PTH and Vitamin D are two major regulators of mineral metabolism. They play critical roles in the maintenance of calcium and phosphate homeostasis as well as the development and maintenance of bone health. PTH and Vitamin D form a tightly controlled feedback cycle, PTH being a major stimulator of vitamin D synthesis in the kidney while vitamin D exerts negative feedback on PTH secretion. The major function of PTH and major physiologic regulator is circulating ionized calcium. The effects of PTH on gut, kidney, and bone serve to maintain serum calcium within a tight range. PTH has a reciprocal effect on phosphate metabolism. In contrast, vitamin D has a stimulatory effect on both calcium and phosphate homeostasis, playing a key role in providing adequate mineral for normal bone formation. Both hormones act in concert with the more recently discovered FGF23 and klotho, hormones involved predominantly in phosphate metabolism, which also participate in this closely knit feedback circuit. Of great interest are recent studies demonstrating effects of both PTH and vitamin D on the cardiovascular system. Hyperparathyroidism and vitamin D deficiency have been implicated in a variety of cardiovascular disorders including hypertension, atherosclerosis, vascular calcification, and kidney failure. Both hormones have direct effects on the endothelium, heart, and other vascular structures. How these effects of PTH and vitamin D interface with the regulation of bone formation are the subject of intense investigation.

Type IIa Na͞Pi cotransporters are expressed in renal proximal brush border and are the major determinants of inorganic phosphate (Pi) reabsorption. Their carboxyl-terminal tail contains information for apical expression, and interacts by means of its three terminal amino acids with several PSD95͞DglA͞ZO-1-like domain (PDZ)-containing proteins. Two of these proteins, NaPi-Cap1 and Na͞H exchanger-regulatory factor 1 (NHERF1), colocalize with the cotransporter in the proximal brush border. We used opossum kidney cells to test the hypothesis of a potential role of PDZ-interactions on the apical expression of the cotransporter. We found that opossum kidney cells contain NaPi-Cap1 and NHERF1 mRNAs. For NHERF1, an apical location of the protein could be documented; this location probably reflects interaction with the cytoskeleton by means of the MERM-binding domain. Overexpression of PDZ domains involved in interaction with the cotransporter (PDZ-1͞ NHERF1 and PDZ-3͞NaPi-Cap1) had a dominant-negative effect, disturbing the apical expression of the cotransporter without affecting the actin cytoskeleton or the basolateral expression of Na͞K-ATPase. These data suggest an involvement of PDZ-interactions on the apical expression of type IIa cotransporters.opossum kidney cells ͉ Na͞H exchanger-regulatory factor 1 ͉ proximal tubules P roximal tubular reabsorption of inorganic phosphate (P i ) plays a key role in P i metabolism (1, 2). Up to 80% of the renal reabsorption of P i is mediated by the brush border membrane (BBM)-associated type IIa Na͞P i -cotransporters (NaPi IIa; refs. 3 and 4; for review, see ref.2). According to their key physiological role, these cotransporters are up-regulated by factors that stimulate renal reabsorption of P i (ref. 5; for review see ref.2), whereas they are down-regulated by phosphaturic factors (refs. 6 and 7; for review, see ref. 2). Their expression is also affected in pathological states associated with P i wasting, such as X-linked hypophosphatemic rickets: a primary defect on the PHEX gene leads by a yet-unknown mechanism to a reduced expression of NaPi IIa (8, 9). Many of the proximal tubular characteristics in terms of P i handling are retained in a cell line derived from opossum kidney (OK) cells. These OK cells contain an endogenous NaPi IIa cotransporter (NaPi4) apically located and regulated by the same hormones and factors as NaPi IIa cotransporters in proximal tubules (6,10,11).NaPi IIa cotransporters are predicted to contain eight transmembrane domains with intracellular N-and C-terminal tails (12). The C-terminal tail contains two signals involved in apical expression: a terminal PSD95͞DglA͞ZO-1-like domain (PDZ)-binding motif (TRL) and an internal determinant (13). The C-terminal tail interacts, by means of TRL residues, with several PDZ-containing proteins, among them NaPi-Cap1 and Na͞H exchanger-regulatory factor 1 (NHERF1; ref. 14). Similar to NaPi IIa, both proteins are located on the BBM of proximal tubules (14, 15). NaPi-Cap1 is a protein of about 500 residues that co...