PICALM, the gene encoding phosphatidylinositol binding clathrin assembly (picalm) protein, plays a key role in endocytosis, a process which regulates the function of cell receptors and synaptic transmission. PICALM is one of the most highly validated Alzheimer’s disease (AD) risk factors. Its role in AD, however, is unknown. A recent genome-wide screen for modifiers of amyloid-b peptide (Aβ) toxicity in yeast has identified the key role of the yeast homologue of PICALM. This study has shown that PICALM efficiently controls Aβ toxicity in yeast, nematode models and mammalian neurons by regulating endocytosis-dependent cell vulnerability to Aβ. Our preliminary data in human and mouse brain show that picalm protein is most abundantly expressed in blood vessels which have been shown to provide a major pathway for Aβ removal from brain into the bloodstream. Therefore, picalm in brain endothelium is ideally situated to participate in Aβ clearance from brain. Interestingly, our pilot data also show significantly reduced picalm expression in brain vessels in AD. Previous findings have established that low density lipoprotein receptor (LRP) in brain endothelium mediates vascular clearance of Aβ from brain via transport across the brain capillary endothelium, a site of the blood-brain barrier (BBB) in vivo. Our preliminary data using human brain endothelial cells show that PICALM is required for rapid endothelial internalization of Aβ after its initial binding to LRP. The current proposal will determine the role of PICALM in regulating internalization and transcellular transport (transcytosis) of LRP-bound Aβ across the endothelial cell wall of the BBB in vitro and in vivo. To test our hypotheses we will use a human model of the BBB and a mouse model of Aβ clearance, both developed in our laboratory. In collaboration with Dr Tanzi we will study the effects of novel PICALM mutations on amyloid-β vascular clearance once the sequence of these functional variants/mutations becomes available. The proposed studies will represent a novel advance in our understanding of the molecular regulation of CNS Aβ homeostasis and will demonstrate a pivotal role of PICALM in controlling brain Aβ.