Type II Na+-Pi cotransporters in osteoblast mineral formation: regulation by inorganic phosphate

Toward understanding the cellular control of vertebrate mineralization: The potential role of mitochondria

This review examines the possible role of mitochondria in maintaining calcium and phosphate ion homeostasis and participating in the mineralization of bone, cartilage and other vertebrate hard tissues. The paper builds on the known structural features of mitochondria and the documented observations in these tissues that the organelles contain calcium phosphate granules. Such deposits in mitochondria putatively form to buffer excessively high cytosolic calcium ion concentrations and prevent metabolic deficits and even cell death. While mitochondria protect cytosolic enzyme systems through this buffering capacity, the accumulation of calcium ions by mitochondria promotes the activity of enzymes of the tricarboxylic acid (TCA/Krebs) cycle, increases oxidative phosphorylation and ATP synthesis, and leads to changes in intramitochondrial pH. These pH alterations influence ion solubility and possibly the transitions and composition in the mineral phase structure of the granules. Based on these considerations, mitochondria are proposed to support the mineralization process by providing a mobile store of calcium and phosphate ions, in smaller cluster or larger granule form, while maintaining critical cellular activities. The rise in the mitochondrial calcium level also increases the generation of citrate and other TCA cycle intermediates that contribute to cell function and the development of extracellular mineral. This paper suggests that another key role of the mitochondrion, along with the effects just noted, is to supply phosphate ions, derived from the breakdown of ATP, to endolysosomes and autophagic vesicles originating in the endoplasmic reticulum and Golgi and at the plasma membrane. These many separate but interdependent mitochondrial functions emphasize the critical importance of this organelle in the cellular control of vertebrate mineralization.

The molecular sociology of NHERF1 PDZ proteins controlling renal hormone-regulated phosphate transport

Parathyroid hormone (PTH) and fibroblast growth factor-23 (FGF23) control extracellular phosphate levels by regulating renal NPT2A-mediated phosphate transport by a process requiring the PDZ scaffold protein NHERF1. NHERF1 possesses two PDZ domains, PDZ1 and PDZ2, with identical core-binding GYGF motifs explicitly recognizing distinct binding partners that play different and specific roles in hormone-regulated phosphate transport. The interaction of PDZ1 and the carboxy-terminal PDZ-binding motif of NPT2A (C-TRL) is required for basal phosphate transport. PDZ2 is a regulatory domain that scaffolds multiple biological targets, including kinases and phosphatases involved in FGF23 and PTH signaling. FGF23 and PTH trigger disassembly of the NHERF1-NPT2A complex through reversible hormone-stimulated phosphorylation with ensuing NPT2A sequestration, down-regulation, and cessation of phosphate absorption. In the absence of NHERF1-NPT2A interaction, inhibition of FGF23 or PTH signaling results in disordered phosphate homeostasis and phosphate wasting. Additional studies are crucial to elucidate how NHERF1 spatiotemporally coordinates cellular partners to regulate extracellular phosphate levels.

The Physiological and Pathological Role of Tissue Nonspecific Alkaline Phosphatase beyond Mineralization

Tissue-nonspecific alkaline phosphatase (TNAP) is a key enzyme responsible for skeletal tissue mineralization. It is involved in the dephosphorylation of various physiological substrates, and has vital physiological functions, including extra-skeletal functions, such as neuronal development, detoxification of lipopolysaccharide (LPS), an anti-inflammatory role, bile pH regulation, and the maintenance of the blood brain barrier (BBB). TNAP is also implicated in ectopic pathological calcification of soft tissues, especially the vasculature. Although it is the crucial enzyme in mineralization of skeletal and dental tissues, it is a logical clinical target to attenuate vascular calcification. Various tools and studies have been developed to inhibit its activity to arrest soft tissue mineralization. However, we should not neglect its other physiological functions prior to therapies targeting TNAP. Therefore, a better understanding into the mechanisms mediated by TNAP is needed for minimizing off targeted effects and aid in the betterment of various pathological scenarios. In this review, we have discussed the mechanism of mineralization and functions of TNAP beyond its primary role of hard tissue mineralization.

The Complexities of Organ Crosstalk in Phosphate Homeostasis: Time to Put Phosphate Sensing Back in the Limelight

Phosphate homeostasis is essential for health and is achieved via interaction between the bone, kidney, small intestine, and parathyroid glands and via intricate processes involving phosphate transporters, phosphate sensors, and circulating hormones. Numerous genetic and acquired disorders are associated with disruption in these processes and can lead to significant morbidity and mortality. The role of the kidney in phosphate homeostasis is well known, although it is recognized that the cellular mechanisms in murine models and humans are different. Intestinal phosphate transport also appears to differ in humans and rodents, with recent studies demonstrating a dominant role for the paracellular pathway. The existence of phosphate sensing has been acknowledged for decades; however, the underlying molecular mechanisms are poorly understood. At least three phosphate sensors have emerged. PiT2 and FGFR1c both act as phosphate sensors controlling Fibroblast Growth Factor 23 secretion in bone, whereas the calcium-sensing receptor controls parathyroid hormone secretion in response to extracellular phosphate. All three of the proposed sensors are expressed in the kidney and intestine but their exact function in these organs is unknown. Understanding organ interactions and the mechanisms involved in phosphate sensing requires significant research to develop novel approaches for the treatment of phosphate homeostasis disorders.

Modulating phosphate consumption, a novel therapeutic approach for the control of cancer cell proliferation and tumorigenesis

Phosphorus, often in the form of inorganic phosphate (Pi), is critical to cellular function on many levels; it is required as an integral component of kinase signaling, in the formation and function of DNA and lipids, and energy metabolism in the form of ATP. Accordingly, crucial aspects of cell mitosis - such as DNA synthesis and ATP energy generation - elevate the cellular requirement for Pi, with rapidly dividing cells consuming increased levels. Mechanisms to sense, respond, acquire, accumulate, and potentially seek Pi have evolved to support highly proliferative cellular states such as injury and malignant transformation. As such, manipulating Pi availability to target rapidly dividing cells presents a novel strategy to reduce or prevent unrestrained cell growth. Currently, limited knowledge exists regarding how modulating Pi consumption by pre-cancerous cells might influence the initiation of aberrant growth during malignant transformation, and if reducing the bioavailability or suppressing Pi consumption by malignant cells could alter tumorigenesis. The concept of targeting Pi-regulated pathways and/or consumption by pre-cancerous or tumor cells represents a novel approach to cancer prevention and control, although current data remains insufficient as to rigorously assess the therapeutic value and physiological relevance of this strategy. With this review, we present a critical evaluation of the paradox of how an element critical to essential cellular functions can, when available in excess, influence and promote a cancer phenotype. Further, we conjecture how Pi manipulation could be utilized as a therapeutic intervention, either systemically or at the cell level, to ultimately suppress or treat cancer initiation and/or progression.

Reactive oxygen species (ROS) generation as an underlying mechanism of inorganic phosphate (Pi)-induced mineralization of osteogenic cells

Reactive Oxygen Species (ROS) are a natural byproduct of oxygen metabolism. At physiological levels, ROS regulate multiple cellular processes like proliferation, migration, and differentiation. Increased levels of ROS are associated with pathological conditions, such as inflammation and vascular calcification, where they elicit cytotoxic effects. These contrasting outcomes of ROS have also been reported in osteogenic precursor cells. However, the role of ROS in committed osteogenic cells has not been investigated. Cytotoxic and physiologic effects have also been demonstrated for extracellular phosphate (Pi). Specifically, in committed osteogenic cells Pi stimulates their major function (mineralization), however in osteogenic precursors and endothelial cells Pi cytotoxicity has been reported. Interestingly, Pi cytotoxic effects have been associated with ROS production in the pathological vascular mineralization. In this study, we investigated a molecular mechanistic link between elevated Pi and ROS production in the context of the mineralization function of committed osteogenic cells. Using committed osteogenic cells, 17IIA11 odontoblast-like cell and MLO-A5 osteoblast cell lines, we have unveil that Pi enhances intracellular ROS production. Furthermore, using a combination of mineralization assays and gene expression analyses, we determined that Pi-induced intracellular ROS supports the physiological mineralization process. In contrast, the exogenous ROS, provided in a form of H2O2, was detrimental for osteogenic cells. By comparing molecular signaling cascades induced by extracellular ROS and Pi, we identified differences in signaling routes that determine physiologic versus toxic effect of ROS on osteogenic cells. Specifically, while both extracellular and Pi-induced intracellular ROS utilize Erk1/2 signaling mediator, only extracellular ROS induces stress-activated mitogen-activated protein kinases P38 and JNK that are associated with cell death. In summary, our results uncovered a physiological role of ROS in the Pi-induced mineralization through the molecular pathway that is distinct from ROS-induced cytotoxic effects.

Effect of DNA/protamine complex paste on bone augmentation of the mandible: A pilot study on dogs

OBJECTIVE

Our previous studies found that a salmon DNA-based scaffold containing protamine promoted bone regeneration of the calvarial defects of rats. The aim of the present pilot study was to examine the influence of the DNA/protamine (DP) complex on bone regeneration of a saddle type, alveolar ridge defects of the dog mandible.

DESIGN

Alveolar ridge defects were performed in the mandibles of five adult female beagles. The following three treatment modalities were randomly allocated: (1) the DP complex paste, (2) a beta-tricalcium phosphate (β-TCP), and (3) a blank (control). Healing of bone defects were evaluated by periapical radiography, micro-computed tomography (micro-CT), and histology.

RESULTS

Periodical radiographic images revealed that a higher percentage of regenerated bone height was consistently achieved in the DP group, as compared with blank controls. All three-dimensional, sagittal, and coronal images of micro-CT showed increased amounts of newly formed bone and a greater bone volume/ tissue volume ratio, as compared with the blank and β-TCP groups. In contrast, there was no significant difference in bone mineral density among the groups. Histological analysis confirmed that the alveolar bone defects were filled with newly formed bone with mature and compact properties in the DP group.

CONCLUSIONS

These findings indicate that the DP complexes enhanced regeneration of vertical alveolar bone defects of the dog mandible.

Expression and function of Slc34 sodium-phosphate co-transporters in skeleton and teeth

Under normal physiological condition, the biomineralization process is limited to skeletal tissues and teeth and occurs throughout the individual's life. Biomineralization is an actively regulated process involving the progressive mineralization of the extracellular matrix secreted by osteoblasts in bone or odontoblasts and ameloblasts in tooth. Although the detailed molecular mechanisms underlying the formation of calcium-phosphate apatite crystals are still debated, it is suggested that calcium and phosphate may need to be transported across the membrane of the mineralizing cell, suggesting a pivotal role of phosphate transporters in bone and tooth mineralization. In this context, this short review describes the current knowledge on the role of Slc34 Na⁺-phosphate transporters in skeletal and tooth mineralization.

Role of αKlotho and FGF23 in regulation of type II Na-dependent phosphate co-transporters

Alpha-Klotho is a member of the Klotho family consisting of two other single-pass transmembrane proteins: βKlotho and γKlotho; αKlotho has been shown to circulate in the blood. Fibroblast growth factor (FGF)23 is a member of the FGF superfamily of 22 genes/proteins. αKlotho serves as a co-receptor with FGF receptors (FGFRs) to provide a receptacle for physiological FGF23 signaling including regulation of phosphate metabolism. The extracellular domain of transmembrane αKlotho is shed by secretases and released into blood circulation (soluble αKlotho). Soluble αKlotho has both FGF23-independent and FGF23-dependent roles in phosphate homeostasis by modulating intestinal phosphate absorption, urinary phosphate excretion, and phosphate distribution into bone in concerted interaction with other calciophosphotropic hormones such as PTH and 1,25-(OH)₂D. The direct role of αKlotho and FGF23 in the maintenance of phosphate homeostasis is partly mediated by modulation of type II Na⁺-dependent phosphate co-transporters in target organs. αKlotho and FGF23 are principal phosphotropic hormones, and the manipulation of the αKlotho-FGF23 axis is a novel therapeutic strategy for genetic and acquired phosphate disorders and for conditions with FGF23 excess and αKlotho deficiency such as chronic kidney disease.

Role of phosphate sensing in bone and mineral metabolism

Inorganic phosphate (P_i) is essential for signal transduction and cell metabolism, and is also an essential structural component of the extracellular matrix of the skeleton. P_i is sensed in bacteria and yeast at the plasma membrane, which activates intracellular signal transduction to control the expression of P_i transporters and other genes that control intracellular P_i levels. In multicellular organisms, P_i homeostasis must be maintained in the organism and at the cellular level, requiring an endocrine and metabolic P_i-sensing mechanism, about which little is currently known. This Review will discuss the metabolic effects of P_i, which are mediated by P_i transporters, inositol pyrophosphates and SYG1-Pho81-XPR1 (SPX)-domain proteins to maintain cellular phosphate homeostasis in the musculoskeletal system. In addition, we will discuss how P_i is sensed by the human body to regulate the production of fibroblast growth factor 23 (FGF23), parathyroid hormone and calcitriol to maintain serum levels of P_i in a narrow range. New findings on the crosstalk between iron and P_i homeostasis in the regulation of FGF23 expression will also be outlined. Mutations in components of these metabolic and endocrine phosphate sensors result in genetic disorders of phosphate homeostasis, cardiomyopathy and familial basal ganglial calcifications, highlighting the importance of this newly emerging area of research.

Salmon DNA Accelerates Bone Regeneration by Inducing Osteoblast Migration

The initial step of bone regeneration requires the migration of osteogenic cells to defective sites. Our previous studies suggest that a salmon DNA-based scaffold can promote the bone regeneration of calvarial defects in rats. We speculate that the salmon DNA may possess osteoinductive properties, including the homing of migrating osteogenic cells. In the present study, we investigated the influence of the salmon DNA on osteoblastic differentiation and induction of osteoblast migration using MG63 cells (human preosteoblasts) in vitro. Moreover, we analyzed the bone regeneration of a critical-sized in vivo calvarial bone defect (CSD) model in rats. The salmon DNA enhanced both mRNA and protein expression of the osteogenesis-related factors, runt-related transcription factor 2 (Runx2), alkaline phosphatase, and osterix (OSX) in the MG63 cells, compared with the cultivation using osteogenic induction medium alone. From the histochemical and immunohistochemical assays using frozen sections of the bone defects from animals that were implanted with DNA disks, many cells were found to express aldehyde dehydrogenase 1, one of the markers for mesenchymal stem cells. In addition, OSX was observed in the replaced connective tissue of the bone defects. These findings indicate that the DNA induced the migration and accumulation of osteogenic cells to the regenerative tissue. Furthermore, an in vitro transwell migration assay showed that the addition of DNA enhanced an induction of osteoblast migration, compared with the medium alone. The implantation of the DNA disks promoted bone regeneration in the CSD of rats, compared with that of collagen disks. These results indicate that the salmon DNA enhanced osteoblastic differentiation and induction of migration, resulting in the facilitation of bone regeneration.

Klotho/fibroblast growth factor 23- and PTH-independent estrogen receptor-α-mediated direct downregulation of NaPi-IIa by estrogen in the mouse kidney

Estrogen treatment causes renal phosphate (Pi) wasting and hypophosphatemia in rats and humans; however, the signaling mechanisms mediating this effect are still poorly understood. To determine the specific roles of estrogen receptor isoforms (ERα and ERβ) and the Klotho pathway in mediating these effects, we studied the effects of estrogen on renal Pi handling in female mice with null mutations of ERα or ERβ or Klotho and their wild type (WT) using balance studies in metabolic cages. Estrogen treatment of WT and ERβ knockout (KO) mice caused a significant reduction in food intake along with increased renal phosphate wasting. The latter resulted from a significant downregulation of NaPi-IIa and NaPi-IIc protein abundance. The mRNA expression levels of both transporters were unchanged in estrogen-treated mice. These effects on both food intake and renal Pi handling were absent in ERα KO mice. Estrogen treatment of Klotho KO mice or parathyroid hormone (PTH)-depleted thyroparathyroidectomized mice exhibited a significant downregulation of NaPi-IIa with no change in the abundance of NaPi-IIc. Estrogen treatment of a cell line (U20S) stably coexpressing both ERα and ERβ caused a significant downregulation of NaPi-IIa protein when transiently transfected with a plasmid containing full-length or open-reading frame (ORF) 3'-untranslated region (UTR) but not 5'-UTR ORF of mouse NaPi-IIa transcript. In conclusion, estrogen causes phosphaturia and hypophosphatemia in mice. These effects result from downregulation of NaPi-IIa and NaPi-IIc proteins in the proximal tubule through the activation of ERα. The downregulation of NaPi-IIa by estrogen involves 3'-UTR of its mRNA and is independent of Klotho/fibroblast growth factor 23 and PTH signaling pathways.

(99m)TC-Methylene diphosphonate uptake at injury site correlates with osteoblast differentiation and mineralization during bone healing in mice

^99mTc-Methylene diphosphonate (^99mTc-MDP) is widely used in clinical settings to detect bone abnormalities. However, the mechanism of ^99mTc-MDP uptake in bone is not well elucidated. In this study, we utilized a mouse tibia injury model, single-photon emission computed tomography (gamma scintigraphy or SPECT), ex vivo micro-computed tomography, and histology to monitor ^99mTc-MDP uptake in injury sites during skeletal healing. In an ex vivo culture system, calvarial cells were differentiated into osteoblasts with osteogenic medium, pulsed with ^99mTc-MDP at different time points, and quantitated for ^99mTc-MDP uptake with a gamma counter. We demonstrated that ^99mTc-MDP uptake in the injury sites corresponded to osteoblast generation in those sites throughout the healing process. The ^99mTc-MDP uptake within the injury sites peaked on day 7 post-injury, while the injury sites were occupied by mature osteoblasts also starting from day 7. ^99mTc-MDP uptake started to decrease 14 days post-surgery, when we observed the highest level of bony tissue in the injury sites. We also found that ^99mTc-MDP uptake was associated with osteoblast maturation and mineralization in vitro. This study provides direct and biological evidence for ^99mTc-MDP uptake in osteoblasts during bone healing in vivo and in vitro.

Estrogen directly and specifically downregulates NaPi-IIa through the activation of both estrogen receptor isoforms (ERα and ERβ) in rat kidney proximal tubule

We have previously demonstrated that estrogen (E2) downregulates phosphate transporter NaPi-IIa and causes phosphaturia and hypophosphatemia in ovariectomized rats. In the present study, we examined whether E2 directly targets NaPi-IIa in the proximal tubule (PT) and studied the respective roles of estrogen receptor isoforms (ERα and ERβ) in the downregulation of NaPi-IIa using both in vivo and an in vitro expression systems. We found that estrogen specifically downregulates NaPi-IIa but not NaPi-IIc or Pit2 in the kidney cortex. Proximal tubules incubated in a "shake" suspension with E2 for 24 h exhibited a dose-dependent decrease in NaPi-IIa protein abundance. Results from OVX rats treated with specific agonists for either ERα [4,4',4″;-(4-propyl-[1H]-pyrazole-1,3,5-triyl) trisphenol, PPT] or ERβ [4,4',4″-(4-propyl-[1H]-pyrazole-1,3,5-triyl) trisphenol, DPN] or both (PPT + DPN), indicated that only the latter caused a sharp downregulation of NaPi-IIa, along with significant phosphaturia and hypophosphatemia. Lastly, heterologous expression studies demonstrated that estrogen downregulated NaPi-IIa only in U20S cells expressing both ERα and ERβ, but not in cells expressing either receptor alone. In conclusion, these studies demonstrate that rat PT cells express both ERα and ERβ and that E2 induces phosphaturia by directly and specifically targeting NaPi-IIa in the PT cells. This effect is mediated via a mechanism involving coactivation of both ERα and ERβ, which likely form a functional heterodimer complex in the rat kidney proximal tubule.

Alpha Klotho and phosphate homeostasis

The Klotho family consists of three single-pass transmembrane proteins—αKlotho, βKlotho and γKlotho. Each of them combines with fibroblast growth factor (FGF) receptors (FGFRs) to form receptor complexes for various FGF’s. αKlotho is a co-receptor for physiological FGF23 signaling and appears essential for FGF23-mediated regulation of mineral metabolism. αKlotho protein also plays a FGF23-independent role in phosphate homeostasis. Animal experimental studies and clinical observations have demonstrated that αKlotho deficiency leads to severe hyperphosphatemia; moderate elevation of αKlotho reduces serum phosphate and extremely high αKlotho induces hypophosphatemia and high-FGF23. αKlotho maintains circulating phosphate in a narrow range by modulating intestinal phosphate absorption, urinary phosphate excretion by the kidney, and phosphate distribution into bone rather than soft tissue in concerted interaction with other calciophosphotropic hormones such as PTH, FGF23, and 1,25-(OH)₂ vitamin D. The role of αKlotho in maintenance of phosphate homeostasis is mediated by direct suppression of Na-dependent phosphate cotransporters in target organs. Therefore, αKlotho manipulation may be a novel strategy for genetic and acquired phosphate disorders and for medical conditions with αKlotho deficiency such as chronic kidney disease in future.

Phosphate enhances reactive oxygen species production and suppresses osteoblastic differentiation

Phosphate has been shown to work as a signaling molecule in several cells including endothelial cells and chondrocytes. However, it is largely unknown how phosphate affects osteoblastic cells. In the present study, we investigated the effects of phosphate on reactive oxygen species (ROS) production and osteoblastic differentiation in murine osteoblastic MC3T3-E1 cells. Phosphate increased production of ROS in MC3T3-E1 cells and the inhibitors of sodium-phosphate cotransporter and NADPH oxidase suppressed ROS production by phosphate. Silencing Nox1 and Nox4 also inhibited the increase of ROS by phosphate. Phosphate also decreased alkaline phosphatase activity induced by bone morphogenetic protein 2 and this inhibition was abrogated by an inhibitor of NADPH oxidase. Furthermore, phosphate decreased the expression of osteoblastic marker genes in MC3T3-E1 cells. These results indicate that phosphate suppresses osteoblastic differentiation at least in part by enhancing ROS production in MC3T3-E1 cells.

An integrated understanding of the physiological response to elevated extracellular phosphate

Recent studies have suggested that changes in serum phosphate levels influence pathological states associated with aging such as cancer, bone metabolism, and cardiovascular function, even in individuals with normal renal function. The causes are only beginning to be elucidated but are likely a combination of endocrine, paracrine, autocrine, and cell autonomous effects. We have used an integrated quantitative biology approach, combining transcriptomics and proteomics to define a multi-phase, extracellular phosphate-induced, signaling network in pre-osteoblasts as well as primary human and mouse mesenchymal stromal cells. We identified a rapid mitogenic response stimulated by elevated phosphate that results in the induction of immediate early genes including c-fos. The mechanism of activation requires FGF receptor signaling followed by stimulation of N-Ras and activation of AP-1 and serum response elements. A distinct long-term response also requires FGF receptor signaling and results in N-Ras activation and expression of genes and secretion of proteins involved in matrix regulation, calcification, and angiogenesis. The late response is synergistically enhanced by addition of FGF23 peptide. The intermediate phase results in increased oxidative phosphorylation and ATP production and is necessary for the late response providing a functional link between the phases. Collectively, the results define elevated phosphate, as a mitogen and define specific mechanisms by which phosphate stimulates proliferation and matrix regulation. Our approach provides a comprehensive understanding of the cellular response to elevated extracellular phosphate, functionally connecting temporally coordinated signaling, transcriptional, and metabolic events with changes in long-term cell behavior.

The kidney sodium-phosphate co-transporter alters bone quality in an age and gender specific manner

Mutations in the kidney NaPiIIa co-transporter are clinically associated with hypophosphatemia, hyperphosphaturia (phosphate wasting), hypercalcemia, nephrolithiasis and bone demineralization. The mouse lacking this co-transporter system was reported to recover its skeletal defects with age, but the "quality" of the bones was not considered. To assess changes in bone quality we examined both male and female NaPiIIa knockout (KO) mice at 1 and 7months of age using micro-computed tomography (micro-CT) and Fourier transform infrared imaging (FTIRI). KO cancellous bones at both ages had greater bone volume fraction, trabecular thickness and lesser structure model index based on micro-CT values relative to age- and sex-matched wildtype animals. There was a sexual-dimorphism in the micro-CT parameters, with differences at 7months seen principally in males. Cortical bone at 1month showed an increase in bone volume fraction, but this was not seen at 7months. Cortical thickness which was elevated in the male and female KO at 1month was lower in the male KO at 7months. FTIRI showed a reduced mineral and acid phosphate content in the male and female KO's bones at 1month with no change in acid phosphate content at 7months. Collagen maturity was reduced in KO cancellous bone at 1month. The observed sexual dimorphism in the micro-CT data may be related to altered phosphate homeostasis, differences in animal growth rates and other factors. These data indicate that the bone quality of the KO mice at both ages differs from the normal and suggests that these bone quality differences may contribute to skeletal phenotype in humans with mutations in this co-transporter.

NHERF1 regulation of PTH-dependent bimodal Pi transport in osteoblasts

Control of systemic inorganic phosphate (Pi) levels is crucial for osteoid mineralization. Parathyroid hormone (PTH) mediates actions on phosphate homeostasis mostly by regulating the activity of the type 2 sodium-phosphate cotransporter (Npt2), and this action requires the PDZ protein NHERF1. Osteoblasts express Npt2 and in response to PTH enhance osteogenesis by increasing mineralized matrix. The regulation of Pi transport in osteoblasts is poorly understood. To address this gap we characterized PTH-dependent Pi transport and the role of NHERF1 in primary mouse calvarial osteoblasts. Under proliferating conditions osteoblasts express Npt2a, Npt2b, PTH receptor, and NHERF1. Npt2a mRNA expression was lower in calvarial osteoblasts from NHERF1-null mice. Under basal conditions Pi uptake in osteoblasts from wild-type mice was greater than that of knockout mice. PTH inhibited Pi uptake in proliferating osteoblasts from wild-type mice, but not in cells from knockout mice. In vitro induction of mineralization enhanced osteoblast differentiation and increased osterix and osteocalcin expression. Contrary to the results with proliferating osteoblasts, PTH increased Pi uptake and ATP secretion in differentiated osteoblasts from wild-type mice. PTH had no effect on Pi uptake or ATP release in differentiated osteoblasts from knockout mice. NHERF1 regulation of PTH-sensitive Pi uptake in proliferating osteoblasts is mediated by cAMP/PKA and PLC/PKC, while modulation of Pi uptake in differentiated osteoblasts depends only on cAMP/PKA signaling. The results suggest that NHERF1 cooperates with PTH in differentiated osteoblasts to increase matrix mineralization. We conclude that NHERF1 regulates PTH that differentially affects Na-dependent Pi transport at distinct stages of osteoblast proliferation and maturation.

Clinical consequences of mutations in sodium phosphate cotransporters

Three families of sodium phosphate cotransporters have been described. Their specific roles in human health and disease have not been defined. Review of the literature reveals that the type II sodium phosphate cotransporters play a significant role in transepithelial transport in a number of tissues including kidney, intestine, salivary gland, mammary gland, and lung. The type I transporters seem to play a major role in renal urate handling and mutations in these proteins have been implicated in susceptibility to gout. The ubiquitously expressed type III transporters play a lesser role in phosphate homeostasis but contribute to cellular phosphate uptake, mineralization, and inflammation. The recognition of species differences in the expression, regulation, and function of these transport proteins suggests an urgent need to find ways to study them in humans.

Phosphate transport kinetics and structure-function relationships of SLC34 and SLC20 proteins

Transport of inorganic phosphate (P(i)) is mediated by proteins belonging to two solute carrier families (SLC20 and SLC34). Members of both families transport P(i) using the electrochemical gradient for Na(+). The role of the SLC34 members as essential players in mammalian P(i) homeostasis is well established, whereas that of SLC20 proteins is less well defined. The SLC34 family comprises the following three isoforms that preferentially cotransport divalent P(i) and are expressed in epithelial tissue: the renal NaPi-IIa and NaPi-IIc are responsible for reabsorbing P(i) in the proximal tubule, whereas NaPi-IIb is more ubiquitously expressed, including the small intestine, where it mediates dietary P(i) absorption. The SLC20 family comprises two members (PiT-1, PiT-2) that preferentially cotransport monovalent P(i) and are expressed in epithelial as well as nonepithelial tissue. The transport kinetics of members of both families have been characterized in detail using heterologous expression in Xenopus oocytes. For the electrogenic NaPi-IIa/b, and PiT-1,-2, conventional electrophysiological techniques together with radiotracer methods have been applied, as well as time-resolved fluorometric measurements that allow new insights into local conformational changes of the protein during the cotransport cycle. For the electroneutral NaPi-IIc, conventional tracer uptake and fluorometry have been used to elucidate its transport properties. The 3-D structures of these proteins remain unresolved and structure-function studies have so far concentrated on defining the topology and identifying sites of functional importance.

Changes in matrix protein gene expression associated with mineralization in the differentiating chick limb-bud micromass culture system

Chick limb-bud mesenchymal stem cells plated in high density culture in the presence of 4 mM inorganic phosphate and vitamin C differentiate and form a mineralizable matrix, resembling that of the chick growth plate. To further elucidate the mechanism that allows these cultures to form physiologic hydroxyapatite deposits, and how the process can be manipulated to gain insight into mineralization mechanisms, we compared gene expression in mineralizing (with 4 mM inorganic phosphate) and non-mineralizing cultures (containing only 1 mM inorganic phosphate) at the start of mineralization (day 11) and after mineralization reached a plateau (day 17) using a chick specific microarray. Based on replicate microarray experiments and K-cluster analysis, several genes associated with the mineralization process were identified, and their expression patterns confirmed throughout the culture period by quantitative RT-PCR. The functions of bone morphogenetic protein 1, BMP1, dentin matrix protein 1, DMP1, the sodium phosphate co-transporter, NaPi IIb, matrix metalloprotease 13. MMP-13, and alkaline phosphatase, along with matrix protein genes (type X collagen, bone sialoprotein, and osteopontin) usually associated with initiation of mineralization are discussed.

Phosphate transporters in renal, gastrointestinal, and other tissues

Inorganic phosphate (Pi) is essential for all living organisms. Bound to organic molecules, Pi fulfills structural, metabolic, and signaling tasks. Therefore, cell growth and maintenance depends on efficient transport of Pi across cellular membranes into the intracellular space. Uptake of Pi requires energy because the substrate is transported against its electrochemical gradient. Till recently, 2 major families of physiologically relevant Pi-specific transporters have been identified: the solute carrier families Slc34 and Slc20. Interestingly, phylogenetic links can be detected between prokaryotic and eukaryotic transporters in both families. Because less complex model organisms are often instrumental in establishing paradigms for protein function in human beings, a brief assessment of Slc34 and Slc20 phylogeny is of interest.

Klotho deficiency causes vascular calcification in chronic kidney disease

Soft-tissue calcification is a prominent feature in both chronic kidney disease (CKD) and experimental Klotho deficiency, but whether Klotho deficiency is responsible for the calcification in CKD is unknown. Here, wild-type mice with CKD had very low renal, plasma, and urinary levels of Klotho. In humans, we observed a graded reduction in urinary Klotho starting at an early stage of CKD and progressing with loss of renal function. Despite induction of CKD, transgenic mice that overexpressed Klotho had preserved levels of Klotho, enhanced phosphaturia, better renal function, and much less calcification compared with wild-type mice with CKD. Conversely, Klotho-haploinsufficient mice with CKD had undetectable levels of Klotho, worse renal function, and severe calcification. The beneficial effect of Klotho on vascular calcification was a result of more than its effect on renal function and phosphatemia, suggesting a direct effect of Klotho on the vasculature. In vitro, Klotho suppressed Na(+)-dependent uptake of phosphate and mineralization induced by high phosphate and preserved differentiation in vascular smooth muscle cells. In summary, Klotho is an early biomarker for CKD, and Klotho deficiency contributes to soft-tissue calcification in CKD. Klotho ameliorates vascular calcification by enhancing phosphaturia, preserving glomerular filtration, and directly inhibiting phosphate uptake by vascular smooth muscle. Replacement of Klotho may have therapeutic potential for CKD.

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

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

Intestinal npt2b plays a major role in phosphate absorption and homeostasis

Intestinal phosphate absorption occurs through both a paracellular mechanism involving tight junctions and an active transcellular mechanism involving the type II sodium-dependent phosphate cotransporter NPT2b (SLC34a2). To define the contribution of NPT2b to total intestinal phosphate absorption, we generated an inducible conditional knockout mouse, Npt2b(-/-) (Npt2b(fl/fl):Cre(+/-)). Npt2b(-/-) animals had increased fecal phosphate excretion and hypophosphaturia, but serum phosphate remained unchanged. Decreased urinary phosphate excretion correlated with reduced serum levels of the phosphaturic hormone FGF23 and increased protein expression of the renal phosphate transporter Npt2a. These results demonstrate that the absence of Npt2b triggers compensatory renal mechanisms to maintain phosphate homeostasis. In animals fed a low phosphate diet followed by acute administration of a phosphate bolus, Npt2b(-/-) animals absorbed approximately 50% less phosphate than wild-type animals, confirming a major role of this transporter in phosphate regulation. In vitro analysis of active phosphate transport in ileum segments isolated from wild-type or Npt2b(-/-) mice demonstrated that Npt2b contributes to >90% of total active phosphate absorption. In summary, Npt2b is largely responsible for intestinal phosphate absorption and contributes to the maintenance of systemic phosphate homeostasis.

Development of monoclonal antibodies specific for the human sodium-dependent phosphate co-transporter NaPi2b

Homeostasis of inorganic phosphate in the human body is maintained by regulated absorption, metabolism, and excretion. Sodium-dependent phosphate transporters (NaPi) mediate the transport of inorganic phosphate (P(i)) in cells in response to dietary phosphate consumption, hormones, and growth factors. NaPi2b is a member of the sodium-dependent phosphate transporter family, with a distinct pattern of expression and regulation. Signaling pathways activated by mitogens, glucocorticoids, and metabolic factors have been implicated in regulating P(i) transport via NaPi2b. Inactivation of NaPi2b function by mutations has been linked to human pathologies, such as pulmonary alveolar microlithiasis. In this study, we describe the generation and characterization of monoclonal antibodies against human NaPi2b. The monoclonal antibodies were shown to recognize specifically transiently overexpressed and endogenous NaPi2b in commonly used immunoassays, including Western blotting, immunoprecipitation, and immunohistochemistry. These properties make them particularly valuable reagents for elucidating NaPi2b function in health and disease.

Genetic evidence of serum phosphate-independent functions of FGF-23 on bone

Maintenance of physiologic phosphate balance is of crucial biological importance, as it is fundamental to cellular function, energy metabolism, and skeletal mineralization. Fibroblast growth factor-23 (FGF-23) is a master regulator of phosphate homeostasis, but the molecular mechanism of such regulation is not yet completely understood. Targeted disruption of the Fgf-23 gene in mice (Fgf-23^−/−) elicits hyperphosphatemia, and an increase in renal sodium/phosphate co-transporter 2a (NaPi2a) protein abundance. To elucidate the pathophysiological role of augmented renal proximal tubular expression of NaPi2a in Fgf-23^−/− mice and to examine serum phosphate–independent functions of Fgf23 in bone, we generated a new mouse line deficient in both Fgf-23 and NaPi2a genes, and determined the effect of genomic ablation of NaPi2a from Fgf-23^−/− mice on phosphate homeostasis and skeletal mineralization. Fgf-23^−/−/NaPi2a^−/− double mutant mice are viable and exhibit normal physical activities when compared to Fgf-23^−/− animals. Biochemical analyses show that ablation of NaPi2a from Fgf-23^−/− mice reversed hyperphosphatemia to hypophosphatemia by 6 weeks of age. Surprisingly, despite the complete reversal of serum phosphate levels in Fgf-23^−/−/NaPi2a^−/−, their skeletal phenotype still resembles the one of Fgf23^−/− animals. The results of this study provide the first genetic evidence of an in vivo pathologic role of NaPi2a in regulating abnormal phosphate homeostasis in Fgf-23^−/− mice by deletion of both NaPi2a and Fgf-23 genes in the same animal. The persistence of the skeletal anomalies in double mutants suggests that Fgf-23 affects bone mineralization independently of systemic phosphate homeostasis. Finally, our data support (1) that regulation of phosphate homeostasis is a systemic effect of Fgf-23, while (2) skeletal mineralization and chondrocyte differentiation appear to be effects of Fgf-23 that are independent of phosphate homeostasis.

Regulation of phosphate homeostasis is a tightly controlled hormonal process involving the intestine, kidneys, and bone, and imbalance of this homeostasis may influence overall mineralization. Fibroblast growth factor-23 (FGF-23) is a circulating hormone produced in the bone that mainly targets the kidneys to control the activity of the sodium/phosphate co-transporters NaPi2a and NaPi2c. These transporters are responsible for actively reabsorbing phosphate ions into the body to maintain physiological serum phosphate levels. Changes in FGF-23 activity lead to human disorders associated with either phosphate wasting or retention. Genetically altered mice in which Fgf-23 activity is lost exhibit severe hyperphosphatemia accompanied by increased NaPi2a activity, and they develop abnormal bone mineralization. Here we describe a new mouse model in which we eliminated NaPi2a from Fgf-23 null mice and show reversal of hyperphosphatemia to hypophosphatemia, suggesting that NaPi2a is the major regulator of phosphate homeostasis. However, the skeletal mineralization defect observed in mice lacking Fgf-23 function remained unchanged in the absence of NaPi2a in these mice. Thus our data indicate that Fgf-23 has a role in controlling bone mineralization independent of systemic phosphate levels.

Effects of the antiestrogen fulvestrant (ICI 182,780) on gene expression of the rat efferent ductules

The efferent ductules express the highest amount of estrogen receptors ESR1 (ERalpha) and ESR2 (ERbeta) within the male reproductive tract. Treatment of rats with the antiestrogen fulvestrant (ICI 182,780) causes inhibition of fluid reabsorption in the efferent ductules, leading to seminiferous tubule atrophy and infertility. To provide a more comprehensive knowledge about the molecular targets for estrogen in the rat efferent ductules, we investigated the effects of ICI 182,780 treatment on gene expression using a microarray approach. Treatment with ICI 182,780 increased or reduced at least 2-fold the expression of 263 and 98 genes, respectively. Not surprisingly, several genes that encode ion channels and macromolecule transporters were affected. Interestingly, treatment with ICI 182,780 markedly altered the expression of genes related to extracellular matrix organization. Matrix metalloproteinase 7 (Mmp7), osteopontin (Spp1), and neuronal pentraxin 1 (Nptx1) were among the most altered genes in this category. Upregulation of Mmp7 and Spp1 and downregulation of Nptx1 were validated by Northern blot. Increase in Mmp7 expression was further confirmed by immunohistochemistry and probably accounted for the decrease in collagen content observed in the efferent ductules of ICI 182,780-treated animals. Downregulation of Nptx1 probably contributed to the extracellular matrix changes and decreased amyloid deposition in the efferent ductules of ICI 182,780-treated animals. Identification of new molecular targets for estrogen action may help elucidate the regulatory role of this hormone in the male reproductive tract.

Phosphate transporters: a tale of two solute carrier families

Phosphate is an essential component of life and must be actively transported into cells against its electrochemical gradient. In vertebrates, two unrelated families of Na+ -dependent P(i) transporters carry out this task. Remarkably, the two families transport different P(i) species: whereas type II Na+/P(i) cotransporters (SCL34) prefer divalent HPO(4)(2-), type III Na(+)/P(i) cotransporters (SLC20) transport monovalent H2PO(4)(-). The SCL34 family comprises both electrogenic and electroneutral members that are expressed in various epithelia and other polarized cells. Through regulated activity in apical membranes of the gut and kidney, they maintain body P(i) homeostasis, and in salivary and mammary glands, liver, and testes they play a role in modulating the P(i) content of luminal fluids. The two SLC20 family members PiT-1 and PiT-2 are electrogenic and ubiquitously expressed and may serve a housekeeping role for cell P(i) homeostasis; however, also more specific roles are emerging for these transporters in, for example, bone mineralization. In this review, we focus on recent advances in the characterization of the transport kinetics, structure-function relationships, and physiological implications of having two distinct Na+/P(i) cotransporter families.

Deciphering PiT transport kinetics and substrate specificity using electrophysiology and flux measurements

Members of the SLC20 family or type III Na(+) -coupled P(i) cotransporters (PiT-1, PiT-2) are ubiquitously expressed in mammalian tissue and are thought to perform a housekeeping function for intracellular P(i) homeostasis. Previous studies have shown that PiT-1 and PiT-2 mediate electrogenic P(i) cotransport when expressed in Xenopus oocytes, but only limited kinetic characterizations were made. To address this shortcoming, we performed a detailed analysis of SLC20 transport function. Three SLC20 clones (Xenopus PiT-1, human PiT-1, and human PiT-2) were expressed in Xenopus oocytes. Each clone gave robust Na(+)-dependent (32)P(i) uptake, but only Xenopus PiT-1 showed sufficient activity for complete kinetic characterization by using two-electrode voltage clamp and radionuclide uptake. Transport activity was also documented with Li(+) substituted for Na(+). The dependence of the P(i)-induced current on P(i) concentration was Michaelian, and the dependence on Na(+) concentration indicated weak cooperativity. The dependence on external pH was unique: the apparent P(i) affinity constant showed a minimum in the pH range 6.2-6.8 of approximately 0.05 mM and increased to approximately 0.2 mM at pH 5.0 and pH 8.0. Xenopus PiT-1 stoichiometry was determined by dual (22)Na-(32)P(i) uptake and suggested a 2:1 Na(+):P(i) stoichiometry. A correlation of (32)P(i) uptake and net charge movement indicated one charge translocation per P(i). Changes in oocyte surface pH were consistent with transport of monovalent P(i). On the basis of the kinetics of substrate interdependence, we propose an ordered binding scheme of Na(+):H(2)PO(4)(-):Na(+). Significantly, in contrast to type II Na(+)-P(i) cotransporters, the transport inhibitor phosphonoformic acid did not inhibit PiT-1 or PiT-2 activity.