The functional cobalamin (vitamin B12)–intrinsic factor receptor is a novel complex of cubilin and amnionless

Abstract

Introduction Cobalamin (vitamin B 12 ) is a coenzyme for the enzymes of intermediate metabolism, methionine synthase, and methylmalonyl-CoA mutase, and deficiency of the vitamin leads to potentially lethal manifestations such as megaloblastic anemia and severe combined degeneration of the central nervous system. Cobalamin deficiency, which is one of the most common vitamin-deficiency diseases, is most often due to failure at a step in the complicated and highly specific gastrointestinal uptake mechanisms for dietary cobalamin rather than an insufficient supply from food. 1 Intrinsic factor (IF) is a glycoprotein produced in the gastric epithelium. It tightly binds to cobalamin in the gastrointestestinal tract, and in the distal small intestine the IF-cobalamin complex is recognized by cubilin, a multiligand apical membrane protein that participates in endocytosis of the complex. 2,3 IF is subsequently degraded in enterocyte lysosomes, and cobalamin is secreted into plasma in complex with transcobalamin-II. 4 Cubilin is a large membrane protein (460 kDa) with a unique set of extracellular protein modules comprising 8 tandem epidermal growth factor domains followed by 27 tandem CUB domains (initially found in complement components C1r/C1s, Uegf, and bone morphogenic protein-1) harboring the IF-cobalamin binding site (CUB domains 5-8). 5,6 Although cubilin has no apparent transmembrane segment or cytoplasmic tail, several studies have shown that binding of IF-cobalamin to cubilin leads to endocytosis of the ligand and recycling of the receptor. 2,3 Besides expression and function in the intestine, cubilin has many-fold higher expression in the apical membrane of kidney proximal tubule and rodent yolk sac epithelial cells. 2,3,7,8 Consistent with this pattern of expression, cubilin is involved in reabsorption of several specific nutrient-carrying proteins from renal glomerular filtrate, including albumin, 9 transferrin, 10 vitamin D-binding protein, 11 and apolipoprotein AI, 12,13 and cubilin has a crucial but not-yet-defined role in early embryonic development of rodents. 14 Evidence to date indicates that the mechanism of cubilin-mediated endocytosis is the same in the various cubilin-expressing epithelia. 2 Accordingly, IF-cobalamin is effectively endocytosed in a cubilin-dependent manner in the proximal tubule 15 and yolk sac, 16 and because these tissues have higher density of cubilin in apical membranes and less luminal proteolytic activity than intestine, they have been the preferred tissues for studying cubilin function. 2,8,[14][15][16] However, it should be noted that ligands other than IF-cobalamin are likely to be physiologically more important in the kidney and yolk sac because little or no gastric IF circulates in plasma. An Inside Blood analysis of this article appears in the front of this issue.Reprints: Søren K. Moestrup, Institute of Medical Biochemistry, University of Aarhus, 8000 Aarhus C, Denmark; e-mail: skm@biobase.dk.The publication costs of this article were defrayed in part by page charge ...

Luminal expression of cubilin is impaired in Imerslund-Grasbeck syndrome with compound AMN mutations in intron 3 and exon 7

“…7 Amnionless (AMN), originally identified as a visceral endoderm specific protein essential for embryonic development, is a 50 kDa transmembrane protein expressed in polarized epithelia that binds tightly to the cubilin N-terminal end. 8,9 Mutations in CUBN and AMN account for 54.8% (23/42) and 23.8% (10/42) of IGS, respectively, whereas mutations in other still unidentified genes account for 21% (9/42). 10 More recently, IGS was caused by a compound heterozygous mutation in CUBN comprising a missense mutation in the paternal allele (c.1010 C>T) and a partial gene deletion in the maternal allele.…”

“…4,5,6 IGS has also been explained as mutations in the cubilin gene (CUBN OMIM # 602997) located on chromosome 10 or in the amnionless gene (AMN OMIM # 605799) located on chromosome 14. [7][8][9][10] These two proteins are parts of the IF-Cbl receptor complex, also called cubam, which is expressed in the ileal mucosa and in the renal proximal tubules. 7,9 Cubilin is a 460 kDa multiple ligand binding protein, which cannot internalize the IF-Cbl complex by itself.…”

“…[7][8][9][10] These two proteins are parts of the IF-Cbl receptor complex, also called cubam, which is expressed in the ileal mucosa and in the renal proximal tubules. 7,9 Cubilin is a 460 kDa multiple ligand binding protein, which cannot internalize the IF-Cbl complex by itself. 7 Amnionless (AMN), originally identified as a visceral endoderm specific protein essential for embryonic development, is a 50 kDa transmembrane protein expressed in polarized epithelia that binds tightly to the cubilin N-terminal end.…”

“…9 But its role in the mechanisms of the Cbl malabsorption of IGS has not yet been documented in humans. In the present study, we describe a compound heterozygous mutation in AMN and its consequence on the activity of intrinsic factor receptor that illustrates the role of amnionless in the luminal expression of cubilin in humans.…”

Genetic heterogeneity of megaloblastic anaemia type 1 in Tunisian patients

“…In Norway, all the studied cases were due to mutations in the AMN gene, whereas cases from Mediterranean regions had mutations in either the CUBN or the AMN gene . It has been demonstrated that AMN functions in the same pathway as cubilin and forms a complex designated cubam, which is essential for endocytosis/trancytosis of several ligands (Fyfe et al 2004).…”