Abstract
In adults, the extent of renal excretion of inorganic phosphate (Pi) is determined by the abundance of the Na-dependent Pi-cotransporter (NaPi-lla; SLC34A1) localized at the brush border membranes of proximal tubular cells. A number of hormonal and metabolic factors, PTH being a paradigm, define the abundance of NaPi-lla. PTH, via apically and basolaterally localized receptors, leads to an internalization and lysosomal degradation of NaPi-lla. In order to understand the mechanisms involved in the apical positioning and the regulation of the NaPi-lla cotransporter, yeast two-hybrid screens have been performed. In a screen, using the C-terminus of NaPi-lla as a bait, among others, two PZD proteins, NHERF1/2 and PDZK1, were identified to interact with the C-terminus of NaPi-lla. These interactions were assigned to specific PDZ domains and to the PDZ binding motif TRL. In addition, applying a membrane split-ubiquitin two-hybrid system, we recently demonstrated that these interactions occur also between full length NaPi-lla. Both, NHERF1 and PDZK1 are expressed in the brush border of proximal tubular cells. Studies performed with NHERF1 and PDZK1 deficient mice demonstrated that ablation of NHERF1 impairs apical localization and regulation of NaPi-lla whereas ablation of PDZK1 was without effect. Furthermore, studies performed with OK-cells indicated that NHERF1 directly or indirectly is involved in the apical positioning of NaPi-lla. As upon PTH stimulation NaPi-lla is internalized but NHERF1/PDZK1 not, the interactions of NaPi-lla with respective PDZ domains are suggested to be regulated. PTH dependent phosphorylation of NHERF1/PDZK1 was investigated in OK-cells and mouse kidney slices. [Jpn J Physiol 55 Suppl:S6 (2005)]
