Rapid downregulation of rat renal Na/P(i) cotransporter in response to parathyroid hormone involves microtubule rearrangement

• Published in: The Journal of clinical investigation 1999-08, Vol.104 (4), p.483-494
• Main Authors: Lötscher, M, Scarpetta, Y, Levi, M, Halaihel, N, Wang, H, Zajicek, H K, Biber, J, Murer, H, Kaissling, B
• Format: Article
• Language: English
• Subjects: Animals; Carrier Proteins - metabolism; Colchicine - pharmacology; Endocytosis - drug effects; Immunohistochemistry; Kidney Tubules, Proximal - drug effects; Kidney Tubules, Proximal - metabolism; Kidney Tubules, Proximal - ultrastructure; Male; Microscopy, Electron; Microtubules - drug effects; Microvilli - drug effects; Microvilli - metabolism; Paclitaxel - pharmacology; Parathyroid Hormone - pharmacology; Peptide Fragments - pharmacology; Phosphates - metabolism; Rats; Rats, Wistar; Sodium - metabolism; Sodium-Phosphate Cotransporter Proteins; Sodium-Phosphate Cotransporter Proteins, Type IIa; Symporters
• ISSN: 0021-9738
• DOI: 10.1172/JCI3208

Renal proximal tubule cells express in their apical brush border membrane (BBM) a Na/P(i) cotransporter type IIa that is rapidly downregulated in response to parathyroid hormone (PTH). We used the rat renal Na/P(i) cotransporter type IIa (NaPi-2) as an in vivo model to assess early cellular events in the rapid downregulation of this transporter. When rats were treated with PTH for 15 minutes, NaPi-2 abundance in the BBM was decreased. In parallel, transporter accumulated in intracellular vesicles. Concomitantly, microtubules (MTs) were found to form dense bundles of apical-to-basal orientation. After 60 minutes of PTH action, the cells were vastly depleted of NaPi-2, whereas their microtubular cytoskeleton had returned to its normal appearance. Prevention of MT rearrangement by taxol resulted in accumulation of NaPi-2 in the subapical cell portion after 15 minutes and a strong delay in depletion of intracellular transporter after 60 minutes of PTH action. Furthermore, the subapical accumulation of NaPi-2 was associated with the expansion of dense apical tubules of the subapical endocytic apparatus (SEA). Depolymerization of MTs by colchicine likewise caused a retardation of intracellular NaPi-2 depletion. These results suggest that NaPi-2 is downregulated in response to PTH through a rapid endocytic process in 2 separate steps: (a) internalization of the transporter into the SEA, and (b) its delivery to degradative organelles by a trafficking mechanism whose efficiency depends on a taxol-sensitive rearrangement of MTs.