Molecular Mechanisms Underlying the Reabsorption of Inorganic Phosphate in Renal Tubules

点击翻译按钮获取中文摘要

Megalin is a multifunctional endocytic receptor expressed in renal proximal tubules, and plays critical roles in the renal uptake of various proteins. We hypothesized that megalin-dependent endocytosis might play a role in renal reabsorption of inorganic phosphate. To address the short-term effects of altered megalin function, we administered a recombinant protein for the soluble form of 39-kD receptor associated protein (RAP) intraperitoneally to 7-wk-old mice. The recombinant protein was prepared as histidine-tagged soluble RAP (aa 39-356) lacking the amino-terminal signal peptide and the carboxyl terminal endoplasmic reticulum (ER) retention signal, which was designated as His-sRAP. After the direct interaction between His-sRAP and megalin was confirmed, mice were given a single intraperitoneal injection of His-sRAP (3.5 mg/dose). Immunostaining and Western blot analyses demonstrated that the uptake of His-sHAP and the accelerated internalization of megalin in proximal tubular cells 1 hour after administration. Interestingly, internalization of the type II sodium/phosphate co-transporter (NaPi-II) was observed. Three sequential administrations of His-sRAP (3.5 mg/dose, three doses with 4-hr intervals) increased the urinary excretion of low molecular weight proteins including vitamin D-binding protein. In addition, the urinary excretion of phosphate was also increased, and the protein level of NaPi-II in the brush border was decreased. Serum 25-hydroxyvitamin D was decreased, while the plasma level of intact parathyroid hormone was not altered. These results suggest that the His-sRAP induced acceleration of megalin-mediated endocytosis caused phosphaturia via altered subcellular distribution of NaPi-II.