GLYCOPROTEIN 330 MEGALIN - PROBABLE ROLE IN RECEPTOR-MEDIATED TRANSPORT OF APOLIPOPROTEIN-J ALONE AND IN A COMPLEX WITH ALZHEIMER-DISEASE AMYLOID-BETA AT THE BLOOD-BRAIN AND BLOOD-CEREBROSPINAL FLUID BARRIERS/

A soluble form of Alzheimer disease amyloid beta-protein (sA beta) is transported in the blood and cerebrospinal fluid mainly complexed with apolipoprotein J (apoJ). Using a well-characterized in situ perfused guinea pig brain model, we recently obtained preliminary evidence that apoJ facilitates transport of sA beta(1-40)-apoJ complexes across the blood-brain barrier and the blood-cerebrospinal fluid barrier, but th e mechanisms remain poorly understood. In the present study, we examin ed the transport process in greater detail and investigated the possib le role of glycoprotein 330 (gp330)/megalin, a receptor for multiple l igands, including apoJ. High-affinity transport systems with a K-m of 0.2 and 0.5 nM were demonstrated for apoJ at the blood-brain barrier a nd the choroid epithelium in vivo, suggesting a specific receptor-medi ated mechanism. The sA beta(1-40)-apoJ complex shared the same transpo rt mechanism and exhibited 2.4- to 10.2-fold higher affinity than apoJ itself. Binding to microvessels, transport into brain parenchyma, and choroidal uptake of both apoJ and sA beta(1-40)-apoJ complexes were m arkedly inhibited (74-99%) in the presence of a monoclonal antibody to gp330/megalin and were virtually abolished by perfusion with the rece ptor-associated protein, which blocks binding of all known ligands to gp330. Western blot analysis of cerebral microvessel with the monoclon al antibody to gp330 revealed a protein with a mass identical to that in extracts of kidney membranes enriched with gp330/megalin, but in mu ch lower concentration. The findings suggest that gp330/megalin mediat es cellular uptake and transport of apoJ and sA beta(1-40)-apoJ comple x at the cerebral vascular endothelium and choroid epithelium.