Cellular ATP synthesis mediated by type III sodium-dependent phosphate transporter Pit-1 is critical to chondrogenesis

CTGF regulates mineralization in human mature chondrocyte by controlling Pit-1 and modulating ANK via the BMP/Smad signalling

OBJECTIVE

Connective tissue growth factor (CTGF) exhibits potent proliferative, differentiated, and mineralizing effects, and is believed to be contribute to cartilage mineralization in Osteoarthritis (OA). However, the underlying mechanism of chondrocyte mineralization induced by CTGF remains obscure. As a key regulator of mineral responses, type III phosphate transporter 1 (Pit-1) has been associated with the pathogenesis of articular mineralization. Therefore, the primary objective of this study was to investigate whether CTGF influences the development of mature chondrocyte mineralization and the underlying mechanisms governing such mineralization.

METHODS

The effect of Connective tissue growth factor (CTGF) on human C-28/I2 chondrocytes were investigated. The chondrocytes were treated with CTGF or related inhibitors, and transfected with Overexpression and siRNA transfection of Type III Phosphate Transporter 1(Pit-1). Subsequently, the cells were subjected to Alizarin red S staining, PiPer Phosphate Assay Kit, Alkaline Phosphatase Diethanolamine Activity Kit, ELISA, RT-PCR or Western blot analysis.

RESULTS

Stimulation with Connective tissue growth factor (CTGF) significantly upregulated the expression of the Type III Phosphate Transporter 1(Pit-1) and mineralization levels in chondrocytes through activation of α5β1 integrin and BMP/Samd1/5/8 signaling pathways. Furthermore, treatment with overexpressed Pit-1 markedly increased the expression of Multipass Transmembrane Ankylosis (ANK) transporter in the cells. The inhibitory effect of CTGF receptor blockade using α5β1 Integrin blocking antibody was demonstrated by significantly suppressed the expression of Pit-1 and ANK transporter, as well as chondrocyte mineralization.

CONCLUSIONS

Our data indicate that Connective tissue growth factor (CTGF) plays a critical role inchondrocyte mineralization, which is dependent on the expression of the Type III Phosphate Transporter 1(Pit-1) and Multipass Transmembrane Ankylosis (ANK) transporter. Consequently, inhibition of CTGF activity may represent a novel therapeutic approach for the management of Osteoarthritis (OA).

Hnrnpk maintains chondrocytes survival and function during growth plate development via regulating Hif1α-glycolysis axis

The harmonious functioning of growth plate chondrocytes is crucial for skeletogenesis. These cells rely on an appropriate intensity of glycolysis to maintain survival and function in an avascular environment, but the underlying mechanism is poorly understood. Here we show that Hnrnpk orchestrates growth plate development by maintaining the appropriate intensity of glycolysis in chondrocytes. Ablating Hnrnpk causes the occurrence of dwarfism, exhibiting damaged survival and premature differentiation of growth plate chondrocytes. Furthermore, Hnrnpk deficiency results in enhanced transdifferentiation of hypertrophic chondrocytes and increased bone mass. In terms of mechanism, Hnrnpk binds to Hif1a mRNA and promotes its degradation. Deleting Hnrnpk upregulates the expression of Hif1α, leading to the increased expression of downstream glycolytic enzymes and then exorbitant glycolysis. Our study establishes an essential role of Hnrnpk in orchestrating the survival and differentiation of chondrocytes, regulating the Hif1α-glycolysis axis through a post-transcriptional mechanism during growth plate development.

Slc20a1b is essential for hematopoietic stem/progenitor cell expansion in zebrafish

Hematopoietic stem and progenitor cells (HSPCs) are able to self-renew and can give rise to all blood lineages throughout their lifetime, yet the mechanisms regulating HSPC development have yet to be discovered. In this study, we characterized a hematopoiesis defective zebrafish mutant line named smu07, which was obtained from our previous forward genetic screening, and found the HSPC expansion deficiency in the mutant. Positional cloning identified that slc20a1b, which encodes a sodium phosphate cotransporter, contributed to the smu07 blood phenotype. Further analysis demonstrated that mutation of slc20a1b affects HSPC expansion through cell cycle arrest at G2/M phases in a cell-autonomous manner. Our study shows that slc20a1b is a vital regulator for HSPC proliferation in zebrafish early hematopoiesis and provides valuable insights into HSPC development.

Phosphate Overload Stimulates Inflammatory Reaction via PiT-1 and Induces Vascular Calcification in Uremia

OBJECTIVE

Vascular calcification (VC) is an important risk factor for cardiovascular disease in maintenance hemodialysis (MHD) patients. Hyperphosphatemia and microinflammation statement are known major contributors to the development of VC; however, the mechanisms are unknown. The aims of this study were to explore the risk factors of VC in MHD patients and to explore whether high phosphate could increase the secretion of inflammatory cytokines via PiT-1 in monocytes.

METHODS

A cross-sectional study was conducted on 65 MHD patients to assess the relevance of coronary artery calcification (CAC), inflammatory factors, serum phosphate, and sodium-dependent phosphate cotransporter (NPT) mRNA expression of peripheral blood mononuclear cells (PBMCs). Multivariate logistic regression analysis was used to analyze the predictors of CAC. The calcification effects of high phosphate (HP), TNF-α, and supernatants of healthy human monocytes treated with HP were further evaluated in cultured HASMCs.

RESULTS

Diabetes, longer dialysis vintage, higher serum TNF-α levels, and PiT-1 mRNA expression of PBMCs) were independent risk factors of CAC in MHD patients. The mRNA levels of PiT-1 in PBMCs were positively correlated with serum phosphate, CAC scores, and Pit-2 mRNA levels of PBMCs. The expressions of TNF-α, IL-6, and PiT-1 in human monocytes were significantly increased in a dose-dependent manner after treatment with HP, which was subsequently inhibited by NPT antagonist phosphonoformic acid. Neither TNF-α alone nor supernatants of monocytes stimulated with HP promoted the expression of osteopontin and Runt-related transcription factor 2 (Runx2) or caused mineralization in human aortic smooth muscle cells, but combined with HP intervention, the calcification effects were markedly increased in human aortic smooth muscle cells and ameliorated by phosphonoformic acid treatment.

CONCLUSION

Hyperphosphatemia directly increased the synthesis and secretion of TNF-α by monocytes may via PiT-1 pathway, resulting in elevated systemic inflammatory response, which may further aggravate VC induced by phosphate overload in MHD patients.

Identification and Characterization of Osmoregulation Related MicroRNAs in Gills of Hybrid Tilapia Under Three Types of Osmotic Stress

Researchers have increasingly suggested that microRNAs (miRNAs) are small non-coding RNAs that post-transcriptionally regulate gene expression and protein translation in organs and respond to abiotic and biotic stressors. To understand the function of miRNAs in osmotic stress regulation of the gills of hybrid tilapia (Oreochromis mossambicus ♀ × Oreochromis urolepis hornorum ♂), high-throughput Illumina deep sequencing technology was used to investigate the expression profiles of miRNAs under salinity stress (S, 25‰), alkalinity stress (A, 4‰) and salinity-alkalinity stress (SA, S: 15‰, A: 4‰) challenges. The results showed that 31, 41, and 27 upregulated and 33, 42, and 40 downregulated miRNAs (P < 0.05) were identified in the salt stress, alkali stress, and saline-alkali stress group, respectively, which were compared with those in the control group (C). Fourteen significantly differently expressed miRNAs were selected randomly and then validated by a quantitative polymerase chain reaction. On the basis of Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analysis, genes related to osmoregulation and biosynthesis were enriched in the three types of osmotic stress. In addition, three miRNAs and three predicted target genes were chosen to conduct a quantitative polymerase chain reaction in the hybrid tilapia and its parents during 96-h osmotic stress. Differential expression patterns of miRNAs provided the basis for research data to further investigate the miRNA-modulating networks in osmoregulation of teleost.

Defining the ATPome reveals cross-optimization of metabolic pathways

Disrupted energy metabolism drives cell dysfunction and disease, but approaches to increase or preserve ATP are lacking. To generate a comprehensive metabolic map of genes and pathways that regulate cellular ATP-the ATPome-we conducted a genome-wide CRISPR interference/activation screen integrated with an ATP biosensor. We show that ATP level is modulated by distinct mechanisms that promote energy production or inhibit consumption. In our system HK2 is the greatest ATP consumer, indicating energy failure may not be a general deficiency in producing ATP, but rather failure to recoup the ATP cost of glycolysis and diversion of glucose metabolites to the pentose phosphate pathway. We identify systems-level reciprocal inhibition between the HIF1 pathway and mitochondria; glycolysis-promoting enzymes inhibit respiration even when there is no glycolytic ATP production, and vice versa. Consequently, suppressing alternative metabolism modes paradoxically increases energy levels under substrate restriction. This work reveals mechanisms of metabolic control, and identifies therapeutic targets to correct energy failure.

Slc20a1/Pit1 and Slc20a2/Pit2 are essential for normal skeletal myofiber function and survival

Low blood phosphate (Pi) reduces muscle function in hypophosphatemic disorders. Which Pi transporters are required and whether hormonal changes due to hypophosphatemia contribute to muscle function is unknown. To address these questions we generated a series of conditional knockout mice lacking one or both house-keeping Pi transporters Pit1 and Pit2 in skeletal muscle (sm), using the postnatally expressed human skeletal actin-cre. Simultaneous conditional deletion of both transporters caused skeletal muscle atrophy, resulting in death by postnatal day P13. smPit1-/-, smPit2-/- and three allele mutants are fertile and have normal body weights, suggesting a high degree of redundance for the two transporters in skeletal muscle. However, these mice show a gene-dose dependent reduction in running activity also seen in another hypophosphatemic model (Hyp mice). In contrast to Hyp mice, grip strength is preserved. Further evaluation of the mechanism shows reduced ERK1/2 activation and stimulation of AMP kinase in skeletal muscle from smPit1-/-; smPit2-/- mice consistent with energy-stress. Similarly, C2C12 myoblasts show a reduced oxygen consumption rate mediated by Pi transport-dependent and ERK1/2-dependent metabolic Pi sensing pathways. In conclusion, we here show that Pit1 and Pit2 are essential for normal myofiber function and survival, insights which may improve management of hypophosphatemic myopathy.

Nephronectin Expression is Inhibited by Inorganic Phosphate in Osteoblasts

Nephronectin (Npnt), an extracellular matrix protein, is known to be a ligand of integrin α8β1, and it has also been known to play critical roles as various organs. In the present study, elevated extracellular inorganic phosphate (Pi) strongly inhibited the expression of Npnt in MC3T3-E1 cells, while the existence of extracellular calcium (Ca) was indispensable for its effect. Furthermore, Pi-induced inhibition of Npnt gene expression was recovered by inhibitors of both sodium-dependent Pi transporter (Pit) and fibroblast growth factor receptors (Fgfrs). These results demonstrated that Npnt gene expression is regulated by extracellular Pi via Pit and Fgfrs.

Phosphate as a Signaling Molecule and Its Sensing Mechanism

In mammals, phosphate balance is maintained by influx and efflux via the intestines, kidneys, bone, and soft tissue, which involves multiple sodium/phosphate (Na+/Pi) cotransporters, as well as regulation by several hormones. Alterations in the levels of extracellular phosphate exert effects on both skeletal and extra-skeletal tissues, and accumulating evidence has suggested that phosphate itself evokes signal transduction to regulate gene expression and cell behavior. Several in vitro studies have demonstrated that an elevation in extracellular Piactivates fibroblast growth factor receptor, Raf/MEK (mitogen-activated protein kinase/ERK kinase)/ERK (extracellular signal-regulated kinase) pathway and Akt pathway, which might involve the type III Na+/Picotransporter PiT-1. Excessive phosphate loading can lead to various harmful effects by accelerating ectopic calcification, enhancing oxidative stress, and dysregulating signal transduction. The responsiveness of mammalian cells to altered extracellular phosphate levels suggests that they may sense and adapt to phosphate availability, although the precise mechanism for phosphate sensing in mammals remains unclear. Unicellular organisms, such as bacteria and yeast, use some types of Pitransporters and other molecules, such as kinases, to sense the environmental Piavailability. Multicellular animals may need to integrate signals from various organs to sense the phosphate levels as a whole organism, similarly to higher plants. Clarification of the phosphate-sensing mechanism in humans may lead to the development of new therapeutic strategies to prevent and treat diseases caused by phosphate imbalance.

An essential role for IGF2 in cartilage development and glucose metabolism during postnatal long bone growth

Postnatal bone growth involves a dramatic increase in length and girth. Intriguingly, this period of growth is independent of growth hormone and the underlying mechanism is poorly understood. Recently, an IGF2 mutation was identified in humans with early postnatal growth restriction. Here, we show that IGF2 is essential for longitudinal and appositional murine postnatal bone development, which involves proper timing of chondrocyte maturation and perichondrial cell differentiation and survival. Importantly, the Igf2 null mouse model does not represent a simple delay of growth but instead uncoordinated growth plate development. Furthermore, biochemical and two-photon imaging analyses identified elevated and imbalanced glucose metabolism in the Igf2 null mouse. Attenuation of glycolysis rescued the mutant phenotype of premature cartilage maturation, thereby indicating that IGF2 controls bone growth by regulating glucose metabolism in chondrocytes. This work links glucose metabolism with cartilage development and provides insight into the fundamental understanding of human growth abnormalities.

Biophysical aspects of biomineralization

During the process of endochondral bone formation, chondrocytes and osteoblasts mineralize their extracellular matrix (ECM) by promoting the synthesis of hydroxyapatite (HA) seed crystals in the sheltered interior of membrane-limited matrix vesicles (MVs). Several lipid and proteins present in the membrane of the MVs mediate the interactions of MVs with the ECM and regulate the initial mineral deposition and posterior propagation. Among the proteins of MV membranes, ion transporters control the availability of phosphate and calcium needed for initial HA deposition. Phosphatases (orphan phosphatase 1, ectonucleotide pyrophosphatase/phosphodiesterase 1 and tissue-nonspecific alkaline phosphatase) play a crucial role in controlling the inorganic pyrophosphate/inorganic phosphate ratio that allows MV-mediated initiation of mineralization. The lipidic microenvironment can help in the nucleation process of first crystals and also plays a crucial physiological role in the function of MV-associated enzymes and transporters (type III sodium-dependent phosphate transporters, annexins and Na⁺/K⁺ ATPase). The whole process is mediated and regulated by the action of several molecules and steps, which make the process complex and highly regulated. Liposomes and proteoliposomes, as models of biological membranes, facilitate the understanding of lipid-protein interactions with emphasis on the properties of physicochemical and biochemical processes. In this review, we discuss the use of proteoliposomes as multiple protein carrier systems intended to mimic the various functions of MVs during the initiation and propagation of mineral growth in the course of biomineralization. We focus on studies applying biophysical tools to characterize the biomimetic models in order to gain an understanding of the importance of lipid-protein and lipid-lipid interfaces throughout the process.

Neuroprotective effect of 5-aminolevulinic acid against low inorganic phosphate in neuroblastoma SH-SY5Y cells

PiT-1 (encoded by SLC20A1) and PiT-2 (encoded by SLC20A2) are type-III sodium-dependent phosphate cotransporters (NaPiTs). Recently, SLC20A2 mutations have been found in patients with idiopathic basal ganglia calcification (IBGC), and were predicted to bring about an inability to transport Pi from the extracellular environment. Here we investigated the effect of low Pi loading on the human neuroblastoma SH-SY5Y and the human glioblastoma A172 cell lines. The results show a different sensitivity to low Pi loading and differential regulation of type-III NaPiTs in these cells. We also examined whether 5-aminolevulinic acid (5-ALA) inhibited low Pi loading-induced neurotoxicity in SH-SY5Y cells. Concomitant application of 5-ALA with low Pi loading markedly attenuated low Pi-induced cell death and mitochondrial dysfunction via the induction of HO-1 by p38 MAPK. The findings provide us with novel viewpoints to understand the pathophysiology of IBGC, and give a new insight into the clinical prevention and treatment of IBGC.

Osteogenic differentiation of human lens epithelial cells might contribute to lens calcification

Calcification of the human lens has been described in senile cataracts and in young patients with congenital cataract or chronic uveitis. Lens calcification is also a major complication of cataract surgery and plays a role in the opacification of intraocular lenses. A cell-mediated process has been suggested in the background of lens calcification, but so far the exact mechanism remained unexplored. Lens calcification shares remarkable similarities with vascular calcification; in both pathological processes hydroxyapatite accumulates in the soft tissue. Vascular calcification is a regulated, cell-mediated process in which vascular cells undergo osteogenic differentiation. Our objective was to investigate whether human lens epithelial cells (HuLECs) can undergo osteogenic transition in vitro, and whether this process contributes to lens calcification. We used inorganic phosphate (Pi) and Ca to stimulate osteogenic differentiation of HuLECs. Osteogenic stimuli (2.5mmol/L Pi and 1.2mmol/L Ca) induced extracellular matrix mineralization and Ca deposition in HuLECs with the critical involvement of active Pi uptake. Osteogenic stimuli almost doubled mRNA expressions of osteo-/chondrogenic transcription factors Runx2 and Sox9, which was accompanied by a 1.9-fold increase in Runx2 and a 5.5-fold increase in Sox9 protein expressions. Osteogenic stimuli induced mRNA and protein expressions of alkaline phosphatase and osteocalcin in HuLEC. Ca content was higher in human cataractous lenses, compared to non-cataractous controls (n=10). Osteocalcin, an osteoblast-specific protein, was expressed in 2 out of 10 cataractous lenses. We conclude that osteogenic stimuli induce osteogenic differentiation of HuLECs and propose that this mechanism might play a role in lens calcification.

Higher serum phosphorus is associated with catabolic/anabolic imbalance in heart failure

BACKGROUND

A higher serum phosphate level is associated with worse outcome. Energy-demanding intracellular transport of phosphate is needed to secure anion bioavailability. In heart failure (HF), energy starvation may modify intracellular and serum levels of phosphate. We analysed determinants of serum phosphates in HF and assessed if catabolic/anabolic balance (CAB) was associated with elevation of serum phosphate.

METHODS

We retrospectively reviewed data from 1029 stable patients with HF and have calculated negative (loss) and positive (gain) components of weight change from the onset of HF till index date. The algebraic sum of these components was taken as CAB. The univariate and multivariable predictors of serum phosphorus were calculated. In quintiles of CAB, we have estimated odds ratios for serum phosphorus above levels previously identified to increase risk of mortality. As a reference, we have selected a CAB quintile with similar loss and gain.

RESULTS

Apart from sex, age, and kidney function, we identified serum sodium, N-terminal fragment of pro-brain-type natriuretic peptide, and CAB as independent predictors of serum phosphorus. The odds for serum phosphorus above thresholds found in literature to increase risk were highest in more catabolic patients. In most catabolic quintile relative to neutral balance, the odds across selected phosphorus thresholds rose, gradually peaking at 1.30 mmol/L with a value of 3.29 (95% confidence interval: 2.00-5.40, P < 0.0001) in an unadjusted analysis and 2.55 (95% confidence interval: 1.38-2.72, P = 0.002) in a fully adjusted model.

CONCLUSIONS

Metabolic status is an independent determinant of serum phosphorus in HF. Higher catabolism is associated with serum phosphorus above mortality risk-increasing thresholds.

Vascular Calcification Revisited: A New Perspective for Phosphate Transport

Elevated serum phosphorus has emerged as a key risk factor for pathologic calcification of cardiovascular structures, or vascular calcification (VC). To prevent the formation of calciumphosphate deposits (CPD), the body uses adenosine-5’-triphosphate (ATP) to synthesize inhibitors of calcification, including proteins and inhibitors of low molecular weight. Extracellular pyrophosphate (PPi) is a potent inhibitor of VC, which is produced during extracellular hydrolysis of ATP. Loss of function in the enzymes and transporters that are involved in the cycle of extracellular ATP, including Pi transporters, leads to excessive deposition of calcium-phosphate salts. Treatment of hyperphosphatemia with Pi-binders and Injection of exogenous PPi are the effective treatments to prevent CPD in the aortic wall. The role of sodium phosphate cotransporters in ectopic calcification is contradictory and not well defined, but their important role in the control of intracellular Pi levels and the synthesis of ATP make them an important target to study.

Amorphous Ca²⁺ polyphosphate nanoparticles regulate the ATP level in bone-like SaOS-2 cells

Polyphosphate (polyP) is a physiologically occurring polyanion that is synthesized especially in bone-forming osteoblast cells and blood platelets. We used amorphous polyP nanoparticles, complexed with Ca(2+), that have a globular size of ∼100 nm. Because polyP comprises inorganic orthophosphate units that are linked together through high-energy phosphoanhydride bonds, we questioned whether the observed morphogenetic effect, elicited by polyP, is correlated with the energy-generating machinery within the cells. We show that exposure of SaOS-2 osteoblast-like cells to polyP results in a strong accumulation of mitochondria and a parallel translocation of the polyP-degrading enzyme alkaline phosphatase to the cell surface. If SaOS-2 cells are activated by the mineralization activation cocktail (comprising β-glycerophosphate, ascorbic acid and dexamethasone) and additionally incubated with polyP, a tenfold intracellular increase of the ATP level occurs. Even more, in those cells, an intensified release of ATP into the extracellular space is also seen. We propose and conclude that polyP acts as metabolic fuel after the hydrolytic cleavage of the phosphoanhydride linkages, which contributes to hydroxyapatite formation on the plasma membranes of osteoblasts.

High phosphorus level leads to aortic calcification via β-catenin in chronic kidney disease

AIMS

Vascular calcification is a risk factor for causing cardiovascular events and has a high prevalence among chronic kidney disease (CKD) patients. However, the molecular mechanism underlying this pathogenic process is still obscure.

METHODS

Vascular smooth muscle cells (VSMCs) were induced by a concentration of phosphorus (Pi) of 2.5 mM, and were subjected to cell calcification analyses. The effect of high Pi on the Wnt/β-catenin pathway was measured using a TOP/FOP-Flash reporter assay. The transcriptional regulation of β-catenin on PIT1 (a type III sodium-dependent phosphate cotransporter) was confirmed by promoter reporter and chromatin immunoprecipitation assays. The 5/6 nephrectomized rat was used as an in vivo model and was fed a high Pi diet to induce aortic calcification. Serum levels of phosphate, calcium, creatine, and blood urea nitrogen were measured, and abdominal aortic calcification was examined.

RESULTS

High Pi induced VSMC calcification, downregulated expression levels of VSMC markers, and upregulated levels of osteogenic markers. High Pi activated the Wnt/β-catenin pathway and β-catenin activity. β-Catenin was involved in the process of high Pi-induced VSMC calcification. Further investigation revealed that β-catenin transcriptionally regulated Pit1, a necessary player in VSMC osteogenic phenotype change and calcification. The in vivo study showed that β-catenin was involved in rat abdominal aortic calcification induced by high Pi. When knockdown expression of β-catenin in the rat model was investigated, we found that aortic calcification was reduced.

CONCLUSION

These results suggest that β-catenin is an important player in high phosphorus level-induced aortic calcification in CKD.

Emerging genetic basis of osteochondritis dissecans

Genome-wide association studies (GWAS) provide an unbiased approach in the identification of genes that increase the risk for osteochondritis dissecans (OCD). Recent GWAS in humans, horses, and pigs are reviewed and genes identified. The identified genes tended to cluster with respect to function and biologic processes. GWAS in humans are a critical next step in the effort to provide a better understanding of the causes of OCD, which will, in turn, allow preventive strategies for treatment of adolescents and young adults who are at risk for the development of degenerative joint disease due to the effects of OCD.

Calcium phosphate-bearing matrices induce osteogenic differentiation of stem cells through adenosine signaling

Synthetic matrices emulating the physicochemical properties of tissue-specific ECMs are being developed at a rapid pace to regulate stem cell fate. Biomaterials containing calcium phosphate (CaP) moieties have been shown to support osteogenic differentiation of stem and progenitor cells and bone tissue formation. By using a mineralized synthetic matrix mimicking a CaP-rich bone microenvironment, we examine a molecular mechanism through which CaP minerals induce osteogenesis of human mesenchymal stem cells with an emphasis on phosphate metabolism. Our studies show that extracellular phosphate uptake through solute carrier family 20 (phosphate transporter), member 1 (SLC20a1) supports osteogenic differentiation of human mesenchymal stem cells via adenosine, an ATP metabolite, which acts as an autocrine/paracrine signaling molecule through A2b adenosine receptor. Perturbation of SLC20a1 abrogates osteogenic differentiation by decreasing intramitochondrial phosphate and ATP synthesis. Collectively, this study offers the demonstration of a previously unknown mechanism for the beneficial role of CaP biomaterials in bone repair and the role of phosphate ions in bone physiology and regeneration. These findings also begin to shed light on the role of ATP metabolism in bone homeostasis, which may be exploited to treat bone metabolic diseases.

Phosphate: an old bone molecule but new cardiovascular risk factor

Phosphate handling in the body is complex and involves hormones produced by the bone, the parathyroid gland and the kidneys. Phosphate is mostly found in hydroxyapatite. however recent evidence suggests that phosphate is also a signalling molecule associated with bone formation. Phosphate balance requires careful regulation of gut and kidney phosphate transporters, SLC34 transporter family, but phosphate signalling in osteoblasts and vascular smooth muscle cells is likely mediated by the SLC20 transporter family (PiT1 and PiT2). If not properly regulated, phosphate imblanace could lead to mineral disorders as well as vascular calcification. In chronic kidney disease-mineral bone disorder, hyperphosphataemia has been consistently associated with extra-osseous calcification and cardiovascular disease. This review focuses on the physiological mechanisms involved in phosphate balance and cell signalling (i.e. osteoblasts and vascular smooth muscle cells) as well as pathological consequences of hyperphosphataemia. Finally, conventional as well as new and experimental therapeutics in the treatment of hyperphosphataemia are explored.

Sodium-dependent phosphate cotransporters and phosphate-induced calcification of vascular smooth muscle cells: redundant roles for PiT-1 and PiT-2

OBJECTIVE

Elevated serum phosphate has emerged as a major risk factor for vascular calcification. The sodium-dependent phosphate cotransporter, PiT-1, was previously shown to be required for phosphate-induced osteogenic differentiation and calcification of cultured human vascular smooth muscle cells (VSMCs), but its importance in vascular calcification in vivo and the potential role of its homologue, PiT-2, have not been determined. We investigated the in vivo requirement for PiT-1 in vascular calcification using a mouse model of chronic kidney disease and the potential compensatory role of PiT-2 using in vitro knockdown and overexpression strategies.

APPROACH AND RESULTS

Mice with targeted deletion of PiT-1 in VSMCs were generated (PiT-1(Δsm)). PiT-1 mRNA levels were undetectable, whereas PiT-2 mRNA levels were increased 2-fold in the vascular aortic media of PiT-1(Δsm) compared with PiT-1(flox/flox) control. When arterial medial calcification was induced in PiT-1(Δsm) and PiT-1(flox/flox) by chronic kidney disease followed by dietary phosphate loading, the degree of aortic calcification was not different between genotypes, suggesting compensation by PiT-2. Consistent with this possibility, VSMCs isolated from PiT-1(Δsm) mice had no PiT-1 mRNA expression, increased PiT-2 mRNA levels, and no difference in sodium-dependent phosphate uptake or phosphate-induced matrix calcification compared with PiT-1(flox/flox) VSMCs. Knockdown of PiT-2 decreased phosphate uptake and phosphate-induced calcification of PiT-1(Δsm) VSMCs. Furthermore, overexpression of PiT-2 restored these parameters in human PiT-1-deficient VSMCs.

CONCLUSIONS

PiT-2 can mediate phosphate uptake and calcification of VSMCs in the absence of PiT-1. Mechanistically, PiT-1 and PiT-2 seem to serve redundant roles in phosphate-induced calcification of VSMCs.

Localization of type-III sodium-dependent phosphate transporter 2 in the mouse brain

Type-III sodium-dependent phosphate transporters 1 and 2 (PiT-1 and PiT-2, respectively) are proteins encoded by SLC20A1 and SLC20A2, respectively. The ubiquitous distribution of PiT-1 and PiT-2 mRNAs in mammalian tissues is in agreement with the housekeeping maintenance of homeostasis of intracellular inorganic phosphate (Pi), which is absorbed from interstitial fluid for normal cellular functions. Recently, mutations of SLC20A2 have been found in patients with idiopathic basal ganglia calcification (IBGC), also known as Fahr's disease. However, the localization of PiT-2 in the brain has not been clarified yet. Therefore, the aim of this study is to clarify the distribution of PiT-2 expression in the mouse brain. Our biochemical and immunohistochemical analyses using a polyclonal antibody (Ab) and a monoclonal Ab showed that PiT-2 was ubiquitously expressed throughout the brain. In terms of the cellular type, PiT-2 was predominantly detected in neurons; it colocalized with β-tubulin III in the cerebral cortex and with calbindin D-28K in Purkinje cells. Additionally, PiT-2 immunopositivity was detected in the microtubule-associated protein 2-positive neuronal dendrites in the cerebral cortex. However, colocalization with PiT-2 immunopositivity was not observed in the synaptophysin-positive nerve terminals. PiT-2 was also expressed in astrocytes and vascular endothelial cells. Our results indicate that PiT-2 plays an important role in the maintenance of cellular Pi homeostasis in neurons, astrocytes, and endothelial cells. This finding is a milestone in the study of the function of PiT-2 in the normal mouse brain and particularly in the brains of patients with Fahr's disease.

The changing face of hypophosphatemic disorders in the FGF-23 era

In the past decade, research in genetic disorders of hypophosphatemia has significantly expanded our understanding of phosphate metabolism. X-linked hypophosphatemia (XLH) is the most common inherited form of rickets due to renal phosphate wasting. Recent understanding of the mechanisms of disease and role of fibroblast growth factor 23 (FGF-23) in XLH and other hypophosphatemic disorders have opened new potential therapeutic avenues. We will discuss the current standard of treatment for XLH as well as promising future directions under study.

Selenium effect on selenoprotein transcriptome in chondrocytes

Selenium is an essential micronutrient and exerts its biological functions predominantly through selenoproteins. Selenium deficiency is associated with cartilage function. This study demonstrated that all 24 selenoprotein transcripts in mouse genome were detectable in ATDC5 chondrocytes except deiodinase 1 (DIO1), DIO2, and selenoprotein V (Sel V), while all 25 selenoprotein transcripts in human genome were detectable in C28/I2 chondrocytes except glutathione peroxidase 6 (GPx6) and DIO1. In addition, gene expression of five selenoproteins (GPx1, Sel H, Sel N, Sel P, and Sel W) was up-regulated and two selenoproteins (SPS2 and Sel O) was down-regulated by sodium selenite (Se) in both ATDC5 and C28/I2 cells. Gene expression of six selenoproteins (TrxR1, Sel I, Sel M, Sel R, Sel S, Sel T) and one selenoprotein (GPx3) was up-regulated by Se in ATDC5 and C28/I2 cells, respectively. Gene expression of one selenoprotein (TrxR2) was down-regulated by Se only in ATDC5 cells. Further transcription inhibition assay showed that both transcriptional and posttranscriptional mechanisms involved in Se-regulated gene expression of GPx1, TrxR1, TrxR2, SPS2, Sel O, and Sel S. However, Se-regulated gene expression of Sel H, Sel I, Sel M, Sel N, Sel P, Sel R, Sel T, and Sel W mainly at posttranscriptional level. Moreover, new protein synthesis inhibition assay indicated that Se-mediated new protein synthesis also played roles in Se-regulated gene expression of GPx1, TrxR1, TrxR2, Sel H, Sel O, Sel P, Sel R, and Sel W. In summary, this study described the selenoprotein transcriptome, Se-regulated selenoproteins and possible mechanisms involved in chondrocytes.

New insights into NPP1 function: lessons from clinical and animal studies

The recent elucidation of rare human genetic disorders resulting from mutations in ectonucleotide pyrophosphotase/phosphodiesterase (ENPP1), also known as plasma cell membrane glycoprotein 1 (PC-1), has highlighted the vital importance of this molecule in human health and disease. Generalised arterial calcification in infants (GACI), a frequently lethal disease, has been reported in recessive inactivating mutations in ENPP1. Recent findings have also linked hypophosphataemia to a lack of NPP1 function. A number of human genetic studies have indicated that NPP1 is a vital regulator that influences a wide range of tissues through various signalling pathways and when disrupted can lead to significant pathology. The function of Enpp1 has been widely studied in rodent models, where both the mutant tiptoe walking (ttw/ttw) mouse and genetically engineered Enpp1(-/-) mice show significant alterations in skeletal and soft tissue mineralisation, calcium/phosphate balance and glucose homeostasis. These models therefore provide important tools with which to study the potential mechanisms underpinning the human diseases associated with altered NPP1. This review will focus on the recent advances in our current knowledge of the actions of NPP1 in relation to bone disease, cardiovascular pathologies and diabetes. A fuller understanding of the mechanisms through which NPP1 exerts its pathological effects may stimulate the development of novel therapeutic strategies for patients at risk from the devastating clinical outcomes associated with disrupted NPP1 function.

Selenium promotes proliferation of chondrogenic cell ATDC5 by increment of intracellular ATP content under serum deprivation

Selenium (Se) is an essential micronutrient, and low Se intake in Se-deficient areas plays roles in an endemic osteochondropathy characterized by chondronecrosis in growth plate and articular cartilage. However, the biological activities of Se on cartilage are largely unknown. In this study, we examined the effects of Se on chondrogenic cell ATDC5 and the possible mechanisms involved. We demonstrated that Se stimulated ATDC5 cell proliferation under serum deprivation but not routine culture. Furthermore, Se promoted G1-phase cell cycle progression along with induction of cyclin D1 expression at the mRNA and protein level. Moreover, Se increased intracellular ATP content and decreased intracellular superoxide anion concentration without affecting intracellular redox status as estimated by ratio of the reduced and oxidized glutathione. In addition, suppression of intracellular ATP synthesis by glycolysis inhibitor or mitochondrial uncoupler both abrogated Se-mediated cyclin D1 induction. These findings suggest Se stimulates proliferation of chondrogenic cell ATDC5 through acceleration of cell cycle progression accompanied with cyclin D1 induction by enhancement of intracellular ATP content. This novel finding provides evidence for a role of Se in cartilage formation and degenerative processes and further supports the relationship between Se status and cartilage function that may lead to better utilization of Se for cartilage homeostasis.

Association of Serum Phosphate and Related Factors in ESRD-Related Vascular Calcification

Vascular calcification is common in ESRD patients and is important in increasing mortality from cardiovascular complications in these patients. Hyperphosphatemia related to chronic kidney disease is increasingly known as major stimulus for vascular calcification. Hyperphosphatemia and vascular calcification become popular discussion among nephrologist environment more than five decades, and many researches have been evolved. Risk factors for calcification are nowadays focused for the therapeutic prevention of vascular calcification with the hope of reducing cardiovascular complications.

Mapping of the minimal inorganic phosphate transporting unit of human PiT2 suggests a structure universal to PiT-related proteins from all kingdoms of life

BACKGROUND

The inorganic (Pi) phosphate transporter (PiT) family comprises known and putative Na(+)- or H(+)-dependent Pi-transporting proteins with representatives from all kingdoms. The mammalian members are placed in the outer cell membranes and suggested to supply cells with Pi to maintain house-keeping functions. Alignment of protein sequences representing PiT family members from all kingdoms reveals the presence of conserved amino acids and that bacterial phosphate permeases and putative phosphate permeases from archaea lack substantial parts of the protein sequence when compared to the mammalian PiT family members. Besides being Na(+)-dependent P(i) (NaP(i)) transporters, the mammalian PiT paralogs, PiT1 and PiT2, also are receptors for gamma-retroviruses. We have here exploited the dual-function of PiT1 and PiT2 to study the structure-function relationship of PiT proteins.

RESULTS

We show that the human PiT2 histidine, H(502), and the human PiT1 glutamate, E(70),--both conserved in eukaryotic PiT family members--are critical for P(i) transport function. Noticeably, human PiT2 H(502) is located in the C-terminal PiT family signature sequence, and human PiT1 E(70) is located in ProDom domains characteristic for all PiT family members.A human PiT2 truncation mutant, which consists of the predicted 10 transmembrane (TM) domain backbone without a large intracellular domain (human PiT2ΔR(254)-V(483)), was found to be a fully functional P(i) transporter. Further truncation of the human PiT2 protein by additional removal of two predicted TM domains together with the large intracellular domain created a mutant that resembles a bacterial phosphate permease and an archaeal putative phosphate permease. This human PiT2 truncation mutant (human PiT2ΔL(183)-V(483)) did also support P(i) transport albeit at very low levels.

CONCLUSIONS

The results suggest that the overall structure of the P(i)-transporting unit of the PiT family proteins has remained unchanged during evolution. Moreover, in combination, our studies of the gene structure of the human PiT1 and PiT2 genes (SLC20A1 and SLC20A2, respectively) and alignment of protein sequences of PiT family members from all kingdoms, along with the studies of the dual functions of the human PiT paralogs show that these proteins are excellent as models for studying the evolution of a protein's structure-function relationship.

Sodium-dependent phosphate cotransporters: lessons from gene knockout and mutation studies

Inorganic phosphate (Pi) is an essential physiological compound, highlighted by the syndromes caused by hypo or hyperphosphatemic states. Hyperphosphatemia is associated with ectopic calcification, cardiovascular disease, and increased mortality in patients with chronic kidney disease (CKD). As phosphate control is not efficient with diet or dialysis, oral Pi binders are used in over 90% of patients with renal failure. However, achieving tight control of serum Pi is difficult, and lower levels of serum Pi (severe hypophosphatemia) do not lead to better outcomes. The inhibition of sodium-dependent Pi (NaPi) transporter would be a preferable method to control serum Pi levels in patients with CKD or patients undergoing dialysis. Three types of NaPi transporters (types I-III) have been identified: solute carrier series SLC17A1 (NPT1/NaPi-I/OATv1), SLC34 (NaPi-IIa, NaPi-IIb, NaPi-IIc), and SLC20 (PiT1, PiT2), respectively. Knockout mice have been created for types I-III NaPi transporters. In this review, we discuss the roles of the NaPi transporters in Pi homeostasis.