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PMID: 8710919 Published · ppublish English Journal Article Research Support, Non-U.S. Gov't

Megalin-mediated endocytosis of transcobalamin-vitamin-B12 complexes suggests a role of the receptor in vitamin-B12 homeostasis.

Proceedings of the National Academy of Sciences of the United States of America ·Vol. 93 ·No. 16 ·1996-08-06 ·Pages 8612-7

Moestrup SK, Birn H, Fischer PB, Petersen CM, Verroust PJ, Sim RB, Christensen EI, Nexø E

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.

MeSH Terms
Animals Cell Membrane/metabolism Cells, Cultured Endocytosis Endodermal Sinus Tumor/metabolism Heymann Nephritis Antigenic Complex Homeostasis Kidney Cortex/metabolism Kidney Tubules, Proximal/metabolism Kinetics Male Membrane Glycoproteins/metabolism,physiology Protein Binding Rabbits Rats Rats, Wistar Receptors, LDL/physiology Transcobalamins/metabolism Vitamin B 12/physiology
Chemicals
Heymann Nephritis Antigenic Complex Membrane Glycoproteins Receptors, LDL Transcobalamins Vitamin B 12
Authors & Affiliations
8 authors, click to expand affiliations / ORCID
Moestrup S K
Department of Medical Biochemistry, Kommunehospitalet University of Aarhus, Denmark.
Birn H
Fischer P B
Petersen C M
Verroust P J
Sim R B
Christensen E I
Nexø E
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Article Info
Journal
Proceedings of the National Academy of Sciences of the United States of America
Abbr.
Proc Natl Acad Sci U S A
ISSN
0027-8424
DOI
10.1073/pnas.93.16.8612
Published
1996-08-06
Pages
8612-7
Language
English
Region
United States
NLM ID
7505876
PMCID
PMC38721
Subset
IM
External Links
PubMed source DOI: 10.1073/pnas.93.16.8612 PMC Full Text View journal details
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