply of essential polyunsaturated fatty acids across an in vitro model of the BBB. 5. Identification of cerebral and brain-microvascular PPAR and LXR target genes via large-scale gene expression profiling in the mouse model.

Nuclear receptors represent a large super family of ligand-dependent transcription factors that regulate the expression of target genes to affect diverse metabolic processes. Two families, peroxisome-proliferator activated receptors (PPARs) and liver X receptors (LXRs), control the transcription of key-genes involved in lipid metabolism, transport, and elimination. PPARs and LXRs are nuclear receptors that control central pathways in (chole)sterol and fatty acid metabolism. The goal of this proposal is to characterize ligand-activated regulation of these nuclear receptors and to define their role as lipid-sensors and regulators of cholesterol and fatty acid metabolism at the blood-brain barrier (BBB). The proposed role of PPARs and LXRs at the BBB is based on the following background: (i) Several neurodegenerative disorders are tightly coupled to functional lipid and lipoprotein metabolism; (ii) Brain function and development rely on a supply with essential polyunsaturated fatty acids from the circulation; (iii) Excess brain cholesterol is eliminated after oxidative conversion to 24(S)OH- cholesterol; (iv) Brain endothelial cells express target genes for nuclear receptors that display gate-keeping functions in peripheral reverse cholesterol transport and/or in fatty acid uptake. The central hypothesis underlying the present application suggests that LXR and PPAR activation regulates reverse cholesterol transport and fatty acid supply at the BBB. Therefore, these nuclear receptors may provide promising drug targets for the treatment of lipid-related neurodegenerative disorders in the near future. In order to validate this hypothesis the following AIMS will be explored: 1. Identification of the most potent endogenous ligands and therapeutic drugs for activation of the nuclear receptors PPARa, b, g and LXRa, b in porcine brain capillary endothelial cells. 2. Evaluation of nuclear receptor-mediated regulation of the expression of specific target genes taking central roles during lipid turnover at the BBB. 3. Effects of (synthetic) agonists for PPARs and LXRs on elimination of cholesterol and 24(S)OH-cholesterol (i.e., on reverse cholesterol transport) across an in vitro model of the BBB. 4. Effects of nuclear receptor ligands on the supply of essential polyunsaturated fatty acids across an in vitro model of the BBB. 5. Identification of cerebral and brain-microvascular PPAR and LXR target genes via large-scale gene expression profiling in the mouse model.