Background. Reabsorption of albumin from the glomerular filtrate occurs via
receptor-mediated endocytosis in the proximal tubule. This process is init
iated by binding of albumin in apical clathrin-coated pits, followed by end
ocytosis and degradation in lysosomes. Although binding sites have been cha
racterized by kinetic studies, the receptors responsible for the binding of
albumin have not been fully identified. Two giant glycoproteins, cubilin a
nd megalin, constitute important endocytic receptors localized to the kidne
y proximal tubule.
Methods. In the present study, we examined the colocalization of cubilin an
d megalin in the endocytic pathway and the relationship between the uptake
of albumin and the expression of cubilin and megalin in opossum kidney (OK)
proximal tubule cells by immunocytochemistry and immunoblotting.
Results. OK cells expressed both cubilin and megalin. The light microscope
labeling patterns for cubilin and megalin were almost identical and were ma
inly located at the surface area of the cells. Cubilin and megalin were als
o shown to colocalize on cell surface microvilli, in coated pits, and in en
docytic compartments at the electron microscope level. Endocytosed bovine s
erum albumin (BSA) was identified exclusively in cells expressing megalin a
nd cubilin. Uptake of BSA-FITC was saturable and inhibited by receptor-asso
ciated protein (RAP) and by intrinsic factor-vitamin B-12 complex (IF-B-12)
at high concentrations. Significant inhibition was also observed by specif
ic antibodies to cubilin, and megalin and cubilin antisense oligonucleotide
s likewise significantly reduced albumin uptake. Egg albumin did not affect
the uptake of BSA.
Conclusion. The present observations suggest that the two receptors cubilin
and megalin are both involved in the endocytic uptake of albumin in renal
proximal tubule cells.