Receptor-mediated endocytosis is responsible for
protein reabsorption in the proximal tubule. This process involves two interacting receptors,
megalin and
cubilin, which form a complex with amnionless. Whether these
proteins function in parallel or as part of an integrated system is not well understood. Here, we report the renal effects of genetic ablation of
cubilin, with or without concomitant ablation of
megalin, using a conditional Cre-loxP system. We observed that proximal tubule cells did not localize amnionless to the plasma membrane in the absence of
cubilin, indicating a mutual dependency of
cubilin and amnionless to form a functional membrane receptor complex. The
cubilin-amnionless complex mediated internalization of
intrinsic factor-
vitamin B12 complexes, but
megalin considerably increased the uptake. Furthermore,
cubilin-deficient mice exhibited markedly decreased uptake of
albumin by proximal tubule cells and resultant
albuminuria. Inactivation of both
megalin and
cubilin did not increase
albuminuria, indicating that the main role of
megalin in
albumin reabsorption is to drive the internalization of
cubilin-
albumin complexes. In contrast,
cubulin deficiency did not affect urinary tubular uptake or excretion of
vitamin D-binding protein (DBP), which binds
cubilin and
megalin. In addition, we observed
cubilin-independent reabsorption of the "specific"
cubilin ligands transferrin, CC16, and
apoA-I, suggesting a role for
megalin and perhaps other receptors in their reabsorption. In summary, with regard to
albumin,
cubilin is essential for its reabsorption by proximal tubule cells, and
megalin drives internalization of
cubilin-
albumin complexes. These genetic models will allow further analysis of protein trafficking in the progression of proteinuric renal diseases.