Megalin-mediated Endocytosis of Transcobalamin-vitamin-B12 Complexes Suggests a Role of the Receptor in Vitamin-B12 Homeostasis

Overview

Journal Proc Natl Acad Sci U S A

Specialty Science

Date 1996 Aug 6

PMID 8710919

Citations 46

Authors

S K Moestrup

H Birn

P B Fischer

C M Petersen

P J Verroust

R B Sim

E I Christensen

E Nexo

Affiliations

Soon will be listed here.

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.

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