Glycoprotein 330/Megalin: Probable Role in Receptor-Mediated Transport of Apolipoprotein J Alone and in a Complex with Alzheimer Disease Amyloid β at the Blood-Brain and Blood-Cerebrospinal Fluid Barriers

A soluble form of Alzheimer disease amyloid β -protein (sAβ ) 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 sAβ 1-40-apoJ complexes across the blood-brain barrier and the blood-cerebrospinal fluid barrier, but the mechanisms remain poorly understood. In the present study, we examined the transport process in greater detail and investigated the possible role of glycoprotein 330 (gp330) / megalin, a receptor for multiple ligands, including apoJ. High-affinity transport systems with a Km of 0.2 and 0.5 nM were demonstrated for apoJ at the blood-brain barrier and the choroid epithelium in vivo, suggesting a specific receptor-mediated mechanism. The sAβ 1-40-apoJ complex shared the same transport 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 sAβ 1-40-apoJ complexes were markedly inhibited (74-99%) in the presence of a monoclonal antibody to gp330/megalin and were virtually abolished by perfusion with the receptor-associated protein, which blocks binding of all known ligands to gp330. Western blot analysis of cerebral microvessels with the monoclonal antibody to gp330 revealed a protein with a mass identical to that in extracts of kidney membranes enriched with gp330 / megalin, but in much lower concentration. The findings suggest that gp330 / megalin mediates cellular uptake and transport of apoJ and sAβ 1-40-apoJ complex at the cerebral vascular endothelium and choroid epithelium.