Deposition of amyloid beta peptide (Abeta) in vascular CNS tissues and the choroid plexus (CP) occurs during normal aging and is accelerated by Alzheimer's disease (AD). Recent studies suggest major roles for the blood-brain barrier (BBB) and the cerebrospinal fluid (CSF) clearance in regulating the concentrations of Abeta in the CNS. The CP has important functions in regulating the levels of several proteins in the CSF and brain, and in cleansing the CSF of waste and potentially cytotoxic substances for brain. The role of CP in Abeta CNS homeostasis is largely unknown. Our preliminary in vivo data in guinea pigs indicate that glycoprotein 330 and possibly some other members of the low density lipoprotein receptor family participate in CP epithelial internalization of lipid-free Abeta/1-40 complexed to apolipoproteins (apo) J and E4, respectively. The CP uptake of blood-borne lipidated apoE3 and apoE4 was up to 4.5-fold greater compared to their respective dilapidated monomers. Circulating unbound free Abeta/1-40 is prevented from crossing the blood-CSF barrier, in contrast to its rapid transport out of the CSF, remarkable accumulation of CNS-derived Abeta by the CP epithelium and significant uptake by leptomeningeal vessels. Preliminary attempts to develop an in vitro transport model of the blood-CSF barrier constructed from cultured human CP epithelial cells confirmed asymmetrical Abeta transcytosis across human CP epithelium consistent with in vivo data. We propose to use in vivo vascular brain/CP and ventriculo-cisternal perfusions in guinea-pigs and an in vitro transport model of human blood-CSF barrier from control and AD subjects to test the hypothesis that the CP regulates Abeta CNS homeostasis by controlling transport in and out of the CSF and brain of lipid-free and lipid-bound apoJ and different apoE isoforms free and complexed to Abeta. We will determine in vivo basolateral CP uptake, metabolism and transport across the blood-CSF barrier of circulating apoJ, E2, E3 and E4, delipidated, lipidated, free and complexed to Abeta (aim 1), and their clearance from the CSF, uptake at the apical side of the CP and sequestration by leptomeningeal and parenchymal microvessels (aim 2). In aims 3 and 4 we will study the CP epithelial basolateral-to-apical and apical-to-basolateral Abeta/apolipoprotein binding, endocytosis and transcytosis using human in vitro model. The role of different lipoprotein and Abeta receptors will be defined. The studies will provide the molecular basis to understand the CP functions involved in the regulation of the CSF and CNS levels of Abeta to minimize its CNS accumulation and pathogenic effects.