Defective coupling of apical PTH receptors to phospholipase C prevents internalization of the Na+-phosphate cotransporter NaPi-IIa in Nherf1-deficient mice

Phosphate reabsorption in the renal proximal tubule occurs mostly via the type IIa Na(+)-phosphate cotransporter (NaP(i)-IIa) in the brush border membrane (BBM). The activity and localization of NaP(i)-IIa are regulated, among other factors, by parathyroid hormone (PTH). NaP(i)-IIa interacts in vitro via its last three COOH-terminal amino acids with the PDZ protein Na(+)/H(+)-exchanger isoform 3 regulatory factor (NHERF)-1 (NHERF1). Renal phosphate reabsorption in Nherf1-deficient mice is altered, and NaP(i)-IIa expression in the BBM is reduced. In addition, it has been proposed that NHERF1 and NHERF2 are important for the coupling of PTH receptors (PTHRs) to phospholipase C (PLC) and the activation of the protein kinase C pathway. We tested the role of NHERF1 in the regulation of NaP(i)-IIa by PTH in Nherf1-deficient mice. Immunohistochemistry and Western blotting demonstrated that stimulation of apical and basolateral receptors with PTH-(1-34) led to internalization of NaP(i)-IIa in wild-type and Nherf1-deficient mice. Stimulation of only apical receptors with PTH-(3-34) failed to induce internalization in Nherf1-deficient mice. Expression and localization of apical PTHRs were similar in wild-type and Nherf1-deficient mice. Activation of the protein kinase C- and A-dependent pathways with 1,2-dioctanoyl-sn-glycerol or 8-bromo-cAMP induced normal internalization of NaP(i)-IIa in wild-type, as well as Nherf1-deficient, mice. Stimulation of PLC activity due to apical PTHRs was impaired in Nherf1-deficient mice. These data suggest that NHERF1 in the proximal tubule is important for PTH-induced internalization of NaP(i)-IIa and, specifically, couples the apical PTHR to PLC.

The association of NHERF adaptor proteins with g protein-coupled receptors and receptor tyrosine kinases

The sodium-hydrogen exchanger regulatory factors (NHERF-1 and NHERF-2) are a family of adaptor proteins characterized by the presence of two tandem PDZ protein interaction domains and a C-terminal domain that binds the cytoskeleton proteins ezrin, radixin, moesin, and merlin. The NHERF proteins are highly expressed in the kidney, small intestine, and other organs, where they associate with a number of transporters and ion channels, signaling proteins, and transcription factors. Recent evidence has revealed important associations between the NHERF proteins and several G protein-coupled receptors such as the beta2-adrenergic receptor, the kappa-opioid receptor, and the parathyroid hormone receptor, as well as growth factor tyrosine kinase receptors such as the platelet-derived growth factor receptor and the epidermal growth factor receptor. This review summarizes the emerging data on the biochemical mechanisms, physiologic outcomes, and potential clinical implications of the assembly and disassembly of receptor/NHERF complexes.

Adenoviral expression of NHERF-1 in NHERF-1 null mouse renal proximal tubule cells restores Npt2a regulation by low phosphate media and parathyroid hormone

Sodium-dependent phosphate transport in NHERF-1(-/-) proximal tubule cells does not increase when grown in a low phosphate media and is resistant to the normal inhibitory effects of parathyroid hormone (PTH). The current experiments employ adenovirus-mediated gene transfer in primary cultures of mouse proximal tubule cells from NHERF-1 null mice to explore the specific role of NHERF-1 on regulated Npt2a trafficking and sodium-dependent phosphate transport. NHERF-1 null cells have decreased sodium-dependent phosphate transport compared with wild-type cells. Infection of NHERF-1 null cells with adenovirus-GFP-NHERF-1 increased phosphate transport and plasma membrane abundance of Npt2a. Adenovirus-GFP-NHERF-1 infected NHERF-1 null proximal tubule cells but not cells infected with adenovirus-GFP demonstrated increased phosphate transport and Npt2a abundance in the plasma membrane when grown in low phosphate (0.1 mM) compared with high phosphate media (1.9 mM). PTH inhibited phosphate transport and decreased Npt2a abundance in the plasma membrane of adenovirus-GFP-NHERF-1-infected NHERF-1 null proximal tubule cells but not cells infected with adenovirus-GFP. Interestingly, phosphate transport is inhibited by activation of protein kinase A and protein kinase C in wild-type proximal tubule cells but not in NHERF-1(-/-) cells. Together, these results highlight the requirement for NHERF-1 for physiological control of Npt2a trafficking and suggest that the Npt2a/NHERF-1 complex represents a unique PTH-responsive pool of Npt2a in renal microvilli.

Na+/H+ Exchanger Regulatory Factor Isoform 1 Overexpression Modulates Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Expression and Activity in Human Airway 16HBE14o- Cells and Rescues ΔF508 CFTR Functional Expression in Cystic Fibrosis Cells*

There is evidence that cystic fibrosis transmembrane conductance regulator (CFTR) interacting proteins play critical roles in the proper expression and function of CFTR. The Na(+)/H(+) exchanger regulatory factor isoform 1 (NHERF1) was the first identified CFTR-binding protein. Here we further clarify the role of NHERF1 in the regulation of CFTR activity in two human bronchial epithelial cell lines: the normal, 16HBE14o-, and the homozygous DeltaF508 CFTR, CFBE41o-. Confocal analysis in polarized cell monolayers demonstrated that NHERF1 distribution was associated with the apical membrane in 16HBE14o- cells while being primarily cytoplasmic in CFBE41o- cells. Transfection of 16HBE14o- monolayers with vectors encoding for wild-type (wt) NHERF1 increased both apical CFTR expression and apical protein kinase A (PKA)-dependent CFTR-mediated chloride efflux, whereas transfection with NHERF1 mutated in the binding groove of the PDZ domains or truncated for the ERM domain inhibited both the apical CFTR expression and the CFTR-dependent chloride efflux. These data led us to hypothesize an important role for NHERF1 in regulating CFTR localization and stability on the apical membrane of 16HBE14o- cell monolayers. Importantly, wt NHERF1 overexpression in confluent DeltaF508 CFBE41o- and DeltaF508 CFT1-C2 cell monolayers induced both a significant redistribution of CFTR from the cytoplasm to the apical membrane and a PKA-dependent activation of CFTR-dependent chloride secretion.

Longitudinal study of urinary excretion of phosphate, calcium, and uric acid in mutant NHERF-1 null mice

NHERF-1 binds numerous renal protein targets, including the proximal tubule transporters Na(+)/H(+) exchanger 3 (NHE3) and Na(+)-phosphate cotransporter 2a (Npt2a). Young NHERF-1(-/-) male mice display defective targeting of Npt2a to apical membranes in the renal proximal tubule and manifest hypophosphatemia and increased urinary excretion of phosphate. The present studies describe the changes in the urinary excretion of phosphate, calcium, uric acid, and sodium in male and female wild-type and NHERF-1 null mice over a time period from 12 to 54 wk of age. Young male and female NHERF-1(-/-) mice demonstrated increased urinary excretion of phosphate and urine phosphate/creatinine ratios. There was an age-related decline in the phosphate/creatinine ratio in mutant mice such that there were no differences between wild-type and NHERF-1(-/-) by 24 to 30 wk of age despite the continued presence of hypophosphatemia. Male and female NHERF-1 null mice also demonstrate increased urine calcium/creatinine and uric acid/creatinine ratios compared with wild-type controls. These studies indicate defects in the renal tubule transport of phosphate, calcium, and uric acid in NHERF-1(-/-) male and female mice that could account for the increased deposition of calcium in the papilla of null mice.

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.

ClC-5: role in endocytosis in the proximal tubule

The proper functioning of the Cl(-) channel, ClC-5, is essential for the uptake of low molecular mass proteins through receptor-mediated endocytosis in the proximal tubule. Dent's disease patients with mutant ClC-5 channels and ClC-5 knockout (KO) mice both have low molecular mass proteinuria. To further understand the function of ClC-5, endocytosis was studied in LLC-PK(1) cells and primary cultures of proximal tubule cells from wild-type (WT) and ClC-5 KO kidneys. Endocytosis in the proximal tubule cells from KO mice was reduced compared with that in WT animals. Endocytosis in WT but not in KO cells was inhibited by bafilomycin A-1 and Cl(-) depletion, whereas endocytosis in both WT and KO cells was inhibited by the NHE3 blocker, S3226. Infection with adenovirus containing WT ClC-5 rescued receptor-mediated endocytosis in KO cells, whereas infection with any of the three disease-causing mutants, myc-W22G-ClC-5, myc-S520P-ClC-5, or myc-R704X-ClC-5, did not. WT and the three mutants all trafficked to the apical surface, as assessed by surface biotinylation. WT-ClC-5 and the W22G mutant were internalized similarly, whereas neither the S520P nor the R704X mutants was. These data indicate that ClC-5 is important for Cl(-) and proton pump-mediated endocytosis. However, not all receptor-mediated endocytosis in the proximal tubule is dependent on ClC-5. There is a significant fraction that can be inhibited by an NHE3 blocker. Our data from the mutants suggest that defective targeting and trafficking of mutant ClC-5 to the endosomes are a major determinant in the lack of normal endocytosis in Dent's disease.

Defective PTH regulation of sodium-dependent phosphate transport in NHERF-1-/- renal proximal tubule cells and wild-type cells adapted to low-phosphate media

The present experiments using primary cultures from renal proximal tubule cells examine two aspects of the regulation of sodium-dependent phosphate transport and membrane sodium-dependent phosphate transporter (Npt2a) expression by parathyroid hormone (PTH). Sodium-dependent phosphate transport in proximal tubule cells from wild-type mice grown in normal-phosphate media averaged 4.4 +/- 0.5 nmol.mg protein(-1).10 min(-1) and was inhibited by 30.5 +/- 8.6% by PTH (10(-7) M). This was associated with a 32.7 +/- 5.2% decrease in Npt2a expression in the plasma membrane. Proximal tubule cells from Na(+)/H(+) exchanger regulatory factor-1 (NHERF-1)(-/-) mice had a lower rate of phosphate transport compared with wild-type cells and a significantly reduced inhibitory response to PTH. Wild-type cells incubated in low-phosphate media for 24 h had a higher rate of phosphate transport compared with wild-type cells grown in normal-phosphate media but a significantly blunted inhibitory response to PTH. These data indicate a role for NHERF-1 in mediating the membrane retrieval of Npt2a and the subsequent inhibition of phosphate transport in renal proximal tubules. These studies also suggest that there is a blunted phosphaturic effect of PTH in cells adapted to low-phosphate media.

The role of NHERF-1 in the regulation of renal proximal tubule sodium-hydrogen exchanger 3 and sodium-dependent phosphate cotransporter 2a

Adaptor proteins containing PDZ interactive domains have been recently identified to regulate the trafficking and activity of ion transporters and channels in epithelial tissue. In the renal proximal tubule, three PDZ adaptor proteins, namely NHERF-1, NHERF-2 and PDZK1, are expressed in the apical membrane, heterodimerize with one another, and, at least in vitro, are capable of binding to NHE3 and Npt2a, two major regulated renal proximal tubule apical membrane transporters. Studies using NHERF-1 null mice have begun to provide insights into the organization of these adaptor proteins and their specific interactions with NHE3 and Npt2a. Experiments using brush border membranes and cultured renal proximal tubule cells indicate a specific requirement for NHERF-1 for cAMP-mediated phosphorylation and inhibition of NHE3. NHERF-1 null mice demonstrate increased urinary excretion of phosphate associated with mistargeting of Npt2a to the apical membrane of renal proximal tubule cells. NHERF-1 null animals challenged with a low phosphate diet and proximal tubule cells from these animals cultured in a low phosphate media fail to adapt as well as wild-type mice. These studies indicate a unique requirement for NHERF-1 in cAMP regulation of NHE3 and in the trafficking of Npt2a.

Receptor Activity-modifying Protein (RAMP) Isoform-specific Regulation of Adrenomedullin Receptor Trafficking by NHERF-1

Receptor activity-modifying proteins (RAMPs 1-3) are single transmembrane accessory proteins critical to various G-protein coupled receptors for plasma membrane expression and receptor phenotype. A functional receptor for the vasodilatory ligand, adrenomedullin (AM), is comprised of RAMP2 or RAMP3 and calcitonin receptor-like receptor (CRLR). It is now known that RAMP3 protein-protein interactions regulate the recycling of the AM2 receptor. The major aim of this study was to identify other interaction partners of RAMP3 and determine their role in CRLR-RAMP3 trafficking. Trafficking of G-protein-coupled receptors has been shown to be regulated by the Na+/H+ exchanger regulatory factor-1 (NHERF-1), an adaptor protein containing two tandem PSD-95/Discs-large/ZO-1 homology (PDZ) domains. In HEK 293T cells expressing the AM2 receptor, the complex undergoes agonist-induced desensitization and internalization. However, in the presence of NHERF-1, although the AM receptor (CRLR/RAMP3) undergoes desensitization, the internalization of the receptor complex is blocked. Overlay assays and mutational analysis indicated that RAMP3 and NHERF-1 interact via a PDZ type I domain on NHERF-1. The internalization of the CRLR-RAMP complex was not affected by NHERF-1 when CRLR was co-expressed with RAMP1 or RAMP2. Mutation of the ezrin/radixin/moesin (ERM) domain on NHERF-1 indicated that NHERF-1 inhibits CRLR/RAMP3 complex internalization by tethering the complex to the actin cytoskeleton. When examined in a primary culture of human proximal tubule cells endogenously expressing the CRLR-RAMP3 complex and NHERF-1, the CRLR-RAMP complex desensitizes but is unable to internalize upon agonist stimulation. Knock-down of either RAMP3 or NHERF-1 by RNA interference technology enabled agonist-induced internalization of the CRLR-RAMP complex. These results, using both endogenous and overexpressed cellular models, indicate a novel function for NHERF-1 and RAMP3 in the internalization of the AM receptor and suggest additional regulatory mechanisms for receptor trafficking.

Expression of TRPC4 channel protein that interacts with NHERF-2 in rat descending vasa recta

The PDZ domain adaptor protein Na+/H+exchanger regulatory factor (NHERF)-2 is expressed in renal medullary descending vasa recta (DVR), although its function has not been defined. Transient receptor potential channels (TRPC) TRPC4 and TRPC5, nonselective cation channels that transport Ca2+, were recently demonstrated to complex with the NHERF proteins. We investigated whether TRPC4 and/or TRPC5 are associated with NHERF-2 in DVR. RT-PCR revealed mRNA for TRPC4 and NHERF-2, but not for TRPC5 or NHERF-1, in microdissected DVR. Immunohistochemical studies demonstrated expression of TRPC4 and NHERF-2 proteins in both the endothelial cells and pericytes. These proteins colocalized in some cells of the DVR. TRPC4 coimmunoprecipitated with NHERF-2 from renal medullary lysates, and NHERF-2 coimmunoprecipitated with TRPC4. TRPC5 was not detected in DVR with the use of immunohistochemistry or in NHERF-2 immunoprecipitates. We conclude that DVR pericytes and endothelia coexpress TRPC4 and NHERF-2 mRNA and protein and that these proteins colocalize and coimmunoprecipitate, indicating a possible physical association. These findings suggest that TRPC4 and NHERF-2 may play a role in interactions related to Ca2+signaling.

NHERF and regulation of the renal sodium-hydrogen exchanger NHE3

The sodium-hydrogen exchanger 3 (NHE3) isoform is the major regulated sodium transporter in the proximal convoluted tubule of the kidney. Study of the regulation of NHE3 by hormonal stimuli has identified a number of PDZ adaptor proteins that form an apical/subapical membrane scaffold that binds NHE3 and facilitates down-regulation of its activity in response to cAMP and activation of protein kinase A. The precise relation of proximal tubule adaptor proteins such as sodium-hydrogen exchanger regulatory factor-1 (NHERF-1), NHERF-2, and PDZ domain-containing-protein-1 (PDZK1) with each other and with protein targets such as NHE3 has been evolving with the development of specific reagents and genetically altered animals. In this review, we trace the discovery of NHERF-1 and NHERF-2, and update our current understanding of the relation between these proteins and the regulation and trafficking of NHE3.

Regulation of ion transport by the NHERF family of PDZ proteins

NHERFs are the best-studied apical PDZ proteins that are highly expressed in epithelial cells. Molecular and cellular studies over the past decade show that NHERFs regulate the targeting or trafficking of ion transporters and other membrane proteins and transduce physiological and pathophysiological signals that regulate ion homeostasis in mammals.

Pyelonephritis and acute renal failure

Bacterial invasion of the renal parenchyma, pyelonephritis, is rarely considered as a primary cause of acute renal failure, particularly in adults. We report two cases of acute renal failure occurring in absence of hypotension, urinary tract obstruction, or nephrotoxic medications that are likely the direct consequence of pyelonephritis. The first case involved a 48-year-old HIV-positive woman who presented with 3 days of nonspecific symptoms and was noted to have acute renal failure. Due to unremitting renal dysfunction, a renal biopsy was performed confirming the diagnosis of bacterial pyelonephritis. The second case, a 33-year-old man with HIV disease, presented with fever and was found to have pyelonephritis by urine culture and ultrasonography. These cases represented initial diagnostic dilemmas for the admitting physicians and demonstrate the varied clinical presentations of acute renal failure as a direct consequence of bacterial infiltration of the renal parenchyma.

Defective Parathyroid Hormone Regulation of NHE3 Activity and Phosphate Adaptation in Cultured NHERF-1-/- Renal Proximal Tubule Cells

The present experiments using primary cultures of renal proximal tubule cells derived from wild-type and NHERF-1 knockout animals examines the regulation of NHE3 by phenylthiohydantoin (PTH) and the regulation of phosphate transport in response to alterations in the media content of phosphate. Forskolin (34.8 +/- 6.2%) and PTH (29.7 +/- 1.8%) inhibited NHE3 activity in wild-type proximal tubule cells but neither forskolin (-3.2 +/- 3.3%) nor PTH (-16.6 +/- 8.1%) inhibited NHE3 activity in NHERF-1(-/-) cells. Using adenovirus-mediated gene transfer, expression of NHERF-1 in NHERF-1(-/-) proximal tubule cells restored the inhibitory response to forskolin (28.2 +/- 3.0%) and PTH (33.2 +/- 3.9%). Compared with high phosphate media, incubation of wild-type cells in low phosphate media resulted in a 36.0 +/- 6.3% higher rate of sodium-dependent phosphate transport and a significant increase in the abundance of Npt2a and PDZK1. NHERF-1(-/-) cells, on the other hand, had lower rates of sodium-dependent phosphate uptake and low phosphate media did not stimulate phosphate transport. Npt2a expression was not affected by the phosphate content of the media in NHERF-1 null cells although low phosphate media up-regulated PDZK1 abundance. Primary cultures of mice proximal tubule cells retain selected regulatory pathways observed in intact kidneys. NHERF-1(-/-) proximal tubule cells demonstrate defective regulation of NHE3 by PTH and indicate that reintroduction of NHERF-1 repairs this defect. NHERF-1(-/-) cells also do not adapt to alterations in the phosphate content of the media indicating that the defect resides within the cells of the proximal tubule and is not dependent on systemic factors.

κ Opioid Receptor Interacts with Na+/H+-exchanger Regulatory Factor-1/Ezrin-Radixin-Moesin-binding Phosphoprotein-50 (NHERF-1/EBP50) to Stimulate Na+/H+ Exchange Independent of Gi/Go Proteins

We previously showed that Na(+)/H(+)-exchanger regulatory factor-1/Ezrin-radixin-moesin-binding phosphoprotein-50 (NHERF-1/EBP50) co-immunoprecipitated with the human kappa opioid receptor (hKOR) and that its overexpression blocked the kappa agonist U50,488H-induced hKOR down-regulation by enhancing recycling. Here, we show that glutathione S-transferase (GST)-hKOR C-tail interacted with purified NHERF-1/EBP50, whereas GST or GST-C-tails of micro or delta opioid receptors did not. GST-hKOR C-tail, but not GST, bound HA-NHERF-1/EBP50 transfected into Chinese hamster ovary cells and endogenous NHERF-1/EBP50 in opossum kidney proximal tubule epithelial cells (OK cells). The PDZ domain I, but not II, of NHERF-1/EBP50 was involved in the interaction. Association of NHERF-1/EBP50 with hKOR C-tail enhanced oligomerization of NHERF-1/EBP50. NHERF-1/EBP50 was previously shown to regulate Na(+)/H(+)-exchanger 3 (NHE3) activities in OK cells. We found stimulation of OK cells with U50,488H significantly enhanced Na(+)/H(+) exchange, which was blocked by naloxone but not by pertussis toxin pretreatment, indicating it is mediated by KORs but independent of G(i)/G(o) proteins. In OKH cells, a subclone of OK cells expressing a much lower level of NHERF-1/EBP50, U50,488H had no effect on Na(+)/H(+) exchange, although it enhanced p44/42 mitogen-activated protein kinase phosphorylation via G(i)/G(o) proteins similar to that in OK cells. Stable transfection of NHERF-1/EBP50 into OKH cells restored the stimulatory effect of U50,488H upon Na(+)/H(+) exchange. Thus, NHERF-1/EBP50 binds directly to KOR, and this association plays an important role in accelerating Na(+)/H(+) exchange. We hypothesize that binding of the KOR to NHERF-1/EBP50 facilitates oligomerization of NHERF-1/EBP50, leading to stimulation of NHE3. This study provides the first direct evidence that a G protein-coupled receptor through association with NHERF-1/EBP-50 stimulates NHE3.

Stimulation of renal Na+ dicarboxylate cotransporter 1 by Na+/H+ exchanger regulating factor 2, serum and glucocorticoid inducible kinase isoforms, and protein kinase B

Renal tubular citrate transport is accomplished by electrogenic Na(+) coupled dicarboxylate transporter NaDC-1, a carrier subjected to regulation by acidosis. Trafficking of the Na(+)/H(+) exchanger NHE3 is controlled by NHE regulating factors NHERF-1 and NHERF-2 and the serum and glucocorticoid inducible kinase SGK1. To test for a possible involvement in NaDC-1 regulation, mRNA encoding NaDC-1 was injected into Xenopus oocytes with or without cRNA encoding NHERF-1, NHERF-2, SGK1, SGK2, SGK3, and/or the constitutively active form of the related protein kinase B ((T308,S473D)PKB). Succinate induced inward currents (I(succ)) were taken as a measure of transport rate. Coexpression of neither NHERF-1 nor NHERF-2 in NaDC-1 expressing oocytes significantly altered I(succ). On the other hand, coexpression of SGK1, SGK3, and (T308,S473D)PKB stimulated I(succ), an effect further stimulated by additional coexpression of NHERF-2 but not of NHERF-1. The action of the kinases and NHERF-2 may link urinary citrate excretion to proximal tubular H(+) secretion.

Expression of NHERF-1, NHERF-2, PDGFR-alpha, and PDGFR-beta in normal human kidneys and in renal transplant rejection

Sodium-hydrogen exchanger regulatory factor-1 and -2 (NHERF-1 and NHERF-2) are adaptor proteins that regulate renal electrolyte transport and interact with the platelet-derived growth factor receptors (PDGFR). The distribution of the NHERF proteins and PDGFR was studied in normal human kidneys and in renal transplant rejection using immunocytochemistry. In normal kidneys, NHERF-1 was detected in proximal tubules. NHERF-2 was detected in glomeruli, peritubular capillaries, and collecting duct principal cells. NHERF-2 was also weakly detected in the proximal tubule. PDGFR-beta was detected in glomeruli but not in tubules while PDGFR-alpha was detected in renal tubules and minimally in glomeruli. Acute and chronic transplant rejection was associated with increased expression of PDGFR-alpha in tubules and expression in the glomeruli. PDGFR-beta expression in the glomeruli was increased in transplant rejection and became detectable in tubules. Expression of NHERF-1 and NHERF-2 was not different in the patient groups. These results indicate that in contrast to the rat, both NHERF isoforms are detected in the human proximal tubule. In renal transplant rejection, there is increased expression of both PDGFR subtypes consistent with a role for PDGF in injury or repair.

A C-terminal PDZ motif in NHE3 binds NHERF-1 and enhances cAMP inhibition of sodium-hydrogen exchange

NHERF-1, a protein adapter containing two tandem PDZ domains, was first identified as an essential cofactor required for the phosphorylation and downregulation of NHE3 activity in response to elevated intracellular cAMP. NHERF-1 contains multiple protein interaction domains, but the mechanism by which it binds NHE3 remains unknown. Yeast two-hybrid analyses demonstrated that the C-terminal sequence, STHM, of NHE3 constitutes a PDZ motif critical for its association with NHERF-1. In this assay, NHE3 bound both PDZ-I and PDZ-II when presented as isolated domains, but mutations of the individual PDZ domains in the full-length NHERF-1 suggested a significant preference of NHE3 for the PDZ-II domain. To investigate NHERF-1/NHE3 association in cells, NHERF-1 complexes were isolated from PS120 cells expressing hexahistidine-tagged NHERF-1 and NHE3 using nickel-NTA-agarose. In these experiments, mutating the C-terminal PDZ motif still allowed NHE3 binding to NHERF-1, suggesting the presence of additional mechanisms or components that stabilized a cellular NHE3/NHERF-1 complex. Transport assays in PS120 cells, however, showed that the C-terminal PDZ motif in NHE3 and a functional PDZ-II domain in NHERF-1 were required for maximal inhibition of sodium-hydrogen exchange in response to forskolin and 8-Br-cAMP. Together, the data suggested that the PDZ interaction between the NHE3 C-terminus and a NHERF-1 PDZ domain enhanced the regulation of sodium-hydrogen exchange by cAMP-elevating hormones.

Localization and interaction of NHERF isoforms in the renal proximal tubule of the mouse

In expression systems and in yeast, Na/H exchanger regulatory factor (NHERF)-1 and NHERF-2 have been demonstrated to interact with the renal brush border membrane proteins NHE3 and Npt2. In renal tissue of mice, however, NHERF-1 is required for cAMP regulation of NHE3 and for the apical targeting of Npt2 despite the presence of NHERF-2, suggesting another order of specificity. The present studies examine the subcellular location of NHERF-1 and NHERF-2 and their interactions with target proteins including NHE3, Npt2, and ezrin. The wild-type mouse proximal tubule expresses both NHERF-1 and NHERF-2 in a distinct pattern. NHERF-1 is strongly expressed in microvilli in association with NHE3, Npt2, and ezrin. Although NHERF-2 can be detected weakly in the microvilli, it is expressed predominantly at the base of the microvilli in the vesicle-rich domain. NHERF-2 appears to associate directly with ezrin and NHE3 but not Npt2. NHERF-1 is involved in the apical expression of Npt2 and the presence of other Npt2-binding proteins does not compensate totally for the absence of NHERF-1 in NHERF-1-null mice. Although NHERF-1 links NHE3 to the actin cytoskeleton through ezrin, the absence of NHERF-1 does not result in a generalized disruption of the architecture of the cell. Thus the mistargeting of Npt2 seen in NHERF-1-null mice likely represents a specific disruption of pathways mediated by NHERF-1 to achieve targeting of Npt2. These findings suggest that the organized subcellular distribution of the NHERF isoforms may play a role in the specific interactions mediating physiological control of transporter function.

NHERF-1 is required for renal adaptation to a low-phosphate diet

The sodium-dependent renal phosphate transporter (Npt2, Na-Pi IIa) is the major regulated phosphate transporter in the renal proximal convoluted tubule. Npt2 associates with a number of PDZ-containing proteins including Na+H+ exchanger regulatory factor-1 (NHERF-1). To determine whether NHERF-1 is involved in the acute regulation of phosphate transport, wild-type and NHERF-1 (-/-) mice were stabilized on a high-phosphate diet and then acutely changed to a low-phosphate diet. At 24 h after the change to a low-phosphate diet, there was a significant decrease in the urinary excretion of phosphate in both groups but the urinary excretion of phosphate in NHERF-1 (-/-) mice was significantly higher than in wild-type animals (1,097 +/- 356 vs. 255 +/- 54 ng/min, P < 0.05). Renal mRNA levels and total cellular Npt2 protein did not differ between the animal groups or in response to the changes in diet. Renal brush-border membrane (BBM) expression of Npt2 protein, however, was lower in NHERF-1 (-/-) mice compared with wild-type. In addition, with both the high- and low-phosphate diets, there was increased detection of Npt2 in submicrovillar domains that were particularly prominent in NHERF-1 (-/-) mice compared with wild-type animals. On the other hand, a change from a low-phosphate diet to a high-phosphate diet was associated with a similar increase in the urinary excretion of phosphate in wild-type and NHERF-1 (-/-) animals. These experiments demonstrate that full renal adaptation to a low-phosphate diet requires NHERF-1, which serves to increase BBM expression of Npt2.

Activation-independent parathyroid hormone receptor internalization is regulated by NHERF1 (EBP50)

Parathyroid hormone (PTH) regulates extracellular calcium homeostasis through the type 1 PTH receptor (PTH1R) expressed in kidney and bone. The PTH1R undergoes beta-arrestin/dynamin-mediated endocytosis in response to the biologically active forms of PTH, PTH-(1-34), and PTH-(1-84). We now show that amino-truncated forms of PTH that do not activate the PTH1R nonetheless induce PTH1R internalization in a cell-specific pattern. Activation-independent PTH1R endocytosis proceeds through a distinct arrestin-independent mechanism that is operative in cells lacking the adaptor protein Na/H exchange regulatory factor 1 (NHERF1) (ezrin-binding protein 50). Using a combination of radioligand binding experiments and quantitative, live cell confocal microscopy of fluorescently tagged PTH1Rs, we show that in kidney distal tubule cells and rat osteosarcoma cells, which lack NHERF1, the synthetic antagonist PTH-(7-34) and naturally circulating PTH-(7-84) induce internalization of PTH1R in a beta-arrestin-independent but dynamin-dependent manner. Expression of NHERF1 in these cells inhibited antagonist-induced endocytosis. Conversely, expression of dominant-negative forms of NHERF1 conferred internalization sensitivity to PTH-(7-34) in cells expressing NHERF1. Mutation of the PTH1R PDZ-binding motif abrogated interaction of the receptor with NHERF1. These mutated receptors were fully functional but were now internalized in response to PTH-(7-34) even in NHERF1-expressing cells. Removing the NHERF1 ERM domain or inhibiting actin polymerization allowed otherwise inactive ligands to internalize the PTH1R. These results demonstrate that NHERF1 acts as a molecular switch that legislates the conditional efficacy of PTH fragments. Distinct endocytic pathways are determined by NHERF1 that are operative for the PTH1R in kidney and bone cells.

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.

Na+-dependent phosphate transporters in the murine osteoclast: cellular distribution and protein interactions

We previously demonstrated that inhibition of Na-dependent phosphate (P(i)) transport in osteoclasts led to reduced ATP levels and diminished bone resorption. These findings suggested that Na/P(i) cotransporters in the osteoclast plasma membrane provide P(i) for ATP synthesis and that the osteoclast may utilize part of the P(i) released from bone resorption for this purpose. The present study was undertaken to define the cellular localization of Na/P(i) cotransporters in the mouse osteoclast and to identify the proteins with which they interact. Using glutathione S-transferase (GST) fusion constructs, we demonstrate that the type IIa Na/P(i) cotransporter (Npt2a) in osteoclast lysates interacts with the Na/H exchanger regulatory factor, NHERF-1, a PDZ protein that is essential for the regulation of various membrane transporters. In addition, NHERF-1 in osteoclast lysates interacts with Npt2a in spite of deletion of a putative PDZ-binding domain within the carboxy terminus of Npt2a. In contrast, deletion of the carboxy-terminal TRL amino acid motif of Npt2a significantly reduced its interaction with NHERF-1 in kidney lysates. Studies in osteoclasts transfected with green fluorescent protein-Npt2a constructs indicated that Npt2a colocalizes with NHERF-1 and actin at or near the plasma membrane of the osteoclast and associates with ezrin, a linker protein associated with the actin cytoskeleton, likely via NHERF-1. Furthermore, we demonstrate by RT/PCR of osteoclast RNA and in situ hybridization that the type III Na/P(i) cotransporter, PiT-1, is also expressed in mouse osteoclasts. To examine the cellular distribution of PiT-1, we infected mouse osteoclasts with a retroviral vector encoding PiT-1 fused to an epitope tag. PiT-1 colocalizes with actin and is present on the basolateral membrane of the polarized osteoclast, similar to that previously reported for Npt2a. Taken together, our data suggest that association of Npt2a with NHERF-1, ezrin, and actin, and of PiT-1 with actin, may be responsible for membrane sorting and regulation of these Na/P(i) cotransporters in the osteoclast.