Parathyroid hormone regulation of type II sodium-phosphate cotransporters is dependent on an A kinase anchoring protein

NHERF1 Loss Upregulates Enzymes of the Pentose Phosphate Pathway in Kidney Cortex

(1) Background: We previously showed Na/H exchange regulatory factor 1 (NHERF1) loss resulted in increased susceptibility to cisplatin nephrotoxicity. NHERF1-deficient cultured proximal tubule cells and proximal tubules from NHERF1 knockout (KO) mice exhibit altered mitochondrial protein expression and poor survival. We hypothesized that NHERF1 loss results in changes in metabolic pathways and/or mitochondrial dysfunction, leading to increased sensitivity to cisplatin nephrotoxicity. (2) Methods: Two to 4-month-old male wildtype (WT) and KO mice were treated with vehicle or cisplatin (20 mg/kg dose IP). After 72 h, kidney cortex homogenates were utilized for metabolic enzyme activities. Non-treated kidneys were used to isolate mitochondria for mitochondrial respiration via the Seahorse XF24 analyzer. Non-treated kidneys were also used for LC-MS analysis to evaluate kidney ATP abundance, and electron microscopy (EM) was utilized to evaluate mitochondrial morphology and number. (3) Results: KO mouse kidneys exhibit significant increases in malic enzyme and glucose-6 phosphate dehydrogenase activity under baseline conditions but in no other gluconeogenic or glycolytic enzymes. NHERF1 loss does not decrease kidney ATP content. Mitochondrial morphology, number, and area appeared normal. Isolated mitochondria function was similar between WT and KO. Conclusions: KO kidneys experience a shift in metabolism to the pentose phosphate pathway, which may sensitize them to the oxidative stress imposed by cisplatin.

Phosphate Transport in Epithelial and Nonepithelial Tissue

Phosphate is an essential nutrient for life and is a critical component of bone formation, a major signaling molecule, and structural component of cell walls. Phosphate is also a component of high-energy compounds (i.e., AMP, ADP, and ATP) and essential for nucleic acid helical structure (i.e., RNA and DNA). Phosphate plays a central role in the process of mineralization, normal serum levels being associated with appropriate bone mineralization, while high and low serum levels are associated with soft tissue calcification. The serum concentration of phosphate and the total body content of phosphate are highly regulated, a process that is accomplished by the coordinated effort of two families of sodium-dependent transporter proteins. The three isoforms of the SLC34 family (SLC34A1-A3) show very restricted tissue expression and regulate intestinal absorption and renal excretion of phosphate. SLC34A2 also regulates the phosphate concentration in multiple lumen fluids including milk, saliva, pancreatic fluid, and surfactant. Both isoforms of the SLC20 family exhibit ubiquitous expression (with some variation as to which one or both are expressed), are regulated by ambient phosphate, and likely serve the phosphate needs of the individual cell. These proteins exhibit similarities to phosphate transporters in nonmammalian organisms. The proteins are nonredundant as mutations in each yield unique clinical presentations. Further research is essential to understand the function, regulation, and coordination of the various phosphate transporters, both the ones described in this review and the phosphate transporters involved in intracellular transport.

Physiological regulation of phosphate by vitamin D, parathyroid hormone (PTH) and phosphate (Pi)

Inorganic phosphate (Pi) is an abundant element in the body and is essential for a wide variety of key biological processes. It plays an essential role in cellular energy metabolism and cell signalling, e.g. adenosine and guanosine triphosphates (ATP, GTP), and in the composition of phospholipid membranes and bone, and is an integral part of DNA and RNA. It is an important buffer in blood and urine and contributes to normal acid-base balance. Given its widespread role in almost every molecular and cellular function, changes in serum Pi levels and balance can have important and untoward effects. Pi homoeostasis is maintained by a counterbalance between dietary Pi absorption by the gut, mobilisation from bone and renal excretion. Approximately 85% of total body Pi is present in bone and only 1% is present as free Pi in extracellular fluids. In humans, extracellular concentrations of inorganic Pi vary between 0.8 and 1.2 mM, and in plasma or serum Pi exists in both its monovalent and divalent forms (H2PO4- and HPO42-). In the intestine, approximately 30% of Pi absorption is vitamin D regulated and dependent. To help maintain Pi balance, reabsorption of filtered Pi along the renal proximal tubule (PT) is via the NaPi-IIa and NaPi-IIc Na+-coupled Pi cotransporters, with a smaller contribution from the PiT-2 transporters. Endocrine factors, including, vitamin D and parathyroid hormone (PTH), as well as newer factors such as fibroblast growth factor (FGF)-23 and its coreceptor α-klotho, are intimately involved in the control of Pi homeostasis. A tight regulation of Pi is critical, since hyperphosphataemia is associated with increased cardiovascular morbidity in chronic kidney disease (CKD) and hypophosphataemia with rickets and growth retardation. This short review considers the control of Pi balance by vitamin D, PTH and Pi itself, with an emphasis on the insights gained from human genetic disorders and genetically modified mouse models.

Phosphate homeostasis disorders

Our understanding of the regulation of phosphate balance has benefited tremendously from the molecular identification and characterization of genetic defects leading to a number of rare inherited or acquired disorders affecting phosphate homeostasis. The identification of the key phosphate-regulating hormone, fibroblast growth factor 23 (FGF23), as well as other molecules that control its production, such as the glycosyltransferase GALNT3, the endopeptidase PHEX, and the matrix protein DMP1, and molecules that function as downstream effectors of FGF23 such as the longevity factor Klotho and the phosphate transporters NPT2a and NPT2c, has permitted us to understand the complex interplay that exists between the kidneys, bone, parathyroid, and gut. Such insights from genetic disorders have allowed not only the design of potent targeted treatment of FGF23-dependent hypophosphatemic conditions, but also provide clinically relevant observations related to the dysregulation of mineral ion homeostasis in health and disease.

PTH and Vitamin D

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.

Role of Na+/H+ exchanger regulatory factor 1 in forward trafficking of the type IIa Na+-Pi cotransporter

Na+/H+ exchanger regulatory factor (NHERF1) plays a critical role in the renal transport of phosphate by binding to Na+-Pi cotransporter (NpT2a) in the proximal tubule. While the association between NpT2a and NHERF1 in the apical membrane is known, the role of NHERF1 to regulate the trafficking of NpT2a has not been studied. To address this question, we performed cell fractionation by sucrose gradient centrifugation in opossum kidney (OK) cells placed in low-Pi medium to stimulate forward trafficking of NpT2a. Immunoblot analysis demonstrated expression of NpT2a and NHERF1 in the endoplasmic reticulum (ER)/Golgi. Coimmunoprecipitation demonstrated a NpT2a-NHERF1 interaction in the ER/Golgi. Low-Pi medium for 4 and 8 h triggered a decrease in NHERF1 in the plasma membrane with a corresponding increase in the ER/Golgi. Time-lapse total internal reflection fluorescence imaging of OK cells placed in low-Pi medium, paired with particle tracking and mean square displacement analysis, indicated active directed movement of NHERF1 at early and late time points, whereas NpT2a showed active movement only at later times. Silence of NHERF1 in OK cells expressing green fluorescent protein (GFP)-NpT2a resulted in an intracellular accumulation of GFP-NpT2a. Transfection with GFP-labeled COOH-terminal (TRL) PDZ-binding motif deleted or wild-type NpT2a in OK cells followed by cell fractionation and immunoprecipitation confirmed that the interaction between NpT2a and NHERF1 was dependent on the TRL motif of NpT2a. We conclude that appropriate trafficking of NpT2a to the plasma membrane is dependent on the initial association between NpT2a and NHERF1 through the COOH-terminal TRL motif of NpT2a in the ER/Golgi and requires redistribution of NHERF1 to the ER/Golgi.

Parathyroid hormone (PTH) decreases sodium-phosphate cotransporter type IIa (NpT2a) mRNA stability

The acute inhibitory effects of parathyroid hormone (PTH) on proximal tubule Na+-K+-ATPase (Na-K) and sodium-dependent phosphate (NaPi) transport have been extensively studied, while little is known about the chronic effects of PTH. Patients with primary hyperparathyroidism, a condition characterized by chronic elevations in PTH, exhibit persistent hypophosphatemia but not significant evidence of salt wasting. We postulate that chronic PTH stimulation results in differential desensitization of PTH responses. To address this hypothesis, we compared the effects of chronic PTH stimulation on Na-Pi cotransporter (Npt2a) expression and Na-K activity and expression in Sprague Dawley rats, transgenic mice featuring parathyroid-specific cyclin D1 overexpression (PTH-D1), and proximal tubule cell culture models. We demonstrated a progressive decrease in brush-border membrane (BBM) expression of Npt2a from rats treated with PTH for 6 h or 4 days, while Na-K expression and activity in the basolateral membranes (BLM) exhibited an initial decrease followed by recovery to control levels by 4 days. Npt2a protein expression in PTH-D1 mice was decreased relative to control animals, whereas levels of Na-K, NHERF-1, and PTH receptor remained unchanged. In PTH-D1 mice, NpT2a mRNA expression was reduced by 50% relative to control mice. In opossum kidney proximal tubule cells, PTH decreased Npt2a mRNA levels. Both actinomycin D and cycloheximide treatment prevented the PTH-mediated decrease in Npt2a mRNA, suggesting that the PTH response requires transcription and translation. These findings suggest that responses to chronic PTH exposure are selectively regulated at a posttranscriptional level. The persistence of the phosphaturic response to PTH occurs through posttranscriptional mechanisms.

Fluorescent ligand-directed co-localization of the parathyroid hormone 1 receptor with the brush-border scaffold complex of the proximal tubule reveals hormone-dependent changes in ezrin immunoreactivity consistent with inactivation

Through binding to parathyroid hormone (PTH), PTH1R interacts with kidney-specific scaffold proteins, including the sodium hydrogen exchanger regulatory factors 1 and 2 (NHERFs), and ezrin. To facilitate in vivo localization, tetramethylrhodamine-labeled PTH (PTH-TMR) was used as a fluorescent probe. In mice, PTH-TMR localizes to luminal surfaces of tubular S1 segments that overlap PTH1R immunostaining, but does not directly overlap with megalin-specific antibodies. PTH-TMR staining directly overlaps with Npt2a in nascent, endocytic vesicles, marking the location of transporter regulation. PKA substrate antibodies display marked staining increases in segments labeled with PTH-TMR, demonstrating a functional effect. In the presence of secondary hyperparathyroidism, PTH-TMR staining is markedly reduced and shifts to co-localizing with megalin. At 15min post-injection, PTH-TMR-labeled vesicles do not co-localize with either NHERF or ezrin, suggesting PTH1R dissociation from the scaffold complex. At the 5min time point, PTH-TMR stains the base of microvilli where it localizes with both NHERF2 and ezrin, and only partially with NHERF1. Strikingly, the bulk of ezrin protein becomes undetectable with the polyclonal, CS3145 antibody, revealing a PTH-induced conformational change in the scaffold. A second ezrin antibody (3C12) is capable of detecting the altered ezrin protein. The CS3145 antibody only binds to the active form of ezrin and fails to recognize the inactive form, while the 3C12 reagent can detect either active or inactive ezrin. Here we show that the PTH1R is part of the ezrin scaffold complex and that acute actions of PTH suggest a rapid inactivation of ezrin in a spatially defined manner.

Ezrin-anchored protein kinase A coordinates phosphorylation-dependent disassembly of a NHERF1 ternary complex to regulate hormone-sensitive phosphate transport

Congenital defects in the Na/H exchanger regulatory factor-1 (NHERF1) are linked to disordered phosphate homeostasis and skeletal abnormalities in humans. In the kidney, these mutations interrupt parathyroid hormone (PTH)-responsive sequestration of the renal phosphate transporter, Npt2a, with ensuing urinary phosphate wasting. We now report that NHERF1, a modular PDZ domain scaffolding protein, coordinates the assembly of an obligate ternary complex with Npt2a and the PKA-anchoring protein ezrin to facilitate PTH-responsive cAMP signaling events. Activation of ezrin-anchored PKA initiates NHERF1 phosphorylation to disassemble the ternary complex, release Npt2a, and thereby inhibit phosphate transport. Loss-of-function mutations stabilize an inactive NHERF1 conformation that we show is refractory to PKA phosphorylation and impairs assembly of the ternary complex. Compensatory mutations introduced in mutant NHERF1 re-establish the integrity of the ternary complex to permit phosphorylation of NHERF1 and rescue PTH action. These findings offer new insights into a novel macromolecular mechanism for the physiological action of a critical ternary complex, where anchored PKA coordinates the assembly and turnover of the Npt2a-NHERF1-ezrin complex.

NHERF-1 and the regulation of renal phosphate reabsoption: a tale of three hormones

The renal excretion of inorganic phosphate is regulated in large measure by three hormones, namely, parathyroid hormone, dopamine, and fibroblast growth factor-23. Recent experiments have indicated that the major sodium-dependent phosphate transporter in the renal proximal tubule, Npt2a, binds to the adaptor protein sodium-hydrogen exchanger regulatory factor-1 (NHERF-1) and in the absence of NHERF-1, the inhibitory effect of these three hormones is absent. From these observations, a new model for the hormonal regulation of renal phosphate transport was developed. The downstream signaling pathways of these hormones results in the phosphorylation of the PDZ 1 domain of NHERF-1 and the dissociation of Npt2a/NHERF-1 complexes. In turn, this dissociation facilitates the endocytosis of Npt2a with a subsequent decrease in the apical membrane abundance of the transporter and a decrease in phosphate reabsorption. The current review outlines the experimental observations supporting the operation of this unique regulatory system.

Functional interaction between CFTR and the sodium-phosphate co-transport type 2a in Xenopus laevis oocytes

Background

A growing number of proteins, including ion transporters, have been shown to interact with Cystic Fibrosis Transmembrane conductance Regulator (CFTR). CFTR is an epithelial chloride channel that is involved in Cystic Fibrosis (CF) when mutated; thus a better knowledge of its functional interactome may help to understand the pathophysiology of this complex disease. In the present study, we investigated if CFTR and the sodium-phosphate co-transporter type 2a (NPT2a) functionally interact after heterologous expression of both proteins in Xenopus laevis oocytes.

Methodology/Findings

NPT2a was expressed alone or in combination with CFTR in X. laevis oocytes. Using the two-electrode voltage-clamp technique, the inorganic phosphate-induced current (IPi) was measured and taken as an index of NPT2a activity. The maximal IPi for NPT2a substrates was reduced when CFTR was co-expressed with NPT2a, suggesting a decrease in its expression at the oolemna. This was consistent with Western blot analysis showing reduced NPT2a plasma membrane expression in oocytes co-expressing both proteins, whereas NPT2a protein level in total cell lysate was the same in NPT2a- and NPT2a+CFTR-oocytes. In NPT2a+CFTR- but not in NPT2a-oocytes, IPi and NPT2a surface expression were increased upon PKA stimulation, whereas stimulation of Exchange Protein directly Activated by cAMP (EPAC) had no effect. When NPT2a-oocytes were injected with NEG2, a short amino-acid sequence from the CFTR regulatory domain that regulates PKA-dependent CFTR trafficking to the plasma membrane, IPi values and NPT2a membrane expression were diminished, and could be enhanced by PKA stimulation, thereby mimicking the effects of CFTR co-expression.

Conclusion/Perspectives

We conclude that when both CFTR and NPT2a are expressed in X. laevis oocytes, CFTR confers to NPT2a a cAMPi-dependent trafficking to the membrane. This functional interaction raises the hypothesis that CFTR may play a role in phosphate homeostasis.

Role of vacuolar ATPase in the trafficking of renal type IIa sodium-phosphate cotransporter

BACKGROUND/AIMS

Total body phosphate homeostasis is regulated by expression of type IIa sodium phosphate cotransporter (NpT2a) in the apical membrane (BBM) of renal proximal tubule cells. NpT2a expression is regulated by dietary phosphate and PTH but the mechanisms for trafficking of the protein are unknown. Based on 2D gel electrophoresis and mass spectroscopy data that changes in dietary phosphate stimulated changes in BBM expression of vacuolar H(+)-ATPase, we hypothesized that vacuolar H(+)-ATPase plays a significant role in regulation of NpT2a in opossum kidney (OK) cells, a model for renal proximal tubule transport.

METHODS

Role of vacuolar H(+)-ATPase was studied in opossum kidney (OK) cells by examining the effect of inhibition of vacuolar H(+)-ATPase on Pi uptake and NpT2a expression.

RESULTS

Pretreatment of OK cells with bafilomycin A(1) and concanamycin A, inhibitors of vacuolar H(+)-ATPases, blocked high phosphate- and PTH-induced degradation of NpT2a, but had no effect on high phosphate or PTH induced inhibition of sodium-dependent phosphate transport. Exposure of the cells to bafilomycin A(1) significantly decreased phosphate transport and apical membrane expression of NpT2a. Treatment with brefeldin A, an inhibitor of Golgi transport, decreased phosphate transport and apical membrane expression of NpT2a while treatment of cells with both brefeldin A and bafilomycin A(1) had no additive effect.

CONCLUSION

We conclude that vacuolar H(+)-ATPase plays a significant role in exocytosis of NpT2a into the apical membrane and in degradation of NpT2a but has no role in endocytosis.

Isolation of proteins and protein complexes by immunoprecipitation

Immunoprecipitation (IP) uses the specificity of antibodies to isolate target proteins (antigens) out of complex sample mixtures. Three different approaches for performing IP will be discussed; traditional (classical) method, oriented affinity method and direct affinity method. The traditional method of incubating the IP antibody with the sample and sequentially binding to Protein A or G agarose beads (resin) facilitates the most efficient target antigen recovery. However, this approach results in the target protein becoming contaminated with the IP antibody that can interfere with downstream analyses. The orientated affinity method uses Protein A or G beads to serve as an anchor to which the IP antibody is crosslinked thereby preventing the antibody from co-eluting with the target protein. Similarly, the direct affinity method also immobilizes the IP antibody except in this case it is directly attached to a chemically activated support. Both methods prevent co-elution of the IP antibody enabling reuse of the immunomatrix. All three approaches have unique advantages and can also be used for co-immunoprecipitation to study protein:protein interactions and investigate the functional proteome.

The A-kinase anchoring protein 15 regulates feedback inhibition of the epithelial Na+ channel

Protein kinase A anchoring proteins or AKAPs regulate the activity of many ion channels. Protein kinase A (PKA) is a well-recognized target of AKAPs, with other kinases now emerging as additional targets. We examined the roles of epithelial-expressed AKAPs in regulating the epithelial Na+ channel (ENaC). Experiments used heterologous expression with AKAP15, AKAP-KL, and AKAP79 in Xenopus oocytes. Experiments were carried out under high and low Na+ conditions, as Na+ loading is known to affect the baseline activity of ENaC in a PKC-dependent mechanism. ENaC activity was unaffected by AKAP79 and AKAP-KL expression. However, oocytes coexpressing AKAP15 exhibited an 80% and 91% reduction in the amiloride-sensitive, whole-cell conductance in high and low Na+ conditions, respectively. The reduced channel activity was unaffected by PKA activation or inhibition, indicating a PKA-independent mechanism. Expression with a membrane-targeting domain, mutant form of AKAP15 (AKAP15m) prevented the decrease of ENaC activity, but only under low Na+ conditions. In high sodium conditions, coexpression with AKAP15m led to an increase of ENaC activity to levels similar to those observed under low Na+. These results indicate that membrane-associated AKAP15 reduces ENaC activity whereas the cytoplasmically associated one may participate in the channel's feedback inhibition by intracellular Na+, a process known to involve PKC. This hypothesis was further confirmed in coexpression experiments, which demonstrated functional and physical interaction between AKAP15 and PKCalpha. We propose that AKAP15 regulates ENaC via a novel PKA-independent pathway.

Pathophysiology of hypercalciuria in children

Urinary excretion of calcium is the result of a complex interplay between three organs-namely, the gastrointestinal tract, bone, and kidney-which is finely orchestrated by multiple hormones. Hypercalciuria is believed to be a polygenic trait and is influenced significantly by diet. This paper briefly reviews calcium handling by the renal tubule in normal and in hereditary disorders as it relates to the pathophysiology of hypercalciuria. The effects of dietary sodium, potassium, protein, calcium, and phosphate on calcium excretion, and the association of hypercalciuria with bone homeostasis is discussed, leading to recommendations on means to address excessive urinary calcium excretion.

REGULATION OF PHOSPHORUS HOMEOSTASIS BY THE TYPE IIA NA/PHOSPHATE COTRANSPORTER

The type IIa Na/phosphate (Pi) cotransporter (Npt2a) is expressed in the brush border membrane (BBM) of renal proximal tubular cells where the bulk of filtered Pi is reabsorbed. Disruption of the Npt2a gene in mice elicits hypophosphatemia, renal Pi wasting, and an 80% decrease in renal BBM Na/Pi cotransport, and led to the demonstration that Npt2a is the target for hormonal and dietary regulation of renal Pi reabsorption. Regulation is achieved by changes in BBM abundance of Npt2a protein and requires the interaction of Npt2a with various scaffolding and regulatory proteins. Molecular studies in patients with renal Pi wasting resulted in the identification of novel regulators of Pi homeostasis: fibroblast growth factor-23 (FGF-23) and a phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX). In mouse models, increased FGF-23 production or loss of Phex function causes hypophosphatemia and decreased renal Pi reabsorption, secondary to decreased BBM Npt2a protein abundance. Thus, Npt2a plays a major role in the maintenance of Pi homeostasis in both health and disease.

Parathyroid hormone regulation of NA+,K+-ATPase requires the PDZ 1 domain of sodium hydrogen exchanger regulatory factor-1 in opossum kidney cells

It was demonstrated that expression of murine sodium hydrogen exchanger regulatory factor (NHERF-1) lacking the ezrin-binding domain blocks parathyroid hormone (PTH) regulation of Na+,K+-ATPase in opossum kidney (OK) cells. The hypothesis that the NHERF-1 PDZ domains contribute to PTH regulation of Na+,K+-ATPase was tested by comparison of PTH regulation of Na+,K+-ATPase in wild-type OK (OK-WT) cells, NHERF-deficient OKH cells, OK-WT transfected with siRNA for NHERF (NHERF siRNA OK-WT), and OKH cells that were stably transfected with full-length NHERF-1 or constructs with mutated PDZ domains. OKH cells and NHERF siRNA OK-WT showed decreased expression of NHERF-1 but equivalent expression of ezrin and Na+,K+-ATPase alpha1 subunit when compared with OK-WT cells. PTH decreased Na+,K+-ATPase activity and stimulated phosphorylation of the Na+,K+-ATPase alpha1 in OK-WT cells but not in NHERF-deficient cells. Rubidium (86Rb) uptake was equivalent in OK-WT, OKH, and OKH cells that were transfected with all but the double PDZ domain mutants. PTH decreased 86Rb uptake significantly in OK-WT but not in OKH cells. PTH also significantly inhibited 86Rb uptake in OKH cells that were transfected with full-length NHERF-1 or NHERF-1 with mutated PDZ 2 but not in OKH cells that were transfected with mutated PDZ 1. Transfection with NHERF expressing both mutated PDZ domains resulted in diminished basal 86Rb uptake that was not inhibited further by PTH. PTH stimulated protein kinase Calpha activity and alpha1 subunit phosphorylation in OK-WT but not in NHERF-deficient cells. Transfection of OKH cells with NHERF constructs that contained an intact PDZ1 domain restored PTH-stimulated protein kinase Calpha activity and alpha1 subunit phosphorylation. These results demonstrate that NHERF-1 is necessary for PTH-mediated inhibition of Na+,K+-ATPase activity and that the inhibition is mediated through the PDZ1, not PDZ2, domain.

Sodium-phosphate cotransporters, nephrolithiasis and bone demineralization

PURPOSE OF REVIEW

We discuss how recent findings obtained in disorders of phosphate metabolism in humans and in animal models have provided insights into the pathogenesis of renal stone formation and bone demineralization.

RECENT FINDINGS

Mice that are null for the sodium-phosphate cotransporter (NPT)2a gene (NPT2a(-/-) mice) exhibit hypophosphataemia, increased urinary phosphate excretion, hypercalciuria and nephrolithiasis, but no bone demineralization. Mice null for the sodium-hydrogen exchanger regulatory factor (NHERF)1 (NHERF1(-/-) mice) also exhibit hypophosphataemia and increased renal phosphate excretion with decreased renal NPT2a expression, but they present with a severe sex-dependent bone demineralization. Heterozygous loss-of-function mutations in the NPT2a gene in humans induce hypophosphataemia, increased urinary phosphate excretion, hypercalciuria, nephrolithiasis in males (to date) and bone demineralization of variable severity in both sexes. Patients and experimental animals with increased circulating levels of fibroblast growth factor 23 present with hypophosphataemia, increased urinary phosphate excretion, inappropriate calcitriol synthesis and rickets/osteomalacia, but no nephrolithiasis except when treated. Low-phosphate diet in spontaneously hypercalciuric rats and disruption of the 1-alpha-hydroxylase gene in NPT2a mice prevent renal stone formation.

SUMMARY

Increased urinary phosphate excretion is a risk factor for renal calcium stone formation when it is associated with hypercalciuria. As yet undefined interplay between NPT2a, NHERF1 and possibly other cotransporters or associated proteins in bone cells may account for the diversity of bone phenotypes observed in disorders of phosphate metabolism with impaired renal phosphate reabsorption. The pathogenesis of both renal stone and bone demineralization appear to be affected by species, sex and mutation type, among other factors.

PDZ interactions and proximal tubular phosphate reabsorption

In adults, the extent of renal reabsorption of P(i) and consequently the extent of urinary excretion of phosphate are to a large extent determined by the abundance of the Na-P(i) 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-P(i) cotransport.

Novel aspects in regulated expression of the renal type IIa Na/Pi-cotransporter

Proximal tubular phosphate (P(i)) reabsorption is a key element in overall phosphate homeostasis; physiologic/pathophysiologic alterations are related to the control of brush border membrane expression (regulated endocytosis) of the type IIa sodium (Na)/phosphate(P(i))-cotransporter (NaPi-IIa). The carboxy terminus of NaPi-IIa contains sequences important for its apical delivery/expression; the last three amino acids are involved in PSD95/DglA/ZO-1 (PDZ) interactions involving NaPi-IIa, Na/H exchanger-regulatory factor 1 (NHERF1/2), and PDZK1/2 (apical scaffold). Regulated endocytosis of NaPi-IIa [e.g., parathyroid hormone (PTH)-induced] is reduced in megalin-deficient mice; internalization occurs via clathrin-coated structures, early endosomes, and finally leads to lysosomal degradation. NaPi-IIa contains, in the third intracellular loop, a sequence motif required for internalization. Different hormonal [e.g., PTH, atrial natriuretic peptide (ANP), also nitric oxide (NO)] and nonhormonal factors activate a variety of intracellular signaling cascades [protein kinase A (PK-A), protein kinase C (PK-C), protein kinase G (PK-G), extracellular receptor kinase (ERK)-1/2] leading (by unknown mechanisms) to NaPi-IIa internalization. Different phosphatonins [e.g., fibroblast growth factor (FGF)-23, frizzled related protein (FRP)-4, matrix extracellularphosphoglycoprotein (MEPE)], associated with different pathophysiologic states of renal P(i)-handling, seem also to control apical expression of NaPi-IIa. Internalization of NaPi-IIa first requires its removal from the apical scaffold. This scaffold can also be considered as a regulatory scaffold containing also protein kinase A (PK-A)-anchoring proteins (AKAPs, ezrin) and the apical PTH receptor. The role of the different components of the regulatory scaffold in regulated endocytosis of NaPi-IIa is at present unknown.

Clathrin-mediated Endocytosis of Na+,K+-ATPase in Response to Parathyroid Hormone Requires ERK-dependent Phosphorylation of Ser-11 within the α1-Subunit

Parathyroid hormone (PTH) inhibits Na(+),K(+)-ATPase activity through protein kinase C- (PKC) and extracellular signal-regulated kinase- (ERK) dependent pathways and increases serine phosphorylation of the alpha(1)-subunit. To determine whether specific serine phosphorylation sites within the Na(+),K(+)-ATPase alpha(1)-subunit are involved in the Na(+),K(+)-ATPase responses to PTH, we examined the effect of PTH in opossum kidney cells stably transfected with wild type rat Na(+),K(+)-ATPase alpha(1)-subunit (WT), serine 11 to alanine mutant alpha(1)-subunit (S11A), or serine 18 to alanine mutant alpha(1)-subunit (S18A). PTH increased phosphorylation and endocytosis of the Na(+),K(+)-ATPase alpha(1)-subunit into clathrin-coated vesicles in cells transfected with WT and S18A rat Na(+),K(+)-ATPase alpha(1)-subunits. PTH did not increase the level of phosphorylation or stimulate translocation of Na(+),K(+)-ATPase alpha(1)-subunits into clathrin-coated vesicles in cells transfected with the S11A mutant. PTH inhibited ouabain-sensitive (86)Rb uptake and Na(+),K(+)-ATPase activity (ouabain-sensitive ATP hydrolysis) in WT- and S18A-transfected opossum kidney cells but not in S11A-transfected cells. Pretreatment of the cells with the PKC inhibitors and ERK inhibitor blocked PTH inhibition of (86)Rb uptake, Na(+),K(+)-ATPase activity, alpha(1)-subunit phosphorylation, and endocytosis in WT and S18A cells. Consistent with the notion that ERK phosphorylates Na(+),K(+)-ATPase alpha(1)-subunit, ERK was shown to be capable of causing phosphorylation of Na(+),K(+)-ATPase alpha(1)-subunit immunoprecipitated from WT and S18A but not from S11A-transfected cells. These results suggest that PTH regulates Na(+),K(+)-ATPase by PKC and ERK-dependent alpha(1)-subunit phosphorylation and that the phosphorylation requires the expression of a serine at the 11 position of the Na(+),K(+)-ATPase alpha(1)-subunit.

Does membrane trafficking play a role in regulating the sodium/hydrogen exchanger isoform 3 in the proximal tubule?

PURPOSE OF REVIEW

The proximal tubule sodium/hydrogen exchanger continuously reabsorbs the bulk of the filtered sodium, controlling salt delivery to the distal nephron which is critical for tubuloglomerular feedback autoregulation and for fine control of salt excretion in the distal nephron. This review focuses on recent studies of the mechanisms of regulation of sodium transport in the proximal tubule, and addresses whether results from studies in proximal tubule cell lines are applicable to the proximal tubule in situ.

RECENT FINDINGS

Recent in-vivo studies provided evidence that sodium/hydrogen exchanger isoform 3 can move into and out of the apical microvilli accompanied by parallel changes in renal sodium transport: the exchanger is retracted from the microvilli in response to hypertension, parathyroid hormone or dopamine treatment and moved into the microvilli in response to sympathetic nervous system stimulation, puromycin aminonucleoside induced nephritic syndrome, and insulin treatment. Studies in cultured opossum kidney proximal tubule cells provided evidence for clathrin coated vesicle mediated, dynamin dependent, cytoskeleton dependent internalization of sodium/hydrogen exchanger isoform 3 from the surface to an endosomal pool in response to dopamine or parathyroid hormone. In the intact proximal tubule there is evidence for a two-step internalization process: (1) from villi to the intermicrovillar cleft region and (2) to a higher density membrane pool that may be either below the microvilli or deep in intermicrovillar clefts. Recent studies have described a significant inactive pool of the exchanger in the intermicrovillar region in vivo that may serve as a storage and recruitable pool.

SUMMARY

The molecular mechanisms responsible for increasing or decreasing sodium transport in the proximal tubule appear to include redistribution of sodium/hydrogen exchanger isoform 3 to or from the microvillar region. Detailed studies in cultured proximal tubule cell lines provide evidence for endocytosis and exocytosis of the exchanger dependent on cytoskeleton and clathrin coated vesicles. In vivo, the apical membrane is differentiated into discrete villar and intermicrovillar membrane domains and the intermicrovillar domain, not observed in cultured cells, may serve as a recruitable storage pool for sodium/hydrogen exchanger isoform 3.

Role of NHERF-1 in regulation of the activity of Na-K ATPase and sodium-phosphate co-transport in epithelial cells

Parathyroid hormone (PTH), acting at least in part through a cAMP signaling pathway, regulates three important transporters in the renal proximal convoluted tubule, namely Na-H exchanger 3, Na-K ATPase, and type IIa sodium phosphate cotransporter (NaPi IIa). The regulation of Na-H exchanger 3 by protein kinase A requires a protein co-factor from the sodium-hydrogen exchanger regulatory factor (NHERF) family of proteins (NHERF-1 and NHERF-2). However, the role of NHERF in PTH regulation of Na-K ATPase and NaPi IIa has not been explored. For studying the role of NHERF-1 on PTH regulation of these transporters, wild-type mNHERF-1 (1-355) or mNHERF-1 (1-325) lacking the ezrin-binding domain were expressed in proximal tubule-derived opossum kidney cells. PTH inhibited Na-K ATPase activity in cells expressing wild-type NHERF-1 associated with increased serine phosphorylation of the alpha subunit of the transporter. By contrast, in cells expressing mNHERF (1-325), the phosphorylation of the alpha subunit of Na-K ATPase was blunted and the activity of the transporter was stimulated in response to PTH. Basal sodium-dependent phosphate transport was lower in cells expressing mNHERF-1 (1-325) as compared with cells expressing mNHERF-1 (1-355). Nonetheless, there were no differences in PTH-associated inhibition of the activity or the decrease in membrane expression of the NaPi IIa in any of the cell lines. These experiments document for the first time an association between NHERF-1 and PTH regulation of Na-K ATPase in epithelial cells. These experiments also suggest that the mechanism for retrieval of NaPi IIa transporters from the apical membrane in response to cAMP does not require NHERF.