Genetic diseases of renal phosphate handling

UNLABELLED: Renal control of systemic phosphate homeostasis is critical as evident from inborn and acquired diseases causing renal phosphate wasting. At least three transport proteins are responsible for renal phosphate reabsorption: NAPI-IIa (SLC34A1), NAPI-IIc (SLC34A3) and PIT-2 (SLC20A2). These transporters are highly regulated by various cellular mechanisms and factors including acid-base status, electrolyte balance and hormones such as dopamine, glucocorticoids, growth factors, vitamin D3, parathyroid hormone and fibroblast growth factor 23 (FGF23). Whether renal phosphate wasting is caused by inactivating mutations in the NAPI-IIa transporter is controversial. Mutations in the NAPI-IIc transporter cause hereditary hypophosphatemic rickets with hypercalciuria. Besides the primary inherited defects, there are also inherited defects in major regulators of phosphate homeostasis that lead to alterations in phosphate handling. Autosomal dominant hypophosphatemic rickets is due to FGF23 mutations leading to resistance against its own degradation. Similarly, inactivating mutations in the PHEX gene, which causes FGF23 inactivation, cause X-linked hypophosphatemia due to renal phosphate losses. In contrast, mutations in galactosamine:polypeptide N-acetyl-galactosaminyltransferase, responsible for O-glycosylation of FGF23, or in klotho, a cofactor for FGF23 signalling result in hyperphosphatemia. Acquired syndromes of renal phosphate wasting, hypophosphatemia and osteomalacia (tumour-associated osteomalacia) can be due to the excessive synthesis or release of phosphaturic factors (FGF23, FGF-7, MEPE and sFRP4) from mesenchymal tumour

Genetic diseases of renal phosphate handling

UNLABELLED: Renal control of systemic phosphate homeostasis is critical as evident from inborn and acquired diseases causing renal phosphate wasting. At least three transport proteins are responsible for renal phosphate reabsorption: NAPI-IIa (SLC34A1), NAPI-IIc (SLC34A3) and PIT-2 (SLC20A2). These transporters are highly regulated by various cellular mechanisms and factors including acid-base status, electrolyte balance and hormones such as dopamine, glucocorticoids, growth factors, vitamin D3, parathyroid hormone and fibroblast growth factor 23 (FGF23). Whether renal phosphate wasting is caused by inactivating mutations in the NAPI-IIa transporter is controversial. Mutations in the NAPI-IIc transporter cause hereditary hypophosphatemic rickets with hypercalciuria. Besides the primary inherited defects, there are also inherited defects in major regulators of phosphate homeostasis that lead to alterations in phosphate handling. Autosomal dominant hypophosphatemic rickets is due to FGF23 mutations leading to resistance against its own degradation. Similarly, inactivating mutations in the PHEX gene, which causes FGF23 inactivation, cause X-linked hypophosphatemia due to renal phosphate losses. In contrast, mutations in galactosamine:polypeptide N-acetyl-galactosaminyltransferase, responsible for O-glycosylation of FGF23, or in klotho, a cofactor for FGF23 signalling result in hyperphosphatemia. Acquired syndromes of renal phosphate wasting, hypophosphatemia and osteomalacia (tumour-associated osteomalacia) can be due to the excessive synthesis or release of phosphaturic factors (FGF23, FGF-7, MEPE and sFRP4) from mesenchymal tumour

Renal localization and regulation by dietary phosphate of the MCT14 orphan transporter

MCT14 is an orphan transporter belonging to the SLC16 transporter family mediating the transport of monocarboxylates, aromatic amino acids, creatine, and thyroid hormones. The expression, tissue localization, regulation, and function of MCT14 are unknown. In mouse MCT14 mRNA abundance is highest in kidney. Using a newly developed and validated antibody, MCT14 was localized to the luminal membrane of the thick ascending limb of the loop of Henle colocalizing in the same cells with uromodulin and NKCC2. MCT14 mRNA and protein was found to be highly regulated by dietary phosphate intake in mice being increased by high dietary phosphate intake at both mRNA and protein level. In order to identify the transport substrate(s), we expressed MCT14 in Xenopus laevis oocytes where MCT14 was integrated into the plasma membrane. However, no transport was discovered for the classic substrates of the SLC16 family nor for phosphate. In summary, MCT14 is an orphan transporter regulated by phosphate and highly enriched in kidney localizing to the luminal membrane of one specific nephron segment

Expression of renal and intestinal Na/Pi cotransporters in the absence of GABARAP

We have recently shown that the abundance of the renal sodium (Na)/inorganic phosphate (Pi) cotransporter NaPi-IIa is increased in the absence of the GABAA receptor-associated protein (GABARAP). Accordingly, GABARAP-deficient mice have a reduced urinary excretion of Pi. However, their circulating levels of Pi do not differ from wild-type animals, suggesting the presence of a compensatory mechanism responsible for keeping serum Pi values constant. Here, we aimed first to identify the molecular basis of this compensation by analyzing the expression of Na/Pi cotransporters known to be expressed in the kidney and intestine. We found that, in the kidney, the upregulation of NaPi-IIa is not accompanied by changes on the expression of either NaPi-IIc or PiT2, the other cotransporters known to participate in renal Pi reabsorption. In contrast, the intestinal expression of NaPi-IIb is downregulated in mutant animals, suggesting that a reduced intestinal absorption of Pi could contribute to maintain a normophosphatemic status despite the increased renal retention. The second goal of this work was to study whether the alterations on the expression of NaPi-IIa induced by chronic dietary Pi are impaired in the absence of GABARAP. Our data indicate that, in response to high Pi diets, GABARAP-deficient mice downregulate the expression of NaPi-IIa to levels comparable to those seen in wild-type animals. However, in response to low Pi diets, the upregulation of NaPi-IIa is greater in the mutant mice. Thus, both the basal expression and the dietary-induced upregulation of NaPi-IIa are increased in the absence of GABARA

NaPi-IIa interacting proteins and regulation of renal reabsorption of phosphate

Control of phosphate (Pi) homeostasis is essential for many biologic functions and inappropriate low levels of Pi in plasma have been suggested to associate with several pathological states, including renal stone formation and stone recurrence. Pi homeostasis is achieved mainly by adjusting the renal reabsorption of Pi to the body's requirements. This task is performed to a major extent by the Na/Pi cotransporter NaPi-IIa that is specifically expressed in the brush border membrane of renal proximal tubules. While the presence of tight junctions in epithelial cells prevents the diffusion and mixing of the apical and basolateral components, the location of a protein within a particular membrane subdomain (i.e., the presence of NaPi-IIa at the tip of the apical microvilli) often requires its association with scaffolding elements which directly or indirectly connect the protein with the underlying cellular cytoskeleton. NaPi-IIa interacts with the four members of the Na+/H+ exchanger regulatory factor family as well as with the GABAA-receptor associated protein . Here we will discuss the most relevant findings regarding the role of these proteins on the expression and regulation of the cotransporter, as well as the impact that their absence has in Pi homeostasi

The SLC34 family of sodium-dependent phosphate transporters

The SLC34 family of sodium-driven phosphate cotransporters comprises three members: NaPi-IIa (SLC34A1), NaPi-IIb (SLC34A2), and NaPi-IIc (SLC34A3). These transporters mediate the translocation of divalent inorganic phosphate (HPO4 2−) together with two (NaPi-IIc) or three sodium ions (NaPi-IIa and NaPi-IIb), respectively. Consequently, phosphate transport by NaPi-IIa and NaPi-IIb is electrogenic. NaPi-IIa and NaPi-IIc are predominantly expressed in the brush border membrane of the proximal tubule, whereas NaPi-IIb is found in many more organs including the small intestine, lung, liver, and testis. The abundance and activity of these transporters are mostly regulated by changes in their expression at the cell surface and are determined by interactions with proteins involved in scaffolding, trafficking, or intracellular signaling. All three transporters are highly regulated by factors including dietary phosphate status, hormones like parathyroid hormone, 1,25-OH2 vitamin D3 or FGF23, electrolyte, and acid-base status. The physiological relevance of the three members of the SLC34 family is underlined by rare Mendelian disorders causing phosphaturia, hypophosphatemia, or ectopic organ calcifications

The SLC34 family of sodium-dependent phosphate transporters

The SLC34 family of sodium-driven phosphate cotransporters comprises three members: NaPi-IIa (SLC34A1), NaPi-IIb (SLC34A2), and NaPi-IIc (SLC34A3). These transporters mediate the translocation of divalent inorganic phosphate (HPO4 2−) together with two (NaPi-IIc) or three sodium ions (NaPi-IIa and NaPi-IIb), respectively. Consequently, phosphate transport by NaPi-IIa and NaPi-IIb is electrogenic. NaPi-IIa and NaPi-IIc are predominantly expressed in the brush border membrane of the proximal tubule, whereas NaPi-IIb is found in many more organs including the small intestine, lung, liver, and testis. The abundance and activity of these transporters are mostly regulated by changes in their expression at the cell surface and are determined by interactions with proteins involved in scaffolding, trafficking, or intracellular signaling. All three transporters are highly regulated by factors including dietary phosphate status, hormones like parathyroid hormone, 1,25-OH2 vitamin D3 or FGF23, electrolyte, and acid-base status. The physiological relevance of the three members of the SLC34 family is underlined by rare Mendelian disorders causing phosphaturia, hypophosphatemia, or ectopic organ calcifications

Expression of phosphate and calcium transporters and their regulators in parotid glands of mice

The concentration of inorganic phosphate (Pi) in plasma is under hormonal control, with deviations from normal values promptly corrected to avoid hyper- or hypophosphatemia. Major regulators include parathyroid hormone (PTH), fibroblast growth factor 23 (FGF-23), and active vitamin D$_{3}$ (calcitriol). This control is achieved by mechanisms largely dependent on regulating intestinal absorption and renal excretion, whose combined actions stabilise plasma Pi levels at around 1–2 mM. Instead, Pi concentrations up to 13 and 40 mM have been measured in saliva from humans and ruminants, respectively, suggesting that salivary glands have the capacity to concentrate Pi. Here we analysed the transcriptome of parotid glands, ileum, and kidneys of mice, to investigate their potential differences regarding the expression of genes responsible for epithelial transport of Pi as well as their known regulators. Given that Pi and Ca$^{2+}$ homeostasis are tightly connected, the expression of genes involved in Ca$^{2+}$ homeostasis was also included. In addition, we studied the effect of vitamin D$_{3}$ treatment on the expression of Pi and Ca$^{2+}$ regulating genes in the three major salivary glands. We found that parotid glands are equipped preferentially with Slc20 rather than with Slc34 Na$^{+}$/Pi cotransporters, are suited to transport Ca$^{2+}$ through the transcellular and paracellular route and are potential targets for PTH and vitamin D$_{3}$ regulation

Expression and regulation of the renal Na/phosphate cotransporter NaPi-IIa in a mouse model deficient for the PDZ protein PDZK1

Inorganic phosphate (Pi) is reabsorbed in the renal proximal tubule mainly via the type-IIa sodium-phosphate cotransporter (NaPi-IIa). This protein is regulated tightly by different factors, among them dietary Pi intake and parathyroid hormone (PTH). A number of PDZ-domain-containing proteins have been shown to interact with NaPi-IIa in vitro, such as Na+/H+ exchanger-3 regulatory factor-1 (NHERF1) and PDZK1. PDZK1 is highly abundant in kidney and co-localizes with NaPi-IIa in the brush border membrane of proximal tubules. Recently, a knock-out mouse model for PDZK1 (Pdzk1−/−) has been generated, allowing the role of PDZK1 in the expression and regulation of the NaPi-IIa cotransporter to be examined in in vivo and in ex vivo preparations. The localization of NaPi-IIa and other proteins interacting with PDZK1 in vitro [Na+/H+ exchanger (NHE3), chloride-formate exchanger (CFEX)/putative anion transporter-1 (PAT1), NHERF1] was not altered in Pdzk1−/− mice. The abundance of NaPi-IIa adapted to acute and chronic changes in dietary Pi intake, but steady-state levels of NaPi-IIa were reduced in Pdzk1−/− under a Pi rich diet. This was paralleled by a higher urinary fractional Pi excretion. The abundance of the anion exchanger CFEX/PAT1 (SLC26A6) was also reduced. In contrast, NHERF1 abundance increased in the brush border membrane of Pdzk1−/− mice fed a high-Pi diet. Acute regulation of NaPi-IIa by PTH in vivo and by PTH and activators of protein kinases A, C and G (PKA, PKC and PKG) in vitro (kidney slice preparation) was not altered in Pdzk1−/− mice. In conclusion, loss of PDZK1 did not result in major changes in proximal tubule function or NaPi-IIa regulation. However, under a Pi-rich diet, loss of PDZK1 reduced NaPi-IIa abundance indicating that PDZK1 may play a role in the trafficking or stability of NaPi-IIa under these condition