Evidence for the role of megalin in renal uptake of transthyretin

The kidney is a major organ for uptake of the thyroid hormone thyroxine (T- 4) and its conversion to the active form, triiodothyronine. In the plasma, one of the T-4 carriers is transthyretin (TTR). In the present study we obs erved that TTR, the transporter of both T-4 and retinol-binding protein, bi nds to megalin, the multiligand receptor expressed on the luminal surface o f various epithelia including the renal proximal tubules. In the kidney, me galin plays an important role in tubular uptake of macromolecules filtered through the glomerulus. To evaluate the importance of megalin for renal upt ake of TTR, we performed binding/uptake assays using immortalized rat yolk sac cells with high expression levels of megalin. Radiolabeled TTR, free as well as in complex with thyroxine or retinol-binding protein, was rapidly taken up by the cells, and the uptake was strongly inhibited by a polyclona l megalin antibody and by the receptor-associated protein, a chaperone-like protein inhibiting ligand binding to megalin. In cell culture, different T TR mutations presented different levels of cell association and degradation , suggesting that the structure of TTR is important for megalin recognition . Both the apo form and the T-4-bound form were taken up by the cells. Anal ysis of urine from patients with Dent's disease, a renal tubular disorder t hat alters receptor-mediated endocytic reabsorption of proteins, identified TTR as an abundant excreted protein. Furthermore, analysis of kidney secti ons of megalin-deficient mice revealed no immunohistochemical TTR labeling in intracellular vesicles in the proximal tubule cells when compared with w ild type control littermates. Taken together, the present data indicate tha t TTR represents a novel megalin Ligand of importance in the thyroid hormon e homeostasis.