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In adults, the extent of renal reabsorption of Pi and consequently the extent of urinary excretion of phosphate are to a large extent determined by the abundance of the Na-Pi cotransporter NaPi-IIa (SLC34A1). Localization of this cotransporter is restricted to the apical membrane of proximal tubular cells, and its abundance is controlled by a number of factors and pathophysiological conditions. To guarantee a proper apical localization and specific regulated endocytosis of NaPi-IIa, an orchestrated pattern of protein interactions has to be envisaged. Attempts to screen for such interacting proteins resulted in the identification of a PDZ domain containing proteins. The purpose of this review is to discuss the roles of these PDZ proteins in proximal tubular Na-Pi cotransport.

Chronic restriction of dietary Pielicits an increased reabsorption of Piin the kidney proximal tubules, which involves a stimulation of apical Na-Picotansport. This adaptation is in part a direct cellular response of which the mechanism(s) are poorly understood. In this study, the impact of dietary Pirestriction on the differential expression of rat kidney cortex mRNAs was visualized to identify gene products regulated by the Pistatus. When kidney cortex mRNAs of rats fed a low- or a high-Pidiet were compared by differential display-polymerase chain reaction (DD-PCR), thirty modulated cDNA bands were observed, of which four were confirmed as being regulated. We focused on one of the upregulated bands, dietary Pi-regulated RNA-1 (diphor-1). A cDNA containing an open reading frame encoding a 52-kDa protein was cloned by library screening. Diphor-1 exhibits a high degree of identity to the Na/H exchanger regulatory factor and to a tyrosine kinase activating protein. Highest expression of diphor-1 mRNA was detected in the kidney (proximal tubules) and in small intestine. Expression experiments showed that diphor-1 specifically increases Na-Picotransport in oocytes of Xenopus laevis coinjected with renal type II Na-Picotransporter cRNA. Further characterizations of diphor-1 will show whether diphor-1 is primarily or secondarily involved in the response to dietary Pi.

Two cD-NAs coding for proximal tubular Na-Pi cotransport (NaPi-2) and Na-SO4 cotransport (NaSi-1) have been transfected by the use of a dexamethasone-inducible vector (pLK-neo) into MDCK and LLC-PK1 cells. By reverse transcription-polymerase chain reaction, expression of corresponding mRNAs was observed after stimulation with dexamethasone only. Similarly, expression of the NaPi-2 protein was detected only after induction with dexamethasone. In transfected Madin-Darby canine kidney (MDCK) cells, dexamethasone induced a large increase of Na-Pi or Na-SO4 cotransport, whereas, in transfected LLC-PK1, cell transport was only minimally expressed. In MDCK cells grown on filter supports, transfected Na-Pi-cotransport activity was equally expressed at both cell surfaces; dual location of expressed NaPi-2 protein was also observed by immunohistochemistry. In contrast, transfected Na-SO4 cotransport activity was predominantly expressed at the apical cell surface of MDCK cells. The results demonstrate that 1), in MDCK cells, the sorting behavior of two proximal tubular cotransport systems seems to be different: apical for Na-SO4 cotransport (NaSi-1) and dual location for Na-Pi cotransport (NaPi-2); and 2) LLC-PK1 cells seem not to be a suitable system to functionally express sodium-dependent cotransport systems for phosphate and sulfate.