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. 2008 Jan 17;22(5):1163–1170. doi: 10.1210/me.2007-0461

Regulation of Parathyroid Hormone Type 1 Receptor Dynamics, Traffic, and Signaling by the Na+/H+ Exchanger Regulatory Factor-1 in Rat Osteosarcoma ROS 17/2.8 Cells

David Wheeler 1, Jose Luis Garrido 1, Alessandro Bisello 1, Yung Kyu Kim 1, Peter A Friedman 1, Guillermo Romero 1
PMCID: PMC2366176  PMID: 18202147

Abstract

The effects of the expression of the Na+/H+ exchanger regulatory factor-1 (NHERF1) on the distribution, dynamics, and signaling properties of the PTH type 1 receptor (PTH1R) were studied in rat osteosarcoma cells ROS 17/2.8. NHERF1 had a dramatic effect on the subcellular distribution of PTH1R, promoting a substantial relocation of the receptor to regions of the plasma membrane located in very close proximity to cytoskeletal fibers. Direct interactions of NHERF1 with the PTH1R and the cytoskeleton were required for these effects, because they were abolished by 1) PTH1R mutations that impair NHERF1 binding, and 2) NHERF1 mutations that impair binding to the PTH1R or the cytoskeleton. NHERF1 reduced significantly the diffusion of the PTH1R by a mechanism that was also dependent on a direct association of NHERF1 with the PTH1R and the cytoskeleton. NHERF1 increased ligand-dependent production of cAMP and induced ligand-dependent rises in intracellular calcium. These effects on calcium were due to increased calcium uptake, as they were blocked by calcium channel inhibitors and by the addition of EGTA to the medium. These calcium effects were abolished by protein kinase A inhibition but phospholipase C inhibition was without effect. Based on these analyses, we propose that, in ROS cells, the presence of NHERF1 induces PTH-dependent calcium signaling by a cAMP-mediated mechanism that involves local protein kinase A-dependent activation of calcium channels.

THE TYPE 1 PTH receptor (PTH1R) is a G protein-coupled receptor that signals primarily through the activation of adenylyl cyclase and the production of cAMP. However, PTH exhibits remarkable cell- and tissue-specific signaling. In most cells and tissues, the PTH1R signals via the activation of cAMP and intracellular Ca2+ release (1,2). In some cells and heterologous expression systems, for instance, the PTH1R activates both adenylyl cyclase and phospholipase C (PLC). In certain instances, however, the PTH1R couples only to a single pathway and does so in a cell-specific manner. For example, in vascular smooth muscle cells, PTH stimulates adenylyl cyclase but not PLC (3,4), whereas in keratinocytes (5,6,7), cardiac myocytes (8,9), and lymphocytes (10,11,12), PLC, but not adenylyl cyclase, is activated. The PTH1R may be influenced by the relative levels of expression of specific G proteins (13). Likewise, N-truncated PTH antagonists, whereas failing to elicit detectable downstream signaling, promote PTH1R internalization in some cell types, but not in others (14,15). Based on the observation that the PTH1R interacts with the Na+/H+ exchanger regulatory factor proteins, NHERF1 and NHERF2 (16,17), a unified model to explain these diverse findings was developed. NHERF1 and NHERF2 are characterized by two tandem N-terminal post-synaptic 95, PSD95 (PDZ) domains that interact primarily with the C-terminal end of many targets, including the PTH1R, regulating their traffic, dynamics, and signaling properties (18,19,20). NHERF1 and NHERF2 also contain an ezrin-radixin-moesin (ERM) binding motif at the C terminus (20). Because of the multivalent nature of NHERF1 and NHERF2, it has been proposed that these proteins act as scaffolds, bringing together specific target molecules, and mediating the interactions of specific proteins with the cytoskeleton (16,17).

Previous work established that NHERF1 expression has dramatic effects on the modulation of the responses of PTH1R to various ligands. Some of these are: 1) reduction of the rate of ligand-induced PTH1R endocytosis (21); 2) reduced production of cAMP (16,17); 3) induction of calcium responses (16,17); and 4) blockade of the receptor internalization effects of N-truncated PTH analogs (14,15). Because calcium responses and cAMP production appear to be inversely correlated, it has been suggested that NHERF1 induces a signaling switch, impairing the coupling of PTH1R with Gs and promoting the activation of PLC by mechanisms linked to the activation of Gi/Go (16,17) or Gq/G11 (13,22). The molecular mechanisms underlying the signaling switch have not been characterized.

Relatively little is known about the mechanisms by which NHERF1 regulates PTH responses in bone cells. NHERF1 is not expressed in rat osteosarcoma ROS 17/2.8 (ROS) cells (16), and in these cells PTH neither activates PLC nor induces calcium-dependent signals. However, PTH(1–34) increased the activity of an activating protein 1-responsive luciferase reporter in ROS cells transfected with NHERF2, suggesting that expression of NHERF2 suffices to induce PKC-dependent responses (16). There is limited additional evidence for a NHERF1 or NHERF2-dependent signaling switch in bone cells. In fact, PLC-independent activation of PKC has been described in bone (23). In this paper we examine the influence of NHERF1 expression on the distribution, dynamics, and signaling events downstream of the PTH1R in ROS cells. Our data show that stable expression of NHERF1 alters significantly the distribution and diffusion of PTH1R, concomitantly inducing the appearance of calcium responses to stimulation with PTH. However, in contrast with the observations made in other cell systems, the induction of calcium responses by NHERF1 in ROS cells is not due to the activation of PLC. We show here that the elevation of intracellular calcium is a consequence of the activation of calcium channels by a mechanism mediated by protein kinase A (PKA) and requiring NHERF1. A novel model for the physiological role of NHERF1 in PTH1R signaling is proposed.

RESULTS

NHERF1 Regulates the Distribution of the PTH1R

Examination of a PTH1R-enhanced green fluorescent protein (EGFP) construct by confocal microscopy revealed a uniform surface distribution of the receptor on the surface of ROS cells (Fig. 1A). Actin fibers were stained with phalloidin-tetramethylrhodamine isothiocyanate (TRITC) (Fig. 1B) to investigate the colocalization of the PTH1R and the cytoskeleton. As shown in Fig. 1C, there was no colocalization of the receptor with actin fibers in the parental ROS cells. To examine the effects of NHERF1 expression on the distribution of the PTH1R, ROS cells were transfected with human NHERF1. NHERF1 expression was verified by Western blotting and immunocytochemistry (data not shown). Stably transfected cells were selected (ROS-NHERF1 cells) and further transfected with PTH1R-EGFP, and the distribution of the receptor was examined. In marked contrast with the results obtained with ROS cells, PTH1R-EGFP was organized around actin fibers (Fig. 1, D−F). To confirm that the interactions of PTH1R and NHERF1 were responsible for the reorganization of the receptor and its colocalization with the actin cytoskeleton, ROS and ROS-NHERF1 cells were transfected with M593A-PTH1R-EGFP (ETVA-PTH1R). This mutated receptor has a defective PDZ-binding motif and does not bind NHERF1 (15). As shown (Fig. 1, G–L), the distribution of ETVA-PTH1R was identical in ROS and ROS-NHERF1 cells. Furthermore, the colocalization with actin fibers was lost in the mutant receptor. Thus, binding of the PTH1R to the actin cytoskeleton requires an intact PDZ-recognition domain and the presence of NHERF1. To confirm that these results were not a consequence of the overexpression of the receptor, the experiments were reproduced by immunocytochemistry using antibodies against the PTH1R. The results shown in Fig S2 (published as supplemental data on The Endocrine Society's Online web site at http://mend.endojournals.org), demonstrate that the endogenous receptor is also organized around actin fibers in ROS-NHERF1 cells.

Figure 1.

Figure 1

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NHERF1 Regulates the Distribution of the PTH1R

ROS and ROS-NHERF1 cells were transfected with PTH1R-EGFP and examined by confocal microscopy. The images shown were obtaining by focusing the microscope onto the plasma membrane adjacent to the coverslip. PTH1R-EGFP (wild-type and the ETVA mutant) images are shown in panels A, D, G, and J. Actin fibers stained with TRITC-phalloidin are shown in panels B, E, H, and K. Panels C, F, I, and L show the merged images. Colocalization of the green and red labels is shown in yellow. wt, Wild type.

NHERF1 Reduces the Mobility of the PTH1R

The diffusion of PTH1R-EGFP was studied in ROS and ROS-NHERF1 cells by image correlation spectroscopy. As shown in Fig. 2, in ROS cells 80% of the PTH1R was readily mobile and diffused with a diffusion coefficient of about 0.15 μm2/sec. In contrast, in ROS-NHERF1 cells, most of the receptor molecules (80%) were immobile, and those receptors that diffused did so with a diffusion coefficient of 0.045 μm2/sec. Importantly, the expression of NHERF1 mutants that do not bind the PTH1R (S1S2, in which the core of the PDZ domains 1 and 2 has been scrambled), or that bind the receptor but fail to attach to the cytoskeleton (ΔERM, where the ERM-binding domain of NHERF1 has been deleted) had no detectable effects on the mobility of the receptor. These data demonstrate that immobilization of the PTH1R by NHERF1 is the result of specific tethering of the receptor to the actin cytoskeleton.

Figure 2.

Figure 2

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NHERF1 Regulates the Diffusion of the PTH1R

ROS and ROS-NHERF1 cells were transfected with PTH1R-EGFP and the diffusion of the PTH1R was examined by image correlation spectroscopy as described in the text. Autocorrelation data were fit to a model assuming a single mobile species. A, The diffusion coefficient was calculated from the autocorrelation data. B, The immobile fraction was calculated from the autocorrelation function fitted to the imaging data. *, Statistically significant differences with all other samples in the data set (P < 0.01; n = 6).

NHERF1 Regulates PTH1R Signaling

It has been proposed that NHERF1 induces a signaling switch in the responses of the PTH1R to hormone (16,17). According to the signaling switch hypothesis, NHERF1 promotes the coupling of the PTH1R to pertussis toxin-sensitive PLC activity with a concomitant reduction in the production of hormone-dependent cAMP. To test this model, we examined the production of cAMP and changes in the intracellular concentration of Ca2+ in ROS and ROS-NHERF1 cells. Figure 3 shows a comparison of the cAMP responses induced by 100 nm PTH(1–34) and forskolin (100 μm) in ROS and ROS-NHERF1 cells. The data demonstrate clearly that NHERF1 expression increased rather than diminished cAMP production. These differences are not due to different levels of adenylyl cyclase expression, because both cell types exhibited comparable responses to forskolin. Radioligand binding studies using the PTH analog [125I][Nle8, 18,Tyr34]PTH(1–34)NH2 demonstrated that ROS and ROS-NHERF1 cells expressed comparable numbers of receptors on the surface (ROS: 10.8 ± 1.6 nmol/mg protein; ROS-NHERF1: 13.6 ± 2.2 nmol/mg protein) (Fig. S1, published as supplemental data).

Figure 3.

Figure 3

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NHERF1 Modulates PTH-Induced cAMP Production

ROS and ROS-NHERF1 cells were prelabeled with tritiated adenine and treated with PTH(1–34) in the presence or absence of IBMX. * and ** denote statistically significant differences with the control and control + IBMX samples (*, P < 0.001; **, P < 0.01; n = 3).

Because the increased cAMP production observed in the ROS-NHERF1 cells was inconsistent with the predictions of the signaling switch model, we also examined PTH(1–34)-induced changes of intracellular Ca2+. The data shown in Fig. 4 demonstrate robust Ca2+ responses to 100 nm PTH(1–34) but only in the ROS-NHERF1 cells (Fig. 4, A and B). These Ca2+ responses were remarkable on two accounts. First, they appeared to be relatively slow in comparison with typical PLC-mediated responses (see Fig. 6B); second, the Ca2+ response was biphasic, with a slow increase in Ca2+ after the initial spike (Fig. 4A). The secondary rise in Ca2+ was not a consequence of reduced cell integrity because it was absent in the parental ROS cells. The Ca2+ responses depicted in Fig. 4A were due to the entry of extracellular Ca2+, as they were blocked by addition of 5 mm EGTA to the extracellular medium 5 min before the addition of PTH(1–34) (Fig. 4C). Furthermore, the voltage-dependent Ca2+-channel blockers nifedipine and verapamil abolished the Ca2+ responses induced by PTH (Fig. 4, D and E).

Figure 4.

Figure 4

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NHERF1 Modulates PTH-Regulated Intracellular Ca2+ by a Mechanism Mediated by Voltage-Dependent Ca2+ Channels

ROS-NHERF1 (A) and ROS (B) cells were preloaded with Fluo4 AM. Cells were treated with 100 nm PTH(1–34), and changes in the intracellular Ca2+ concentration were recorded from the changes in fluorescence at 1-sec intervals for up to 20 min. Actual [Ca2+] concentrations were determined as described in Materials and Methods. C, ROS-NHERF1 cells were pretreated with 5 mm EGTA 5 min before stimulation with PTH (1–34). D, ROS-NHERF1 cells were pretreated with 5 μm nifedipine 15 min before the experiment. E, ROS-NHERF1 cells were pretreated with 10 μm verapamil 15 min before the experiment. The figure shows representative traces of experiments that were reproduced at least three times. Calcium traces from 8–12 cells were recorded simultaneously in each experiment.

Figure 6.

Figure 6

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PTH-Induced, NHERF1-Dependent Ca2+ Entry Requires the Binding of PTH1R and NHERF1 to the Cytoskeleton

ROS and ROS-NHERF1 cells were transiently transfected with ΔERM-NHERF1 24 h before the experiments. The cells were stimulated with 100 nm PTH(1–34), and the intracellular [Ca2+] was monitored as described in Fig. 4.

To further define the mechanism of PTH-induced Ca2+ entry, PLC activity was inhibited with U73122 (24). Even at rather high concentrations (10 μm), U73122 had only minor effects on the PTH-induced entry of Ca2+ in ROS-NHERF1 cells (Fig. 5A). At these concentrations, U73122 completely blocked carbachol-induced Ca2+ responses (Fig. 5B). These studies strongly suggest that PLC activation was not involved in PTH-stimulated Ca2+ entry. To test the involvement of a classical cAMP-PKA-dependent mechanism, the cells were pretreated with the PKA inhibitor H89 (10 μm, 10 min) before exposure to the ligand. As shown in Fig. 5C, H89 abolished PTH-induced Ca2+ entry. Consistent with this, treatment of the cells with forskolin elicited a transient Ca2+ response in ROS-NHERF1 cells (Fig. 5D). Remarkably, ROS cells did not display an equivalent response to forskolin, indicating that NHERF1 is required for cAMP-dependent Ca2+ entry. These data strongly suggest that PKA activation is necessary, but not sufficient, for the activation of Ca2+ entry.

Figure 5.

Figure 5

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PTH-Dependent Ca2+ Entry Is Mediated by PKA Activation and Independent of PLC Activity

Calcium entry experiments were performed as described for Fig. 4. A, Cells were preincubated with 10 μm U73122 15 min before the addition of PTH. B, Cells were preincubated with 15 μm U73122 15 min before treatment with carbachol. C, H89 (15 μm) was added to the cells 20 min before stimulation with PTH(1–34). D, Cells were stimulated with forskolin (20 μm) after preincubation with 250 μm IBMX for 20 min.

The actual role of NHERF1 in the generation of the Ca2+ responses observed in ROS-NHERF1 cells was further explored using a C-terminal NHERF1 truncation mutant missing most of the ERM-binding motif (ΔERM). As shown in Fig. 6A, ΔERM did not elicit any Ca2+ responses, unlike full-length NHERF1. Furthermore, transfection of ROS-NHERF1 cells with the ΔERM mutant blocked the Ca2+ responses induced by PTH treatment. We conclude from these results that the binding of PTH1R to the cytoskeleton is required for the observed NHERF1 effects on Ca2+ entry.

DISCUSSION

There are multiple signaling pathways downstream of the PTH1R. Abundant evidence shows the activation of cAMP- and Ca2+-mediated responses, and the nature and magnitude of these responses are cell and tissue dependent (2,13,14,16,17,22,25). These remarkable variations in the signals triggered by PTH have received substantial attention in the last few years. Some insight as to the origins of these variations emerged from the discovery of the NHERF1/EBP50 family of scaffolding proteins. Mahon et al. (16) reported that NHERF2, a protein very closely related to NHERF1, reduced cAMP responses to PTH and promoted the activation of PLC by a Pertussis toxin-sensitive mechanism. These observations were extended to NHERF1 (17). Thus, a signaling switch mechanism was proposed. According to this model, the two PDZ domains of NHERF1/2 interact with the C terminus of the PTH1R and PLCβ, while tethering this complex to the cytoskeleton. The formation of this complex presumably switches the signaling mode of the PTH1R from a Gs-based activation of adenylyl cyclase to a Gi-dependent inhibition of cAMP production and activation of PLCβ. Thus, the PTH1R, in the presence of NHERF1, would signal via the activation of Ca2+-dependent processes.

The generality of the signaling switch model has not been well established. In fact, several lines of evidence suggest that the proposed NHERF1-dependent Gs-to-Gi coupling change is not necessary for the stimulation of substantial Ca2+-dependent responses to PTH. For instance, some cells that express NHERF1 at very high levels, such as osteoblast-like UMR-106 cells (our unpublished observations), produce large amounts of cAMP in response to PTH (26). Furthermore, several G protein-coupled receptors primarily linked to Gs and cAMP production elicit Ca2+ entry via the activation of voltage-dependent Ca2+ channels (27). Finally, work with signaling-selective PTH analogs demonstrated that PKC-dependent pathways may be activated by the PTH1R without concomitant activation of PLC-dependent phosphatidylinositol hydrolysis (23), suggesting PLC-independent Ca2+-dependent responses. In this study we address the mechanism by which NHERF1 induces PTH-dependent Ca2+ signaling in a well-established bone cell model, the rat osteosarcoma ROS 17/2.8 cell line.

Remarkably, the present results diverge substantially from those reported for other cell systems (13,16,22,25). Our data indicate that NHERF1 increases PTH-dependent cAMP accumulation, and that this effect is accompanied by Ca2+ entry. Importantly, NHERF1 is absolutely required for PTH-induced Ca2+ entry in ROS cells. However, the entry of Ca2+ into ROS-NHERF1 cells is qualitatively different from the responses reported in opossum kidney (OK) cells (17). Whereas the response to PTH appears as a single Ca2+ spike that has a duration of less than 50 sec in OK cells, the PTH response in ROS-NHERF1 cells is biphasic, with an initial spike that has a duration of 200–250 sec and a second, sustained phase that appears to plateau after 15 min (Figs. 4A and Fig. 5A). Furthermore, the mechanisms by which PTH induces Ca2+ entry in the ROS-NHERF1 cells differs from that in OK cells. In OK cells, Gi-dependent activation of PLC appears to be involved in the pathway, whereas in ROS-NHERF1 cells the mechanism is PKA mediated.

An important question is: because PTH increases cAMP levels in both ROS and ROS-NHERF1 cells, why is Ca2+ entry observed exclusively in the latter? We propose that the answer to this question lies in the subcellular distribution and membrane dynamics of the PTH1R and its signaling targets in cells that express NHERF1. First, NHERF1 is not serving a simple scaffolding role by bringing together proteins targeted by NHERF1's PDZ domains. This is evident from the fact that ΔERM-NHERF1 fails to transduce Ca2+ signals (Fig. 6A). These data clearly demonstrate that the attachment of NHERF1 to the cytoskeleton is absolutely required for its role in the promotion of Ca2+ entry in ROS-NHERF1 cells. This conclusion is further strengthened by the fact that ΔERM-NHERF1 inhibits the effects of the expression of the full-length NHERF1, acting as a dominant-negative mutant (Fig. 6B). Second, in the absence of IBMX (i.e. under physiological conditions) very little cAMP accumulates in ROS cells. In fact, the data shown in Fig. 3 suggest that, 20 min after the addition of PTH, the levels of cAMP are indistinguishable from the basal state in the parental ROS cells. In contrast, even in the absence of IBMX, PTH elicits cAMP production to some detectable degree in ROS-NHERF1 cells. Third, the PTH1R distribution and diffusion data shown in Figs. 1 and 2 demonstrate clearly that NHERF1 tethers the PTH1R to the cytoskeleton, promoting the accumulation of the receptor in the vicinity of actin fibers, where the PTH1R is immobilized. Therefore, even though the average intracellular concentration of cAMP may not substantially increase after stimulation, the local concentration of cAMP may rise dramatically in the vicinity of the cytoskeletal fibers. And finally, as shown in Fig. 5D, forskolin cannot induce Ca2+ entry in the absence of NHERF1. Taken together, these data clearly suggest that the scaffolding function of NHERF1 is an absolute requirement for the cAMP-dependent entry of Ca2+ in ROS cells. Furthermore, several known A kinase-anchoring proteins link PKA to the cytoskeleton, such as ezrin (28), which also interacts with NHERF1 via the C-terminal ERM-binding motif of NHERF1. We propose an alternative model for PTH-dependent activation of Ca2+ entry in cells that express NHERF1/2. According to this model, NHERF1 forms a scaffolding complex with the PTH1R and ERM proteins, such as ezrin or radixin. This complex accumulates along actin fibers running parallel to the surface of the cell. Upon the activation of the receptor, Gs-dependent adenylyl cyclase produces high local concentrations of cAMP, which, in turn, activate PKA locally, inducing the phosphorylation of voltage-dependent Ca2+ channels, thus allowing Ca2+ entry. PTH-dependent PLC activation may occur or not, but our data suggest that PLC activity is not required for the Ca2+ entry process.

MATERIALS AND METHODS

ROS and ROS-NHERF1 Cells

ROS 17/2.8 cells were grown in DMEM/F12 medium supplemented with 10% fetal calf serum. ROS-NHERF1 cells were generated by transfection of the parental cell line with human NHERF1 subcloned in the multiple cloning site of the plasmid pCDNA3.1/Hygro (Stratagene, La Jolla, CA). Stable transfectants were selected with hygromycin (up to 150 μg/ml) over a period of 4 wk. The expression of NHERF1 was determined by Western blotting using a specific anti-NHERF1 antibody (Upstate Biotechnology, Inc., Lake Placid, NY). ROS-NHERF1 cells were further characterized by measuring the levels of expression of PTH1R and the dissociation constant (Kd) for PTH(1–34). Both cell lines expressed similar levels of PTH1R (ROS: 10.8 ± 1.6 nmol/mg protein; ROS-NHERF1: 13.6 ± 2.2 nmol/mg protein). Likewise, the Kd of PTH(1–34) was the same in both cell lines (see Fig S1).

Immunocytochemistry

Cells were cultured on glass coverslips, transfected with wild-type PTH1R-EGFP or ETVA-PTH1R-EGFP, and allowed to grow until 80% confluence. The coverslips were washed in PBS and fixed for 20 min in 4% paraformaldehyde in PBS at 4 C. Cells were permeabilized with 5% nonfat milk, 0.1% Triton X-100 for 1 h at 4 C and then stained with phalloidin-TRITC (3 nm) overnight at 4 C. For the detection of endogenous receptors, untransfected cells were simultaneously incubated with anti-PTH1R antibody E-17 (Santa Cruz Biotechnology, Inc., Santa Cruz, CA) and a dilution of 1:100. Bound antibody was detected with an Alexa488-conjugated antigoat IgG. The cells were then washed four times with PBS, mounted with gelvatol, and examined by confocal microscopy using an Olympus Fluoview 1000 instrument (Olympus Corp., Lake Success, NY). Colocalization was measured using the image correlation plug-ins of Image J.

Adenylate Cyclase Activity

cAMP accumulation was determined as described previously (14). Briefly, cells cultured in 24-well plates were labeled with 0.5 μCi of [3H]adenine for 2 h. The cells were then treated with vehicle or 100 nm PTH(1–34) for 5 min. IBMX (250 μm) was included where indicated. The reaction was terminated by addition of 1 m trichloroacetic acid followed by neutralization with KOH. cAMP was isolated by the two-column method (29).

Measurement of the Diffusion Coefficient of the PTH1R

These studies were done by total internal reflection-image correlation spectroscopy as described previously (21). The technique is based on the analysis of the correlation of an image with itself after a certain lag time τ. This correlation is a function of the mobility of the fluorescent molecules (30,31). Cells were transfected with PTH1R-EGFP and examined 24 h after transfection. Cell images were collected by total internal reflection microscopy at 200-msec intervals. Up to 300 images were obtained for each experiment. Fluorescence loss due to photobleaching of the sample was almost negligible. The image data were exported to ImageJ and analyzed using a plug-in specifically written to calculate the autocorrelation function of the data (21). The resulting autocorrelation data were exported into GraphPad Prism and fit to a single species two-dimensional diffusion model [G(τ) = K(1 + τ/τd)−1 + G0 where τd is the characteristic time constant, K is a proportionality factor, and GD is a term that accounts for spatial autocorrelation]. The diffusion coefficient was calculated from the Stokes-Einstein equation [D = r2/(4τd)].

Determination of Intracellular Ca2+

ROS and ROS-NHERF1 cells cultured in Mattek dishes were loaded with the calcium-sensitive dye Fluo4 AM in serum-free medium for 20–30 min. The final loading concentration of the dye was 4 μm. All calcium measurements were done using DMEM/F12 containing 20 mm HEPES (pH 7.4) at 37 C in an Olympus IX70 inverted microscope equipped with a temperature-controlled chamber (Harvard Apparatus, Inc., South Natick, MA) and a Orca ER camera (Hamamatsu Photonic Systems Corp., Bridgewater, NJ). The intracellular Ca2+ concentration was determined using the expression F = [Fmax − Fmin)/(1 + ([Ca2+]/Kd)] where F is the measured fluorescence intensity, Fmax is the fluorescence measured after addition of ionomycin (which equilibrates the interior of the cell with the extracellular medium), Fmin is the fluorescence measured after addition of 10 mm EGTA, and Kd is the dissociation equilibrium constant of the dye-Ca2+ complex (0.19 μm) (32).

Statistical Analysis

All curve-fitting analyses were done using GraphPad Prism (GraphPad Software, Inc., San Diego, CA). All experiments reported were reproduced at least three times and done by triplicate. The calcium concentration traces shown in Figs. 4–6 are representative, and some cell-to-cell variations were observed. Statistical comparisons of multiple samples were done by ANOVA followed by posttest comparisons using Tukey's method. Pairs of samples were compared using Student's t tests.

Supplementary Material

[Supplemental Data]
mend_me.2007-0461_index.html (907B, html)

Footnotes

This work was supported by Grant DK-69998 from the National Institutes of Health (to P.A.F.), and an internal grant from the Office of the Senior Vice Chancellor for the Health Sciences, University of Pittsburgh (to G.R.).

Disclosure Statement: All authors have nothing to disclose.

First Published Online January 17, 2008

Abbreviations: EGFP, Enhanced green fluorescent protein; ERM, ezrin-radixin-moesin; IBMX, 3-isobutyl-1-methylxanthine; NHERF1, Na+/H+ exchanger regulatory factor-1; PDZ, post-synaptic 95, PSD95; PKA, protein kinase A; PLC, phospholipase C; PTH1R, PTH type 1 receptor; TRITC, tetramethylrhodamine isothiocyanate.

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