Glycoprotein 330/Megalin (LRP-2) Has Low Prevalence as mRNA and Protein in Brain Microvessels and Choroid Plexus

Prior studies indicated that glycoprotein 330 (gp330)/megalin mediates transcytosis of apolipoprotein J (apoJ) with Alzheimer's amyloide-peptide (Aβ) across the vascular membranes of the central nervous system (CNS). Here we show the presence of gp330/megalin mRNA and gp330-like immunoepitopes in brain capillaries and choroid plexus and their absence from brain parenchyma. By polymerase chain reaction (PCR) we estimated 1.2 × 105molecules (1 pg) of gp330/megalin mRNA/μg total brain capillary RNA, which is 3% of that in kidney RNA. However, gp330 mRNA was not detected byin situhybridization in vascular CNS tissue, presumably because of low transcript prevalence. The ratio of gp330 protein:RNA was 17-fold higher in choroid plexus vs brain capillaries, which implies tissue specific regulation of the protein and mRNA prevalence. We conclude that gp330/megalin mRNA and protein are expressed in brain capillaries and choroid plexus in small amounts that are consistent with the observed activities of this endocytosing receptor in the regulation of apoJ and Aβ uptake by the CNS.