Cubilin, a high affinity receptor for fibroblast growth factor 8, is required for cell survival in the developing vertebrate head.

Cubilin (Cubn) is a multiligand endocytic receptor critical for the intestinal absorption of vitamin B12 and renal protein reabsorption. During mouse development, Cubn is expressed in both embryonic and extra-embryonic tissues, and Cubn gene inactivation results in early embryo lethality most likely due to the impairment of the function of extra-embryonic Cubn. Here, we focus on the developmental role of Cubn expressed in the embryonic head. We report that Cubn is a novel, interspecies-conserved Fgf receptor. Epiblast-specific inactivation of Cubn in the mouse embryo as well as Cubn silencing in the anterior head of frog or the cephalic neural crest of chick embryos show that Cubn is required during early somite stages to convey survival signals in the developing vertebrate head. Surface plasmon resonance analysis reveals that fibroblast growth factor 8 (Fgf8), a key mediator of cell survival, migration, proliferation, and patterning in the developing head, is a high affinity ligand for Cubn. Cell uptake studies show that binding to Cubn is necessary for the phosphorylation of the Fgf signaling mediators MAPK and Smad1. Although Cubn may not form stable ternary complexes with Fgf receptors (FgfRs), it acts together with and/or is necessary for optimal FgfR activity. We propose that plasma membrane binding of Fgf8, and most likely of the Fgf8 family members Fgf17 and Fgf18, to Cubn improves Fgf ligand endocytosis and availability to FgfRs, thus modulating Fgf signaling activity.

Overlapping expression patterns of the multiligand endocytic receptors cubilin and megalin in the CNS, sensory organs and developing epithelia of the rodent embryo.

Cubilin and megalin are multiligand epithelial endocytic receptors well characterized in the adult kidney and ileum where they form a complex essential for protein, lipid and vitamin uptake. Although inactivation of the megalin gene leads to holoprosencephaly and administration of anti-cubilin antibodies induces fetal resorptions or cranio-facial malformations their function in the developing embryo remains unclear. We recently showed that both proteins are strongly expressed by the maternal-fetal interfaces and the neuroepithelium of the early rodent embryo where they co-localize and form a complex important for nutrient uptake. The aim of the present study was the further investigation of cubilin expression at later developmental stages of the rodent embryo and its correlation to that of megalin. Immunohistochemical and in situ hybridization analysis showed striking similarities in the spatial and temporal expression patterns of cubilin and megalin. The electrophoretic mobility of both proteins was identical to that of the adult as revealed by Western blot analysis. Cubilin and megalin were strongly expressed in the sensory organs, the central nervous system, the respiratory and urogenital tracts as well as in the thymus, parathyroids and thyroid. In each site, the expression mainly concerned epithelial structures and correlated with the onset of epithelial induction. Depending on the site, a decreased or restricted expression was observed by the end of the gestation for both proteins.