Lack of megalin expression in adult human terminal ileum suggests megalin-independent cubilin/amnionless activity during vitamin B₁₂absorption

Abstract

Cubilin plays an essential role in terminal ileum and renal proximal tubules during absorption of vitamin B12 and ligands from the glomerular ultrafiltrate. Cubilin is coexpressed with amnionless, and cubilin and amnionless are mutually dependent on each other for correct processing to the plasma membrane upon synthesis. Patients with defects in either protein suffer from vitamin B12‐malabsorption and in some cases proteinuria. Cubilin lacks a transmembrane region and signals for endocytosis and is dependent on a transmembrane coreceptor during internalization. Amnionless has been shown to be able to mediate internalization of cubilin in a cell‐based model system. Cubilin has additionally been suggested to function together with megalin, and a recent study of megalin‐deficient patients indicates that uptake of cubilin ligands in the kidney is critically dependent on megalin. To further investigate the potential role of amnionless and megalin in relation to cubilin function in terminal ileum and vitamin B12 uptake, we initiated a study of CUBN/cubilin, AMN/amnionless, and LRP2/megalin expression in adult human terminal ileum. Our study is the first to reveal the expression pattern of cubilin, amnionless, and megalin in adult human terminal ileum, where cubilin and amnionless localize to the epithelial cells. Surprisingly, we did not detect any megalin protein in adult terminal ileum and consistently, only extremely low amounts of LRP2 mRNA. Our data therefore advocate that cubilin and amnionless act independently of megalin in adult terminal ileum and that the cubilin‐megalin interdependency accordingly should be considered as tissue and ligand specific.

Structural assembly of the megadalton-sized receptor for intestinal vitamin B12 uptake and kidney protein reabsorption

“…Cubam and megalin co-localise in the endocytic apparatus and appear to be functionally dependent on each other, at least in the kidney 43 , 44 . However, megalin is not required for cubam surface expression 16 and the fact that megalin seems less expressed in the intestine 45 suggests that cubam is capable of functioning independently of megalin. The extraordinary sizes of receptor ecto-domains in both megalin and cubam are remarkable and may be advantageous for optimal catching and endocytosis of multiple ligands from the extracellular fluids.…”

Decreased Brain Levels of Vitamin B12 in Aging, Autism and Schizophrenia

“…Megalin also facilitates endocytosis of the TCR [ 45 ] and TCR knock out mice show a deficit in brain B12 concentrations accompanied by DNA hypomethylation [ 46 ]. A recent study showed that LRP2 is robustly expressed in the distal ileum during fetal and early postnatal development, but is not expressed in adult human ileum [ 47 ], indicating an age-dependent decline in its expression. Thus a decline in megalin and/or TCR activity could underlie the lower brain levels of vitamin B 12 we observed in elderly subjects.…”

The role of albumin receptors in regulation of albumin homeostasis: Implications for drug delivery

“…However, cubilin may also associate with the protein amnionless, which has been shown to be required for cubilin internalization and transport to the cell surface [178]. A recent study shows that cubilin is expressed in human terminal ileum in the absence of megalin but instead depends on association with amnionless, a finding that strongly suggests that cubilin and amnionless may operate independently of megalin [179]. The functional impact of the these proteins in uptake and transport of albumin at the different body sites and tissues are not well studied, except for its role in reabsorption in the kidneys, as discussed below.…”