ApoA-I and apoJ modulate amyloid beta metabolism at the blood-brain barrier

The overproduction and accumulation of the amyloidogenic product of amyloid precursor protein (APP), amyloid- beta peptide (A-beta) in the brain is gaining significant support as the primary causative agent of Alzheimer`s disease (AD). Strikingly, A-beta accumulation in cerebral capillaries (causing cerebral amyloid angiopathy, CAA) significantly correlates with common AD criteria, implying a thus far underestimated role of the blood brain barrier (BBB) in the pathogenesis of the disease. In addition, it has become increasingly evident that specific, in the periphery mainly high-density lipoprotein (HDL)-associated apolipoproteins are involved in protective mechanisms not only against atherosclerosis, but also against AD development. Although striking evidence exists for a close connection between peripheral and cerebral apolipoprotein-, cholesterol-, and APP/A-beta metabolism, information about the interconnecting mechanisms occurring in or mediated by brain capillary endothelial cells (BCEC) - the anatomical basis of the BBB - is lacking. The central hypothesis of the present proposal is thus that the BBB plays an important role in the pathogenesis of AD, (i) because it may control the clearance of A-beta peptides from the brain; (ii) it restricts exchange of (lipoprotein) cholesterol between the circulation and the brain; and (iii) both, APP and apolipoproteins - namely apoA-I and apoJ - are synthesized by BCEC. The goals of the present proposal are to identify the roles of apoA-I and apoJ in APP/Aß synthesis and turnover at the BBB in vitro and in vivo by using a combination of established in vitro BBB models consisting of primary porcine (p) and murine (m)BCEC, and the appropriate animal models (3xTg-AD mice, APP-tg mice, apoA-I-tg, apoA-I-ko and apoJ-ko mice). To achieve these goals we will address the following specific research questions: 1. Is pBCEC-derived apoA-I and/or apoJ involved in cholesterol and APP/Aß metabolism at the BBB in vitro? 2. In which way(s) do plasma-derived apoA-I and apoJ modulate cholesterol and APP/Aß metabolism and transport at the BBB in vitro? 3. What is the impact of apoA-I and/or apoJ knock-out or overexpression of human apoA-I in cerebrovascular APP/Aß metabolism and cognitive performance in AD model mice? 4. Which (novel) cellular receptors and/or binding proteins relate to apolipoprotein metabolism and are involved in Ab turnover at the BBB?

The present project investigated functions of two abundant proteins, namely apolipoprotein (apo)A-I and apoJ in the processes of amyloid peptide (A) production as well as elimination from the brain by distinct endothelial cells forming the blood-brain barrier (BBB). ApoA-I is the major component of high-density lipoproteins (HDL) in the blood and is under intense investigation by cardiovascular researchers for its ability to protect against heart disease. However, several studies over the last decade suggest that apoA-I might also help defend the brain against cognitive deficits associated with Alzheimers-like pathology. Alzheimers disease (AD) is the most common cause of dementia and is associated with neuropathological hallmarks such as neurofibrillary tangles and amyloid senile plaques. Increasing evidence shows that overproduction and accumulation of Aß plays a pivotal role in the pathogenesis of AD. Strikingly, Aß accumulation in cerebral capillaries (causing cerebral amyloid angiopathy, CAA) significantly correlates with common AD criteria, thus implying an underestimated role of the blood-brain barrier (BBB) in the pathogenesis of AD. Apolipoproteins are crucial transporters of not only cholesterol but also Aß in the central nervous system (CNS). Another apolipoprotein, also present in HDL but particularly abundant in the brain is apoJ (clusterin). ApoJ levels are increased in AD brains and in plasma of CAA patients. ApoJ may bind, prevent fibrillization, and enhance clearance of Aß. Our studies revealed that porcine brain capillary endothelial cells (pBCEC) synthesize and release apoA-I and apoJ and actively contribute to Aß synthesis. Further, our studies showed that modulation of cellular cholesterol homeostasis using certain pharmacological or natural compounds significantly affects Aß synthesis and transport at the BBB in vitro and in AD model mice. We found that, thereby, apoA-I, apoJ and other HDL-associated proteins contribute to redirect Aß synthesis and processing towards a beneficial and non-amyloidogenic pathway. The identification of proteins that bind Aß and modulate its aggregation, elimination across the BBB, and cyto/neurotoxicity will provide a platform for the development of potential novel approaches for preventative and therapeutic intervention in AD.