Megalin binds and mediates cellular internalization of folate binding protein

Abstract

Folate is an essential vitamin involved in a number of biological processes. High affinity folate binding proteins (FBPs) exist both as glycosylphosphatidylinositol‐linked, membrane associated folate binding proteins and as soluble FBPs in plasma and some secretory fluids such as milk, saliva and semen. The function and significance of FBPs are unresolved, however, it has been suggested that they may facilitate folate uptake, e.g. during suckling. The present study shows that megalin, a large, multiligand endocytic receptor and member of the low‐density lipoprotein‐receptor family, is able to bind and mediate cellular uptake of FBP. Surface plasmon resonance analysis shows binding of bovine and human milk FBP to immobilized megalin, but not to low density lipoprotein receptor related protein. Binding of 125I‐labeled folate binding protein (FBP) to sections of kidney proximal tubule, known to express high levels of megalin, is inhibitable by excess unlabeled FBP and by receptor associated protein, a known inhibitor of binding to megalin. Immortalized rat yolk sac cells, representing an established model for studying megalin‐mediated uptake, reveal 125I‐labeled FBP uptake which is inhibited by receptor associated protein and by antimegalin antibodies. Microinjection of 125I‐labeled FBP into renal tubules in vivo shows proximal tubular uptake by endocytosis. Megalin is expressed in several absorptive epithelia, including intestine and kidney proximal tubule, and thus the present findings provide a mechanism for intestinal and renal endocytic uptake of soluble FBP.

LRP2 mediates folate uptake in the developing neural tube

“…A functional link between LRP2 activity and endocytic folate uptake in vitro in kidney sections has been suggested previously (Birn et al, 2005). The binding of soluble folate receptor 1 (sFOLR1; also known as FBP1 or FBP, for folate binding protein) to immobilized LRP2 was shown by surface plasmon resonance analysis, as well as by binding of sFOLR1 to sections of kidney cortex and uptake of sFOLR1 by BN-16 cells.…”

“…A functional link between LRP2 activity in the kidney and endocytic uptake of folic acid and the folate receptor has been suggested (Birn et al, 2005). Additional evidence for the role of LRP2 in folate uptake in embryonic tissue comes from a study by Gelineau-van-Waes and colleagues, showing that components of the LRP2-cubilin receptor complex are upregulated in SLC19a1-deficient embryos (Gelineau-van Waes et al, 2008b).…”

Embryonic development in the reduced folate carrier knockout mouse is modulated by maternal folate supplementation

“…Megalin (Lrp2) is a low-density lipoprotein receptor-related protein that is coexpressed with cubilin in many absorptive epithelia (i.e., neuroepithelium and yolk sac) of developing embryos that binds and internalizes multiple ligands, including the soluble folate binding protein (Folr1) and sonic hedgehog (Shh), among others (Barth and Argraves, 2001;Birn et al, 2002;Christensen and Birn, 2002;Moestrup et al, 1996;Morales et al, 2006). In the absence of RFC1, the primary route of folate uptake by the yolk sac visceral endoderm may therefore be via the GPI-anchored folate receptor, Folr1, or via the soluble form of the folate receptor bound to megalin (Birn et al, 2005).…”

Renal Tubular Reabsorption of Folate Mediated by Folate Binding Protein 1

“…Whereas total protein binding of folate in rat serum has been estimated to be approximately 20% over a wide range of concentrations (20), the concentration of the approximately 35 kD, high-affinity plasma folate binding protein secreted from hemopoietic cells (11,12), is assumed to be more constant and thus carrying a larger fraction of total plasma folate at low folate concentration. Filtration of this folate-protein complex is followed by tubular reabsorption, most likely by a megalin-mediated process (39), providing a potential mechanism of tubular folate uptake that is independent of membrane-associated folbp1 and may play a role at low plasma folate concentrations. Finally, it cannot be excluded that other, very low capacity reabsorptive mechanisms may operate under low-folate conditions.…”