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TRPM7 silencing attenuates Mg2+ influx in cardiac myoblasts, H9c2 cells
The Journal of Physiological Sciences volume 70, Article number: 47 (2020)
TRPM7, a member of the melastatin subfamily of transient receptor potential channels, is suggested to be a potential candidate for a physiological Mg2+ channel. However, there is no direct evidence of Mg2+ permeation through endogenous TRPM7. To determine the physiological roles of TRPM7 in intracellular Mg2+ homeostasis, we measured the cytoplasmic free Mg2+ concentration ([Mg2+]i) in TRPM7-silenced H9c2 cells. [Mg2+]i was measured in a cluster of 8–10 cells using the fluorescent indicator, furaptra. TRPM7 silencing did not change [Mg2+]i in Ca2+-free Tyrode’s solution containing 1 mM Mg2+. Increasing the extracellular Mg2+ to 92.5 mM raised [Mg2+]i in control cells (1.56 ± 0.19 mM) at 30 min, while this effect was significantly attenuated in TRPM7-silenced cells (1.12 ± 0.07 mM). The Mg2+ efflux driven by Na+ gradient was unaffected by TRPM7 silencing. These results suggest that TRPM7 regulates the rate of Mg2+ influx in H9c2 cells, although cytoplasmic Mg2+ homeostasis at basal conditions is unaffected by TRPM7 silencing.
The importance of intracellular Mg2+ has been widely recognized. Mg2+ is essential for protein synthesis, the regulation of ion channels, and as a co-factor in over 600 enzymatic reactions, many of which affect cellular functions and viability . Thus, cytoplasmic free Mg2+ concentration ([Mg2+]i) should be kept in physiological range, but the molecules responsible for Mg2+ influx pathway remain to be identified. In cardiac myocytes, several candidates, such as transient receptor potential melastatin subfamily member 7 (TRPM7, non-selective cation channel) and magnesium transporter 1 (MagT1, Mg2+-selective channel), have been proposed [2, 3]. The properties of these channels have been investigated by heterologous overexpression systems [4, 5], and the data driven by these types of studies may not always be associated with the physiological functions of endogenous Mg2+-related channels.
The low rate of Mg2+ influx under physiological conditions makes it difficult to identify endogenous Mg2+ channels. We previously quantified Mg2+ entry in rat ventricular myocytes, and found that the rate of Mg2+ influx was altered by TRPM7 modulators in a concentration-dependent manner with EC50 (half-maximal effective concentration) values comparable with those reported for the TRPM7 channel activities [6, 7]. To extend these pharmacological studies suggesting that TRPM7 functions as a physiological pathway of Mg2+ influx in native cardiac myocytes, we investigated the rate of Mg2+ transport of the cardiac myoblasts (H9c2) transfected with TRPM7-targeted shRNA. Some of the results have been reported in abstract form .
H9c2 (2-1) rat-cardiac myoblast cells from ECACC were cultured on glass-bottom dishes (D11130H, Matsunami-Glass, Osaka) with DMEM including 1.8 mM CaCl2 and 0.8 mM MgSO4 (D6429; Sigma-Aldrich, St. Louis, MO) supplemented with 10% FBS, 100 U/mL penicillin G and 0.1 mg/mL streptomycin, in the 37 °C, 5% CO2 humidified incubator.
Cells were transfected by lipofectamine 2000 (Thermo Fisher Scientific, Waltham, MA) with plasmid DNA encoding shRNA of rat Trpm7 (shTRPM7) or non-targeting shRNA (shControl), and GFP gene as a marker of introduction (Sure Silencing shRNA plasmids purchased from Qiagen, Hilden, Germany). The shRNA-Trpm7 target sequence was 5′-AGCGTTTGACCAGCTTATCCTTA-3 while that for the non-targeting shRNA was 5′-GGAATCTCATTCGATGCATAC -3′. Three days after transfection, cells were used for experiments.
Cell viability and transfection efficiency
Cells (1 × 105/mL/well) were plated onto a 12-well plate with DMEM (10% FBS) one day before transfection. On several days after transfection, the cells were dissociated with TrypLE™ Express Enzyme (Gibco, Thermo Fisher Scientific), and counted with the hemocytometer to assess viability using 0.4% Trypan Blue Solution (Gibco). We also counted GFP-positive cells using the same chamber through fluorescence microscopy to check transfection efficiency.
Quantitative real-time PCR
Total RNA was isolated from H9c2 using the SV total RNA isolation system (Promega, Madison, WI) and converted to cDNA using the high-capacity reverse transcription kit (Applied Biosystems, Foster City, CA) according to manufactures’ protocols. The expression of mRNA of TRPM7 was determined by quantitative real-time polymerase chain reaction (Applied Biosystems 7500 Real-time PCR system) using TaqMan probe sets (Thermo Fisher Scientific) for rat TRPM7 (Rn00586779_m1) relative to rat glyceraldehyde-3-phosphate dehydrogenase (GAPDH, Rn99999916_s1).
The cells were homogenized by sonication in lysis buffer (150 mM Na2CO3, 1 mM EDTA-Na). These samples were mixed with sodium dodecyl sulfate (SDS) buffer (0.5 M Tris–HCl, SDS, glycerol, BPB, and β-mercaptoethanol) and boiled at 100℃ for 5 min. Proteins were separated by SDS-PAGE (6% for TRPM7, 10% for GAPDH) and electrically transferred onto a polyvinylidene difluoride membrane. The membrane was incubated with an anti-TRPM7 antibody (ab109438; Abcam Biochemicals, Bristol, UK) or an anti-GAPDH antibody (SC-25778; Santa Cruz Biotechnology, Dallas, TX, USA) followed by peroxidase-conjugated anti-rabbit IgG antibody (CST#7074S; Cell Signaling Technology, Danvers, MA, USA). The positive bands were visualized using the SuperSignal™ West Dura Extended Duration Substrate (Thermo Fisher Scientific) and the enhanced chemiluminescence system of Chemi DOC™, and then analyzed with software, imageLab4.1™ (Bio-Rad Laboratories, Hercules, CA).
Measurements and analysis of furaptra signals
The instruments and procedures for the measurements of furaptra (mag-fura-2)-fluorescence signals from cells have been described previously [7, 9]. Briefly, H9c2 cells on the glass-bottom culture dish was placed on the stage of an inverted microscope (TE300; Nikon, Tokyo) and was superfused with Ca2+-free Tyrode’s solution (see Solutions). The intracellular fluorescence was alternately excited with 350 nm and 382 nm light beams, and the fluorescence at 500 nm (25 nm bandwidth) was detected from the cluster of 8–10 cells including 4–5 cells labeled GFP. The area of illumination by excitation light was limited to the cluster size with an aperture diaphragm. After measurement of the background fluorescence from the cluster, cells were loaded with 5 μM furaptra AM (Invitrogen, Life Technologies, Carlsbad, CA) by incubation in Ca2+-free Tyrode’s solution for 14 min at room temperature, and the acetoxy methyl (AM) ester was washed out with Ca2+-free Tyrode’s solution for 10 min. Subsequent fluorescence measurements were carried out at 25 °C under Ca2+-free conditions to minimize possible cell damage and interference in the furaptra fluorescence caused by Ca2+ overloading of the cells.
The ratio of furaptra fluorescence intensities excited at 382 and 350 nm after background subtraction, R = F(382)/F(350), was converted to [Mg2+]i according to the equation:
where KD is the dissociation constant, and Rmin and Rmax are R values at zero [Mg2+] and saturating [Mg2+], respectively. We used the parameter values previously estimated in rat ventricular myocytes (KD = 5.30 mM, Rmax = 0.223  and Rmin = 0.967 ).
Influx of Ni2+ was monitored by fluorescence quenching of intracellular furaptra as previously described . We measured the decrease in furaptra fluorescence intensity excited at 350 nm (an isosbestic wavelength for Mg2+) induced by substitution of 1 mM Ni2+ for Mg2+ of Ca2+-free Tyrode’s solution at 25 °C.
Analysis of the rate of Mg2+ influx and efflux
After furaptra loading, the cells were initially perfused with high-Mg2+solution for 30 min, then were reperfused with Ca2+-free Tyrode’s solution for 30 min. [Mg2+]i was measured at ~ 5-min intervals during the perfusion. Because the [Mg2+]i rise is likely caused by the influx of Mg2+, we analyzed the rate of increase in [Mg2+]i as Mg2+ influx rate by Mg2+-loading.
The rate of Na+-dependent Mg2+ efflux was analyzed as described previously . In brief, the cells were incubated in Mg2+-loading solution (see Solutions) for varying periods up to 2 h, until [Mg2+]i was elevated to 1.5 mM or higher. Introduction of extracellular Na+ by perfusion of Ca2+-free Tyrode’s solution induced the Na+-dependent Mg2+ efflux. The initial rate of decrease in [Mg2+]i was estimated by linear regression of data points spanning for 120 s (30–150 s after solution exchange).
Ca2+-free Tyrode’s solution contained (mM): 135 NaCl, 5.4 KCl, 1.0 MgCl2, 0.33 NaH2PO4, 5.0 glucose, 10 HEPES and 0.1 K2EGTA (pH 7.40 at 25 °C by NaOH). High-Mg2+solution contained 68.5 mM MgCl2 and 24 mM MgMs2 in place of 135 mM NaCl and 1.0 mM MgCl2 of Ca2+-free Tyrode’s solution. Mg2+-loading solution contained 24 mM [Mg2+], which was prepared by substitution of 135 mM NaCl of Ca2+-free Tyrode’s solution with 101 mM NMDG-Cl (N-methyl-D-glucamine titrated HCl), 19.6 mM MgCl2 and 6.0 mM MgMs2.
Linear and nonlinear least-squares curve fitting was performed with the program Origin (Ver. 9.1, Origin Lab, Northampton, MA, USA). Statistical values are expressed as the mean ± SE. Differences between groups were analyzed by Student’s t-test or two-way repeated measure ANOVA with the significance level set at p < 0.05 (Ver. 26, IBM SPSS statistics).
In Ca2+-free Tyrode’s solution, [Mg2+]i of TRPM7-silenced H9c2 cells (shTRPM7 cells) was 0.99 ± 0.05 mM, which was similar to that of shControl cells (0.98 ± 0.08 mM) (Fig. 1A). The levels of [Mg2+]i in shControl cells were raised gradually by perfusion with high-Mg2+ solution (92.5 mM), and then partially recovered by reperfusion of Ca2+-free Tyrode’s solution containing 1 mM Mg2+. On the other hand, the increment in [Mg2+]i (1.12 ± 0.07 mM) in shTRPM7 cells was significantly smaller than that of shControl cells (1.56 ± 0.19 mM, p = 0.034) (Fig. 1A).
We confirmed whether TRPM7 was silenced in the shTRPM7 cells compared with the shControl cells. Figure 1B shows that the number of cells was not changed at least up to 144 h after transfection of both of plasmids (shControl 1.15 ± 0.12 × 105 and shTRPM7 0.98 ± 0.12 × 105 at 72 h after transfection, n = 3, p = 0.38). There was no significant difference of cell viability between shControl cells and shTRPM7 up to 144 h (91.8 ± 2.1 and 89.6 ± 3.8%, respectively, at 72 h after transfection, n = 3, p = 0.63).
Since the transfection efficiency reached about 50% in 72 h (Fig. 1B), we used the cells for experiments three days after transfection. After RNA interference with Trpm7 shRNA, the relative expression level of Trpm7 mRNA was significantly reduced to 41 ± 3% in shTRPM7 cells compared with shControl cells (Fig. 1C). The expression level of TRPM7 protein was decreased to 58.7% (Fig. 1D). To further confirm the efficiency of TRPM7 silencing, we estimated TRPM7 channel activity using Ni2+ influx monitored by quenching of intracellular furaptra. Figure 1E shows that Ni2+ quenching of fluorescence in shTRPM7 cells was significantly smaller than that of shControl cells (p = 0.009, two-way repeated measures ANOVA), suggesting impairment of TRPM7 channel activity on the plasma membrane of shTRPM7 cells.
We further investigated the effect of TRPM7 silencing on Mg2+ efflux, as shown in Fig. 2. Excessive Mg2+ in rat ventricular myocytes is mainly extruded with a Na+-gradient-dependent Mg2+-efflux system . The Na+-dependent Mg2+ efflux was induced by introduction of extracellular Na+ to the Mg2+-loaded cells (see Methods). The initial rate of decrease in [Mg2+]i was, on average, 1.02 ± 0.06 μM/s in shTRPM7 cells (n = 5) and 1.02 ± 0.21 μM/s in shControl cells (n = 5); these values were not significantly different.
TRPM7 channel has been characterized in heterologous overexpression systems, as Mg2+-permeable channel which is inactivated by intercellular free Mg2+, and its Mg2+ sensitivity is regulated by in vivo factors such as nucleotides and oxidative stress [12, 13]. Regarding cardiac myocytes, a TRPM7-like current was demonstrated in rat, guinea pig and pig ventricular myocytes [14, 15], and human atrial myocytes [3, 16]. Sah et al. described the importance of TRPM7 in maintaining cardiac automaticity in the sinoatrial node . They also reported that TRPM7 was critical for cardiogenesis based on the results of cardiac myocyte-targeted knockout mice .
The present study focuses on the involvement of TRPM7 as an Mg2+ influx pathway in cardiac myocytes. Since we have studied on Mg2+-regulation mechanisms in adult rat ventricular myocytes [2, 6, 7], we initially tried to knockdown TRPM7 in primary cultures of these cells using shRNA (+ GFP) transfection with adenovirus vector. However, it was not possible to detect a significant decrease in TRPM7-like currents in shTRPM7 (GFP positive) ventricular myocytes, probably because of protein turnover of endogenous TRPM7. Since the H9c2 cell line, myoblasts derived from rat heart, have been widely used as in vitro model of cardiac myocytes [19, 20], we used H9c2 cells to investigate the roles of TRPM7 in Mg2+ regulation.
The rate of increase in [Mg2+]i by inflow of extracellular Mg2+ is very slow . A long-time soaking in high-Mg+ solutions caused damage and morphological changes in H9c2 cells. We therefore used extremely high (92.5 mM)-Mg2+ solution to observe Mg2+ influx in a short period (30 min). TRPM7 silencing significantly decreases the rate of Mg2+ influx of the cells at high [Mg2+]o, although basal [Mg2+]i appears to still be maintained at normal [Mg2+]o. TRPM7 silencing had little effect on the activity of the Na+-dependent Mg2+ efflux. The results indicate that TRPM7 serves as a physiological Mg2+ influx pathway, but it might not be the sole key player to maintain basal [Mg2+]i in H9c2 cells.
The necessity of TRPM7 for cellular Mg2+ homeostasis in DT40 (chicken B lymphocytes) has been described by Schmitz et al. . The authors also reported that deletion of TRPM7 in DT40-upregulated expression of MagT1 compensates for the Mg2+ deficiency . In contrast, Jin and coworkers reported that tissue-specific deletion of TRPM7 in T lymphocytes of mice , and that in neural stem cells  does not alter total Mg contents of these cells. Castiglioni et al. reported that TRPM7/MagT1 co-silencing does not affect the intracellular total Mg content in human mesenchymal stem cells . Maintenance of total Mg content even in the absence of TRPM7 [24,25,26] seems in line with little change in basal [Mg2+]i with TRPM7 silencing found in the present study. Physiological roles of TRPM7 should not be denied, as reduction of Mg2+ influx via TRPM7 might be compensated by other channels/transporters with upregulated expression levels, particularly in immature cell types. Thus, Mg2+ homeostasis is likely regulated by TRPM7 in cooperation with other channels/transporters, which varies in cell types.
Although the TRPM7 gene silencing was slightly less than half in this study, it suppressed the Mg2+ influx induced by extracellular high Mg2+. To the best of our knowledge, this is the first report demonstrating that extracellular Mg2+ passes through endogenous TRPM7. This study also indicates that TRPM7 silencing inhibits the relatively rapid influx of Mg2+ driven by large concentration gradient of the ion across the cell membrane. It is then tempting to speculate that TRPM7 may exert a prominent role when [Mg2+]i falls rapidly. In cardiac myocytes, total Mg content or [Mg2+]i is known to markedly decrease under pathological conditions, such as hypoxia–reoxygenation  and heart failure . Low [Mg2+]i should activate TRPM7, and the channel may provide a rapid supply of Mg2+ from the extracellular space for recovery of [Mg2+]i.
Our results suggest that TRPM7 appreciably participates in Mg2+ influx, but it might not be indispensable for Mg2+ homeostasis in H9c2 cells. It seems reasonable to assume that Mg2+ homeostasis is concertedly regulated by several Mg2+-permeant channel/transporters including TRPM7.
Availability of data and materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
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We thank Mary Shibuya for reading the manuscript.
This work was supported by Tokyo medical university and JSPS KAKENHI grants; 15K08188 (MT), 20K11518 (MT).
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Tashiro, M., Konishi, M., Kobayashi, R. et al. TRPM7 silencing attenuates Mg2+ influx in cardiac myoblasts, H9c2 cells. J Physiol Sci 70, 47 (2020). https://doi.org/10.1186/s12576-020-00772-z
- Cardiac myoblast