title: Megalin-mediated endocytosis of transcobalamin-vitamin-B12 complexes suggests a role of the receptor in vitamin-B12 homeostasis.
abstract: Kidney cortex is a main target for circulating vitamin B12 (cobalamin) in complex with transcobalamin (TC). Ligand blotting of rabbit kidney cortex with rabbit 125I-TC-B12 and human TC-57Co-B12 revealed an exclusive binding to megalin, a 600-kDa endocytic receptor present in renal proximal tubule epithelium and other absorptive epithelia. The binding was Ca2+ dependent and inhibited by receptor-associated protein (RAP). Surface plasmon resonance analysis demonstrated a high-affinity interaction between purified rabbit megalin and rabbit TC-B12 but no measurable affinity of the vitamin complex for the homologous alpha 2-macroglobulin receptor (alpha 2MR)/low density lipoprotein receptor related protein (LRP). 125I-TC-B12 was efficiently endocytosed in a RAP-inhibitable manner in megalin-expressing rat yolk sac carcinoma cells and in vivo microperfused rat proximal tubules. The radioactivity in the tubules localized to the endocytic compartments and a similar apical distribution in the proximal tubules was demonstrated after intravenous injection of 125I-TC-B12. The TC-B12 binding sites in the proximal tubule epithelium colocalized with megalin as shown by ligand binding to cryosections of rat kidney cortex, and the binding was inhibited by anti-megalin polyclonal antibody, EDTA, and RAP. These data show a novel nutritional dimension of megalin as a receptor involved in the cellular uptake of vitamin B12. The expression of megalin in absorptive epithelia in the kidney and other tissues including yolk sac and placenta suggests a role of the receptor in vitamin B12 homeostasis and fetal vitamin B12 supply.
authors.affiliation: Department of Medical Biochemistry, Kommunehospitalet University of Aarhus, Denmark.
authors.forename: S K
authors.initials: SK
authors.lastname: Moestrup
authors.forename: H
authors.initials: H
authors.lastname: Birn
authors.forename: P B
authors.initials: PB
authors.lastname: Fischer
authors.forename: C M
authors.initials: CM
authors.lastname: Petersen
authors.forename: P J
authors.initials: PJ
authors.lastname: Verroust
authors.forename: R B
authors.initials: RB
authors.lastname: Sim
authors.forename: E I
authors.initials: EI
authors.lastname: Christensen
authors.forename: E
authors.initials: E
authors.lastname: Nexø
mesh_terms: D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D004705:Endocytosis; D018240:Endodermal Sinus Tumor / Q000378:metabolism; D026541:Heymann Nephritis Antigenic Complex; D006706:Homeostasis; D007672:Kidney Cortex / Q000378:metabolism; D007687:Kidney Tubules, Proximal / Q000378:metabolism; D007700:Kinetics; D008297:Male; D008562:Membrane Glycoproteins / Q000378:metabolism / Q000502:physiology*; D011485:Protein Binding; D011817:Rabbits; D051381:Rats; D017208:Rats, Wistar; D011973:Receptors, LDL / Q000502:physiology; D014155:Transcobalamins / Q000378:metabolism*; D014805:Vitamin B 12 / Q000502:physiology*
chemical_list: D026541:Heymann Nephritis Antigenic Complex; D008562:Membrane Glycoproteins; D011973:Receptors, LDL; D014155:Transcobalamins; D014805:Vitamin B 12