Acute down-regulation of sodium-dependent phosphate transporter NPT2a involves predominantly the cAMP/PKA pathway as revealed by signaling-selective parathyroid hormone analogs

Induced pluripotent stem cells derived renal tubular cells from a patient with pseudohypoparathyroidism and its response to parathyroid hormone stimulation

INTRODUCTION

Creating induced pluripotent stem cells (iPSCs) from somatic cells of patients with genetic diseases offers a pathway to generate disease-specific iPSCs carrying genetic markers. Differentiating these iPSCs into renal tubular cells can aid in understanding the pathophysiology of rare inherited renal tubular diseases through cellular experiments.

MATERIALS AND METHODS

Two Japanese patients with Pseudohypoparathyroidism (PHP), a 49-year-old woman and a 71-year-old man, were studied. iPSC-derived tubular cells were established from their peripheral blood mononuclear cells (PBMCs). We examined changes in intracellular and extracellular cyclic adenosine monophosphate (cAMP) levels in these cells in response to parathyroid hormone (PTH) stimulation.

RESULTS

Renal tubular cells, differentiated from iPSCs of a healthy control (648A1), showed a PTH-dependent increase in both intracellular and extracellular cAMP levels. However, the renal tubular cells derived from the PHP patients' iPSCs showed inconsistent changes in cAMP levels upon PTH exposure.

CONCLUSION

We successfully created disease-specific iPSCs from PHP patients' PBMCs, differentiated them into tubular cells, and replicated the distinctive response of the disease to PTH in vitro. This approach could enhance our understanding of the pathophysiology of inherited renal tubular diseases and contribute to developing effective treatments.

Regulation of intracellular cAMP levels in osteocytes by mechano-sensitive focal adhesion kinase via PDE8A

Osteocytes are the primary mechano-sensitive cell type in bone. Mechanical loading is sensed across the dendritic projections of osteocytes leading to transient reductions in focal adhesion kinase (FAK) activity. Knowledge regarding the signaling pathways downstream of FAK in osteocytes is incomplete. We performed tyrosine-focused phospho-proteomic profiling in osteocyte-like Ocy454 cells to identify FAK substrates. Gsα, parathyroid hormone receptor (PTH1R), and phosphodiesterase 8A (PDE8A), all proteins associated with cAMP signaling, were found as potential FAK targets based on their reduced tyrosine phosphorylation in both FAK- deficient or FAK inhibitor treated cells. Real time monitoring of intracellular cAMP levels revealed that FAK pharmacologic inhibition or gene deletion increased basal and GPCR ligand-stimulated cAMP levels and downstream phosphorylation of protein kinase A substrates. Mutating FAK phospho-acceptor sites in Gsα and PTH1R had no effect on PTH- or FAK inhibitor-stimulated cAMP levels. Since FAK inhibitor treatment augmented cAMP levels even in the presence of forskolin, we focused on potential FAK substrates downstream of cAMP generation. Indeed, PDE8A inhibition mimicked FAK inhibition at the level of increased cAMP, PKA activity, and expression of cAMP-regulated target genes. In vitro kinase assay showed that PDE8A is directly phosphorylated by FAK while immunoprecipitation assays revealed intracellular association between FAK and PDE8A. Thus, FAK inhibition in osteocytes acts synergistically with signals that activate adenylate cyclase to increase intracellular cAMP. Mechanically-regulated FAK can modulate intracellular cAMP levels via effects on PDE8A. These data suggest a novel signal transduction mechanism that mediates crosstalk between mechanical and cAMP-linked hormonal signaling in osteocytes.

Highly biased agonism for GPCR ligands via nanobody tethering

Ligand-induced activation of G protein-coupled receptors (GPCRs) can initiate signaling through multiple distinct pathways with differing biological and physiological outcomes. There is intense interest in understanding how variation in GPCR ligand structure can be used to promote pathway selective signaling ("biased agonism") with the goal of promoting desirable responses and avoiding deleterious side effects. Here we present an approach in which a conventional peptide ligand for the type 1 parathyroid hormone receptor (PTHR1) is converted from an agonist which induces signaling through all relevant pathways to a compound that is highly selective for a single pathway. This is achieved not through variation in the core structure of the agonist, but rather by linking it to a nanobody tethering agent that binds with high affinity to a separate site on the receptor not involved in signal transduction. The resulting conjugate represents the most biased agonist of PTHR1 reported to date. This approach holds promise for facile generation of pathway selective ligands for other GPCRs.

Endosomal signaling via cAMP in parathyroid hormone (PTH) type 1 receptor biology

Compartmentalization of GPCR signaling is an emerging topic that highlights the physiological relevance of spatial bias in signaling. The parathyroid hormone (PTH) type 1 receptor (PTH1R) was the first GPCR described to signal via heterotrimeric G-protein and cAMP from endosomes after β-arrestin mediated internalization, challenging the canonical GPCR signaling model which established that signaling is terminated by receptor internalization. More than a decade later, many other GPCRs have been shown to signal from endosomes via cAMP, and recent studies have proposed that location of cAMP generation impacts physiological outcomes of GPCR signaling. Here, we review the extensive literature regarding PTH1R endosomal signaling via cAMP, the mechanisms that regulate endosomal generation of cAMP, and the implications of spatial bias in PTH1R physiological functions.

Highly biased agonism for GPCR ligands via nanobody tethering

Ligand-induced activation of G protein-coupled receptors (GPCRs) can initiate signaling through multiple distinct pathways with differing biological and physiological outcomes. There is intense interest in understanding how variation in GPCR ligand structure can be used to promote pathway selective signaling ("biased agonism") with the goal of promoting desirable responses and avoiding deleterious side effects. Here we present a new approach in which a conventional peptide ligand for the type 1 parathyroid hormone receptor (PTHR1) is converted from an agonist which induces signaling through all relevant pathways to a compound that is highly selective for a single pathway. This is achieved not through variation in the core structure of the agonist, but rather by linking it to a nanobody tethering agent that binds with high affinity to a separate site on the receptor not involved in signal transduction. The resulting conjugate represents the most biased agonist of PTHR1 reported to date. This approach holds promise for facile generation of pathway selective ligands for other GPCRs.

Molecular Mechanisms of PTH/PTHrP Class B GPCR Signaling and Pharmacological Implications

The classical paradigm of G protein-coupled receptor (GPCR) signaling via G proteins is grounded in a view that downstream responses are relatively transient and confined to the cell surface, but this notion has been revised in recent years following the identification of several receptors that engage in sustained signaling responses from subcellular compartments following internalization of the ligand-receptor complex. This phenomenon was initially discovered for the parathyroid hormone (PTH) type 1 receptor (PTH1R), a vital GPCR for maintaining normal calcium and phosphate levels in the body with the paradoxical ability to build or break down bone in response to PTH binding. The diverse biological processes regulated by this receptor are thought to depend on its capacity to mediate diverse modes of cyclic adenosine monophosphate (cAMP) signaling. These include transient signaling at the plasma membrane and sustained signaling from internalized PTH1R within early endosomes mediated by PTH. Here we discuss recent structural, cell signaling, and in vivo studies that unveil potential pharmacological outputs of the spatial versus temporal dimension of PTH1R signaling via cAMP. Notably, the combination of molecular dynamics simulations and elastic network model-based methods revealed how precise modulation of PTH signaling responses is achieved through structure-encoded allosteric coupling within the receptor and between the peptide hormone binding site and the G protein coupling interface. The implications of recent findings are now being explored for addressing key questions on how location bias in receptor signaling contributes to pharmacological functions, and how to drug a difficult target such as the PTH1R toward discovering nonpeptidic small molecule candidates for the treatment of metabolic bone and mineral diseases.

Kidney glycolysis serves as a mammalian phosphate sensor that maintains phosphate homeostasis

How phosphate levels are detected in mammals is unknown. The bone-derived hormone fibroblast growth factor 23 (FGF23) lowers blood phosphate levels by reducing kidney phosphate reabsorption and 1,25(OH)2D production, but phosphate does not directly stimulate bone FGF23 expression. Using PET scanning and LC-MS, we found that phosphate increases kidney-specific glycolysis and synthesis of glycerol-3-phosphate (G-3-P), which then circulates to bone to trigger FGF23 production. Further, we found that G-3-P dehydrogenase 1 (Gpd1), a cytosolic enzyme that synthesizes G-3-P and oxidizes NADH to NAD+, is required for phosphate-stimulated G-3-P and FGF23 production and prevention of hyperphosphatemia. In proximal tubule cells, we found that phosphate availability is substrate-limiting for glycolysis and G-3-P production and that increased glycolysis and Gpd1 activity are coupled through cytosolic NAD+ recycling. Finally, we show that the type II sodium-dependent phosphate cotransporter Npt2a, which is primarily expressed in the proximal tubule, conferred kidney specificity to phosphate-stimulated G-3-P production. Importantly, exogenous G-3-P stimulated FGF23 production when Npt2a or Gpd1 were absent, confirming that it was the key circulating factor downstream of glycolytic phosphate sensing in the kidney. Together, these findings place glycolysis at the nexus of mineral and energy metabolism and identify a kidney-bone feedback loop that controls phosphate homeostasis.

How useful is an oral calcium load test for diagnosing recurrent calcium stone formers?

Hypercalciuria is the main risk factor for recurrent calcium urolithiasis. The goal of our study is to determinate how useful an oral calcium load test is for stone formers to classify different forms of hypercalciuria in pathogenetic categories defined as renal or absorptive according to the current knowledge. Between June 2013 and February 2016, a prospective study was carried out on 117 documented recurrent hypercalciuric stone formers undergoing an oral calcium load test modified from the original description by Pak. After 2 days of calcium-restricted diet, urine and blood were analyzed at baseline and 120 min after receiving orally 1 g of calcium. Total and ionized calcium, parathyroid hormone from serum and urine calcium and creatinine were assessed in order to divide patients in three groups as previously described: resorptive, absorptive, and renal hypercalciuria. This allowed the identification of 19, 39, 34 and 33 patients with normocalcemic primary hyperparathyroidism (NPHPT), renal hypercalciuria aka renal calcium leak (RCL), absorptive hypercalciuria (AH) and unidentified cause, respectively. Patients with NPHPT (who required parathyroidectomy) experienced a lower PTH decrease (41.41 ± 12.82 vs. 54.06 ± 13.84% p < 0.01), higher beta-crosslaps, as well as lower TmP/GFR and distal third radius bone mineral density. RCL resulted in increased fasting urine calcium-to-creatinine ratio (Uca/Cr), i.e., > 0.37 mmol/mmol), without hyperparathyroidism. AH was diagnosed by the presence of ΔUCa/Cr > 0.60 mmol/mmol between baseline and 120 min without any other anomaly. For all remaining patients, results were inconclusive due to the lack of sufficient increase in serum calcium or because the cause of lithogenesis could not be clearly identified. The oral calcium load test is useful in nearly 80% of patients by identifying the different forms of hypercalciuria causing urolithiasis and by guiding treatment, including parathyroid surgery.

Activation of PTH1R alleviates epididymitis and orchitis through Gq and β-arrestin-1 pathways

Inflammation in the epididymis and testis contributes significantly to male infertility. Alternative therapeutic avenues treating epididymitis and orchitis are expected since current therapies using antibiotics have limitations associated to side effects and are commonly ineffective for inflammation due to nonbacterial causes. Here, we demonstrated that type 1 parathyroid hormone receptor (PTH1R) and its endogenous agonists, parathyroid hormone (PTH) and PTH-related protein (PTHrP), were mainly expressed in the Leydig cells of testis as well as epididymal epithelial cells. Screening the secretin family G protein-coupled receptor identified that PTH1R in the epididymis and testis was down-regulated in mumps virus (MuV)- or lipopolysaccharide (LPS)-induced inflammation. Remarkably, activation of PTH1R by abaloparatide (ABL), a Food and Drug Administration-approved treatment for postmenopausal osteoporosis, alleviated MuV- or LPS-induced inflammatory responses in both testis and epididymis and significantly improved sperm functions in both mouse model and human samples. The anti-inflammatory effects of ABL were shown to be regulated mainly through the Gq and β-arrestin-1 pathway downstream of PTH1R as supported by the application of ABL in Gnaq± and Arrb1-/- mouse models. Taken together, our results identified an important immunoregulatory role for PTH1R signaling in the epididymis and testis. Targeting to PTH1R might have a therapeutic effect for the treatment of epididymitis and orchitis or other inflammatory disease in the male reproductive system.

Comparable Initial Engagement of Intracellular Signaling Pathways by Parathyroid Hormone Receptor Ligands Teriparatide, Abaloparatide, and Long-Acting PTH

Multiple analogs of parathyroid hormone, all of which bind to the PTH/PTHrP receptor PTH1R, are used for patients with osteoporosis and hypoparathyroidism. Although ligands such as abaloparatide, teriparatide (hPTH 1-34 [TPTD]), and long-acting PTH (LA-PTH) show distinct biologic effects with respect to skeletal and mineral metabolism endpoints, the mechanistic basis for these clinically-important differences remains incompletely understood. Previous work has revealed that differential signaling kinetics and receptor conformation engagement between different PTH1R peptide ligands. However, whether such acute membrane proximal differences translate into differences in downstream signaling output remains to be determined. Here, we directly compared short-term effects of hPTH (1-34), abaloparatide, and LA-PTH in multiple cell-based PTH1R signaling assays. At the time points and ligand concentrations utilized, no significant differences were observed between these three ligands at the level of receptor internalization, β-arrestin recruitment, intracellular calcium stimulation, and cAMP generation. However, abaloparatide showed significantly quicker PTH1R recycling in washout studies. Downstream of PTH1R-stimulated cAMP generation, protein kinase A regulates gene expression via effects on salt inducible kinases (SIKs) and their substrates. Consistent with no differences between these ligands on cAMP generation, we observed that hPTH (1-34), abaloparatide, and LA-PTH showed comparable effects on SIK2 phosphorylation, SIK substrate dephosphorylation, and downstream gene expression changes. Taken together, these results indicate that these PTH1R peptide agonists engage downstream intracellular signaling pathways to a comparable degree. It is possible that differences observed in vivo in preclinical and clinical models may be related to pharmacokinetic factors. It is also possible that our current in vitro systems are insufficient to perfectly match the complexities of PTH1R signaling in bona fide target cells in bone in vivo. © 2020 American Society for Bone and Mineral Research © 2020 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

Effects of ORAI calcium release-activated calcium modulator 1 (ORAI1) on neutrophil activity in dairy cows with subclinical hypocalcemia1

Hypocalcemia in dairy cows is often associated with inflammation-related disorders such as metritis and mastitis. The protein encoded by the Ca2+ release-activated calcium modulator 1 (ORAI1) gene is a membrane Ca2+ channel subunit that is activated when Ca2+ stores are depleted. Polymorphonuclear neutrophils (PMNL) have a crucial role in the defense against infection through migration, adhesion, chemotaxis, phagocytosis, and reactive oxygen species (ROS) production in response to pathogens. Whether hypocalcemia affects the activity of PMNL and if ORAI1 is involved remains unknown. To address this, PMNL were isolated at 3 d of calving from dairy cows diagnosed as clinically healthy (n = 20, CONTROL) or with plasma concentration of calcium < 2.0 mmol/L as a criterion for diagnosis of subclinical hypocalcemia (n = 20, HYPOCAL). PMNL isolated from both groups of cows were treated with or without the sarcoendoplasmic Ca2+ ATPase inhibitor thapsigargin, Ca2+ ionophore Ionomycin, and ORAI1 blocker 2APB. The intracellular Ca2+ concentration, ORAI1 abundance, ROS, phagocytosis rate, migration, and adhering capacity of treated PMNL were evaluated. Some of the in vitro assays also included use of small interfering ORAI1 RNA (siORAI1), 100 nM 1,25(OH)2D3, or 100 nM parathyroid hormone (PTH). Intracellular Ca2+ concentration was markedly lower in HYPOCAL. In addition, ORAI1 was detected in PMNL plasma membrane via FACS and was markedly lower in cows with HYPOCAL. Migration, adhesion capacity, and phagocytosis rate of PMNL were lower in response to HYPOCAL. Furthermore, plasma and PMNL concentration of nucleosome assembly protein (NAP2) and pro-platelet basic protein (CXCL7) was markedly lower with HYPOCAL. All these changes were associated with lower ROS production by PMNL. Thapsigargin and ionomycin treatment in vitro increased ORAI1 expression, migration of PMNL, adhering capacity, phagocytosis rate, and ROS production; conversely, those effects were abrogated by siORAI1 and ORAI1 inhibitor 2APB treatment. Also cytosolic Ca2+ concentration and ORAI1 abundance were increased by 1,25(OH)2D3 and PTH supplementation. Overall, the data indicate that failure of PMNL to uptake Ca2+ due to downregulation of ORAI1 during subclinical hypocalcemia is a factor contributing to impaired PMNL function. In addition, plasma PTH or 1,25(OH)2D3 could regulate ORAI1 and also participate in the regulation of PMNL activity.

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.

Pharmacological Npt2a Inhibition Causes Phosphaturia and Reduces Plasma Phosphate in Mice with Normal and Reduced Kidney Function

BACKGROUND

The kidneys play an important role in phosphate homeostasis. Patients with CKD develop hyperphosphatemia in the later stages of the disease. Currently, treatment options are limited to dietary phosphate restriction and oral phosphate binders. The sodium-phosphate cotransporter Npt2a, which mediates a large proportion of phosphate reabsorption in the kidney, might be a good therapeutic target for new medications for hyperphosphatemia.

METHODS

The authors assessed the effects of the first orally bioavailable Npt2a inhibitor (Npt2a-I) PF-06869206 in normal mice and mice that had undergone subtotal nephrectomy (5/6 Nx), a mouse model of CKD. Dose-response relationships of sodium, chloride, potassium, phosphate, and calcium excretion were assessed in response to the Npt2a inhibitor in both groups of mice. Expression and localization of Npt2a/c and levels of plasma phosphate, calcium, parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF-23) were studied up to 24-hours after Npt2a-I treatment.

RESULTS

In normal mice, Npt2a inhibition caused a dose-dependent increase in urinary phosphate (ED₅₀ approximately 21 mg/kg), calcium, sodium and chloride excretion. In contrast, urinary potassium excretion, flow rate and urinary pH were not affected dose dependently. Plasma phosphate and PTH significantly decreased after 3 hours, with both returning to near baseline levels after 24 hours. Similar effects were observed in the mouse model of CKD but were reduced in magnitude.

CONCLUSIONS

Npt2a inhibition causes a dose-dependent increase in phosphate, sodium and chloride excretion associated with reductions in plasma phosphate and PTH levels in normal mice and in a CKD mouse model.

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.

Hypoparathyroidism and the Kidney

Hypocalcemia and hyperphosphatemia are the pathognomonic biochemical features of hypoparathyroidism, and result directly from lack of parathyroid hormone (PTH) action on the kidney. In the absence of PTH action, the renal mechanisms transporting calcium and phosphate reabsorption deregulate, resulting in hypocalcemia and hyperphosphatemia. Circulating calcium negatively regulates PTH secretion. Hypocalcemia causes neuromuscular disturbances ranging from epilepsy and tetany to mild paresthesia. Circulating phosphate concentration does not directly regulate PTH secretion. Hyperphosphatemia is subclinical, but chronically promotes ectopic mineralization disease. Vitamin D-thiazide treatment leads to ectopic mineralization and renal damage. PTH treatment has the potential for fewer side effects.

CFTR-mediated anion secretion across intestinal epithelium-like Caco-2 monolayer under PTH stimulation is dependent on intermediate conductance K+ channels

Parathyroid hormone (PTH), a pleiotropic hormone that maintains mineral homeostasis, is also essential for controlling pH balance and ion transport across renal and intestinal epithelia. Optimization of luminal pH is important for absorption of trace elements, e.g., calcium and phosphorus. We have previously demonstrated that PTH rapidly stimulated electrogenic [Formula: see text] secretion in intestinal epithelial-like Caco-2 monolayers, but the underlying cellular mechanism, contributions of other ions, particularly Cl^- and K⁺, and long-lasting responses are not completely understood. Herein, PTH and forskolin were confirmed to induce anion secretion, which peaked within 1-3 min (early phase), followed by an abrupt decay and plateau that lasted for 60 min (late phase). In both early and late phases, apical membrane capacitance was increased with a decrease in basolateral capacitance after PTH or forskolin exposure. PTH also induced a transient increase in apical conductance with a long-lasting decrease in basolateral conductance. Anion secretion in both phases was reduced under [Formula: see text]-free and/or Cl^--free conditions or after exposure to carbonic anhydrase inhibitor (acetazolamide), CFTR inhibitor (CFTRinh-172), Na⁺/H⁺ exchanger (NHE)-3 inhibitor (tenapanor), or K⁺ channel inhibitors (BaCl₂, clotrimazole, and TRAM-34; basolateral side), the latter of which suggested that PTH action was dependent on basolateral K⁺ recycling. Furthermore, early- and late-phase responses to PTH were diminished by inhibitors of PI3K (wortmannin and LY-294002) and PKA (PKI 14-22). In conclusion, PTH requires NHE3 and basolateral K⁺ channels to induce [Formula: see text] and Cl^- secretion, thus explaining how PTH regulated luminal pH balance and pH-dependent absorption of trace minerals.

Acute Parathyroid Hormone Injection Increases C-Terminal but Not Intact Fibroblast Growth Factor 23 Levels

The acute effects of parathyroid hormone (PTH) on fibroblast growth factor 23 (FGF23) in vivo are not well understood. After a single subcutaneous PTH (1-34) injection (50 nmol/kg) in mice, FGF23 levels were assessed in plasma using assays that measure either intact alone (iFGF23) or intact/C-terminal FGF23 (cFGF23). Furthermore, FGF23 messenger RNA (mRNA) and protein levels were assessed in bone. In addition, we examined the effects of PTH treatment on FGF23 production in vitro using differentiated calvarial osteocyte-like cells. cFGF23 levels increased by three- to fivefold within 2 hours following PTH injection, which returned to baseline by 4 hours. In contrast, iFGF23 levels remained unchanged for the first 2 hours, yet declined to ∼60% by 6 hours and remained suppressed before returning to baseline after 24 hours. Using homozygous mice for an autosomal dominant hypophosphatemic rickets-FGF23 mutation or animals treated with a furin inhibitor, we showed that cFGF23 and iFGF23 levels increased equivalently after PTH injection. These findings are consistent with increased FGF23 production in bone, yet rapid cleavage of the secreted intact protein. Using primary osteocyte-like cell cultures, we showed that PTH increased FGF23 mRNA expression through cyclic adenosine monophosphate/protein kinase A, but not inositol triphosphate/protein kinase C signaling; PTH also increased furin protein levels. In conclusion, PTH injection rapidly increases FGF23 production in bone in vivo and in vitro. However, iFGF23 is rapidly degraded. At later time points through an unidentified mechanism, a sustained decrease in FGF23 production occurs.

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.

GPCR Signaling and Trafficking: The Long and Short of It

Emerging findings disclose unexpected components of G protein-coupled receptor (GPCR) signaling and cell biology. Select GPCRs exhibit classical signaling, that is restricted to cell membranes, as well as newly described persistent signaling that depends on internalization of the GPCR bound to β-arrestins. Termination of non-canonical endosomal signaling requires intraluminal acidification and sophisticated protein trafficking machineries. Recent studies reveal the structural determinants of the trafficking chaperones. This review summarizes advances in GPCR signaling and trafficking with a focus on the parathyroid hormone receptor (PTHR) as a prototype, and on the actin-sorting nexin 27 (SNX27)-retromer tubule (ASRT) complex, an endosomal sorting hub responsible for recycling and preservation of cell surface receptors. The findings are integrated into a model of PTHR trafficking with implications for signal transduction, bone growth, and mineral ion metabolism.

Prolonged Pharmacokinetic and Pharmacodynamic Actions of a Pegylated Parathyroid Hormone (1-34) Peptide Fragment

Polyethylene glycol (PEG) addition can prolong the pharmacokinetic and pharmacodynamic actions of a bioactive peptide in vivo, in part by impeding rates of glomerular filtration. For parathyroid hormone (PTH) peptides, pegylation could help in exploring the actions of the hormone in the kidney; e.g., in dissecting the relative roles that filtered versus blood-borne PTH play in regulating phosphate transport. It could also lead to potential alternate forms of treatment for hypoparathyroidism. We thus synthesized the fluorescent pegylated PTH derivative [Lys¹³ (tetramethylrhodamine {TMR}), Cys³⁵ (PEG-20,000 Da)]PTH(1-35) (PEG-PTH^TMR ) and its non-pegylated counterpart [Lys¹³ (TMR), Cys³⁵ ]PTH(1-35) (PTH^TMR ) and assessed their properties in cells and in mice. In PTHR1-expressing HEK-293 cells, PEG-PTH^TMR and PTH^TMR exhibited similar potencies for inducing cAMP signaling, whereas when injected into mice, the pegylated analog persisted much longer in the circulation (>24 hours versus ∼ 1 hour) and induced markedly more prolonged calcemic and phosphaturic responses than did the non-pegylated control. Fluorescence microscopy analysis of kidney sections obtained from the injected mice revealed much less PEG-PTH^TMR than PTH^TMR on the luminal brush-border surfaces of renal proximal tubule cells (PTCs), on which PTH regulates phosphate transporter function, whereas immunostained phosphorylated PKA substrate, a marker of cAMP signaling, was increased to similar extents for the two ligands and for each, was localized to the basolateral portion of the PTCs. Pegylation of a bioactive PTH peptide thus led to prolonged pharmacokinetic/pharmacodynamic properties in vivo, as well as to new in vivo data that support a prominent role for PTH action at basolateral surfaces of renal proximal tubule cells. © 2016 American Society for Bone and Mineral Research.

Convergent Signaling Pathways Regulate Parathyroid Hormone and Fibroblast Growth Factor-23 Action on NPT2A-mediated Phosphate Transport

Parathyroid hormone (PTH) and FGF23 are the primary hormones regulating acute phosphate homeostasis. Human renal proximal tubule cells (RPTECs) were used to characterize the mechanism and signaling pathways of PTH and FGF23 on phosphate transport and the role of the PDZ protein NHERF1 in mediating PTH and FGF23 effects. RPTECs express the NPT2A phosphate transporter, αKlotho, FGFR1, FGFR3, FGFR4, and the PTH receptor. FGFR1 isoforms are formed from alternate splicing of exon 3 and of exon 8 or 9 in Ir-like loop 3. Exon 3 was absent, but mRNA containing both exons 8 and 9 is present in cytoplasm. Using an FGFR1c-specific antibody together with mass spectrometry analysis, we show that RPTECs express FGFR-β1C. The data are consistent with regulated FGFR1 splicing involving a novel cytoplasmic mechanism. PTH and FGF23 inhibited phosphate transport in a concentration-dependent manner. At maximally effective concentrations, PTH and FGF23 equivalently decreased phosphate uptake and were not additive, suggesting a shared mechanism of action. Protein kinase A or C blockade prevented PTH but not FGF23 actions. Conversely, inhibiting SGK1, blocking FGFR dimerization, or knocking down Klotho expression disrupted FGF23 actions but did not interfere with PTH effects. C-terminal FGF23(180-251) competitively and selectively blocked FGF23 action without disrupting PTH effects. However, both PTH and FGF23-sensitive phosphate transport were abolished by NHERF1 shRNA knockdown. Extended treatment with PTH or FGF23 down-regulated NPT2A without affecting NHERF1. We conclude that FGFR1c and PTHR signaling pathways converge on NHERF1 to inhibit PTH- and FGF23-sensitive phosphate transport and down-regulate NPT2A.

Parathyroid Hormone-Related Protein, Its Regulation of Cartilage and Bone Development, and Role in Treating Bone Diseases

Although parathyroid hormone-related protein (PTHrP) was discovered as a cancer-derived hormone, it has been revealed as an important paracrine/autocrine regulator in many tissues, where its effects are context dependent. Thus its location and action in the vasculature explained decades-long observations that injection of PTH into animals rapidly lowered blood pressure by producing vasodilatation. Its roles have been specified in development and maturity in cartilage and bone as a crucial regulator of endochondral bone formation and bone remodeling, respectively. Although it shares actions with parathyroid hormone (PTH) through the use of their common receptor, PTHR1, PTHrP has other actions mediated by regions within the molecule beyond the amino-terminal sequence that resembles PTH, including the ability to promote placental transfer of calcium from mother to fetus. A striking feature of the physiology of PTHrP is that it possesses structural features that equip it to be transported in and out of the nucleus, and makes use of a specific nuclear import mechanism to do so. Evidence from mouse genetic experiments shows that PTHrP generated locally in bone is essential for normal bone remodeling. Whereas the main physiological function of PTH is the hormonal regulation of calcium metabolism, locally generated PTHrP is the important physiological mediator of bone remodeling postnatally. Thus the use of intermittent injection of PTH as an anabolic therapy for bone appears to be a pharmacological application of the physiological function of PTHrP. There is much current interest in the possibility of developing PTHrP analogs that might enhance the therapeutic anabolic effects.

Ablation of the Stimulatory G Protein α-Subunit in Renal Proximal Tubules Leads to Parathyroid Hormone-Resistance With Increased Renal Cyp24a1 mRNA Abundance and Reduced Serum 1,25-Dihydroxyvitamin D

PTH regulates serum calcium, phosphate, and 1,25-dihydroxyvitamin D (1,25(OH)2D) levels by acting on bone and kidney. In renal proximal tubules (PTs), PTH inhibits reabsorption of phosphate and stimulates the synthesis of 1,25(OH)2D. The PTH receptor couples to multiple G proteins. We here ablated the α-subunit of the stimulatory G protein (Gsα) in mouse PTs by using Cre recombinase driven by the promoter of type-2 sodium-glucose cotransporter (Gsα(Sglt2KO) mice). Gsα(Sglt2KO) mice were normophosphatemic but displayed, relative to controls, hypocalcemia (1.19 ±0.01 vs 1.23 ±0.01 mmol/L; P < .05), reduced serum 1,25(OH)2D (59.3 ±7.0 vs 102.5 ±12.2 pmol/L; P < .05), and elevated serum PTH (834 ±133 vs 438 ±59 pg/mL; P < .05). PTH-induced elevation in urinary cAMP excretion was blunted in Gsα(Sglt2KO) mice (2- vs 4-fold over baseline in controls; P < .05). Relative to baseline in controls, PTH-induced reduction in serum phosphate tended to be blunted in Gsα(Sglt2KO) mice (-0.39 ±0.33 vs -1.34 ±0.36 mg/dL; P = .07). Gsα(Sglt2KO) mice showed elevated renal vitamin D 24-hydroxylase and bone fibroblast growth factor-23 (FGF23) mRNA abundance (∼3.4- and ∼11-fold over controls, respectively; P < .05) and tended to have elevated serum FGF23 (829 ±76 vs 632 ±60 pg/mL in controls; P = .07). Heterozygous mice having constitutive ablation of the maternal Gsα allele (E1(m-/+)) (model of pseudohypoparathyroidism type-Ia), in which Gsα levels in PT are reduced, also exhibited elevated serum FGF23 (474 ±20 vs 374 ±27 pg/mL in controls; P < .05). Our findings indicate that Gsα is required in PTs for suppressing renal vitamin D 24-hydroxylase mRNA levels and for maintaining normal serum 1,25(OH)2D.

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.

Identification of an RNA-binding protein that is phosphorylated by PTH and potentially mediates PTH-induced destabilization of Npt2a mRNA

Parathyroid hormone (PTH) is a key regulator of the expression and function of the type IIa sodium-phosphate cotransporter (Npt2a), the protein responsible for regulated renal phosphate reabsorption. We previously showed that PTH induces rapid decay of Npt2a mRNA through posttranscriptional mechanisms. We hypothesized that PTH-induced changes in RNA-binding protein (RBP) activity mediate the degradation of Npt2a mRNA. To address this aim, we treated opossum kidney (OK) cells, a PTH-sensitive proximal tubule cell culture model, with 100 nM PTH for 30 min and 2 h, followed by mass spectrometry characterization of the PTH-stimulated phosphoproteome. We identified 1,182 proteins differentially phosphorylated in response to PTH, including 68 RBPs. Preliminary analysis identified a phospho-RBP, hnRNPK-homology-type-splicing regulatory protein (KSRP), with predicted binding sites for the 3'-untranslated region (UTR) of Npt2a mRNA. Western blot analysis confirmed expression of KSRP in OK cells and showed PTH-dependent translocation to the nucleus. Immunoprecipitation of KSRP from control and PTH-treated cells followed by RNA isolation and RT-quantitative PCR analysis identified Npt2a mRNA from both control and PTH-treated KSRP pulldowns. Knockdown of KSRP followed by PTH treatment showed that KSRP is required for mediating PTH-stimulated reduction in sodium/hydrogen exchanger 3 mRNA, but not Npt2a mRNA. We conclude that 1) PTH is a major regulator of both transcription and translation, and 2) KSRP binds Npt2a mRNA but its role in PTH regulation of Npt2a mRNA is not clear.

International Union of Basic and Clinical Pharmacology. XCIII. The parathyroid hormone receptors--family B G protein-coupled receptors

The type-1 parathyroid hormone receptor (PTHR1) is a family B G protein-coupled receptor (GPCR) that mediates the actions of two polypeptide ligands; parathyroid hormone (PTH), an endocrine hormone that regulates the levels of calcium and inorganic phosphate in the blood by acting on bone and kidney, and PTH-related protein (PTHrP), a paracrine-factor that regulates cell differentiation and proliferation programs in developing bone and other tissues. The type-2 parathyroid hormone receptor (PTHR2) binds a peptide ligand, called tuberoinfundibular peptide-39 (TIP39), and while the biologic role of the PTHR2/TIP39 system is not as defined as that of the PTHR1, it likely plays a role in the central nervous system as well as in spermatogenesis. Mechanisms of action at these receptors have been explored through a variety of pharmacological and biochemical approaches, and the data obtained support a basic "two-site" mode of ligand binding now thought to be used by each of the family B peptide hormone GPCRs. Recent crystallographic studies on the family B GPCRs are providing new insights that help to further refine the specifics of the overall receptor architecture and modes of ligand docking. One intriguing pharmacological finding for the PTHR1 is that it can form surprisingly stable complexes with certain PTH/PTHrP ligand analogs and thereby mediate markedly prolonged cell signaling responses that persist even when the bulk of the complexes are found in internalized vesicles. The PTHR1 thus appears to be able to activate the Gα(s)/cAMP pathway not only from the plasma membrane but also from the endosomal domain. The cumulative findings could have an impact on efforts to develop new drug therapies for the PTH receptors.

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.

Renal phosphate wasting in the absence of adenylyl cyclase 6

Parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF-23) enhance phosphate excretion by the proximal tubule of the kidney by retrieval of the sodium-dependent phosphate transporters (Npt2a and Npt2c) from the apical plasma membrane. PTH activates adenylyl cyclase (AC) through PTH 1 receptors and stimulates the cAMP/PKA signaling pathway. However, the precise role and isoform(s) of AC in phosphate homeostasis are not known. We report here that mice lacking AC6 (AC6(-/-)) have increased plasma PTH and FGF-23 levels compared with wild-type (WT) mice but comparable plasma phosphate concentrations. Acute activation of the calcium-sensing receptor or feeding a zero phosphate diet almost completely suppressed plasma PTH levels in both AC6(-/-) and WT mice, indicating a secondary cause for hyperparathyroidism. Pharmacologic blockade of FGF receptors resulted in a comparable increase in plasma phosphate between genotypes, whereas urinary phosphate remained significantly higher in AC6(-/-) mice. Compared with WT mice, AC6(-/-) mice had reduced renal Npt2a and Npt2c protein abundance, with approximately 80% of Npt2a residing in lysosomes. WT mice responded to exogenous PTH with redistribution of Npt2a from proximal tubule microvilli to intracellular compartments and lysosomes alongside a PTH-induced dose-response relationship for fractional phosphate excretion and urinary cAMP excretion. These responses were absent in AC6(-/-) mice. In conclusion, AC6 in the proximal tubule modulates cAMP formation, Npt2a trafficking, and urinary phosphate excretion, which are highlighted by renal phosphate wasting in AC6(-/-) mice.

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+).

Critical role of parathyroid hormone (PTH) receptor-1 phosphorylation in regulating acute responses to PTH

Agonist-induced phosphorylation of the parathyroid hormone (PTH) receptor 1 (PTHR1) regulates receptor signaling in vitro, but the role of this phosphorylation in vivo is uncertain. We investigated this role by injecting "knock-in" mice expressing a phosphorylation-deficient (PD) PTHR1 with PTH ligands and assessing acute biologic responses. Following injection with PTH (1-34), or with a unique, long-acting PTH analog, PD mice, compared with WT mice, exhibited enhanced increases in cAMP levels in the blood, as well as enhanced cAMP production and gene expression responses in bone and kidney tissue. Surprisingly, however, the hallmark hypercalcemic and hypophosphatemic responses were markedly absent in the PD mice, such that paradoxical hypocalcemic and hyperphosphatemic responses were observed, quite strikingly with the long-acting PTH analog. Spot urine analyses revealed a marked defect in the capacity of the PD mice to excrete phosphate, as well as cAMP, into the urine in response to PTH injection. This defect in renal excretion was associated with a severe, PTH-induced impairment in glomerular filtration, as assessed by the rate of FITC-inulin clearance from the blood, which, in turn, was explainable by an overly exuberant systemic hypotensive response. The overall findings demonstrate the importance in vivo of PTH-induced phosphorylation of the PTHR1 in regulating acute ligand responses, and they serve to focus attention on mechanisms that underlie the acute calcemic response to PTH and factors, such as blood phosphate levels, that influence it.

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

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

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

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

Phosphate and FGF-23

Fibroblast growth factor (FGF)-23 is probably the most important regulator of serum phosphate and calcitriol (1,25(OH)₂D₃) levels. It is secreted by osteocytes and osteoblasts in response to oral phosphate loading or increased serum 1,25(OH)₂D₃ levels. In human chronic kidney disease (CKD), plasma FGF-23 appears to be a sensitive biomarker of abnormal renal phosphate handling, as FGF-23 levels increase during early stages of kidney malfunction. In humans and animals with CKD, elevated FGF-23 levels increase fractional phosphate excretion, reduce serum phosphate levels, and reduce 1α-hydroxylase activity, which reduces 1,25(OH)₂D₃ formation thereby increasing parathyroid hormone (PTH) secretion. FGF-23 thus has a key adaptive role in maintaining normophosphatemia. Plasma FGF-23 continues to increase as CKD progresses, increasing by orders of magnitude in end-stage renal disease. At the same time, responsiveness to FGF-23 declines as the number of intact nephrons declines, which is associated with reduced expression of Klotho, the co-receptor required for FGF-23 signaling. In late CKD, FGF-23 cannot reduce serum phosphate levels, and abnormally high plasma FGF-23 concentrations appear to exert unwarranted off-target effects, including left ventricular hypertrophy, faster CKD progression, and premature mortality. Lowering serum phosphate levels through the use of oral phosphate binders and/or long-acting PTH agents may reduce FGF-23 levels in early CKD stages, thereby limiting off-target effects, which may improve patient outcomes.