Cementum is a mineralized tissue, similar in composition and properties to bone, that is synthesized by cementoblasts during tooth root formation and plays an essential role in anchoring the tooth to the surrounding alveolar bone. Examination of cementum development in ank/ank mutant mice revealed cementoblasts to be exceptionally sensitive to extracellular inorganic pyrophosphate (ePPi) [1]. The mouse progressive ankylosis protein (ANK) is a multipass transmembrane protein shown to regulate PPi transport from the cell interior to the extracellular milieu [2]. The ank/ank nonsense mutation therefore results in increased intracellular pyrophosphate (iPPi) and decreased ePPi as well as disruption in the overall ratio of available inorganic phosphate (Pi) and PPi. Mice carrying ank/ank have been used as a model for studying arthritis, and they present a phenotype including ectopic calcifications on articular surfaces and in synovial fluids, joint space narrowing, and osteophyte formation leading to fusion, joint immobility, and a characteristic flat-footed gait [2–4]. Upon studying the dentition, we found that ank/ank mutant mice also exhibited a >10-fold increase in cementum thickness, while maintaining apparently normal dimensions in the adjacent periodontal ligament (PDL) space, dentin, and alveolar bone. Another mouse model for PPi imbalance is the tiptoe walking (ttw or Enpp−/−) mutant mouse carrying a mutation in the Pc-1 gene that, like ank/ank, results in decreased ePPi [1]. The ttw PC-1 mutant mouse exhibits hypermineralization abnormalities including ossification of the vertebral column and joints [5–7]. Interestingly, a tooth phenotype similar to that in ank/ank was also observed in PC-1 mutant mice. While ANK and PC-1 regulate accumulation of ePPi, tissue nonspecific alkaline phosphatase (TNAP) catalyzes PPi breakdown to Pi. TNAP deficiency in humans and mice results in a condition of hypophosphatasia, marked by skeletal hypomineralization, skeletal abnormalities, and often a rickets-like condition [8–11]. TNAP deficiency also causes a dental phenotype distinguished by defective acellular cementum formation or no cementum at all, with the consequence of premature tooth loss [12, 13]. Clearly, cementum formation is contingent on careful regulation of PPi and Pi in the vicinity of the tooth root.

The aim of this study was to examine the role of Pi and PPi in regulating genes associated with cementoblast activity and function. Though mechanisms of Pi/PPi regulation for particular genes are being investigated [14, 15], much remains to be explored, including the effects of phosphate on a broader range of genes and, more specifically, genes controlling behavior of cells within the tooth region. This is particularly of interest in light of the observation that in ank/ank mutant mice the cementum thickness dramatically increases, while surrounding alveolar bone and dentin tissues are apparently unchanged [1], suggesting different regulatory influences for cementum and adjacent mineralized tissues in response to this PPi imbalance.

Materials and Methods

Cell Culture

Immortalized murine cementoblasts (OCCM-30) were maintained in Dulbecco’s modified Eagle medium (DMEM) supplemented with 10% (v/v) fetal bovine serum (FBS) and penicillin, streptomycin, and L-glutamine (100 units/mL, 100 μg/mL, and 2 mM, respectively; all tissue culture reagents from Invitrogen/GIBCO BRL, Carlsbad, CA). Cells were incubated at 37°C in an atmosphere of 5% CO2. Isolation and characterization of OCCM-30 cementoblasts have been previously described [16] and were approved by the University Committee on Use and Care of Animals (University of Michigan, Ann Arbor, MI) and in compliance with state and federal laws.

Cell Proliferation Experiments

OCCM-30 cells were plated in 35 mm dishes at a concentration of 2 × 103/cm2 in DMEM with 10% FBS. One day after seeding, baseline cell counts were taken and media were changed to DMEM with 5% FBS and treatment added. Phosphate was added at concentrations of 3, 5, and 7 mM, and media were changed every 2–3 days during the course of the experiment. Cell counts were taken by hemacytometer at days 1, 3, and 6 after initial treatment.

Gene Expression Experiments

OCCM-30 cells were plated in 60 mm dishes at a concentration of 2.6 × 104 cells/cm2 and maintained in DMEM with 10% FBS. Upon reaching confluence, media were changed to DMEM with 5% FBS and experimental treatments were added. Total RNA was isolated by Trizol reagent (Invitrogen/GIBCO BRL) 48 hours after initial addition of Pi. Phosphate was added to media at doses of 0.1–10.0 mM. Doses were based on our own observations as well as concentrations used by other researchers exploring the osteoblast response to Pi [17, 18]. A stock solution of 100 mM Pi was made in DMEM at a pH of 7.4 and filter-sterilized.

For time-course experiments, cells were seeded and grown to confluence as described above. Following treatment with 5 mM Pi, RNA was isolated at 1, 6, 24, and 48 hours. To determine the requirement of Pi entry into cells for cellular effects, media were supplemented with 3 mM phosphonoformic acid (PFA, or foscarnet) in addition to 5 mM Pi treatment for 24 hours. Total RNA was then harvested and analyzed as described above.

Real-time Reverse-Transcriptase Polymerase Chain Reaction

For real-time reverse-transcriptase polymerase chain reaction (RT-PCR) analysis, RNA was DNAse-treated (DNA-freeTM; Ambion, Austin, TX) and cDNA was synthesized from 1.0 μg total RNA with a cDNA synthesis kit for RT-PCR (either First-Strand AMV or Transcriptor kit; Roche Diagnostic, Indianapolis, IN). Two microliters of the resulting cDNA product were used per 20 μL reaction in the Lightcycler system (Roche Diagnostics, Mannheim, Germany). PCRs were carried out with the DNA Master SYBR Green I kit (Roche Diagnostic, Indianapolis, IN), with a total volume of 20 μL. Primers were designed by Lightcycler probe design software (Roche, Germany). A Basic Local Alignment Search Tool (BLAST) search of GenBank was performed on the primer sequences to ensure specificity, and melting curve analysis of products was additionally performed to ensure specificity. Expression was analyzed for genes including bone sialoprotein (Bsp), osteopontin (Opn), dentin matrix protein-1 (DMP-1), osteocalcin (Ocn), type I collagen (Col1), progressive ankylosis (Ank), plasma cell membrane glycoprotein-1 (Pc-1), tnap, and Pit1 type III sodium-dependent phosphate cotransporter (Pit1), with glyceraldehyde-3-phosphate dehydrogenase (Gapdh) serving as a housekeeping/reference gene for normalization. The amplification profile used on the Lightcycler was 95/0, 55/7, and 72/20 (temperature [°C]/time [seconds]) and 35-40 cycles. Primer sequences are listed in Table 1. All primers were used at a concentration of 0.5 μM (except DMP-1, used at 0.25 μM to optimize PCR conditions) in 3 mM MgCl2.

Table 1 Real-time PCR primer sequences
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Relative quantification of PCR products was achieved by using LightCycler Relative Quantification Software, version 1.0 (Roche Diagnostics, Germany) to compare amplification of the target gene of interest to that of Gapdh as a reference gene, with calibrator normalization and amplification efficiency correction.

Statistical Analysis

Data from proliferation, dose-response, and PFA experiments were analyzed by analysis of variance (ANOVA) to determine differences among treatments, with further pairwise comparisons made by the Student-Newman-Keuls (SNK) test. P ≤ 0.05 was used to indicate significance. Sigmastat 3.1 (Systat Software, Inc., Point Richmond, CA) was used for ANOVA statistical testing.

For analysis of the time-course experiment, PCR data from four independent experiments were transformed by calculating the log of the ratio of treated vs. untreated samples for a given time point. The transformation was performed in order to control for variation in passage-to-passage levels of basal expression in cells. Statistical significance of the Pi treatment effect was assessed by applying a t-test to the log-transformed ratios (Pi/No treatment) for each gene at each time point. P ≤ 0.05 was used to indicate significance. Excel (Microsoft, Redmond, WA) was used for statistical analysis of time-course experiments.

Results

Effect of Phosphate on OCCM-30 Cementoblast Proliferation

The effect of Pi on cell proliferation was assessed to determine if changes in proliferation could be associated with the regulation of gene expression. OCCM-30 cells were treated with 3, 5, or 7 mM Pi (or untreated control) in DMEM plus 5% FBS, and cells were counted by hemacytometer on days 0, 1, 3, and 6, where day 0 was the first day of treatment. Until day 3, all treatments resulted in equivalent cell numbers. However, by day 6, cells receiving the highest dose of Pi (7 mM) exhibited decreased proliferation, while the 5 mM Pi-treated and untreated plates yielded the same numbers of cells. Cell numbers with the lowest dose of 3 mM Pi were slightly higher at day 6 (Fig. 1).

Fig. 1
figure 1

Effect of phosphate on OCCM-30 cementoblast proliferation. Cementoblasts (OCCM-30) were plated in DMEM (10% FBS) and 1 day after seeding (day 0) switched to 5% FBS DMEM with or without 3, 5, or 7 mM Pi. Total cell number per plate was determined by hemacytometer on days 0, 1, 3, and 6; and average cell numbers per plate are represented on the graph ± standard error. *P < 0.05, treated vs. untreated control. Results from six experiments. □ NT, no treatment, ◊ 3mM Pi, △ 5mM Pi, × 7mM Pi.

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Effect of Phosphate on OCCM-30 Cementoblast Gene Expression: Dose Response

Pi treatment affected expression of genes associated with cementoblast differentiation and with mineral regulation (Bsp, Opn, Dmp-1, Ocn, and Col1). These genes were regulated in a dose-response fashion by Pi treatments in the range of 0.1–10 mM over the course of 48 hours. A dose of 5 mM Pi was observed to be generally very effective at eliciting a cell response, while there were concerns that doses of 7 and 10 mM may have created stressful conditions for cells (see proliferation experiment results, Fig. 1). The highest two doses were not used in subsequent time-course experiments. Among the small integrin-binding ligand, N-linked glycoprotein (SIBLING) family genes, considerable increases were observed in Opn (>300% of control) and Dmp-1 (>3,000% of control) with 5 mM Pi, while Bsp expression was downregulated by about 25%. To the authors’ knowledge, this is the first report of in vitro expression of Dmp-1 by cementoblasts. SIBLING member dentin sialophosphoprotein (Dspp) expression was also observed in cementoblasts, with indications of a similar upregulation of Dspp by Pi; however, overall Dspp expression was extremely low in these cells, and results were somewhat variable (data not shown). Other genes important in matrix synthesis and regulation of mineralization were downregulated, with Ocn substantially declining by about 85% and Col1 being affected with a decrease of about 50% (Fig. 2).

Fig. 2
figure 2

Effect of phosphate on OCCM-30 cementoblast gene expression: dose response. Confluent cementoblasts (OCCM-30) were cultured in DMEM (5% FBS) plus Pi at doses of 0.1–10 mM for 48 hours. Total RNA was extracted and used to synthesize cDNA, and quantitative real-time PCR was used to check expression of differentiation- and mineralization-related genes, including Bsp, Opn, Dmp-1, Ocn, and Col1. Results were normalized to GAPDH as a reference gene and calibrator sample. Results are presented as percent of untreated control (NT) on the y axis, with standard error indicated by error bars. *P < 0.05, treated vs. untreated control. Results from three experiments.

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Effect of Phosphate on OCCM-30 Cementoblast Gene Expression: Time Course

Having established that several genes associated with cementoblast differentiation/mineralization were regulated by Pi, the next step was to determine the time required to realize these regulatory effects. A dose of 5 mM Pi was chosen for these experiments because this dose delivered a robust response for the genes of interest but did not seem be associated with the potentially stressful conditions of higher Pi doses. Time-course experiments indicated that these genes responded relatively quickly to phosphate regulation, within 6–24 hours (Fig. 3). Patterns of expression varied from gene to gene, as has been previously described for OCCM-30 cells (and osteoblasts), with variations in expression over time in untreated samples as well.

Fig. 3
figure 3

Effect of phosphate on OCCM-30 cementoblast gene expression: time course. Cementoblasts (OCCM-30) were plated and cultured as described in the dose-response experiments (above), with a Pi dose of 5 mM. Total RNA was extracted at 1, 6, 24, and 48 hours and used to synthesize cDNA; quantitative real-time PCR was used to check gene expression. Treated samples are indicated by Pi and untreated controls by NT. Expression levels analyzed included genes for differentiation and mineralization and Pi/PPi homeostasis. Results are presented as percent of untreated control, where control is NT at 1 hour, with standard error indicated by error bars. *P < 0.05 difference of log-transformed Pi-treated vs. NT control data from same time point. Results from four experiments.

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Time-course experiments also indicated rapid regulation (by 6 hours) of genes concerned with Pi/PPi handling and homeostasis, including the mouse Ank gene, Pc-1, Tnap, and Pit1 (Fig. 3). Strong upregulation was observed in Ank (300% of control), PC-1 (700% of control), and Pit1 (200% of control), while Tnap expression greatly decreased (ultimately by 85%). OCCM-30 cementoblasts are differentiated cells that do not require ascorbic acid (AA) for induction of differentiation/mineralization genes, as do some precursor cells such as the murine osteoprogenitor cells MC3T3-E1 [19]. Nonetheless, experiments with added AA (50 μg/mL) were performed; and while some basal gene expression was higher with AA (as has been previously observed), response to Pi for all genes analyzed followed the same trend with or without AA (data not shown).

Effect of Phosphate Transport Inhibition on OCCM-30 Cementoblast Gene Expression

In order to determine whether the observed effects on gene expression were Pi-specific, entry of Pi into cells was blocked by treatment with the competitive inhibitor of Na-Pi transport PFA, also called foscarnet. With addition of 3 mM PFA concurrent with Pi exposure for 24 hours, gene regulation by Pi was substantially diminished (Opn and Dmp-1 shown here), indicating that Pi uptake is indeed necessary to realize these transcriptional changes. While induction of gene expression by Pi was not completely abolished, there was a considerable dampening of response by blocking Pi uptake. PFA addition did not affect basal cell expression, as indicated by unaltered Gapdh levels between PFA-treated and untreated samples (data not shown) (Fig. 4).

Fig. 4
figure 4

Effect of phosphate transport inhibition on OCCM-30 cementoblast gene expression. Cementoblasts (OCCM-30) were plated as described in the dose-response experiment (above). Prior to addition of Pi treatment (0.1–10 mM) at time 0 hours, cells were pretreated for 30 min with 3 mM PFA, also known as foscarnet, an inhibitor of Na-Pitransport into the cell. Pi doses and PFA remained in the culture media until total RNA was isolated 24 hours after treatment and used to synthesize cDNA and perform quantitative real-time PCR. Shown here are effects of PFA treatment on expression of the SIBLING family genes Opn and Dmp-1 vs. untreated controls (NT). Results are presented as percent of untreated control on the y axis, with standard error indicated by error bars. Results from three experiments.

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Discussion

Our results support cell regulatory properties of Pi, as reported by other groups using other cell types. We now report additional effects of Pi on cementoblast cell behavior and indicate that the changes in genes associated with the mature cementoblast may be unique to this cell phenotype. Opn, Dmp-1, Ank, Pc-1, and Pit1 mRNA levels were increased, while Bsp, Ocn, Col1, and Tnap mRNA levels were decreased (Figs. 2 and 3). Most of these genes responded within 6 hours, suggesting a relatively rapid response to Pi treatment (Fig. 3). Importantly, it was shown that Pi must enter the cell to cause the observed changes in gene expression (Fig. 4). No effects of 3 or 5 mM Pi on proliferation were observed, though 7 mM Pi at day 6 notably decreased proliferative potential (Fig. 1).

Phosphate Regulates Differentiation/Mineralization Genes

The genes regulated by Pi in cementoblasts could be split into at least two “functional” groups: (1) positive/negative regulators of differentiation/mineralization, and (2) phosphate/pyrophosphate homeostasis genes. In the first group, notably, several members of the SIBLING family were regulated by Pi in cementoblasts, including Opn and Dmp-1 (both increased) and Bsp (decreased) (Figs. 2 and 3A). The SIBLING family is comprised of genes encoding extracellular matrix proteins from mineralized tissues that are located on human chromosome 4 (mouse chromosome 5) and have in common the presence of an arginine-glycine-aspartate (RGD) integrin binding domain, exon organizational features, and similarity in posttranslational modifications, including glycosylation and phosphorylation [20–22]. In addition to functioning as extracellular matrix proteins involved in the regulation of hydroxyapatite mineral initiation and growth, SIBLING members have been implicated in cell attachment, local activation of matrix metalloproteinases, extracellular matrix maturation, wound healing, and protection of cells from alternative complement pathway lysis, among other functions [23–25].

SIBLING family transcripts Bsp, Opn, and Dmp-1 were shown to be regulated by Pi in these studies. Dspp expression was also detected in OCCM-30 cementoblasts, and there was some evidence of Pi regulation of this SIBLING member; however, expression was extremely low, leading to variable results (data not shown). OPN, in its phosphorylated form, is a potent inhibitor of hydroxyapatite crystal growth [26]; conversely, many studies support roles for BSP and DMP-1 as nucleators/promoters of mineral formation [27, 28]. There is evidence that DSPP cleavage products DSP and DPP, as well as DMP-1, are important in controlling dentin formation as they are expressed during dentinogenesis [29] and DMP-1-deficient mice exhibit reduced and hypomineralized dentin [30, 31]. Currently, a role for these genes/proteins in cementogenesis has not been established. Thus, future studies into the roles of DSP, DPP, and DMP-1 in cementogenesis should be considered. A recent study by Takano et al. [32] showed a potential effect of DSP on cellular cementum formation in rats and guinea pigs, and we now report in vitro cementoblast expression of SIBLING members Dmp-1 and Dspp. The modulation of SIBLING expression, while striking, seems in sum to be a mixed signal with regard to differentiation and mineralization.

Phosphate Regulates Pi/PPi Homeostasis Genes

Phosphate homeostasis genes in OCCM-30 cementoblasts were shown to respond to Pi regulation. Ank and Pc-1, which increase PPi in the extracellular environment through different processes, are both strongly upregulated in response to increased Pi (Fig. 3). Tnap, one of the primary enzymes in mineralized tissues for liberation of Pi from PPi, was convincingly downregulated (Fig. 3), possibly part of a negative feedback mechanism. Terkeltaub’s group [33] observed that Pc-1 expression was decreased in Tnap−/− mice and showed in MC3T3-E1 and calvarial osteoblasts that increased TNAP enzyme activity significantly increased PC-1 function; here, we show that the mechanism was likely via Pi, by observing the same reciprocal relationship between Tnap and Pc-1 on the mRNA level. Additionally, we show that Ank is regulated by Pi in a manner comparable to Pc-1. Similarly, in ank/ank and ttw/ttw mutant mice, in which mutated ANK and PC-1 proteins decrease ePPi, OPN is also reported to be substantially decreased [34]. TNAP function is normal in these animals, so decreased ePPi may also mean decreased Pi (because through the action of TNAP, PPi becomes a pool available for creation of Pi) and, subsequently, low OPN expression.

In addition to changes in Ank, Pc-1, and Tnap, the type III sodium-dependent phosphate transporter Pit1 was upregulated by addition of Pi (Fig. 3B). Pit1 is highly expressed in MC3T3-E1 preosteoblast cells, and its expression increased during differentiation, suggesting a role in osteoblast calcification of the extracellular matrix [35]. Additionally, we here show Pit1 expression in cementoblasts and further demonstrate upregulation in response to Pi, likely facilitating Pi uptake to the cell interior, where regulation of other genes may be the result. When Pi uptake into cells is blocked, as with inclusion of the Pi transport inhibitor foscarnet, gene regulation by Pi is substantially diminished (Fig. 4).

Regulation of Pi/PPi by Cementoblasts

The sum result of the changes in ANK, PC-1, and TNAP would be increased ePPi, perhaps an attempt to compensate for the Pi excess and return the Pi/PPi ratio back to the balance of homeostasis. We hypothesize that in addition to absolute levels of Pi and PPi available in the extracellular pool, the relative ratio of Pi/PPi may be important. When exposed to excess Pi, the cellular response is to compensate with greater ePPi through increased PC-1 and ANK and decreased TNAP. The cell may also compensate through modulating Pi levels, such as by increased Pi uptake via Pit1. After one mechanism compensates, levels of Pi and PPi may approach normal over time, and gene expression may again relax to a constitutive level. In this way, the tightly regulated and dynamic antagonism of Pi vs. PPi continues. Indeed, Millan’s group [7] showed that by crossing Tnap knockout (Akp2) and Pc-1 knockout (Enpp1) mice, in vitro and in vivo mineralizing capabilities of osteoblasts were returned to normal and ePPi levels in the double knockout were comparable to wild type, possibly with the facilitation of ANK or other factors. The cementum from these animals is currently under examination. The importance of the Pi/PPi ratio in both mineralized tissue and pathological calcifications is explored in depth in a series of well-conceived experiments by Murshed et al. [36], highlighting the working relationships between ANK, PC-1, and TNAP, as well as other proteins involved in Pi metabolism and regulation of mineralization.

Role of Cementoblasts

The reaction by cementoblasts to excess Pi is to decrease transcripts for the differentiation genes Bsp and Ocn while increasing the hydroxyapatite inhibitor Opn and potentially increasing ePPi via Ank, Pc-1, and Tnap regulation. In total, these results may be an indication that mature cementoblasts, once their role in cementum formation has been fulfilled, promote periodontal homeostasis by self-regulating their mineralization capacity, even when conditions (such as high Pi) favor hydroxyapatite formation. A similar response has been evoked by treating this same cementoblast cell line with the differentiation factor BMP-2, causing these mature cells to downregulate Bsp and Ocn and reduce in vitro mineral nodule formation [37]. Additionally, when treated with a bisphosphonate (specifically, etidronate), which alters proliferation, differentiation, and gene expression in osteoblasts [38, 39], these cementoblasts responded with downregulation of Cbfa1 and Ocn and upregulation of Opn, further indicating a responsive but reactive metabolism relative to nearby osteoblasts [40].

These results with cementoblasts in vitro alone do not explain in vivo observations of a hypercementosis phenotype in ank/ank and ttw/ttw mutant mice. ePPi is decreased as a result of both of these mutations, so the ratio of Pi/PPi may increase, favoring hydroxyapatite formation rather than suppression. When treated with Pi in vitro, cementoblasts responded by regulating transcripts in a way that would decrease or deemphasize mineralization and increase ePPi. Cementoblast behavior in vivo is probably more complex than previously thought, reflecting interactions with other cells and signaling molecules during development.

Phosphate as a Signaling Molecule

Phosphate present in the extracellular milieu of mineralizing cells (cementoblasts, osteoblasts, odontoblasts, etc.) may serve as a signal for a shift in cell function/differentiation, in addition to acting as one of the building blocks for hydroxyapatite mineral. Studies by several groups have begun to explore the mechanisms of Pi and PPi regulation of cell gene expression (for a thorough review of PPi pathophysiology, see [41]). Pi was first shown to be a direct inducer of Opn expression in MC3T3-E1 preosteoblast cells [42] and hypothesized to act as a signal to cue these precursor cells to undergo cellular changes necessary for differentiation and matrix mineralization [14, 18, 43]. The OCCM-30 cementoblasts used in the present study, however, are more differentiated cells and not necessarily primed to respond in the same way to molecular signals from an earlier developmental stage. In fact, these mature cells have been shown to downregulate differentiation- and mineralization-associated gene transcripts such as Bsp and Ocn when exposed to prodifferentiation signals such as BMP-2 [37], while the putative precursor dental follicle cell can be induced to differentiate to a mineralizing phenotype by BMP-2 [44]. Therefore, if mature cementoblasts respond to a Pi signal, it may not be a shift toward differentiation and need not be interpreted as such.

Pihas additionally been shown to be an apoptogen for chondrocytes and osteoblasts in the 1–10 mM range, possibly a signal for the clearing of terminally differentiated cells, with osteoblasts showing relatively less sensitivity to Pi treatment than tibial chondrocytes [17, 45]. This histological clearing would accomplish the removal of cells that had served their purpose (e.g., chondrocytes in the tibial growth plate) or that needed to be removed for tissue remodeling (osteoblasts or osteocytes during bone turnover). While cementoblasts have been found to be phenotypically very similar to osteoblasts [46, 47] and cementum is similar to both bone and dentin in composition (all are carbonated hydroxyapatite), cementum is not known to undergo significant remodeling during its lifetime. In fact, cementum seems to be extremely resistant to resorption by osteo-/odontoclasts, as evidenced by the relative rarity of cementum resorption and its resistance to resorption even when adjacent alveolar bone is being actively resorbed, as in conditions of advanced periodontal disease. Cementoblasts may play a protective role against root resorption, and we have shown a high production of osteo-protegerin (Opg) in cementoblasts in vitro, suggesting that this role may include active production of antiresorptive factors [48, 49]. Therefore, cementoblasts may not respond to such cues for apoptosis and may be less sensitive (than chondrocytes and osteoblasts) or not sensitive to Pi as an apoptosis signal. Other possible roles for Pi include those proposed by George’s group [50] and Shapiro’s group [51, 52], which further implicate a potential role for Ca2+ ions in conjunction with Pi-mediated signaling events, a facet that we did not examine here.

Phosphate and the Cementoblast

What emerges is a portrait of the cementoblast as a sophisticated cell operating in a complex environment and Pi as a multitasking ion operating in mineralized tissue formation (Fig. 5). Along with calcium, Pi is a necessary building block for hydroxyapatite formation that is released during mineral tissue turnover. While ePPi is a potent inhibitor of hydroxyapatite formation, it is also present as a pool for Pi creation via the action of enzymes such as TNAP. In this way, several proteins regulate local concentrations of Pi and PPi, increasing, decreasing, and transporting Pi and PPi. Here, we show that Pi regulates genes involved in Pi/PPi homeostasis and those known to be important in the maturation and mineralizing functions of cementoblasts and other mineralizing cells, such as SIBLING family genes. During formation, Pi may be an important regulator of cementoblast function, including areas of maturation and regulation of matrix mineralization. In addition to elucidating factors important to cementum development, there is potential for these findings to be extended to the creation of favorable conditions for cementum regeneration.

Fig. 5
figure 5

Model for the regulation of cementoblast cell behavior by Pi. Pi and calcium (Ca) are the building blocks for hydroxyapatite (HA) mineral in the body and released during mineral tissue turnover. ePPi is a potent inhibitor of HA formation. Several proteins regulate the local concentrations of Pi and PPi, including the Pit1 Na-Pi cotransporter TNAP that catalyzes conversion of PPi to Pi, the progressive ankylosis protein that regulates PPi transfer from the intracellular space (iPPi) to the extracellular (ePPi) milieu, and PC-1 plasma cell membrane glycoprotein that catalyzes PPi production from triphosphates (PPP). Here, we show that Pi addition can regulate genes involved in Pi/PPi homeostasis (Pit1, TnapP, Ank, Pc-1) and those known to be important in the maturation and mineralizing functions of cementoblasts and other mineralizing cells (Bsp, Opn, Dmp-1, Ocn, Col1).

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