- Original Paper
Expression of the TRPM6 in mouse placental trophoblasts; potential role in maternal–fetal calcium transport
The Journal of Physiological Sciences volume 67, pages 151–162 (2017)
The placenta is required to transport calcium (Ca2+) from mother to fetus during fetal bone mineralization. In an attempt to clarify the molecular basis of Ca2+ entry for this transport, we identified TRPM6 as a candidate for apical Ca2+ entry pathway. TRPM6 mRNA increased during the last 4 days of pregnancy, coinciding with fetal bone mineralization in mice. TRPM6 mRNA and protein was localized in the trophoblasts in labyrinth where the maternal–fetal Ca2+ transport occurs. In patch-clamp recordings, we observed TRPM6/TRPM7-like currents in mouse trophoblasts after starting fetal bone mineralization but not before mineralization. Plasma membrane Ca2+ permeability was significantly increased in TRPM6/TRPM7 expressed HEK293 cells under physiological Mg2+ and ATP concentration but not in TRPM6 or TRPM7 homomer-expressing cells. These results suggest that TRPM6 is functionally expressed in mouse placental trophoblasts, implicating in maternal–fetal Ca2+ transport likely with TRPM7, which might enable to sustain fetal bone mineralization.
Calcium (Ca2+) has essential roles in fetal growth and development. Fetal blood Ca2+ concentration is maintained for the function and development of excitatory cells including the nervous system and heart, as well as other non-excitable cells. In parallel, Ca2+ is used for fetal bone mineralization. Many reports have shown that fetal blood Ca2+ levels are higher than maternal levels when fetal bones are mineralized [1–3]. These indicate that Ca2+ is actively transported from mother to fetus via the placenta in late pregnancy. However, the molecular mechanism and the regulation of the maternal–fetal Ca2+ transport remains unclear.
In the placenta, trophoblasts play the main role in maternal–fetal nutrient transport including Ca2+. In the placenta during late pregnancy, Ca2+ is thought to be transported through a transcellular pathway rather than a paracellular route due to the uphill nature of the Ca2+ gradient. There are three molecular processes involved in the transcellular pathway: (1) Ca2+ enters into the cell using an electrochemical gradient likely through Ca2+-permeable cation channels; (2) Ca2+ binds to calbindin D9K, an intracellular Ca2+ buffer to avoid an increase of the intracellular free Ca2+ concentration; (3) Ca2+ is extruded mainly via plasma membrane Ca2+-ATPase (PMCA) [4, 5]. Among these processes, it is not yet clear which Ca2+ channel is responsible for the Ca2+ entry process. In a previous study, the TRPV6 (transient receptor potential vanilloid 6) knockout mouse exhibited a decrease in maternal–fetal Ca2+ transport . However, the majority of Ca2+ transport was still remained in these mice. This result suggested that there is another key factor for the Ca2+ entry pathway. In this study, we focused on TRPM6 (transient receptor potential melastatin 6) as another candidate for this pathway. The expression pattern of TRPM6 in mouse placenta was temporally and spatially consistent with such a role. Membrane current and Ca2+-permeability assays under physiological conditions suggested the presence of endogenous TRPM6-derived Ca2+-entry in mouse placental trophoblasts. These suggest that TRPM6 contributes to the Ca2+ entry for the maternal–fetal Ca2+ transport.
Materials and methods
All animal experiments were performed with C57BL/6NCr pregnant mice (SLC Japan). All procedures were conducted in accordance with the policies of the Institutional Animal Care and Use Committee, National Institute for Physiological Sciences.
Molecular cloning of mouse TRPM6 and TRPM7 cDNA
Mouse kidney cDNA was synthesized using the Superscript III kit (Life Technologies Corporation, Carlsbad, CA, USA) and poly-A-rich RNA from mouse kidney (Takara, Japan). PCR reactions with mouse TRPM6 or TRPM7-specific primers were performed with 10 μl of 5× Phusion HF buffer, 4 μl dNTPs (2.5 mM), 1 μl mouse kidney cDNA, and 0.5 μl Phusion DNA polymerase (Thermo Fisher Scientific, Waltham, MA, USA), using the following temperature cycles: 98 °C for 30 s, then ten cycles of 98 °C for 10 s, 68 °C for 3 min, and 72 °C for 3 min, then 30 cycles of 98 °C for 10 s and 72 °C for 3 min. After elongation at 72 °C for 10 min, PCR products were purified using a gel extraction kit (Takara, Japan) from 0.7 % agarose gel electrophoresis and used as template for secondary PCR; 10 μl of 5× Phusion HF buffer, 4 μl dNTPs, mouse TRPM6 or TRPM7-specific primers, purified PCR fragments, and 0.5 μl of Phusion DNA polymerase. The cycles for secondary PCR were as follows: 98 °C for 30 s, then 30 cycles at 98 °C for 10 s and 72 °C for 3 min, then elongation at 72 °C for 10 min. The secondary PCR products were again purified, and introduced into the pcDNA3.1 vector (Life Technologies) after digestion using Bam HI and Not I (NEB, USA). Sequences were confirmed using a DNA sequence analyzer (ABI, USA). The GenBank accession numbers are as follows; mouse TRPM6 (NM_153417), mouse TRPM7 (NM_021450).
Whole placenta from pregnant mouse at 14 or 18 days post-fertilization were resected from anesthetized mice. Total RNA was isolated using a Sepasol I kit (Nacalai, Japan). First-strand cDNA was synthesized using a Revertra Ace kit (Toyobo, Japan). Conventional polymerase chain reaction (PCR) was performed using rTaq polymerase (Takara, Japan) with specific primer sets for all mouse TRPMs, TRPVs, and TRPA1 (Table S1). The PCR products were visualized by agarose gel electrophoresis with 2 % agarose (Ultrapure agarose, Life Technologies, USA). Plasmid with partial cDNA fragment of each TRP channels was used to verify PCR reactions. GAPDH was used as a housekeeping gene.
Quantitative PCR analysis was performed using a StepOne analyzer (Life Technologies, USA) as described previously . The temperature profile consisted of 40 cycles of denaturation (95 °C, 15 s), annealing (60 °C, 30 s), and elongation (72 °C, 1 min). Melting curve analysis was performed at the end of each assay to discriminate specific from non-specific signal. To quantify the amount of cDNA, purified PCR products with known concentration were serially diluted and used as standards. The ∆∆Ct method was applied to compare the expression levels from each time point. Beta-actin was used as a housekeeping gene. The expression level of β-actin was consistent between gestational ages.
In situ hybridization
In situ hybridization was performed as described previously [6, 8]. The placenta, along with the extraplacental yolk sac, was resected from anesthetized mice at 18 days post-fertilization. The tissue was frozen with OCT compound (Sakura, Japan), cut using a cryostat (CM3050S, Leica, Germany) at −20 °C with 20-μm thickness. Sections were incubated in PBS with 4 % paraformaldehyde (PFA) for 2 h at room temperature and washed twice with 2 × SSC. The sections were then treated with Tris–EDTA containing 5 μg/ml of proteinase K at 37 °C for 15 min with stirring. Those sections were incubated with 4 % PFA again, washed twice with 2 × SSC, and incubated with 0.1 M triethanolamine solution (pH 8.0) with 0.5 % acetic anhydride for 10 min at room temperature with stirring. After washing with Milli-Q water and treated with hybridization solution (50 % deionized formamide, 4 × SSC, 2 × Denhardt’s solution) at 50 °C for 2 h, sections were treated with the same hybridization buffer containing 1 % dextran sulfate and a denatured, digoxigenin-labeled RNA probe at 50 °C overnight in a humidified chamber. Sections were washed with 2 × SSC with 50 % formamide for 30 min at 37 °C with stirring, then treated with 2 × SSC with 1 U/ml RNase T1 (Roche, Switzerland) for 30 min at 37 °C with stirring. Next, sections were washed twice with 2 × SSC, 0.1 % SDS for 10 min at 55 °C, and twice with 0.2 × SSC, 0.1 % SDS for 10 min at 55 °C. After incubation with 1 % blocking reagent in maleic acid buffer (Roche, Switzerland), sections were treated with anti-digoxigenin-AP Fab fragments (1:1000, Roche, Switzerland) in the blocking solution for 2 h at room temperature in a humidified chamber. Sections were washed twice with maleic acid buffer with 0.2 % Tween-20 for 10 min with stirring, equilibrated in ALP detection buffer (0.1 M Tris–HCl, pH 9.5, 0.1 M NaCl, 50 mM MgCl2), and incubated in a substrate solution (Roche, Switzerland) with polyvinyl alcohol and levamisole at 30 °C overnight. After washing, sections were mounted with Fluoromount (Diagnostic BioSystems, USA).
Sense and antisense RNA probes were synthesized from plasmid containing mouse TRPM6 cDNA inserted into pCR2.1 (Life Technologies) using a digoxigenin RNA labeling kit (Roche, Switzerland). Plasmid containing mouse TRPM6 cDNA was linearized and cRNA probes were synthesized in vitro from fragments containing 472 bp of TRPM6 (nt. 1958-2430, NM_153417). The digoxigenin labeling was confirmed by dot-blot experiments.
Human embryonic kidney-derived 293T cells (HEK293T) were maintained in DMEM with 10 % heat-inactivated FBS, 100 units/ml penicillin and streptomycin, 2 mM l-glutamine, and 5 % CO2 at 37 °C. HEK293T cells were transfected with the appropriate plasmids using Lipofectamine reagent (Life Technologies). To prevent Ca2+ overload into the cells through TRPM6/TRPM7 heteromers, ruthenium red (2 μM) was added into the medium just after the transfection and compared with cells without ruthenium red treatment. After 18-h incubation, cells were washed with ice-cold PBS and lysed in 500 μl TNE buffer (10 mM Tris–HCl, pH 7.4, 150 mM NaCl, 1 mM EDTA, 1 mM Na3VO4) with protease inhibitors (Complete, Roche) and 1 % NP-40. After homogenizing, proteins were centrifuged at 10,000 × g for 60 min at 4 °C. Supernatant (membrane fraction) was quantified and kept at −20 °C until use. SDS-PAGE was performed with the protein sample (2 μg) in 7.5 % polyacrylamide gel, and then transferred into a PVDF membrane. The membrane was blocked overnight at 4 °C with 3 % BSA in PBS-T and then incubated with rabbit anti-mouse TRPM6 antibody (Alomone Labs, 1/300 dilution in 3 % BSA in PBS-T) with or without antigen peptides. After three washes with PBS-T, the membrane was incubated with anti-rabbit IgG HRP (CST, 1/3000 dilution) for 30 min at room temperature, then signal was visualized by ECL prime kit (Amersham) and FLA3000mini (Fujifilm).
Immunohistochemistry was performed as described previously . Pregnant mice at 18 days post-fertilization were anesthetized and the placenta was resected. The tissue was frozen with OCT compound (Sakura, Japan) and stored at −80 °C until use. Sections (8 μm) were cut by a cryostat (CM3050S, Leica, Germany), fixed with 10 % buffered formaldehyde for 10 min at room temperature. After washing with PBS 3 times, sections were incubated with 5 % normal goat serum with 0.05 % Triton X-100 (Sigma, USA) at 4 °C overnight for blocking. Sections were treated at 4 °C overnight with primary antibody; rabbit anti-mouse TRPM6 (Alomone Labs, Israel) with 1/50 dilution. After washing three times with PBS containing 0.05 % Triton X-100, sections were incubated with secondary antibody (Alexa488, Molecular Probes, Eugene, Oregon, USA) with 1/100 dilution for 90 min at room temperature. After washing three times with PBS containing 0.05 % Triton X-100, sections were mounted with Fluoromout (DBS, USA) and analyzed by using a fluorescent microscope (BZ-9000, Keyence, Japan).
Isolation of mouse primary trophoblasts
Mouse primary trophoblasts were isolated as described previously with some modifications . Pregnant mice (C57BL/6NCr, 14 or 18 days post-fertilization) were anesthetized and the placenta was dissected. The tissue was cut into pieces using scissors and incubated in Medium 199 (Life Technologies) containing 1 mg/ml collagenase (Sigma, USA) with 20 μg/ml DNase (Life Technologies) at 37 °C for 1 h with shaking. After the collagenase treatment, cells were mixed by pipetting (P1000, Gilson, France) 30 times, filtered with a 250 μm followed by 60-μm filter. After washing with NCTC-109 (Life Technologies), primary trophoblasts were cultured with NCTC-109 containing Glutamax, 1.65 mM cysteine, 10 % fetal bovine serum, and penicillin/streptomycin (Life Technologies) until use (from 5 to 10 h for patch-clamp recording, 18–24 h for Ca2+ imaging). Culture dishes were coated with poly-l-lysine (Sigma) before use.
Measurement of intracellular Ca2+ concentrations
Primary cultures of mouse trophoblasts or HEK293T cells were loaded with the fluorescent Ca2+ indicator Fura-2 (5 μM, Fura-2-acetoxymethyl ester, Life Technologies) in NCTC-109 medium (trophoblasts) or DMEM medium (HEK293T) at 37 °C. Cytosolic Ca2+ concentrations were measured in a standard bath solution containing 150 mM NaCl, 5 mM KCl, 1.8 mM CaCl2, 1.2 mM MgCl2, 25 mM HEPES (pH 7.4 adjusted with NaOH), and 10 mM glucose; ratiometric imaging was performed with Fura-2 at 340 and 380 nm, and the emitted light signal was read at 510 nm with a CCD camera (CoolSnap ES, Roper Scientific/Photometrics, Fairfax, USA). The ratio of F340/F380 was calculated and acquired with an imaging processing system (IP-Lab, Scanalytics Inc., USA) and ImageJ (https://imagej.nih.gov/ij/). Changes in the delta ratio were calculated by subtracting basal values from peak values, and all the values were normalized with that of ionomycin (5 μM, Sigma, USA).
Plasma membrane Ca2+ permeability was estimated by calculating an increase in intracellular Ca2+ concentration after extracellular Ca2+ application . Initial Ca2+ levels were measured before the application. Basal Ca2+ levels were determined when cells were treated with 2 mM EDTA solution without Ca2+. Next, solution containing 2 mM Ca2+ was superfused. An intracellular Ca2+ increase, which was an indicator of plasma membrane Ca2+ permeability, was determined and normalized with that of ionomycin value.
Whole-cell patch-clamp recording
Whole-cell patch-clamp recordings were carried out as described previously with some modifications . Mouse primary trophoblasts derived from pregnant mouse at 14 or 18 days after fertilization were maintained in NCTC-109 medium until recording (for 3–8 h). Human embryonic kidney-derived 293 (HEK293) T cells were transfected with 1.0 μg of empty vector pcDNA3.1 (mock), mouse TRPM6, TRPM7, or both TRPM6 and TRPM7 (each 0.5 μg), using Lipofectamine Plus Reagent (Life Technologies). Whole-cell patch-clamp recordings were performed 6–10 h after starting the culture (trophoblasts) or 14–24 h after transfection (HEK293T). Standard bath solution for TRPM6/7 contained 130 mM NaCl, 5 mM KCl, 10 mM HEPES, pH 7.4, 1 mM MgCl2, 2 mM CaCl2, 27 mM mannitol. The pipette solution contained 100 mM Cs aspartate, 20 mM CsCl, 2.93 mM CaCl2, 10 mM HEPES, pH 7.4, 4 mM Na2ATP, and 10 mM BAPTA. Data were sampled at 10 kHz and filtered at 4 kHz for analysis using an Axon 700B amplifier with pCLAMP software (Axon Instruments, USA). Membrane potential was clamped at -60 mV, and voltage ramp-pulses from −110 mV to +110 mV (400 ms) were applied every 5 s. All experiments were performed at room temperature.
Data are represented as the mean ± SEM. Statistical analysis was performed using the Student’s t test, or one-way ANOVA with Bonferroni’s post hoc test. Differences with p values of less than 0.05 were considered significant.
TRPM6 as a candidate Ca2+ entry channel of placental trophoblasts
In an attempt to clarify the molecular basis of maternal–fetal Ca2+ transport during fetal bone mineralization, we focused on TRP channels, since it had been reported that ruthenium red, a broad TRP channel blocker inhibits Ca2+ uptake activity in placental trophoblasts . To determine which TRP channel is involved, we first examined the expression levels in mice of all TRPV, TRPM, and TRPA channels in mouse placenta at 14 or 18 days post-fertilization (before or after initiation of fetal bone mineralization , Fig. S1). Among these TRP channels, we found that the expression of TRPM6 mRNA significantly increased (p < 0.01, n = 7, Student’s t test) (Fig. 1a). In contrast, expression level of TRPM7 mRNA was not significantly changed (Fig. 1b). TRPV6 was found to be expressed and increased as previously described  (Fig. S1). In this study, we focus on TRPM6 for further analysis.
Localization of TRPM6 in mouse placenta
We performed in situ hybridization in mouse placenta at 18 days post-fertilization to investigate the localization of TRPM6 mRNA. As a positive control, we used antisense probe of β-actin, which stained almost all the cells in the placenta (Fig. 1c), whereas a staining without DIG-labeled probes did not show any significant signal (Fig. 1d). TRPM6 mRNA was mainly localized in the labyrinthine trophoblasts (Fig. 1e, g). These signals were not observed when we used sense probe (Fig. 1f, h). We also performed immunohistochemistry and Western blotting to determine the protein expression of TRPM6 in the placenta at 18 days post-fertilization. In Western blotting, we observed a ~230-kDa band, which was decreased under antigen treatment (Fig. 2a; lane 3, Fig. 2b; lane 3, theoretical molecular weight of mouse TRPM6 is 233 kDa) but not in mock transfected HEK293T cells (lane 2). However, we could not get clear band in cells expressing TRPM6 and TRPM7 (lane 4). We hypothesized that this was because of the Ca2+ overload through these overexpressed TRPM6/TRPM7 heteromers, and cells might be dead as reported previously in the case of TRPV5 mutants whose intracellular Ca2+-dependent inactivation was impaired . Indeed, when we added ruthenium red (2 μM) in the medium after transfection to prevent Ca2+ overload through overexpressed TRPM6/TRPM7 heteromers, we could observe strong band in cells overexpressing TRPM6 and TRPM7 (lane 7). We could also observe a strong band in TRPM6-expressing cells, probably because of the partial heteromer formation of TRPM6 with endogenously expressing TRPM7 in HEK293T cells (lane 6). In immunohistochemistry, we used cytokeratin 7 (CK7) as a marker for trophoblasts (Fig. 2g, h). Although strong signals were seen in the nucleus, which were most likely false positives, TRPM6 signals were observed at the plasma membrane and cytosol in trophoblasts in labyrinth, colocalizing with CK7 (Fig. 2c, d, i–n). These signals were also seen in trophoblasts in other regions but expression levels were smaller compared to labyrinth. This observation was comparable to the result of in situ hybridization (Fig. 1e, g). Moreover, TRPM6 signals were decreased under the antigen treatment in antibody absorption tests (Fig. 2e, f). These results strongly suggest that TRPM6 is expressed in trophoblasts.
Ca2+ permeability in mouse primary trophoblasts and HEK293T cells
Our next question was the functional significance of TRPM6 in mouse placenta. For this purpose, we cultured mouse primary trophoblasts . In our culture condition, RT-PCR experiments indicated that placental lactogen II (a marker for placental trophoblasts), TRPM6 and TRPM7 were positive, but no signals were detected for TRPV1 or TRPV3 (Fig. 3a). These results suggest that the majority of these primary cells are trophoblasts. By using these primary cells, we measured plasma membrane Ca2+ permeability using the Ca2+ indicator Fura-2 . We used 2-aminoethoxydiphenyl borate (2-APB) with various concentrations to discriminate activities derived from TRPM6, TRPM7, TRPM6/TRPM7 heteromers, or other channels such as TRPV2 or TRPV6. These channels have been reportedly activated or inhibited by 2-APB with different EC50/IC50 values. For example, TRPM6 is activated by 2-APB with an EC50 value of 205 ± 10 μM . On the other hand, TRPM7 is inhibited with an IC50 value of 178 ± 14 μM . The Ca2+ permeability of our primary culture cells was significantly higher with application of 2 mM 2-APB (p < 0.05, n = 11–12, Student’s t test, Fig. 3b–d) but not in lower concentrations, suggesting that the Ca2+ permeability was mediated by TRPM6/TRPM7 heteromer, whose reported EC50 value was 1.6 ± 0.1 mM . There might be a small contribution of TRPV2 (EC50 = 22 μM) , or TRPV6 (2-APB inhibits TRPV6, IC50 = 68.66 μM) . Indeed, the expression level of TRPV2 mRNA was not changed much between 14 and 18 days post-fertilization (Fig. S1). Furthermore, TRPV2 activator lysophosphatidyl choline (LPC, 30 μM) did not increase the Ca2+ uptake, which supports the idea that the contribution of TRPV2 was small (data not shown).
It has been reported that TRPM6/TRPM7 heteromer exhibits channel activity under physiological concentrations of intracellular Mg2+ and ATP, but TRPM6 homomer and TRPM7 homomer do not, since homomers of TRPM6 or TRPM7 are blocked by physiological concentration of extracellular and intracellular Mg2+ . To confirm this, we investigated plasma membrane Ca2+ permeability in a HEK293T cells expressing these channels . Actually, Ca2+ permeability was significantly higher compared to the mock-transfected control when TRPM6 was co-expressed with TRPM7 (p < 0.001, n = 20, Student’s t test), but not with TRPM6 or TRPM7 alone (Fig. 3e). We assumed that TRPM6/TRPM7-expressing cells should have higher intracellular Ca2+ concentration in a steady-state condition as well because extracellular solution contained 2 mM Ca2+. We also compared the Fura-2 ratios in these cells in a steady state. However, we could not detect a significant difference between these cells (data not shown).
Patch-clamp recordings in mouse primary trophoblasts
We performed whole-cell patch-clamp experiments in mouse primary trophoblasts to confirm the functional significance of TRPM6/TRPM7 heteromer. We used intracellular and extracellular divalent cation-free conditions to enhance TRPM7-like currents. At 14 days post-fertilization, we observed Mg2+-inhibitable outwardly rectifying currents, which were significantly inhibited by 0.1 mM 2-APB (p = 0.025, one-way ANOVA with Bonferroni post hoc test, n = 7, Fig. 4a, c ). This suggests that these currents were mediated by TRPM7 homomer . We did not observe TRPV2-like currents (EC50 for 2-APB is 22 μM), again supporting the supposition that there was a small contribution of TRPV2 in mouse placental trophoblasts, at least in our condition. We also observed a fast inhibition and slow activation kinetics of 2-APB with higher concentrations (2 mM, Fig. 4a) as previously reported in cloned mouse TRPM7 . For statistics, we used the values at the fast inhibition phase. In contrast, 2-APB-inhibitable currents were not observed at 18 days post-fertilization. Membrane currents were even activated significantly by 0.5 mM 2-APB (p = 0.047, n = 5, one-way ANOVA with Bonferroni post hoc test, Fig. 4b, c) as previously reported in the TRPM6/TRPM7-coexpressing cells .
Patch-clamp recordings in TRPM6 and TRPM7 expressing HEK293T cells
We cloned mouse TRPM6 and TRPM7 cDNAs to perform whole-cell patch-clamp recording in HEK293T cells. Figure 5 shows representative time traces of membrane currents from mock (a), TRPM7 alone (b), and combined TRPM6 and TRPM7 (c) transfected HEK293T cells. We did not observe any differences between mock and TRPM6 (data not shown), as previously reported [18, 19]. Indeed, in our hands, TRPM7 currents showed almost the same 2-APB responses (Fig. 5b, d) as observed in trophoblasts at 14 days post-fertilization (Fig. 4a, c); 2-APB (0.1 mM) inhibited those currents (p = 0.002, n = 7, one-way ANOVA with Bonferroni post hoc test) and higher concentration (2 mM) exhibited the fast-inhibition and slow-activation kinetics. When TRPM6 was expressed with TRPM7, currents showed similar 2-APB responses (Fig. 5c, d) as observed in trophoblasts at 18 days post-fertilization (Fig. 4b, c); 2-APB (0.5 mM) activated those currents (p = 0.013, n = 4, one-way ANOVA with Bonferroni post hoc test).
In this study, we focused on TRPM6, a Ca2+-permeable cation channel. Our finding is that (1) TRPM6 mRNA and protein are localized in the labyrinthine trophoblasts, which have been reported to be important for the maternal–fetal Ca2+ transport. (2) The TRPM6 expression increased coinciding with fetal bone mineralization. (3) We observed endogenous TRPM6/TRPM7-like currents and intracellular Ca2+ increases likely through endogenous TRPM6/TRPM7 in mouse placental trophoblasts from 18 days, but not from 14 days post-fertilization. These results suggest that TRPM6 is functionally expressed in mouse placental trophoblasts playing a role in Ca2+ transport from mother to fetus in order to sustain fetal bone mineralization.
However, we could not observe TRPM6 homomer-like currents or Ca2+ uptake activities in placental trophoblasts. Even in a heterologous expression system in HEK293T cells, we could never observe TRPM6 homomer-like currents. To our knowledge, there were still no reports showing endogenous TRPM6 homomer, and only one vector construct can be used for the heterologous expression of TRPM6 homomer . Indeed, TRPM6 homomer is not likely to be functional in physiological Mg2+ and ATP concentrations (Fig. 3e). Taken together, it is likely that TRPM6 is functional when it forms a complex with TRPM7. We hypothesize that TRPM6/TRPM7 is the molecular identity of the apical Ca2+ entry channels in the placenta in order to sustain the maternal–fetal Ca2+ transport for fetal bone mineralization.
It was reported that the Ca2+ uptake in human primary trophoblasts was inhibited by a broad TRP channel inhibitor ruthenium red and also by extracellular Mg2+ but not by voltage-gated Ca2+ channel inhibitors . The authors speculated that the Ca2+ uptake was mediated by TRPV5 or TRPV6 or both. However, the expression of TRPV5 was restricted in the distal tubule cells of the kidney, and the TRPV5 expression in the placenta was extremely small throughout the placental development  (Fig. S1). In this study, we have found mRNA, protein, and functional expression of TRPM6/TRPM7 heteromer in mouse trophoblasts. Membrane currents were inhibited by extracellular Mg2+ and activated by 2-APB, as previously reported in exogenously expressed TRPM6/TRPM7 heteromer [15, 18] (Fig. 5). These results strongly support our above-mentioned hypothesis.
We performed Ca2+ imaging experiments to examine plasma membrane Ca2+ permeability in a physiological extracellular and intracellular divalent cation concentration. In this condition, if 2-APB could increase plasma membrane Ca2+ permeability by increasing TRPM6/TRPM7 activity, we could observe it as an increased intracellular Ca2+ concentration and actually this was the case (Fig. 3b–d). On the contrary, we could not observe any significant difference in the cells expressing TRPM6 or TRPM7 alone, most likely because these channels were blocked by physiological concentration of intracellular Mg2+ . We speculate that this is the physiological meaning of TRPM6/TRPM7. Next, we performed whole-cell patch-clamp recordings to see how the endogenous magnesium-inhibitable, TRPM7-like currents would change during late pregnancy, under extracellular and intracellular divalent cation-free conditions (intracellular ATP was used for a strong buffer for Mg2+). This condition allowed us to see TRPM7-like currents more clearly, although this was not a very physiological one. In this case, we could observe a marked difference in membrane currents between 14 and 18 days post-fertilization as described in Fig. 4. The activation by 2-APB was observed only in positive potentials, most likely because we used divalent cation-free conditions. If we use physiological concentration of extracellular Ca2+, there would be no currents in 14-day post-fertilization and it would be hard to see a difference in the aspect of TRPM7-like currents.
Regarding the plasma membrane Ca2+ permeability, we compared intracellular Ca2+ concentration among the mock-transfected cells and cells expressing TRPM6, TRPM7, or both, in a steady state. However, we could not find statistical difference of those. We hypothesized that it was because of the variation of the amount of intracellular Ca2+ buffers to avoid continuous increase of intracellular free Ca2+ concentration, which was known to be cytotoxic. Actually, in Western blotting we could confirm the cytotoxicity when TRPM6 was expressed with TRPM7 (Fig. 2a, lane 4 and 7). This result suggested that in a steady state, cells with higher intracellular free Ca2+ concentrations might be dead because of a Ca2+ overload, which might be other indirect evidence for an increase in plasma membrane Ca2+ permeability by TRPM6/TRPM7 heteromers.
It has been reported that Trpm6 or Trpm7 knockout mice (−/−) are embryonic lethal, and a statistically lower weight for Trpm6 −/− embryo compared to Trpm6 +/− or Trpm6 +/+ at 16.5, 17.5, and 18.5 days post-fertilization [20–22] was also found. Although there are so far no reports showing Ca2+ or Mg2+ composition in these embryos, these results suggest a defect of bone mineralization in Trpm6 −/− embryo due to a defect of the maternal–fetal Ca2+ and Mg2+ transport in the placenta. Future studies should focus on the contribution of TRPM6 in the maternal–fetal Ca2+ as well as Mg2+ transport. It has been shown that blood [Mg2+] was significantly lower in adult Trpm6 +/− mice compared to Trpm6 +/+ mice, like in hypomagnesemic patients with TRPM6 mutations [23, 24]. Interestingly, it has also been shown that blood [Ca2+] was lower under high Mg2+ conditions in adult Trpm6 +/− mice, suggesting that Mg2+ could compete with Ca2+ at the intestinal Ca2+ entry pathway , which was likely through a transcellular route under luminal low calcium condition. We speculate that there is a common Ca2+ and Mg2+ entry pathway in intestinal epithelium as well at least in part.
Intrauterine growth restriction (IUGR) is one of the major health problems worldwide. It has also been reported that a low birth weight resulting from IUGR might be a risk factor for other diseases such as diabetes and hypertension after growth . Although poor maternal–fetal transport of nutrition including Ca2+ might be one of the promising causes of IUGR , in most cases the molecular mechanisms of IUGR is still not understood. In human placenta, syncytiotrophoblasts play a role in the maternal–fetal nutrient transport including Ca2+. Based on our results, we propose that TRPM6 plays a role in the maternal–fetal Ca2+ transport in human syncytiotrophoblasts cooperatively with TRPV6. Smaller embryonic weight in Trpm6 −/−  as well as Trpv6 −/−  might support this hypothesis. In Trpv6 −/− mice, the maternal–fetal Ca2+ transport still remained 60 % , probably due to the compensatory role of TRPM6 in terms of Ca2+ transport. One can speculate that TRPM6/TRPM7 heteromer might be important for balky transport of divalent cations, whereas TRPV6 might be critical for more Ca2+-selective transport for a fine tuning of Ca2+ level. The association between these genes encoding these channels and human diseases including IUGR would facilitate the understanding of the functional significance of these channels.
In summary, we found that TRPM6 is functionally expressed in mouse placental trophoblasts, most likely with TRPM7. The spatio-temporal expression pattern suggests that TRPM6 plays a role in the maternal–fetal Ca2+ transport in order to sustain fetal bone mineralization. Future studies need to focus on the molecular mechanism of regulation of the maternal–fetal transport by fetal demands.
Pitkin RM (1985) Calcium metabolism in pregnancy and the perinatal period: a review. Am J Obstet Gynecol 151:99–109
Stulc J (1997) Placental transfer of inorganic ions and water. Physiol Rev 77:805–836
Twardock AR (1967) Placental transfer of calcium and strontium in the guinea pig. Am J Physiol 213:837–842
Hoenderop JG, Nilius B, Bindels RJ (2005) Calcium absorption across epithelia. Physiol Rev 85:373–422
Suzuki Y, Landowski CP, Hediger MA (2008) Mechanisms and regulation of epithelial Ca2+ absorption in health and disease. Annu Rev Physiol 70:257–271
Suzuki Y, Kovacs CS, Takanaga H, Peng JB, Landowski CP, Hediger MA (2008) Calcium channel TRPV6 is involved in murine maternal–fetal calcium transport. J Bone Miner Res 23:1249–1256
Mizuno H, Suzuki Y, Watanabe M, Sokabe T, Yamamoto T, Hattori R et al (2014) Potential role of transient receptor potential (TRP) channels in bladder cancer cells. J Physiol Sci 64:305–314
Miyamoto T, Mochizuki T, Nakagomi H, Kira S, Watanabe M, Takayama Y et al (2014) Functional role for Piezo1 in stretch-evoked Ca(2)(+) influx and ATP release in urothelial cell cultures. J Biol Chem 289:16565–16575
Watanabe M, Suzuki Y, Uchida K, Miyazaki N, Murata K, Matsumoto S et al (2015) Trpm7 Protein Contributes to Intercellular Junction Formation in Mouse Urothelium. J Biol Chem 290:29882–29892
Yamaguchi M, Ogren L, Endo H, Thordarson G, Kensinger R, Talamantes F (1992) Epidermal growth factor stimulates mouse placental lactogen I but inhibits mouse placental lactogen II secretion in vitro. Proc Natl Acad Sci USA 89:11396–11400
Loh NY, Bentley L, Dimke H, Verkaart S, Tammaro P, Gorvin CM et al (2013) Autosomal dominant hypercalciuria in a mouse model due to a mutation of the epithelial calcium channel, TRPV5. PLoS ONE 8:e55412
Moreau R, Daoud G, Bernatchez R, Simoneau L, Masse A, Lafond J (2002) Calcium uptake and calcium transporter expression by trophoblast cells from human term placenta. Biochim Biophys Acta 1564:325–332
Sommer B, Bickel M, Hofstetter W, Wetterwald A (1996) Expression of matrix proteins during the development of mineralized tissues. Bone 19:371–380
Nilius B, Weidema F, Prenen J, Hoenderop JG, Vennekens R, Hoefs S et al (2003) The carboxyl terminus of the epithelial Ca(2+) channel ECaC1 is involved in Ca(2+)-dependent inactivation. Pflugers Arch 445:584–588
Li M, Jiang J, Yue L (2006) Functional characterization of homo- and heteromeric channel kinases TRPM6 and TRPM7. J Gen Physiol 127:525–537
Neeper MP, Liu Y, Hutchinson TL, Wang Y, Flores CM, Qin N (2007) Activation properties of heterologously expressed mammalian TRPV2: evidence for species dependence. J Biol Chem 282:15894–15902
Kovacs G, Montalbetti N, Simonin A, Danko T, Balazs B, Zsembery A et al (2012) Inhibition of the human epithelial calcium channel TRPV6 by 2-aminoethoxydiphenyl borate (2-APB). Cell Calcium 52:468–480
Zhang Z, Yu H, Huang J, Faouzi M, Schmitz C, Penner R et al (2014) The TRPM6 kinase domain determines the Mg.ATP sensitivity of TRPM7/M6 heteromeric ion channels. J Biol Chem 289:5217–5227
Schmitz C, Dorovkov MV, Zhao X, Davenport BJ, Ryazanov AG, Perraud AL (2005) The channel kinases TRPM6 and TRPM7 are functionally nonredundant. J Biol Chem 280:37763–37771
Walder RY, Yang B, Stokes JB, Kirby PA, Cao X, Shi P et al (2009) Mice defective in Trpm6 show embryonic mortality and neural tube defects. Hum Mol Genet 18:4367–4375
Woudenberg-Vrenken TE, Sukinta A, van der Kemp AW, Bindels RJ, Hoenderop JG (2011) Transient receptor potential melastatin 6 knockout mice are lethal whereas heterozygous deletion results in mild hypomagnesemia. Nephron Physiol 117:p11–p19
Jin J, Desai BN, Navarro B, Donovan A, Andrews NC, Clapham DE (2008) Deletion of Trpm7 disrupts embryonic development and thymopoiesis without altering Mg2+ homeostasis. Science 322:756–760
Schlingmann KP, Weber S, Peters M, Niemann Nejsum L, Vitzthum H, Klingel K et al (2002) Hypomagnesemia with secondary hypocalcemia is caused by mutations in TRPM6, a new member of the TRPM gene family. Nat Genet 31:166–170
Walder RY, Landau D, Meyer P, Shalev H, Tsolia M, Borochowitz Z et al (2002) Mutation of TRPM6 causes familial hypomagnesemia with secondary hypocalcemia. Nat Genet 31:171–174
Morton JS, Cooke CL, Davidge ST (2016) In utero origins of hypertension: mechanisms and targets for therapy. Physiol Rev 96:549–603
Baczyk D, Kingdom JC, Uhlen P (2011) Calcium signaling in placenta. Cell Calcium 49:350–356
We thank members of the Division of Cell Signaling, Okazaki Institute for Integrative Bioscience (National Institute for Physiological Sciences, Japan) for scientific discussion and technical assistance. This work was supported by a Grant-in-Aid for Scientific Research to YS from the Ministry of Education, Culture, Sports, Science and Technology in Japan.
Conflict of interest
The authors declare that they have no conflicts of interest.
Electronic supplementary material
Below is the link to the electronic supplementary material.
Rights and permissions
About this article
Cite this article
Suzuki, Y., Watanabe, M., Saito, C.T. et al. Expression of the TRPM6 in mouse placental trophoblasts; potential role in maternal–fetal calcium transport. J Physiol Sci 67, 151–162 (2017). https://doi.org/10.1007/s12576-016-0449-0
- Maternal–fetal calcium transport