Inactivating mutations of Phex cause X-linked hypophosphatemia (XLH) by increasing levels of a circulating phosphaturic factor. FGF23 is a candidate for this phosphaturic factor. Elevated serum FGF23 levels correlate with the degree of hypophosphatemia in XLH, suggesting that loss of Phex function in this disorder results in either diminished degradation and/or increased biosynthesis of FGF23. To establish the mechanisms whereby Phex regulates FGF23, we assessed Phex-dependent hydrolysis of recombinant FGF23 in vitro and measured fgf23 message levels in the Hyp mouse homologue of XLH. In COS-7 cells, overexpression of FGF23 resulted in its degradation into N-and C-terminal fragments by an endogenous decanoyl-Arg-Val-Lys-Arg-chloromethyl ketone-sensitive furin-type convertase. Phex-dependent hydrolysis of full-length FGF23 or its N-and C-terminal fragments could not be demonstrated in the presence or absence of decanoyl-Arg-Val-Lys-Arg-chloromethyl ketone in COS-7 cells expressing Phex and FGF23. In a reticulolysate system, apparent cleavage of FGF23 occurred with wild-type Phex, the inactive Phex-3M mutant, and vector controls, indicating nonspecific metabolism of FGF23 by contaminating enzymes. These findings suggest that FGF23 is not a direct Phex substrate. In contrast, by real-time reverse transcriptase PCR, the levels of fgf23 transcripts were highest in bone, the predominant site of Phex expression. In addition, Hyp mice displayed a bonerestricted increase in fgf23 transcripts in association with inactivating Phex mutations. Increased expression of fgf23 was also observed in Hyp-derived osteoblasts in culture. These findings suggest that Phex, possibly through the actions of unidentified Phex substrates or other downstream effectors, regulates fgf23 expression as part of a potential hormonal axis between bone and kidney that controls systemic phosphate homeostasis and mineralization. X-linked hypophosphatemia (XLH)1 is a disorder characterized by defective calcification of cartilage and bone, growth retardation, impaired renal tubular reabsorption of phosphate, aberrant regulation of 1,25(OH) 2 D 3 production, and resistance to phosphorus and vitamin D therapy (1). XLH is caused by inactivating mutations of PHEX (2-5), a member of the M13 family of type II cell surface zinc-dependent proteases that include neprilysin, endothelin-converting enzymes 1 and 2 (6, 7), KELL (8), and DINE/X-converting enzyme (9, 10). The mouse Phex cDNA sequence is highly homologous to that of humans (11,12), and inactivating mutations of Phex are identified in several mouse homologues of XLH, including Hyp, Gy, and Ska1 mice (10,13,14).Current data indicate that Phex regulates the production and/or degradation of a systemic phosphaturic hormone, referred to as phosphatonin (15). The presence of phosphatonin in XLH/Hyp was detected by parabiosis experiments in which Hyp mice transferred the phosphaturic phenotype to normal mice (16). Studies in parathyroidectomized Hyp mice eliminated parathyroid hormone as the responsible phosph...

The identities of the apical Cl−/base exchangers in kidney proximal tubule and cortical collecting duct (CCD) cells remain unknown. Pendrin (PDS), which is expressed at high levels in the thyroid and its mutation causes Pendred's syndrome, is shown to be an anion exchanger. We investigated the renal distribution of PDS and its function. Our results demonstrate that pendrin mRNA expression in the rat kidney is abundant and limited to the cortex. Proximal tubule suspensions isolated from kidney cortex were highly enriched in pendrin mRNA. Immunoblot analysis studies localized pendrin to cortical brush-border membranes. Nephron segment RT-PCR localized pendrin mRNA to proximal tubule and CCD. Expression studies in HEK-293 cells demonstrated that pendrin functions in the Cl−/OH−, Cl−/HCO3 −, and Cl−/formate exchange modes. The conclusion is that pendrin is an apical Cl−/base exchanger in the kidney proximal tubule and CCD and mediates Cl−/OH−, Cl−/HCO3 −, and Cl−/formate exchange.

Several modes of HCO 3؊ transport occur in the kidney, including Na ؉ -independent Cl/HCO ؊ and sensitivity to inhibition by 4,4-diisothiocyanatostilbene-2,2-disulfonic acid) suggested a strategy for cloning the other transporters based on structural similarity with the AE family. An expressed sequence tag encoding part of a protein that is related to the known anion exchangers was identified in the GenBank™ expressed sequence tag data base and used to design an oligonucleotide probe. This probe was used to screen a human kidney cDNA library. Several clones were identified, isolated, and sequenced. Two overlapping cDNA clones were spliced together to form a 7.6-kilobase cDNA that contained the entire coding region of a novel protein. Based on the deduced amino acid sequence, the cDNA encodes a protein with a M r of 116,040. The protein has 29% identity with human brain AE3. Northern blot analysis reveals that the 7.6-kilobase mRNA is highly expressed in kidney and pancreas, with detectable levels in brain. Functional studies in transiently transfected HEK-293 cells demonstrate that the cloned transporter mediates Na ؉ :HCO 3 ؊ cotransport.