Parathyroid hormone inhibits renal phosphate transport by phosphorylation of serine 77 of sodium-hydrogen exchanger regulatory factor-1

FGF23 and Vitamin D Metabolism

Apart from its phosphaturic action, the bone-derived hormone fibroblast growth factor-23 (FGF23) is also an essential regulator of vitamin D metabolism. The main target organ of FGF23 is the kidney, where FGF23 suppresses transcription of the key enzyme in vitamin D hormone (1,25(OH)₂D) activation, 1α-hydroxylase, and activates transcription of the key enzyme responsible for vitamin D degradation, 24-hydroxylase, in proximal renal tubules. The circulating concentration of 1,25(OH)₂D is a positive regulator of FGF23 secretion in bone, forming a feedback loop between kidney and bone. The importance of FGF23 as regulator of vitamin D metabolism is underscored by the fact that in the absence of FGF23 signaling, the tight control of renal 1α-hydroxylase fails, resulting in overproduction of 1,25(OH)₂D in mice and men. During recent years, big strides have been made toward a more complete understanding of the mechanisms underlying the FGF23-mediated regulation of vitamin D metabolism, especially at the genomic level. However, there are still major gaps in our knowledge that need to be filled by future research. Importantly, the intracellular signaling cascades downstream of FGF receptors regulating transcription of 1α-hydroxylase and 24-hydroxylase in proximal renal tubules still remain unresolved. The purpose of this review is to highlight our current understanding of the molecular mechanisms underlying the regulation of vitamin D metabolism by FGF23, and to discuss the role of these mechanisms in physiology and pathophysiology. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

Importance of Dietary Phosphorus for Bone Metabolism and Healthy Aging

Inorganic phosphate (Pi) plays a critical function in many tissues of the body: for example, as part of the hydroxyapatite in the skeleton and as a substrate for ATP synthesis. Pi is the main source of dietary phosphorus. Reduced bioavailability of Pi or excessive losses in the urine causes rickets and osteomalacia. While critical for health in normal amounts, dietary phosphorus is plentiful in the Western diet and is often added to foods as a preservative. This abundance of phosphorus may reduce longevity due to metabolic changes and tissue calcifications. In this review, we examine how dietary phosphorus is absorbed in the gut, current knowledge about Pi sensing, and endocrine regulation of Pi levels. Moreover, we also examine the roles of Pi in different tissues, the consequences of low and high dietary phosphorus in these tissues, and the implications for healthy aging.

Interaction between serum FGF-23 and PTH in renal phosphate excretion, a case-control study in hypoparathyroid patients

BACKGROUND

phosphate homeostasis is mediated through complex counter regulatory feed-back balance between parathyroid hormone, FGF-23 and 1,25(OH)2D. Both parathyroid hormone and FGF-23 regulate proximal tubular phosphate excretion through signaling on sodium- phosphate cotransporters II_a and II_c. However, the interaction between these hormones on phosphate excretion is not clearly understood. We performed the present study to evaluate whether the existence of sufficient parathyroid hormone is necessary for full phosphaturic function of FGF-23 or not.

METHODS

In this case-control study, 19 patients with hypoparathyroidism and their age- and gender-matched normal population were enrolled. Serum calcium, phosphate, alkaline phosphatase,parathyroid hormone, FGF-23, 25(OH)D, 1,25(OH)2D and Fractional excretion of phosphorous were assessed and compared between the two groups, using SPSS software.

RESULTS

The mean serum calcium and parathyroid hormone level was significantly lower in hypoparathyroid patients in comparison with the control group (P < 0.001 and P < 0.001, respectively). We found high serum level of phosphate and FGF-23 in hypoparathyroid patients compared to the control group (P < 0.001 and P < 0.001, respectively). However, there was no significant difference in Fractional excretion of phosphorous or 1,25OH2D level between the two groups. There was a positive correlation between serum FGF-23 and Fractional excretion of phosphorous just in the normal individuals (P < 0.001, r = 0.79).

CONCLUSIONS

Although the FGF-23 is a main regulator of urinary phosphate excretion but the existence of sufficient parathyroid hormone is necessary for the full phosphaturic effect of FGF-23.

Mechanisms of phosphate transport

Over the past 25 years, successive cloning of SLC34A1, SLC34A2 and SLC34A3, which encode the sodium-dependent inorganic phosphate (P_i) cotransport proteins 2a-2c, has facilitated the identification of molecular mechanisms that underlie the regulation of renal and intestinal P_i transport. P_i and various hormones, including parathyroid hormone and phosphatonins, such as fibroblast growth factor 23, regulate the activity of these P_i transporters through transcriptional, translational and post-translational mechanisms involving interactions with PDZ domain-containing proteins, lipid microdomains and acute trafficking of the transporters via endocytosis and exocytosis. In humans and rodents, mutations in any of the three transporters lead to dysregulation of epithelial P_i transport with effects on serum P_i levels and can cause cardiovascular and musculoskeletal damage, illustrating the importance of these transporters in the maintenance of local and systemic P_i homeostasis. Functional and structural studies have provided insights into the mechanism by which these proteins transport P_i, whereas in vivo and ex vivo cell culture studies have identified several small molecules that can modify their transport function. These small molecules represent potential new drugs to help maintain P_i homeostasis in patients with chronic kidney disease - a condition that is associated with hyperphosphataemia and severe cardiovascular and skeletal consequences.

Phosphate acts directly on the calcium-sensing receptor to stimulate parathyroid hormone secretion

Extracellular phosphate regulates its own renal excretion by eliciting concentration-dependent secretion of parathyroid hormone (PTH). However, the phosphate-sensing mechanism remains unknown and requires elucidation for understanding the aetiology of secondary hyperparathyroidism in chronic kidney disease (CKD). The calcium-sensing receptor (CaSR) is the main controller of PTH secretion and here we show that raising phosphate concentration within the pathophysiologic range for CKD significantly inhibits CaSR activity via non-competitive antagonism. Mutation of residue R62 in anion binding site-1 abolishes phosphate-induced inhibition of CaSR. Further, pathophysiologic phosphate concentrations elicit rapid and reversible increases in PTH secretion from freshly-isolated human parathyroid cells consistent with a receptor-mediated action. The same effect is seen in wild-type murine parathyroid glands, but not in CaSR knockout glands. By sensing moderate changes in extracellular phosphate concentration, the CaSR represents a phosphate sensor in the parathyroid gland, explaining the stimulatory effect of phosphate on PTH secretion.

Elevated inorganic phosphate levels promote excessive parathyroid hormone secretion, which contributes to the aetiology of secondary hyperparathyroidism. Here, the authors show that phosphate directly inhibits the calcium-sensing receptor, the main regulator of parathyroid hormone secretion.

1,25-Dihydroxyvitamin D Maintains Brush Border Membrane NaPi2a and Attenuates Phosphaturia in Hyp Mice

Phosphate homeostasis is critical for many cellular processes and is tightly regulated. The sodium-dependent phosphate cotransporter, NaPi2a, is the major regulator of urinary phosphate reabsorption in the renal proximal tubule. Its activity is dependent upon its brush border localization that is regulated by fibroblast growth factor 23 (FGF23) and PTH. High levels of FGF23, as are seen in the Hyp mouse model of human X-linked hypophosphatemia, lead to renal phosphate wasting. Long-term treatment of Hyp mice with 1,25-dihydroxyvitamin D (1,25D) or 1,25D analogues has been shown to improve renal phosphate wasting in the setting of increased FGF23 mRNA expression. Studies were undertaken to define the cellular and molecular basis for this apparent FGF23 resistance. 1,25D increased FGF23 protein levels in the cortical bone and circulation of Hyp mice but did not impair FGF23 cleavage. 1,25D attenuated urinary phosphate wasting as early as one hour postadministration, without suppressing FGF23 receptor/coreceptor expression. Although 1,25D treatment induced expression of early growth response 1, an early FGF23 responsive gene required for its phosphaturic effects, it paradoxically enhanced renal phosphate reabsorption and NaPi2a protein expression in renal brush border membranes (BBMs) within one hour. The Na-H+ exchange regulatory factor 1 (NHERF1) is a scaffolding protein thought to anchor NaPi2a to the BBM. Although 1,25D did not alter NHERF1 protein levels acutely, it enhanced NHERF1-NaPi2a interactions in Hyp mice. 1,25D also prevented the decrease in NHERF1/NaPi2a interactions in PTH-treated wild-type mice. Thus, these investigations identify a novel role for 1,25D in the hormonal regulation of renal phosphate handling.

Phosphate binding by sucroferric oxyhydroxide ameliorates renal injury in the remnant kidney model

Recent clinical studies indicate that the disturbed phosphate metabolism in chronic kidney disease (CKD) may facilitate kidney injury; nonetheless, the causal role of phosphate in CKD progression remains to be elucidated. Here, we show that intestinal phosphate binding by sucroferric oxyhydroxide (SF) ameliorates renal injury in the rat remnant kidney model. Sprague-Dawley rats received 5/6 nephrectomy (RK) and had a normal chow or the same diet containing SF (RK + SF). RK rats showed increased plasma FGF23 and phosphate levels, which were suppressed by SF administration. Of note, albuminuria in RK rats was significantly ameliorated by SF at both 4 and 8 weeks. SF also attenuated glomerulosclerosis and tubulointerstitial injury. Moreover, several different approaches confirmed the protective effects on podocytes, explaining the attenuation of glomerulosclerosis and albuminuria observed in this study. As a possible mechanism, we found that SF attenuated renal inflammation and fibrosis in RK rats. Interestingly, von Kossa staining of the kidney revealed calcium phosphate deposition in neither RK nor RK + SF rats; however, plasma levels of calciprotein particles were significantly reduced by SF. These data indicate that latent positive phosphate balance accelerates CKD progression from early stages, even when overt ectopic calcification is absent.

Parathyroid hormone initiates dynamic NHERF1 phosphorylation cycling and conformational changes that regulate NPT2A-dependent phosphate transport

Na⁺-H⁺ exchanger regulatory factor-1 (NHERF1) is a PDZ protein that scaffolds membrane proteins, including sodium-phosphate co-transport protein 2A (NPT2A) at the plasma membrane. NHERF1 is a phosphoprotein with 40 Ser and Thr residues. Here, using tandem MS analysis, we characterized the sites of parathyroid hormone (PTH)-induced NHERF1 phosphorylation and identified 10 high-confidence phosphorylation sites. Ala replacement at Ser⁴⁶, Ser¹⁶², Ser¹⁸¹, Ser²⁶⁹, Ser²⁸⁰, Ser²⁹¹, Thr²⁹³, Ser²⁹⁹, and Ser³⁰² did not affect phosphate uptake, but S290A substitution abolished PTH-dependent phosphate transport. Unexpectedly, Ser²⁹⁰ was rapidly dephosphorylated and rephosphorylated after PTH stimulation, and we found that protein phosphatase 1α (PP1α), which binds NHERF1 through a conserved VxF/W PP1 motif, dephosphorylates Ser²⁹⁰ Mutating ²⁵⁷VPF²⁵⁹ eliminated PP1 binding and blunted dephosphorylation. Tautomycetin blocked PP1 activity and abrogated PTH-sensitive phosphate transport. Using fluorescence lifetime imaging (FLIM), we observed that PTH paradoxically and transiently elevates intracellular phosphate. Added phosphate blocked PP1α-mediated Ser²⁹⁰ dephosphorylation of recombinant NHERF1. Hydrogen-deuterium exchange MS revealed that β-sheets in NHERF1's PDZ2 domain display lower deuterium uptake than those in the structurally similar PDZ1, implying that PDZ1 is more cloistered. Dephosphorylated NHERF1 exhibited faster exchange at C-terminal residues suggesting that NHERF1 dephosphorylation precedes Ser²⁹⁰ rephosphorylation. Our results show that PP1α and NHERF1 form a holoenzyme and that a multiprotein kinase cascade involving G protein-coupled receptor kinase 6A controls the Ser²⁹⁰ phosphorylation status of NHERF1 and regulates PTH-sensitive, NPT2A-mediated phosphate uptake. These findings reveal how reversible phosphorylation modifies protein conformation and function and the biochemical mechanisms underlying PTH control of phosphate transport.

Phosphaturia in kidney stone formers: Still an enigma

Calcium kidney stones are common worldwide. Most are idiopathic and composed of calcium oxalate. Calcium phosphate is present in around 80% and may initiate stone formation. Stone production is multifactorial with a polygenic genetic contribution. Phosphaturia is found frequently among stone formers but until recently received scant attention. This review examines possible mechanisms for the phosphaturia and its relevance to stone formation from a wide angle. There is a striking lack of clinical data. Phosphaturia is associated, but not correlated, with hypercalciuria, increased 1,25 dihydroxy-vitamin D [1,25 (OH)₂D], and sometimes evidence of disturbances in proximal renal tubular function. Phosphate reabsorption in the proximal renal tubules requires tightly regulated interaction of many proteins. Paracellular flow through intercellular tight junctions is the major route of phosphate absorption from the intestine and can be reduced therapeutically in hyperphosphatemic patients. In monogenic defects stones develop when phosphaturia is associated with hypercalciuria, generally explained by increased 1,25 (OH)₂D production in response to hypophosphatemia. Calcification does not occur in disorders with increased FGF23 when phosphaturia occurs in isolation and 1,25 (OH)₂D is suppressed. Candidate gene studies have identified mutations in the phosphate transporters, but in few individuals. One genome-wide study identified a polymorphism of the phosphate transporter gene SLC34A4 associated with stones. Others did not find mutations obviously linked to phosphate reabsorption. Future genetic studies should have a wide trawl and should focus initially on groups of patients with clearly defined phenotypes. The global data should be pooled.

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.

NaPi-IIa interacting partners and their (un)known functional roles

The sorting and stabilization of proteins at specific subcellular domains depend upon the formation of networks build up by specific protein-protein interactions. In addition, protein networks also ensure the specificity of many regulatory processes by bringing together regulatory molecules with their targets. Whereas the success on the identification of protein-protein interactions is (up to a point) technology-driven, the assignment of functional roles to specific partners remains a major challenge. This review summarizes the work that led to the identification of partners of the Na⁺/phosphate cotransporter NaPi-IIa as well as the effects of the interactions in the expression and/or regulation of the cotransporter.

Mechanisms and Regulation of Intestinal Phosphate Absorption

States of hypo- and hyperphosphatemia have deleterious consequences including rickets/osteomalacia and renal/cardiovascular disease, respectively. Therefore, the maintenance of appropriate plasma levels of phosphate is an essential requirement for health. This control is executed by the collaborative action of intestine and kidney whose capacities to (re)absorb phosphate are regulated by a number of hormonal and metabolic factors, among them parathyroid hormone, fibroblast growth factor 23, 1,25(OH)₂ vitamin D₃ , and dietary phosphate. The molecular mechanisms responsible for the transepithelial transport of phosphate across enterocytes are only partially understood. Indeed, whereas renal reabsorption entirely relies on well-characterized active transport mechanisms of phosphate across the renal proximal epithelia, intestinal absorption proceeds via active and passive mechanisms, with the molecular identity of the passive component still unknown. The active absorption of phosphate depends mostly on the activity and expression of the sodium-dependent phosphate cotransporter NaPi-IIb (SLC34A2), which is highly regulated by many of the factors, mentioned earlier. Physiologically, the contribution of NaPi-IIb to the maintenance of phosphate balance appears to be mostly relevant during periods of low phosphate availability. Therefore, its role in individuals living in industrialized societies with high phosphate intake is probably less relevant. Importantly, small increases in plasma phosphate, even within normal range, associate with higher risk of cardiovascular disease. Therefore, therapeutic approaches to treat hyperphosphatemia, including dietary phosphate restriction and phosphate binders, aim at reducing intestinal absorption. Here we review the current state of research in the field. © 2017 American Physiological Society. Compr Physiol 8:1065-1090, 2018.

Evidence for a role of PDZ domain-containing proteins to mediate hypophosphatemia in calcium stone formers

Background

Hypophosphatemia (HYP) is common among calcium stone formers (SFs) and in rare cases is associated with mutations in sodium-phosphate cotransporters or in Na+/H+ exchanger regulatory factor 1 (NHERF1), but the majority of cases are unexplained. We hypothesized that reduced sodium-phosphate cotransporter activity mediated via NHERF1 or a similar PDZ domain-containing protein, causes HYP. If so, other transport activities controlled by NHERF1, such as NHE3 and URAT1, might be reduced in HYP.

Methods

To test this idea, we analyzed two large but separate sets of 24-h urine samples and paired serums of 2700 SFs from the University of Chicago and 11 073 SFs from Litholink, a national laboratory. Patients were divided into quintiles based on serum phosphate.

Results

Males were more common in the lowest phosphate tiles in both datasets. Phosphate excretion did not vary across the quintiles, excluding diet as a cause of HYP. Tubule maximum (Tm) phosphate per unit glomerular filtration rate decreased and fractional excretion increased with decreasing phosphate quintiles, indicating reduced tubule phosphate reabsorption was responsible for HYP. Urine pH and serum chloride increased with decreasing serum phosphate, suggesting a coordinate change in NHE3 activity. Serum uric acid and Tm uric acid decreased significantly with decreasing serum phosphate, while uric acid excretion did not vary. Conclusion. HYP in SFs results from decreased tubule phosphate reabsorption and, being associated with related changes in other proximal tubule transporters, may arise from alterations in or signaling to PDZ-containing proteins.

NHERF1 Between Promises and Hopes: Overview on Cancer and Prospective Openings

Na+/H+ exchanger regulatory factor 1 (NHERF1) is a scaffold protein, with two tandem PDZ domains and a carboxyl-terminal ezrin-binding (EB) region. This particular sticky structure is responsible for its interaction with different molecules to form multi-complexes that have a pivotal role in a lot of diseases. In particular, its involvement during carcinogenesis and cancer progression has been deeply analyzed in different tumors. The role of NHERF1 is not unique in cancer; its activity is connected to its subcellular localization. The literature data suggest that NHERF1 could be a new prognostic/predictive biomarker from breast cancer to hematological cancers. Furthermore, the high potential of this molecule as therapeutical target in different carcinomas is a new challenge for precision medicine. These evidences are part of a future view to improving patient clinical management, which should allow different tumor phenotypes to be treated with tailored therapies. This article reviews the biology of NHERF1, its engagement in different signal pathways and its involvement in different cancers, with a specific focus on breast cancer. It also considers NHERF1 potential role during inflammation related to most human cancers, designating new perspectives in the study of this "Janus-like" protein.

Physiological Actions of Fibroblast Growth Factor-23

Fibroblast growth factor-23 (FGF23) is a bone-derived hormone suppressing phosphate reabsorption and vitamin D hormone synthesis in the kidney. At physiological concentrations of the hormone, the endocrine actions of FGF23 in the kidney are αKlotho-dependent, because high-affinity binding of FGF23 to FGF receptors requires the presence of the co-receptor αKlotho on target cells. It is well established that excessive concentrations of intact FGF23 in the blood lead to phosphate wasting in patients with normal kidney function. Based on the importance of diseases associated with gain of FGF23 function such as phosphate-wasting diseases and chronic kidney disease, a large body of literature has focused on the pathophysiological consequences of FGF23 excess. Less emphasis has been put on the role of FGF23 in normal physiology. Nevertheless, during recent years, lessons we have learned from loss-of-function models have shown that besides the paramount physiological roles of FGF23 in the control of 1α-hydroxylase expression and of apical membrane expression of sodium-phosphate co-transporters in proximal renal tubules, FGF23 also is an important stimulator of calcium and sodium reabsorption in distal renal tubules. In addition, there is an emerging role of FGF23 as an auto-/paracrine regulator of alkaline phosphatase expression and mineralization in bone. In contrast to the renal actions of FGF23, the FGF23-mediated suppression of alkaline phosphatase in bone is αKlotho-independent. Moreover, FGF23 may be a physiological suppressor of differentiation of hematopoietic stem cells into the erythroid lineage in the bone microenvironment. At present, there is little evidence for a physiological role of FGF23 in organs other than kidney and bone. The purpose of this mini-review is to highlight the current knowledge about the complex physiological functions of FGF23.

Pleiotropic Actions of FGF23

Fibroblast growth factor-23 (FGF23) is a bone-derived hormone, mainly produced by osteoblasts and osteocytes in response to increased extracellular phosphate and circulating vitamin D hormone. Endocrine FGF23 signaling requires co-expression of the ubiquitously expressed FGF receptor 1 (FGFR1) and the co-receptor α-Klotho (Klotho). In proximal renal tubules, FGF23 suppresses the membrane expression of the sodium-phosphate cotransporters Npt2a and Npt2c which mediate urinary reabsorption of filtered phosphate. In addition, FGF23 suppresses proximal tubular expression of 1α-hydroxylase, the key enzyme responsible for vitamin D hormone production. In distal renal tubules, FGF23 signaling activates with-no-lysine kinase 4, leading to increased renal tubular reabsorption of calcium and sodium. Therefore, FGF23 is not only a phosphaturic but also a calcium- and sodium-conserving hormone, a finding that may have important implications for the pathophysiology of chronic kidney disease. Besides these endocrine, Klotho-dependent functions of FGF23, FGF23 is also an auto-/paracrine suppressor of tissue-nonspecific alkaline phosphatase transcription via Klotho-independent FGFR3 signaling, leading to local inhibition of mineralization through accumulation of pyrophosphate. In addition, FGF23 may target the heart via an FGFR4-mediated Klotho-independent signaling cascade. Taken together, there is emerging evidence that FGF23 is a pleiotropic hormone, linking bone with several other organ systems.

Klotho Lacks an FGF23-Independent Role in Mineral Homeostasis

Fibroblast growth factor-23 (FGF23) is a bone-derived hormone regulating vitamin D hormone production and renal handling of minerals by signaling through an FGF receptor/αKlotho (Klotho) receptor complex. Whether Klotho has FGF23-independent effects on mineral homeostasis is a controversial issue. Here, we aimed to shed more light on this controversy by comparing male and female triple knockout mice with simultaneous deficiency in Fgf23 and Klotho and a nonfunctioning vitamin D receptor (VDR) (Fgf23/Klotho/VDR) with double (Fgf23/VDR, Klotho/VDR, and Fgf23/Klotho) and single Fgf23, Klotho, and VDR mutants. As expected, 4-week-old Fgf23, Klotho, and Fgf23/Klotho knockout mice were hypercalcemic and hyperphosphatemic, whereas VDR, Fgf23/VDR, and Klotho/VDR mice on rescue diet were normocalcemic and normophosphatemic. Serum levels of calcium, phosphate, and sodium did not differ between 4-week-old triple Fgf23/Klotho/VDR and double Fgf23/VDR or Klotho/VDR knockout mice. Notably, 3-month-old Fgf23/Klotho/VDR triple knockout mice were indistinguishable from double Fgf23/VDR and Klotho/VDR compound mutants in terms of serum calcium, serum phosphate, serum sodium, and serum PTH, as well as urinary calcium and sodium excretion. Protein expression analysis revealed increased membrane abundance of sodium-phosphate co-transporter 2a (NaPi-2a), and decreased expression of sodium-chloride co-transporter (NCC) and transient receptor potential cation channel subfamily V member 5 (TRPV5) in Fgf23/Klotho/VDR, Fgf23/VDR, and Klotho/VDR mice, relative to wild-type and VDR mice, but no differences between triple and double knockouts. Further, ex vivo treatment of live kidney slices isolated from wild-type and Klotho/VDR mice with soluble Klotho did not induce changes in intracellular phosphate, calcium or sodium accumulation assessed by two-photon microscopy. In conclusion, our data suggest that the main physiological function of Klotho for mineral homeostasis in vivo is its role as co-receptor mediating Fgf23 action. © 2017 American Society for Bone and Mineral Research.

Roles of NHERF Family of PDZ-Binding Proteins in Regulating GPCR Functions

Multicellular organisms are equipped with an array of G-protein-coupled receptors (GPCRs) that mediate cell-cell signaling allowing them to adapt to environmental cues and ultimately survive. This is mechanistically possible through complex intracellular GPCR machinery that encompasses a vast network of proteins. Within this network, there is a group called scaffolding proteins that facilitate proper localization of signaling proteins for a quick and robust GPCR response. One protein family within this scaffolding group is the PSD-95/Dlg/ZO-1 (PDZ) family which is important for GPCR localization, internalization, recycling, and downstream signaling. Although the PDZ family of proteins regulate the functions of several receptors, this chapter focuses on a subfamily within the PDZ protein family called the Na⁺/H⁺ exchanger regulatory factors (NHERFs). Here we extensively review the predominantly characterized roles of NHERFs in renal phosphate absorption, intestinal ion regulation, cancer progression, and immune cell functions. Finally, we discuss the future perspectives and possible clinical application of targeting NHERFs in several disorders.

FGF23-Klotho signaling axis in the kidney

Fibroblast growth factor-23 (FGF23) is a bone-derived hormone protecting against the potentially deleterious effects of hyperphosphatemia by suppression of phosphate reabsorption and of active vitamin D hormone synthesis in the kidney. The kidney is one of the main target organs of FGF23 signaling. The purpose of this review is to highlight the recent advances in the area of FGF23-Klotho signaling in the kidney. During recent years, it has become clear that FGF23 acts independently on proximal and distal tubular epithelium. In proximal renal tubules, FGF23 suppresses phosphate reabsorption by a Klotho dependent activation of extracellular signal-regulated kinase-1/2 (ERK1/2) and of serum/glucocorticoid-regulated kinase-1 (SGK1), leading to phosphorylation of the scaffolding protein Na⁺/H⁺ exchange regulatory cofactor (NHERF)-1 and subsequent internalization and degradation of sodium-phosphate cotransporters. In distal renal tubules, FGF23 augments calcium and sodium reabsorption by increasing the apical membrane expression of the epithelial calcium channel TRPV5 and of the sodium-chloride cotransporter NCC through a Klotho dependent activation of with-no-lysine kinase-4 (WNK4). In proximal and distal renal tubules, FGF receptor-1 is probably the dominant FGF receptor mediating the effects of FGF23 by forming a complex with membrane-bound Klotho in the basolateral membrane. The newly described sodium- and calcium-conserving functions of FGF23 may have major implications for the pathophysiology of diseases characterized by chronically increased circulating FGF23 concentrations such as chronic kidney disease.

Heterotrimeric G proteins in the control of parathyroid hormone actions

Parathyroid hormone (PTH) is a key regulator of skeletal physiology and calcium and phosphate homeostasis. It acts on bone and kidney to stimulate bone turnover, increase the circulating levels of 1,25 dihydroxyvitamin D and calcium and inhibit the reabsorption of phosphate from the glomerular filtrate. Dysregulated PTH actions contribute to or are the cause of several endocrine disorders. This calciotropic hormone exerts its actions via binding to the PTH/PTH-related peptide receptor (PTH1R), which couples to multiple heterotrimeric G proteins, including G_s and G_q/11 Genetic mutations affecting the activity or expression of the alpha-subunit of G_s, encoded by the GNAS complex locus, are responsible for several human diseases for which the clinical findings result, at least partly, from aberrant PTH signaling. Here, we review the bone and renal actions of PTH with respect to the different signaling pathways downstream of these G proteins, as well as the disorders caused by GNAS mutations.

Fgf23 and parathyroid hormone signaling interact in kidney and bone

Fibroblast growth factor-23 (FGF23) is a bone-derived hormone, suppressing renal phosphate reabsorption and vitamin D hormone synthesis in proximal tubules, and stimulating calcium reabsorption in distal tubules of the kidney. Here, we analyzed the long term sequelae of deficient Fgf23 signaling on bone and mineral metabolism in 9-month-old mice lacking both Fgf23 or Klotho and a functioning vitamin D receptor (VDR). To prevent hypocalcemia in VDR deficient mice, all mice were kept on a rescue diet enriched with calcium, phosphate, and lactose. VDR mutants were normocalcemic and normophosphatemic, and had normal tibial bone mineral density. Relative to VDR mutants, Fgf23/VDR and Klotho/VDR compound mutants were characterized by hypocalcemia, hyperphosphatemia, and very high serum parathyroid hormone (PTH). Despite ∼10-fold higher serum PTH levels in compound mutants, urinary excretion of phosphate and calcium as well as osteoclast numbers in bone remained unchanged relative to VDR mutants. The increase in plasma cAMP after hPTH(1-34) injection was similar in all genotypes. However, a 5-day infusion of hPTH(1-34) via osmotic minipumps resulted in reduced phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) in bone and kidney of Fgf23/VDR and Klotho/VDR compound mutants, relative to VDR and WT controls. Similarly, the PTH-mediated ERK1/2 phosphorylation was reduced in primary osteoblasts isolated from Fgf23 and Klotho deficient mice, but was restored by concomitant treatment with recombinant FGF23. Collectively, our data indicate that the phosphaturic, calcium-conserving, and bone resorption-stimulating actions of PTH are blunted by Fgf23 or Klotho deficiency. Hence, FGF23 may be an important modulator of PTH signaling in bone and kidney.

Acute Adaption to Oral or Intravenous Phosphate Requires Parathyroid Hormone

Phosphate (Pi) homeostasis is regulated by renal, intestinal, and endocrine mechanisms through which Pi intake stimulates parathyroid hormone (PTH) and fibroblast growth factor-23 secretion, increasing phosphaturia. Mechanisms underlying the early adaptive phase and the role of the intestine, however, remain ill defined. We investigated mineral, endocrine, and renal responses during the first 4 hours after intravenous and intragastric Pi loading in rats. Intravenous Pi loading (0.5 mmol) caused a transient rise in plasma Pi levels and creatinine clearance and an increase in phosphaturia within 10 minutes. Plasma calcium levels fell and PTH levels increased within 10 minutes and remained low or high, respectively. Fibroblast growth factor-23, 1,25-(OH)₂-vitamin D₃, and insulin concentrations did not respond, but plasma dopamine levels increased by 4 hours. In comparison, gastric Pi loading elicited similar but delayed phosphaturia and endocrine responses but did not affect plasma mineral levels. Either intravenous or gastric loading led to decreased expression and activity of renal Pi transporters after 4 hours. In parathyroidectomized rats, however, only intravenous Pi loading caused phosphaturia, which was blunted and transient compared with that in intact rats. Intravenous but not gastric Pi loading in parathyroidectomized rats also led to higher creatinine clearance and lower plasma calcium levels but did not reduce the expression or activity of Pi transporters. This evidence suggests that an intravenous or intestinal Pi bolus causes rapid phosphaturia through mechanisms requiring PTH and downregulation of renal Pi transporters but does not support a role of the intestine in stimulating renal clearance of Pi.

Update on FGF23 and Klotho signaling

Fibroblast growth factor-23 (FGF23) is a bone-derived hormone known to suppress phosphate reabsorption and vitamin D hormone production in the kidney. Klotho was originally discovered as an anti-aging factor, but the functional role of Klotho is still a controversial issue. Three major functions have been proposed, a hormonal function of soluble Klotho, an enzymatic function as glycosidase, and the function as an obligatory co-receptor for FGF23 signaling. The purpose of this review is to highlight the recent advances in the area of FGF23 and Klotho signaling in the kidney, in the parathyroid gland, in the cardiovascular system, in bone, and in the central nervous system. During recent years, major new functions of FGF23 and Klotho have been discovered in these organ systems. Based on these novel findings, FGF23 has emerged as a pleiotropic endocrine and auto-/paracrine factor influencing not only mineral metabolism but also cardiovascular function.

Phosphate-a poison for humans?

Maintenance of phosphate balance is essential for life, and mammals have developed a sophisticated system to regulate phosphate homeostasis over the course of evolution. However, due to the dependence of phosphate elimination on the kidney, humans with decreased kidney function are likely to be in a positive phosphate balance. Phosphate excess has been well recognized as a critical factor in the pathogenesis of mineral and bone disorders associated with chronic kidney disease, but recent investigations have also uncovered toxic effects of phosphate on the cardiovascular system and the aging process. Compelling evidence also suggests that increased fibroblastic growth factor 23 and parathyroid hormone levels in response to a positive phosphate balance contribute to adverse clinical outcomes. These insights support the current practice of managing serum phosphate in patients with advanced chronic kidney disease, although definitive evidence of these effects is lacking. Given the potential toxicity of excess phosphate, the general population may also be viewed as a target for phosphate management. However, the widespread implementation of dietary phosphate intervention in the general population may not be warranted due to the limited impact of increased phosphate intake on mineral metabolism and clinical outcomes. Nonetheless, the increasing incidence of kidney disease or injury in our aging society emphasizes the potential importance of this issue. Further work is needed to more completely characterize phosphate toxicity and to establish the optimal therapeutic strategy for managing phosphate in patients with chronic kidney disease and in the general population.

FGF23 regulation of renal tubular solute transport

PURPOSE OF REVIEW

Fibroblast growth factor-23 (FGF23) is a bone-derived hormone known to suppress phosphate reabsorption in the kidney. The purpose of this review was to highlight the recent advances in the area of FGF23-regulated solute transport in the kidney.

RECENT FINDINGS

Recent evidence suggests that FGF23 suppresses phosphate reabsorption in renal proximal tubular epithelium by a Klotho-dependent, FGF receptor (FGFR)-1 and FGFR4-mediated signaling mechanism that may also involve Janus kinase 3. Moreover, it was recently established that FGF23 signaling in the distal renal tubule targets with-no-lysine kinase-4 (WNK4), a key molecule in the regulation of solute transport in the distal nephron. By targeting WNK4, FGF23 has been shown to increase the membrane abundance of the epithelial calcium channel TRPV5 and of the sodium-chloride cotransporter NCC, resulting in augmented renal calcium and sodium reabsorption.

SUMMARY

Significant progress has been made in the further characterization of the signaling pathways involved in the FGF23-induced inhibition of phosphate transport in proximal tubular epithelium, and major new functions of FGF23 in solute transport have been discovered in distal renal tubules. The calcium- and sodium-conserving functions of FGF23 may have major implications for the pathophysiology of cardiovascular diseases. VIDEO ABSTRACT.

The PDZ Protein Na+/H+ Exchanger Regulatory Factor-1 (NHERF1) Regulates Planar Cell Polarity and Motile Cilia Organization

Directional flow of the cerebrospinal fluid requires coordinated movement of the motile cilia of the ependymal epithelium that lines the cerebral ventricles. Here we report that mice lacking the Na⁺/H⁺ Exchanger Regulatory Factor 1 (NHERF1/Slc9a3r1, also known as EBP50) develop profound communicating hydrocephalus associated with fewer and disorganized ependymal cilia. Knockdown of NHERF1/slc9a3r1 in zebrafish embryos also causes severe hydrocephalus of the hindbrain and impaired ciliogenesis in the otic vesicle. Ultrastructural analysis did not reveal defects in the shape or organization of individual cilia. Similar phenotypes have been described in animals with deficiencies in Wnt signaling and the Planar Cell Polarity (PCP) pathway. We show that NHERF1 binds the PCP core genes Frizzled (Fzd) and Vangl. We further show that NHERF1 assembles a ternary complex with Fzd4 and Vangl2 and promotes translocation of Vangl2 to the plasma membrane, in particular to the apical surface of ependymal cells. Taken together, these results strongly support an important role for NHERF1 in the regulation of PCP signaling and the development of functional motile cilia.

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.

Regulation of renal phosphate handling: inter-organ communication in health and disease

In this review, we focus on the interconnection of inorganic phosphate (Pi) homeostasis in the network of the bone-kidney, parathyroid-kidney, intestine-kidney, and liver-kidney axes. Such a network of organ communication is important for body Pi homeostasis. Normalization of serum Pi levels is a clinical target in patients with chronic kidney disease (CKD). Particularly, disorders of the fibroblast growth factor 23/klotho system are observed in early CKD. Identification of phosphaturic factors from the intestine and liver may enhance our understanding of body Pi homeostasis and Pi metabolism disturbances in CKD patients.

Characterization of FGF23-Dependent Egr-1 Cistrome in the Mouse Renal Proximal Tubule

Fibroblast growth factor 23 (FGF23) is a potent regulator of phosphate (Pi) and vitamin D homeostasis. The transcription factor, early growth response 1 (egr-1), is a biomarker for FGF23-induced activation of the ERK1/2 signaling pathway. We have shown that ERK1/2 signaling blockade suppresses renal egr-1 gene expression and prevents FGF23-induced hypophosphatemia and 1,25-dihydroxyvitamin D (1,25(OH)2D) suppression in mice. To test whether egr-1 itself mediates these renal actions of FGF23, we administered FGF23 to egr-1-/- and wild-type (WT) mice. In WT mice, FGF23 induced hypophosphatemia and suppressed expression of the renal Na/Pi cotransporters, Npt2a and Npt2c. In FGF23-treated egr-1-/- mice, hypophosphatemic response was greatly blunted and Na/Pi cotransporter expression was not suppressed. In contrast, FGF23 induced equivalent suppression of serum 1,25(OH)2D concentrations by suppressing renal cyp27b1 and stimulating cyp24a1 mRNA expression in both groups of mice. Thus, downstream of receptor binding and ERK1/2 signaling, we can distinguish the effector pathway that mediates FGF23-dependent inhibition of Pi transport from the pathway that mediates inhibition of 1,25(OH)2D synthesis in the kidney. Furthermore, we demonstrate that the hypophosphatemic effect of FGF23 is significantly blunted in Hyp/egr-1-/- mice; specifically, serum Pi concentrations and renal Npt2a and Npt2c mRNA expression are significantly higher in Hyp/egr-1-/- mice than in Hyp mice. We then characterized the egr-1 cistrome in the kidney using ChIP-sequencing and demonstrate recruitment of egr-1 to regulatory DNA elements in proximity to several genes involved in Pi transport. Thus, our data demonstrate that the effect of FGF23 on Pi homeostasis is mediated, at least in part, by activation of egr-1.

Retromer and sorting nexins in endosomal sorting

The evolutionarily conserved endosomal retromer complex rescues transmembrane proteins from the lysosomal degradative pathway and facilitates their recycling to other cellular compartments. Retromer functions in conjunction with numerous associated proteins, including select members of the sorting nexin (SNX) family. In the present article, we review the molecular architecture and cellular roles of retromer and its various functional partners. The endosomal network is a crucial hub in the trafficking of proteins through the cellular endomembrane system. Transmembrane proteins, here termed cargos, enter endosomes by endocytosis from the plasma membrane or by trafficking from the trans-Golgi network (TGN). Endosomal cargo proteins face one of the two fates: retention in the endosome, leading ultimately to lysosomal degradation or export from the endosome for reuse ('recycling'). The balance of protein degradation and recycling is crucial to cellular homoeostasis; inappropriate sorting of proteins to either fate leads to cellular dysfunction. Retromer is an endosome-membrane-associated protein complex central to the recycling of many cargo proteins from endosomes, both to the TGN and the plasma membrane (and other specialized compartments, e.g. lysosome-related organelles). Retromer function is reliant on a number of proteins from the SNX family. In the present article, we discuss this inter-relationship and how defects in retromer function are increasingly being linked with human disease.

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.

Regulation of hormone-sensitive renal phosphate transport

Phosphate is essential for growth and maintenance of the skeleton and for generating high-energy phosphate compounds. Evolutionary adaptation to high dietary phosphorous in humans and other terrestrial vertebrates involves regulated mechanisms assuring the efficient renal elimination of excess phosphate. These mechanisms prominently include PTH, FGF23, and Vitamin D, which directly and indirectly regulate phosphate transport. Disordered phosphate homeostasis is associated with pathologies ranging from kidney stones to kidney failure. Chronic kidney disease results in hyperphosphatemia, an elevated calcium×phosphate product with considerable morbidity and mortality, mostly associated with adverse cardiovascular events. This chapter highlights recent findings and insights regarding the hormonal regulation of renal phosphate transport along with imbalances of phosphate balance due to acquired or inherited diseases states.

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.

Phosphate transporters of the SLC20 and SLC34 families

Transport of inorganic phosphate (Pi) across the plasma membrane is essential for normal cellular function. Members of two families of SLC proteins (SLC20 and SLC34) act as Na(+)-dependent, secondary-active cotransporters to transport Pi across cell membranes. The SLC34 proteins are expressed in specific organs important for Pi homeostasis: NaPi-IIa (SLC34A1) and NaPi-IIc (SLC34A3) fulfill essential roles in Pi reabsorption in the kidney proximal tubule and NaPi-IIb (SLC34A2) mediates Pi absorption in the gut. The SLC20 proteins, PiT-1 (SLC20A1), PiT-2 (SLC20A2) are expressed ubiquitously in all tissues and although generally considered as "housekeeping" transport proteins, the discovery of tissue-specific activity, regulatory pathways and gene-related pathophysiologies, is redefining their importance. This review summarizes our current knowledge of SLC20 and SLC34 proteins in terms of their basic molecular characteristics, physiological roles, known pathophysiology and pharmacology.

Activation of a non-cAMP/PKA signaling pathway downstream of the PTH/PTHrP receptor is essential for a sustained hypophosphatemic response to PTH infusion in male mice

PTH increases urinary Pi excretion by reducing expression of two renal cotransporters [NaPi-IIa (Npt2a) and NaPi-IIc (Npt2c)]. In contrast to acute transporter regulation that is cAMP/protein kinase A dependent, long-term effects require phospholipase C (PLC) signaling by the PTH/PTHrP receptor (PPR). To determine whether the latter pathway regulates Pi through Npt2a and/or Npt2c, wild-type mice (Wt) and animals expressing a mutant PPR incapable of PLC activation (DD) were tested in the absence of one (Npt2a(-/-) or Npt2c(-/-)) or both phosphate transporters (2a/2c-dko). PTH infusion for 8 days caused a rapid and persistent decrease in serum Pi in Wt mice, whereas serum Pi in DD mice fell only transiently for the first 2 days. Consistent with these findings, fractional Pi excretion index was increased initially in both animals, but this increase persisted only when the PPR Wt was present. The hypophosphatemic response to PTH infusion was impaired only slightly in PPR Wt/Npt2c(-/-) or DD/Npt2c(-/-) mice. Despite lower baselines, PTH infusion in PPR Wt/Npt2a(-/-) mice decreased serum Pi further, an effect that was attenuated in DD/Npt2a(-/-) mice. Continuous PTH had no effect on serum Pi in 2a/2c-dko mice. PTH administration increased serum 1,25 dihydroxyvitamin D3 levels in Wt and DD mice and increased levels above the elevated baseline with ablation of either but not of both transporters. Continuous PTH elevated serum fibroblast growth factor 23 and blood Ca(2+) equivalently in all groups of mice. Our data indicate that PLC signaling at the PPR contributes to the long-term effect of PTH on Pi homeostasis but not to the regulation of 1,25 dihydroxyvitamin D3, fibroblast growth factor 23, or blood Ca(2+).

Roles for NHERF1 and NHERF2 on the regulation of C3a receptor signaling in human mast cells

Background

The anaphylatoxin C3a binds to the G protein coupled receptor (GPCR, C3aR) and activates divergent signaling pathways to induce degranulation and cytokine production in human mast cells. Adapter proteins such as the Na⁺/H⁺ exchange regulatory factor (NHERF1 and NHERF2) have been implicated in regulating functions of certain GPCRs by binding to the class I PDZ (PSD-95/Dlg/Zo1) motifs present on their cytoplasmic tails. Although C3aR possesses a class I PDZ motif, the possibility that it interacts with NHERF proteins to modulate signaling in human mast cells has not been determined.

Methodology/Principal Findings

Using reverse transcription PCR and Western blotting, we found that NHERF1 and NHERF2 are expressed in human mast cell lines (HMC-1, LAD2) and CD34⁺-derived primary human mast cells. Surprisingly, however, C3aR did not associate with these adapter proteins. To assess the roles of NHERFs on signaling downstream of C3aR, we used lentiviral shRNA to stably knockdown the expression of these proteins in human mast cells. Silencing the expression of NHERF1 and NHERF2 had no effect on C3aR desensitization, agonist-induced receptor internalization, ERK/Akt phosphorylation or chemotaxis. However, loss of NHERF1 and NHERF2 resulted in significant inhibition of C3a-induced mast cell degranulation, NF-κB activation and chemokine production.

Conclusion/Significance

This study demonstrates that although C3aR possesses a class I PDZ motif, it does not associate with NHERF1 and NHERF2. Surprisingly, these proteins provide stimulatory signals for C3a-induced degranulation, NF-κB activation and chemokine generation in human mast cells. These findings reveal a new level of complexity for the functional regulation of C3aR by NHERFs in human mast cells.

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.

FGF23 acts directly on renal proximal tubules to induce phosphaturia through activation of the ERK1/2-SGK1 signaling pathway

Fibroblast growth factor-23 (FGF23) is a bone-derived endocrine regulator of phosphate homeostasis which inhibits renal tubular phosphate reabsorption. Binding of circulating FGF23 to FGF receptors in the cell membrane requires the concurrent presence of the co-receptor αKlotho. It is still controversial whether αKlotho is expressed in the kidney proximal tubule, the principal site of phosphate reabsorption. Hence, it has remained an enigma as to how FGF23 downregulates renal phosphate reabsorption. Here, we show that renal proximal tubular cells do express the co-receptor αKlotho together with cognate FGF receptors, and that FGF23 directly downregulates membrane expression of the sodium-phosphate cotransporter NaPi-2a by serine phosphorylation of the scaffolding protein Na(+)/H(+) exchange regulatory cofactor (NHERF)-1 through ERK1/2 and serum/glucocorticoid-regulated kinase-1 signaling.

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.

The association of bone mineral density and parathyroid hormone with serum magnesium in adult patients with sickle-cell anaemia

INTRODUCTION

Bone disorders including osteopenia and osteoporosis are a frequent cause of morbidity in sickle-cell disease (SCD). Magnesium (Mg) regulates some biological processes important in bone remodelling. We aimed to investigate whether serum Mg levels (sMg) may have an impact on bone mineral density (BMD) in sickle-cell anaemia (SCA).

MATERIAL AND METHODS

Sixty adults with SCA in steady-state and 20 age- and race-matched healthy blood donors were included in the study. The BMD was evaluated with respect to minerals and biochemical indices of bone metabolism. Multivariate analysis was performed to determine the factors influencing BMD.

RESULTS

The mean sMg concentration was 0.64 ±0.06 (reference range 0.7-1.2 mmol/l) for 34% of the population, and 0.86 ±0.08 mmol/l for 66%. There were significant differences between Mg groups and controls in BMD, phosphorus (PO(4)), parathyroid hormone (PTH) (p = 0.011, p = 0.011 and p = 0.0001 respectively) and osteocalcin (OC) (p = 0.030) levels. The sMg was found to be associated positively with serum calcium (Ca), PTH and OC (r = 0.585; r = 0.436; r = 0.351 respectively, all at p < 0.05), and negatively with PO(4) (r = -0.312; p < 0.05). Multivariate analysis demonstrated that only PTH (p < 0.05) was an independent factor for BMD. Moreover, it identified sMg, OC, and CTX as independent factors for PTH (all p < 0.05).

CONCLUSIONS

These results indicate that serum Mg may be a co-contributing factor in causing low BMD. However, other possible aetiologies including decreased PTH and increased bone turnover certainly play a role. Based on the present data, it is prudent to monitor sMg routinely in this patient population and treat the condition whenever possible.