TRPM7 Provides an Ion Channel Mechanism for Cellular Entry of Trace Metal Ions

Abstract

Trace metal ions such as Zn²⁺, Fe²⁺, Cu²⁺, Mn²⁺, and Co²⁺ are required cofactors for many essential cellular enzymes, yet little is known about the mechanisms through which they enter into cells. We have shown previously that the widely expressed ion channel TRPM7 (LTRPC7, ChaK1, TRP-PLIK) functions as a Ca²⁺- and Mg²⁺-permeable cation channel, whose activity is regulated by intracellular Mg²⁺ and Mg²⁺·ATP and have designated native TRPM7-mediated currents as magnesium-nucleotide–regulated metal ion currents (MagNuM). Here we report that heterologously overexpressed TRPM7 in HEK-293 cells conducts a range of essential and toxic divalent metal ions with strong preference for Zn²⁺ and Ni²⁺, which both permeate TRPM7 up to four times better than Ca²⁺. Similarly, native MagNuM currents are also able to support Zn²⁺ entry. Furthermore, TRPM7 allows other essential metals such as Mn²⁺ and Co²⁺ to permeate, and permits significant entry of nonphysiologic or toxic metals such as Cd²⁺, Ba²⁺, and Sr²⁺. Equimolar replacement studies substituting 10 mM Ca²⁺ with the respective divalent ions reveal a unique permeation profile for TRPM7 with a permeability sequence of Zn²⁺ ≈ Ni²⁺ >> Ba²⁺ > Co²⁺ > Mg²⁺ ≥ Mn²⁺ ≥ Sr²⁺ ≥ Cd²⁺ ≥ Ca²⁺, while trivalent ions such as La³⁺ and Gd³⁺ are not measurably permeable. With the exception of Mg²⁺, which exerts strong negative feedback from the intracellular side of the pore, this sequence is faithfully maintained when isotonic solutions of these divalent cations are used. Fura-2 quenching experiments with Mn²⁺, Co²⁺, or Ni²⁺ suggest that these can be transported by TRPM7 in the presence of physiological levels of Ca²⁺ and Mg²⁺, suggesting that TRPM7 represents a novel ion-channel mechanism for cellular metal ion entry into vertebrate cells.

INTRODUCTION

Aside from Ca²⁺, whose fundamental role in countless cellular processes is well recognized (Berridge et al., 1999), and Mg²⁺, which plays an important role in the modulation of ion channels, receptors, G proteins, and effector enzymes (Volpe and Vezu, 1993; Romani and Scarpa, 2000), the trace metal ions Fe²⁺, Zn²⁺, Cu²⁺, Mn²⁺, and Co²⁺ function as cofactors vital to the catalytic function of diverse types of enzymes involved in virtually every major cellular process (Hediger, 1997; Nelson, 1999). There exists a duality to metal ions: while undeniably crucial for cellular function, if they accumulate above trace levels, these ions are highly toxic. This is particularly true for zinc, which is one of the most abundant transition metals in the brain (Weiss et al., 2000; Koh, 2001). Excessive vesicular release of Zn²⁺ is thought to play a key role in neuronal cell death following ischemia (Choi and Koh, 1998). In addition to its acute neurotoxicity, zinc may contribute to the pathogenesis of chronic neurodegenerative conditions such as Alzheimer's disease, where it is found to accumulate in amyloid plaques (Lee et al., 2002).

Although considerable progress has been made toward understanding intracellular Ca²⁺ homeostasis, transport processes, and homeostatic mechanisms, the regulation of intracellular concentrations of other divalent metal ions remains poorly understood (Hediger, 1997; Nelson, 1999; Romani and Scarpa, 2000). So far, only few proteins with established roles in trace metal ion homeostasis are known: (a) the natural resistance–associated macrophage proteins (NRAMPs),* a family of proton-coupled divalent cation transporters of varying selectivity (Gunshin et al., 1997; Knopfel et al., 2000; Picard et al., 2000) and two more zinc-specific families of proteins, (b) the CDFs (cation diffusion facilitator), a family that includes the ZnT proteins, which are thought to be involved in zinc export and vesicular zinc transport (Cousins and McMahon, 2000; Harris, 2002), and the ZIPs (ZRT/IRT-related protein), a family that functions in zinc influx into cytoplasm (Gaither and Eide, 2000, 2001). However, there may be additional metal ion transport mechanisms available to cells.

Most divalent ions block ion fluxes through Ca²⁺-permeable ion channels, with the exception of Sr²⁺ and Ba²⁺, which are usually more permeable than Ca²⁺. Nevertheless, various ion channels have been implicated in transporting divalent ions other than Ca²⁺. For example, early reports on insect muscle membranes indicated permeation of Mn²⁺, Cd²⁺, Zn²⁺, and beryllium (Be²⁺) but not Co²⁺, Ni²⁺, or Mg²⁺ through calcium channels (Fukuda and Kawa, 1977). Later, endplate channels were shown to conduct trace metal ions including Co²⁺, Ni²⁺, Mg²⁺, and Cd²⁺ (Adams et al., 1980). Furthermore, voltage-dependent and receptor-mediated influx pathways may allow some Mn²⁺ influx, which can be detected as fluorescence quench of fura-2 (Merritt et al., 1989; Jacob, 1990; Shibuya and Douglas, 1992; Fasolato et al., 1993a,b). The store-operated Ca²⁺ current I_CRAC has a very small Mn²⁺ permeability that can only be resolved electrophysiologically when using isotonic Mn²⁺ solutions (Hoth and Penner, 1993). Zn²⁺ permeation has been suggested for AMPA/kainate glutamate receptors (Weiss and Sensi, 2000) and voltage-dependent Ca²⁺ channels have been reported to carry small Zn²⁺ currents (Kerchner et al., 2000). Of the TRP-related ion channels, the vanilloid receptor VR1 has been suggested to have some limited Mg²⁺ permeability (Caterina et al., 1997) and the epithelial calcium channel ECaC1 (CaT2) has been reported to conduct Cd²⁺ (Peng et al., 2000). However, in most of these cases, the transport rates of divalent cations are a fraction of those achieved by the main permeant ion species and rarely large enough to be resolved by electrophysiological recordings.

TRPM7 (Nadler et al., 2001; Runnels et al., 2001), a widely expressed member of the TRPM family of ion channels (Harteneck et al., 2000; Montell et al., 2002) is a cation channel that is regulated by intracellular levels of Mg·ATP and is strongly activated when Mg·ATP falls below 1 mM, thereby producing a current that we have designated MagNuM for magnesium-nucleotide–regulated metal ion current (Nadler et al., 2001; Hermosura et al., 2002). Our initial analyses demonstrated that TRPM7 is similarly permeable to both of the dominant divalent cations Ca²⁺ and Mg²⁺. We here report that TRPM7 is generally permeable to metal ions, exhibiting particularly high permeation of Zn²⁺, but also permeating other essential divalent cations, including Mg²⁺, Mn²⁺, and Co²⁺ as well as the nonphysiological or toxic metal ions Ba²⁺, Sr²⁺, Ni²⁺, and Cd²⁺. These results suggest that TRPM7 acts as a ubiquitous metal ion influx pathway, the existence of which may have important implications for many physiological and pathological contexts.

MATERIALS AND METHODS

Cells

HEK-293 cells transfected with the FLAG-murineTRPM7/pCDNA4/TO construct were grown on glass coverslips with DMEM medium supplemented with 10% fetal bovine serum, with 10% FBS, blasticidin (5 μg/ml), and zeocin (0.4 mg/ml). TRPM7 expression was induced 1 d before use by adding 1 μg/ml tetracycline to the culture medium. Patch-clamp and fura-2 measurements were done 16–24 h postinduction. See Nadler et al. (2001).

Solutions

Cells grown on glass coverslips were transferred to the recording chamber and kept in a standard modified Ringer's solution of the following composition (in mM): NaCl 145, KCl 2.8, CaCl₂ 1, MgCl₂ 2, glucose 10, HEPES·NaOH 10, pH 7.2, with osmolarity typically ranging from 298–308 mOsm. In some experiments, CaCl₂ and MgCl₂ concentrations were adjusted as indicated in the text and figure legends. Intracellular pipette-filling solutions contained (in mM): Cs-glutamate 145, NaCl 8, MgCl₂ 1, Cs-BAPTA 10, HEPES·CsOH, pH 7.2 adjusted with CsOH. In some experiments, 10 mM Cs-BAPTA was replaced with 200 μM fura-2 and in others 3 mM Mg·ATP was added. For equimolar ion-substitution experiments, cells were bathed in a standard Ringer's solution containing 10 mM CaCl₂ and no added MgCl₂. This solution was temporarily replaced by an identical solution that contained 10 mM of the metal-chloride salt of the relevant test ion. Isotonic solutions were prepared from Cl⁻ salts of the relevant metal ions in distilled water at concentrations of 120 mM (osmolarities, 295–310 mOsm; pH, 5.1–5.5). The effects of pH were assessed in TRPM7-overexpressing cells by applying isotonic choline-Cl solutions (150 mM) without divalent ions and buffered to various pH values using 10 mM TRIS and HCl. These solutions completely inhibited inward currents through TRPM7 down to pH values of 4 (n = 5, unpublished data), indicating that the tested isotonic metal solutions (pH = 5.1–5.5) are not contaminated with proton currents. For the case of Zn²⁺, we also prepared a choline-Cl–based solution that lacked any monovalent cations. To obtain an adequately buffered, nonprecipitating Zn²⁺ solution, a 150 mM choline-Cl solution was pH adjusted to 7.6 using TRIS base and HCl, followed by addition of 10 mM ZnCl₂, yielding a final pH of 5.5. We also performed Zn²⁺-substitution experiments where the cells were kept in a solution containing (in mM): choline-Cl 160, TRIS 10, and CaCl₂ 1, pH 7.3. Cells were then superfused with an identical solution, except that 1 mM CaCl₂ was replaced by 1 mM ZnCl₂ (Fig. 5 A, open circles). Solution changes were performed by pressure ejection from a wide-tipped pipette.

Figure 5.

Permeation of zinc. (A) Whole-cell currents were recorded in HEK-293 cells overexpressing TRPM7. Closed circles indicate cells that were kept in standard external solution containing 10 mM Ca²⁺ and 0 mM Mg²⁺, and subsequently exposed to a choline-Cl₂ based solution containing 10 mM Zn²⁺ (n = 8). Open circles show cells that were kept in a solution that contained 160 mM choline-Cl, 10 mM TRIS, and 1 mM Ca²⁺ (n = 5). During the time indicated by the black bar, cells were superfused with an otherwise identical solution, except that 1 mM Ca²⁺ was replaced with 1 mM Zn²⁺. Average inward and outward currents at −80 and 80 mV were normalized so that the inward and outward current amplitudes immediately preceding the solution change were set to 1. (B) High-resolution current records from a representative cell showing current-voltage relationships just before (control) and immediately after application of 10 mM Zn²⁺ solution. (C) Whole-cell currents were recorded in WT HEK-293 cells with experimental protocol as in (Fig. 2). Two datasets were generated (a) MagNuM was activated in the absence of Mg·ATP (n = 12) and (b) MagNuM was suppressed by 4 mM Mg²⁺ (n = 5) Datasets were averaged and subtracted to compensate for Zn²⁺ inhibition of background currents (see text). Note the different Y-axis scaling. (D) Average inward and outward currents at −80 and 80 mV, respectively, recorded in HEK-293 cells overexpressing TRPM7 in standard external solution containing 10 mM Ca²⁺ without Mg²⁺. 10 mM Gd31 (n = 3) or La31 (n = 6) were applied as indicated. Note the different Y-axis scaling.

Patch-clamp Experiments

Patch-clamp experiments were performed in the tight-seal whole-cell configuration at 21–25°C. High-resolution current recordings were acquired by a computer-based patch-clamp amplifier system (EPC-9, HEKA). Patch pipettes had resistances between 2–4 MΩ after filling with the standard intracellular solution. Immediately following establishment of the whole-cell configuration, voltage ramps of 50 ms duration spanning the voltage range of −100 to 100 mV were delivered from a holding potential of 0 mV at a rate of 0.5 Hz over a period of 300–400 s. All voltages were corrected for a liquid junction potential of 10 mV between external and internal solutions when using glutamate as intracellular anion. Currents were filtered at 2.9 kHz and digitized at 100 μs intervals. Capacitive currents and series resistance were determined and corrected before each voltage ramp using the automatic capacitance compensation of the EPC-9. In HEK-293 cells overexpressing TRPM7, data were analyzed without correcting for leak currents, since there is a significant level of basal TRPM7 conductance at the time of whole-cell establishment (break-in). The low-resolution temporal development of currents at a given potential was extracted from individual ramp current records by measuring the current amplitudes at voltages of −80 mV and 80 mV, respectively. Where applicable, statistical errors of averaged data are given as means ± SEM with n determinations and statistical significance was assessed by Student's t test.

Fluorescence Measurements and Divalent-induced Quench of Fura-2

Cytosolic calcium concentration of patch-clamped cells was monitored at a rate of 5 Hz with a photomultiplier-based system using a monochromatic light source (TILL Photonics) tuned to excite fura-2 fluorescence at 360 and 390 nm for 20 ms each. Emission was detected at 450–550 nm with a photomultiplier whose analogue signals were sampled and processed by the X-Chart software package (HEKA). Fluorescence ratios were translated into free intracellular calcium concentration based on calibration parameters derived from patch-clamp experiments with calibrated calcium concentrations. A standard Ringer's solution was used except that Ca²⁺ was increased to 2 mM. In patch-clamp experiments, 200 μM Fura-2 was added to the standard intracellular solution without BAPTA. In some cases, 3 mM Mg·ATP was added to suppress TRPM7. For Mn²⁺-, Co²⁺-, or Ni²⁺-induced fluorescence quenching, intact cells were loaded with 5 μM fura-2-AM for 45–60 min at 37°C. Fluorescence recordings were initially made in an external solution that was nominally Mg²⁺ free but contained 3 mM extracellular Ca²⁺. Quenching of the 360 nm signal (the Ca²⁺-independent wavelength of fura-2) was assessed during application of a Ringer's solution containing either 1 mM of MnCl₂, CoCl₂, or NiCl₂ plus both CaCl₂ and MgCl₂ (1 mM each) via a wide-tipped, pressure-controlled application pipette.

RESULTS

TRPM7 Provides a Significant Ca²⁺ Entry Pathway

As we have shown previously with electrophysiological recordings that TRPM7 allows permeation of Ca²⁺ (Nadler et al., 2001), we assayed [Ca²⁺]i using fura-2 to ascertain that the channel could indeed function as a significant Ca²⁺ influx pathway to cause changes in cytosolic Ca²⁺ levels. To this end, HEK-293 cells overexpressing TRPM7 were kept in a bath solution with 2 mM each of Ca²⁺ and Mg²⁺ and were perfused via the patch pipette with a Cs-glutamate–based internal solution containing 200 μM fura-2 and no other exogenous Ca²⁺ buffers. The simultaneous development of magnesium-nucleotide regulated metal current (MagNuM) was monitored under whole-cell patch-clamp by delivering, from a holding potential of 0 mV, repetitive voltage ramps that spanned −100 to 100 mV over 50 ms at a rate of 0.5 Hz. When Mg·ATP was absent in the pipette, MagNuM rapidly activated, as witnessed by the increase in both inward and outward currents (Fig. 1 A) as well as its characteristic highly nonlinear current-voltage (I-V) relationship (Fig. 1 B, dotted trace). In parallel, fluorescence measurements revealed a steady increase of [Ca²⁺]i (Fig. 1 C) that was due to Ca²⁺ influx, since [Ca²⁺]i transiently increased during periodic 5-s hyperpolarizations to −80 mV (n = 6). By contrast, in cells where the internal solution contained 3 mM Mg·ATP, there was little change in TRPM7 activity and [Ca²⁺]i remained similarly steady under these conditions. An example I-V curve under these conditions is shown in Fig. 1 B (solid trace). The progressive decrease in amplitude of hyperpolarization-driven changes in [Ca²⁺]i observed under these conditions is likely due to increased Ca²⁺ buffering as fura-2 equilibrates with the cytosol at its final concentration of 200 μM. Control uninduced cells not overexpressing TRPM7 perfused with ATP-free solutions (dotted trace in Fig. 1 C) behaved very much like TRPM7 overexpressing cells perfused with 3 mM ATP, suggesting that the increase in [Ca²⁺]i is not simply due to compromised pumping activity (although this may account for the small increase observed toward the end of the experiment). Thus, it appears that at physiologic concentrations of extracellular Ca²⁺ and Mg²⁺, activation of MagNuM allows significant Ca²⁺ entry in cells overexpressing TRPM7.

Figure 1.

TRPM7 is an influx pathway for Ca²⁺. Simultaneous whole-cell patch-clamp recordings of MagNuM and fura-2 measurements of [Ca²⁺]i in HEK-293 cells overexpressing TRPM7. (A) Average inward and outward MagNuM currents at −80 and 80 mV, respectively, in cells perfused with Cs-glutamate–based internal solution in the absence of Mg·ATP (filled circles, n = 6) and with 3 mM Mg·ATP (open circles, n = 6). Note the different Y-axis scaling. (B) Representative high-resolution current record obtained in response to a 50-ms voltage ramp from −100 to 100 mV from a cell dialyzed with 0 Mg·ATP or 3 mM Mg·ATP, respectively, showing the characteristic signature of MagNuM (strong outward rectification at potentials above 40 mV and suppression by increased Mg·ATP concentrations). Note that data are not leak-corrected which reveals a reversal potential of −33.2 mV ± 2.4 mV (n = 9) under conditions where intracellular calcium is weakly buffered. (C) Average intracellular Ca²⁺ signals recorded from cells patched in A showing a steady rise in [Ca²⁺]i in the absence of Mg·ATP. In contrast, [Ca²⁺]i remains at steady basal levels when TRPM7 is blocked by 3 mM Mg·ATP or in control uninduced HEK-293 cells not overexpressing the channel (dotted line, n = 5).

Equimolar Substitution of 10 mM Extracellular Ca²⁺ by Transition Metals

To assess the permeation of other divalent ions, we measured ionic currents in ion-substitution experiments, where 10 mM Ca²⁺ was replaced with 10 mM of other divalent cations. In these experiments, HEK-293 cells overexpressing TRPM7 were kept in a bath solution containing 10 mM Ca²⁺, without Mg²⁺, and development of TRPM7-mediated current was monitored by whole-cell patch-clamp. When the current had reached maximal amplitude, cells were transiently exposed to an extracellular solution containing 10 mM of the test cation applied for a period of 60 s. As illustrated in Fig. 2 , this resulted in characteristic changes of TRPM7-mediated inward and outward currents. To better compare the responses and compensate for variations in expression levels, we normalized both outward and inward currents such that the current magnitudes just before substituting Ca²⁺ were set to 1. The changes in inward current are therefore relative to the current magnitude of 10 mM Ca²⁺.

Figure 2.

Equimolar substitution of 10 mM Ca²⁺ by transition metals. Whole-cell currents were recorded in HEK-293 cells overexpressing TRPM7 kept in a bath containing 10 mM Ca²⁺, without Mg²⁺, and exposed for 60 s to an otherwise identical external solution where 10 mM Ca²⁺ was equimolarly replaced by the test cation. Average inward and outward currents at −80 and 80 mV were scaled so that the inward and outward current amplitudes immediately preceding the solution change were set to 1. (A) Exposure to 10 mM Zn²⁺ (n = 5) and 10 mM Ni²⁺ (n = 9) caused a large increase of the inward current, with a block of the outward current. (B) Left, 10 mM Co²⁺ (n = 3), 10 mM Mg²⁺ (n = 5), and 10 mM Mn²⁺ (n = 5) caused a slight to moderate increase of the inward current, with a block of the outward current. Right, 10 mM Ba²⁺ (n = 6), 10 mM Sr²⁺ (n = 3), and 10 mM Cd²⁺ (n = 3) caused a slight to moderate increase of the inward current, and increase of the outward current. Note that the first 150 s of whole-cell time are not shown. (C) Representative high-resolution current record obtained in response to a 50-ms voltage ramp from −100 to 100 mV from a cell before (200 s whole-cell time, dotted control) and during (250 s whole-cell time, thick continuous line) application of 10 mM Ni²⁺ showing the right shift in reversal potential during application. Note that data are not leak-corrected, which reveals an average control reversal potential of −7.2 mV ± 2.7 mV (n = 39) under conditions where intracellular calcium is buffered to zero. The black open box indicates the area shown enlarged in the inset to the left. (D) The bottom panel shows the rank order of permeation through TRPM7 based on percentage increase (±SEM) of the inward current when carrying the test cation relative to the current magnitude at 10 mM Ca²⁺. The top panel plots effects on the outward current as percent increase or inhibition (±SEM) for each divalent cation.

As shown in Fig. 2 A, Zn²⁺ and Ni²⁺ caused a large increase of the inward current, combined with a block of outward currents. For both Zn²⁺ and Ni²⁺, the increase in inward current was greater than threefold compared with Ca²⁺ (Fig. 2 D), whereas the inhibition of outward currents was less pronounced for Zn²⁺ than for Ni²⁺. Ba²⁺ was able to sustain significant inward currents (Fig. 2 B, right, and D), but without suppression of outward currents. Co²⁺, Mg²⁺, and Mn²⁺ are characterized by more modest (less than double) increases of inward currents, but were again accompanied by a block of outward currents (Fig. 2 B, left, and D). The strongest block of outward currents was caused by Mg²⁺, which we have previously shown is a strong independent suppressor of MagNuM currents (Nadler et al., 2001; Hermosura et al., 2002). Ba²⁺, Sr²⁺, and Cd²⁺ application increased the outward current with only slight to moderate increases of the inward current (Fig. 2, C and D).

In the experiments described above, where we substituted 10 mM Ca²⁺ with other divalent ions, one would expect the reversal potential (E_rev) to shift at least to some degree toward depolarized potentials. Indeed, this could be observed for all ion species investigated. Compared with control E_rev values (−7.2 ± 2.7 mV; n = 39), Ba²⁺ and Ni²⁺ had the strongest effect in shifting E_rev by 19 ± 7, (n = 5) and 15 mV ± 3 (n = 6), respectively. At 10 mM concentration, the other ion species investigated had less dramatic effects, ranging between 2 mV ± 0.7 mV for Cd²⁺ (n = 5) and 8.5 mV ± 4.7 mV for Co²⁺ (n = 5). Example I-V curves are shown in Fig. 2 C. Here a cell was superfused with a 10 mM Ni²⁺-containing extracellular Mg²⁺-free solution. The currents just before application (at 200 s whole-cell time) and during application (at 230 s) are shown. It can be seen that there is a remarkable increase in inward currents with some suppression of the outward current. This is accompanied by a clear depolarizing shift of E_rev, as detailed by the magnified inset. However, as will be discussed later, a quantitative assessment of reversal potentials under these conditions is rather difficult and error prone, because the current is highly nonlinear and has a very shallow slope around E_rev, so that relatively small changes in TRPM7 current and/or Ca²⁺-dependent conductances may shift it.

Permeation Profile of Transition Metals in Isotonic Solutions

The above experiments reveal the surprising finding that TRPM7 is not only permeable to Ca²⁺ ions, but in fact even more permeable to a range of divalent transition metal ions and their permeation yields considerable macroscopic inward currents at negative membrane potentials. It should be noted, however, that the above experiments cannot unequivocally rule out possible ion–ion interactions that would allow the cotransport of, e.g., Na⁺ ions along with divalent ions. To avoid such complications, we next studied permeation of divalent ions in isotonic extracellular solutions of each metal ion. Isotonic solutions of 120 mM of each divalent ion species yielded appropriate osmolarities within 10% of the standard bath and pipette solutions and pH values in the range of 5.1 to 5.5, which do not compromise accurate assessment of inward currents carried by TRPM7 (see materials and methods). However, Zn²⁺ and Cd²⁺ had to be excluded from this analysis, as isotonic solutions of Zn²⁺ yielded pH values below 3 and 120 mM Cd²⁺ solutions were hypo-osmotic.

In this series of experiments, the cells were initially bathed in the standard external solution containing 1 mM Ca²⁺ and 2 mM Mg²⁺. At 200 s, when TRPM7 current had reached its full amplitude, isotonic solutions of each metal ion were applied for 60 s via a puffer pipette. The development of inward and outward currents before, during, and after application of selected isotonic divalent cation solutions are shown in Fig. 3 A for the case of Ca²⁺ and Mg²⁺. The experiments illustrate that isotonic Ca²⁺ and Mg²⁺ solutions induce an increase in inward currents through TRPM7, while at the same time there is strong suppression of outward currents. We next performed essentially identical experiments for a range of divalent metal ions and illustrate the average responses in Fig. 3, B–D. Example I-V curves measured before (200 s whole-cell time) and during isotonic Ni²⁺ application (at 230 s) are shown in Fig. 3 E. Note the dramatic increase in inward currents, the substantial reduction of the outward currents, and the significant depolarizing shift of E_rev, which is detailed in the data inset of the graph. When analyzing these data for the relative increase of inward currents, it can be seen that, with the exception of Mg²⁺, the rank order of permeation under isotonic conditions (Fig. 4, A and B) is virtually identical with the permeation sequence observed in the 10 mM ion substitution experiments (Fig. 2 D).

Figure 3.

Permeation of transition metals in isotonic solutions. Whole-cell currents were recorded in HEK-293 cells overexpressing TRPM7 in standard external solution containing 1 mM Ca²⁺ and 2 mM Mg²⁺, and subsequently exposed to an isotonic solution of the test cation for 60 s. Average inward and outward currents at −80 and 80 mV were scaled so that the inward and outward current amplitudes immediately preceding the solution change were set to 1. (A–D) Exposure to isotonic Ca²⁺ (n = 4), Mg²⁺ (n = 5), Co²⁺ (n = 3), Mn²⁺ (n = 5), Ba²⁺ (n = 3), Sr²⁺ (n = 7), and Ni²⁺ (n = 4) elicits characteristic responses for each cation. Data for Ca²⁺ and Mg²⁺ reprinted in normalized form by permission from Nature [Nadler et al., 2001], copyright 2001 Macmillan Publishers Ltd. (E) Representative high-resolution current records obtained in response to a 50-ms voltage ramp from −100 to 100 mV from a cell before (control at 200-s whole-cell time) and during (230-s whole-cell time) exposure to isotonic Ni²⁺ illustrating the right shift in the reversal potential. Note that data are not leak-corrected which reveals an average control reversal potential of −7.2 ± 2.7 mV (n = 39) under conditions where intracellular calcium is buffered to zero. The black open box indicates the area shown enlarged in the inset to the left.

Figure 4.

(A) The rank order of permeation through TRPM7 based on percentage increase (±SEM) of the peak inward current. Note that increases are relative to 1 mM Ca²⁺. (B) Corresponding effects on the outward current as percent inhibition (±SEM). (C) Relative shift of reversal potential (E_rev) to control E_rev (see below) induced by application of the respective isotonic divalent solutions. Data are sorted according to the rank order of inward current (A). The average control reversal potential before application was −7.5 mV ± 5 mV (n = 31) as indicated in Fig. 3 E. (D) Mn²⁺-induced quench of fura-2 fluorescence at 360 nm excitation in HEK-293 cells induced to overexpress TRPM7 (n = 3) and transfected cells that remained uninduced (n = 3). Cells were kept in an extracellular solution supplemented with 3 mM Ca²⁺ and 0 Mg²⁺. The application contained 1 mM Mn²⁺, 1 mM Ca²⁺, and 1 mM Mg²⁺ as divalent ions. (E) Ni²⁺-induced quench of fura-2 fluorescence at 360 nm excitation in HEK-293 cells induced to overexpress TRPM7 (n = 4) and control cells that remained uninduced (n = 3). Cells were kept in an extracellular solution as described in Fig. 4 D. The application contained standard solution with 1 mM Ni²⁺, 1 mM Ca²⁺, and 1 mM Mg²⁺ as divalent ions. (F) Co²⁺-induced quench of fura-2 fluorescence at 360 nm excitation in HEK-293 overexpressing TRPM7 (n = 3) and in uninduced control cells (n = 3). Cells were kept in an extracellular solution as described in Fig. 4 D. The application contained standard solution with 1 mM Co²⁺, 1 mM Ca²⁺, and 1 mM Mg²⁺ as divalent ions.

The analysis of E_rev changes induced by isotonic ion application reveals that, similarly to the 10 mM ion substitution experiments, Ni²⁺ induced the strongest depolarizing shift (35.4 ± 2.7 mV, n = 4, P >> 0.001; Fig. 4 C). Similarly, Mg²⁺ caused the weakest potential change (+10 ± 8 mV, n = 4; Fig. 4 C). The effects of the other ions ranked in between those two extremes. It should be noted that the data used for current and reversal potential analysis were not leak subtracted and are relative measurements.

Physiological Ca²⁺ Levels Do Not Impede Trace Metal Permeation Through TRPM7

TRPM7 mediates low levels of homeostatic plasma membrane Ca²⁺ fluxes and several Ca²⁺ influx pathways are known to also allow permeation of Mn²⁺ ions (Shuttleworth and Thompson, 1999). Therefore, we hypothesized that TRPM7, if it carried Mn²⁺ as well, may underlie experimental observations of unstimulated Mn²⁺ entry that lead to fluorescence quenching of fura-2 in resting cells (Mertz et al., 1990; Shuttleworth and Thompson, 1996). If this were the case, it would also hand us a tool to test whether the presence of physiological Ca²⁺ and Mg²⁺ concentrations, the major divalent ion species in extracellular fluids, affect the permeation of other divalent ions through TRPM7 channels.

To test these hypotheses, HEK-293 cells overexpressing TRPM7 as well as control cells were loaded with fura-2-AM and bathed in the standard external solution containing 3 mM Ca²⁺ and 0 mM Mg²⁺. Fluorescence was monitored at 360 nm excitation (the isosbestic or Ca²⁺-independent wavelength of fura-2) and after 20 s, an external solution that maintained the total divalent concentration but contained 1 mM Mn²⁺, 1 mM Ca²⁺, and 1 mM Mg²⁺ was applied for 180 s. As shown in Fig. 4 D, the application of 1 mM Mn²⁺ caused a pronounced quench of the 360 nm signal in HEK-293 cells induced to overexpress TRPM7 (n = 3), whereas Mn²⁺-induced quench of the 360-nm signal in uninduced cells, which express only low levels of endogenous TRPM7, was much less dramatic (n = 3). Linear regression over the initial 60 s of Mn²⁺ exposure yielded a fourfold higher rate of Mn²⁺-induced quench of fura-2 fluorescence in induced cells (28%/min) as compared with uninduced cells (6.6%/min). Similar experiments were performed for Ni²⁺, where an external solution containing 1 mM Ni²⁺, 1 mM Ca²⁺, and 1 mM Mg²⁺ was applied for 180 s. Fig. 4 E confirms that Ni²⁺ too can permeate TRPM7 in the presence of Ca²⁺ and Mg²⁺. Here, the fura-2-induced fluorescence quench increases about twofold from 10%/min in uninduced cells (n = 3) to 20%/min in TRPM7 overexpressing cells (n = 4). Finally, using Co²⁺ in the presence of Ca²⁺ and Mg²⁺ also increases the rate of fura-2 fluorescence quenching (by about threefold) in TRPM7 overexpressing cells (17%, n = 3; Fig. 4 F) versus uninduced controls (5.4%, n = 3). These results clearly suggest that basal, unstimulated TRPM7 activity in intact cells constitutes an important pathway for Mn²⁺, Co²⁺, and Ni²⁺ entry that is significantly augmented by TRPM7 overexpression and can occur in the presence of physiological levels of Ca²⁺ and Mg²⁺.

Permeation of Zn²⁺and Trivalent Ion Species

Based on its physiological and pathophysiological importance, we wanted to ascertain that Zn²⁺ permeation of TRPM7 occurs under experimental conditions in which it was the only permeable ion species available to carry inward currents through TRPM7 channels. As the pH of isotonic Zn²⁺ solutions precluded such experiments, we used the same experimental approach as in Fig. 2, but tested Zn²⁺ permeation by applying 10 mM Zn²⁺ in a choline-Cl–based extracellular solution (see materials and methods). As illustrated in Fig. 5 A, this resulted in a significant enhancement of inward currents in HEK-293 cells overexpressing TRPM7, indicating that Zn²⁺ is indeed a highly permeable ion species of TRPM7. The current-voltage relationships of TRPM7 before and during Zn²⁺ application are illustrated in Fig. 5 B, demonstrating the increase in inward current at negative potentials and the decrease in outward currents at positive potentials, consistent with significant Zn²⁺ permeation. To further confirm Zn²⁺ permeation in the absence of other permeant ion species, we performed experiments where TRPM7-overexpressing cells were kept in a choline-Cl–based solution supplemented with 1 mM Ca²⁺ instead of our standard external solution (see materials and methods; Fig. 5 A, open circles, n = 5). At the time indicated, patch-clamped cells were then superfused with 1 mM Zn²⁺ and 0 mM Ca²⁺ in an otherwise identical choline-Cl solution. The results show that even at 1 mM concentration, Zn²⁺ alone is more effective in supporting inward current than equimolar Ca²⁺.

We also attempted to measure Zn²⁺ permeation under more physiological conditions, i.e., in the presence of physiological Ca²⁺ and Mg²⁺ concentrations. To this end, we allowed TRPM7 currents to develop in standard extracellular solutions containing 1 mM Ca²⁺ and 2 mM Mg²⁺ and applied the same solution, but additionally contained 1 mM Zn²⁺. Under these conditions, we again observed a clear increase in inward current and a concomitant small decrease in outward current (Fig. 5 C), suggesting additive permeation of Zn²⁺ ions in the presence of Ca²⁺ and Mg²⁺.

Finally, to address the issue of whether Zn²⁺ permeation is also characteristic of native, endogenously expressed TRPM7, we recorded MagNuM in wild-type (WT) HEK-293 cells under identical conditions as the ion-substitution experiments in Fig. 2, with 10 mM Zn²⁺ as the replacement ion. Since Zn²⁺ inhibits resting background currents unrelated to TRPM7, we corrected for this effect by first measuring Zn²⁺ effects in cells where MagNuM was completely suppressed by pipette solutions containing 4 mM Mg²⁺ (n = 5). These data were averaged and subtracted from the average dataset obtained from cells where MagNuM was activated by omission of pipette Mg·ATP (n = 12). As can be seen in Fig. 5 D, we observed a significant increase in inward current when applying Zn²⁺, indicating that Zn²⁺ permeation is a genuine physiological property of native TRPM7 channels.

We also tested for permeation of trivalent metal ions La³⁺ and Gd³⁺ (Fig. 5 E). We first assessed concentrations that normally completely suppress voltage- or store-operated Ca²⁺ channels and found that 10 μM of either Gd³⁺ or La³⁺ were rather ineffective in suppressing inward or outward currents carried by TRPM7 (<5% inhibition, n = 3–4, unpublished data). However, at 10 mM, both ions almost completely inhibited inward currents (n = 5 each), although the effects on outward currents were markedly different. Provided that the inhibitory action of Gd³⁺, and to a smaller degree that of La³⁺, is mediated at the cytosolic side, there may in fact be a very small permeation of this ion that cannot be unambiguously resolved at the macroscopic current level.

DISCUSSION

The main result of the present study is that the widely expressed ion channel TRPM7, which has previously been shown to conduct both Ca²⁺ and Mg²⁺ (Nadler et al., 2001) is permeable to a variety of divalent cations with a permeability sequence of Zn²⁺ ≈ Ni²⁺ >> Ba²⁺ > Co²⁺ > Mg²⁺ ≥ Mn²⁺ ≥ Sr²⁺ ≥ Cd²⁺ ≥ Ca²⁺ and they do so even in the presence of physiological levels of Ca²⁺ and Mg²⁺. Our results therefore suggest that TRPM7 represents a novel and unique ion-channel pathway for cellular divalent ion transport.

TRPM7 Constitutes a Significant Ca²⁺ Entry Pathway

Previous electrophysiological characterizations of TRPM7 have established that the channel is capable of sustaining small inward currents carried by Ca²⁺ ions (Nadler et al., 2001; Runnels et al., 2001). We have extended this analysis in the present work by combined fluorescence and patch-clamp measurements (see Fig. 1), which demonstrate that TRPM7 can drive significant changes in [Ca²⁺]i after depletion of intracellular Mg·ATP in cells that overexpress the protein. In native cells, where expression levels of endogenous TRPM7 is smaller, the contribution of MagNuM may be more subtle and may not lead to measurable changes of free [Ca²⁺]i. However, given that TRPM7 is constitutively active to a small degree, even in the presence of physiological Mg·ATP, it can well serve a function of a homeostatic mechanism for basal, background influx of Ca²⁺ that could be used to maintain the filling state of intracellular stores. The notion that MagNuM constitutes a background pathway for divalent cation fluxes is supported by the fluorescence quenching experiments illustrated in Fig. 4, where a small basal entry of Mn²⁺, Co²⁺, and Ni²⁺ is observed in intact uninduced cells, which is presumably due to native MagNuM and is augmented significantly in cells overexpressing TRPM7. From this, it seems reasonable to assume that the same pathway can support a steady, low-level transport of Ca²⁺ ions, which under extreme conditions, such as prolonged ischemia, can lead to a persistent strong activation of TRPM7 that ultimately may contribute to Ca²⁺ overload and cell death.

Equimolar Substitution of 10 mM Extracellular Ca²⁺ by Transition Metals

In general, all tested divalent metal ions were at least as efficient and some even considerably more so than Ca²⁺ in permeating TRPM7 at negative membrane potentials. Zn²⁺ and Ni²⁺ caused the largest increases of the inward current, which were greater than threefold compared with Ca²⁺. We also observed significant Ba²⁺ inward currents, while Co²⁺, Mg²⁺, and Mn²⁺ exhibit more modest (less than double) increases of inward currents and Sr²⁺ and Cd²⁺ still supported slightly larger inward currents than Ca²⁺ itself. Taken together, the changes in inward current observed under 10 mM ion substitution experiments are most consistent with the interpretation that TRPM7 is as or more permeable to each of the divalent cations than Ca²⁺.

At the same time, most of these ions produced a significant block of outward currents, the strongest one caused by Mg²⁺, which we have shown previously is a potent intracellular suppressor of MagNuM currents (Nadler et al., 2001; Hermosura et al., 2002). In contrast, Ba²⁺, Sr²⁺, and Cd²⁺, while still supporting inward currents, actually increase outward currents. The outward currents at positive potentials represent currents of monovalent ions at potentials where divalent ions do not experience sufficient driving force to enter the cell and therefore no longer impede monovalent outward fluxes (Hille, 1992). Their behavior is a complex function of several factors including (a) the Nernst equilibrium potential of divalent ions, which determines the degree of permeation block imposed by divalent ions at positive membrane potentials, and (b) inhibitory effects of the relevant divalent ion at the intracellular side of the channel after it has gained access to the cytosol. The latter effects are particularly relevant for Mg²⁺, which plays an important role as a cofactor in the gating of TRPM7 and therefore can inactivate MagNuM if allowed to accumulate intracellularly. A similar inhibition is also seen with Ca²⁺, which is the second major physiological divalent cation to permeate TRPM7. However, under our experimental conditions, the inhibitory action of [Ca²⁺]i may be more limited due to the inclusion of 10 mM BAPTA and may be only noticeable under the weakly buffered conditions during calcium signaling measurements (e.g., Fig. 1). Based on the inhibitory actions of a number of divalent ions, we propose putative divalent binding site(s) at the cytosolic side of TRPM7 that modify channel gating and/or cause channel block. Different divalent ions may possess different affinities to this site, resulting in the observed inhibition of outward currents. Thus, the efficacy of any divalent ion to block outward currents would be the net result of its degree of permeation, its ability to accumulate in the cytosol in the presence of BAPTA, and its inhibitory potency on the channel.

Permeation of Transition Metals in Isotonic Solutions

In general agreement with the 10 mM substitution experiments, application of isotonic divalent solutions resulted in increased inward currents and inhibition of outward currents. The permeation sequence of divalent ions, based on peak inward currents, is similar to the sequence we observed with 10 mM ion-substitution experiments. Thus, in the absence of data for isotonic Zn²⁺, Co²⁺ and Mn²⁺ are the best-permeating physiologically relevant metal ions, followed by Ca²⁺ and Mg²⁺. Among the toxic or nonphysiologic metals, Ni²⁺ is the most permeable, followed by Ba²⁺ and Sr²⁺. This permeation sequence, in combination with the facilitated transport of trace metals over Ca²⁺ and Mg²⁺, is unprecedented for any ion channel presently known. TRPM7 must therefore be considered an important influx pathway for both essential and toxic metal ions. With respect to outward currents, isotonic solutions invariably caused a complete or almost complete inhibition, with the exception of Ba²⁺ and Sr²⁺, which were less effective in doing so. As in the equimolar substitution experiments, here too the efficacy of permeation and its rank order would be a net result of the three factors discussed above (permeation, intracellular ion accumulation and negative feedback on the channel). In most cases, the currents recovered close to their original sizes within ∼150 s, suggesting that none of the divalent ion species exert any lasting effect on channel integrity.

Together, both datasets (10 mM equimolar replacements and isotonic substitutions) are complementary in that they address interpretation issues that arise under either experimental condition. For example, the acidic nature of isotonic divalent solutions prevented us to conduct experiments with isotonic Zn²⁺. This was not a concern with the 10 mM replacement experiments. Most importantly, however, the two datasets show that in the case of TRPM7, the rank order of the permeation sequence is essentially independent of the divalent ion concentration and the presence of monovalent cations (Na⁺). The latter aspect is in marked contrast to observations made with other channels that are able to conduct divalent ions other than Ca²⁺.

Trace Metal–induced Current Augmentation Shifts E_rev

Ion permeation ratios are usually calculated based on reversal potentials using Nernst and Goldmann-Hodgkin-Katz (GHK) equations (Hille, 1992), which make the important implicit assumption that ions can permeate independently. Applying this formalism to TRPM7 currents, which normally reverse around 0 mV, would lead to the conclusion that the channel is nonselective (Runnels et al., 2001). However, since TRPM7 conducts divalent ions into the cell and monovalent ions out of the cell (Nadler et al., 2001), the independence rule is questionable and, therefore, correct permeability ratios based on GHK-based calculations do not adequately describe this channel's permeability. Nevertheless, if other ions preferentially permeate TRPM7 over Ca²⁺, one would expect a depolarizing shift in the reversal potential when applying the 10 mM equimolar or isotonic replacement solutions. Indeed, analyses of reversal potentials revealed that all divalent ions caused a shift in E_rev toward more positive potentials. The rank order of the E_rev values under isotonic conditions fairly well reflected the rank order observed for the inward currents, with Ni²⁺ inducing the strongest change (35 mV) and Mg²⁺ the weakest (10 mV). This indicates that unlike observations made in other channels (Adams et al., 1980) the apparent permeability of TRPM7 for divalent ions is independent of their concentration.

Reversal potential measurements of TRPM7 are clearly very difficult, because the current is highly nonlinear and has a very shallow slope around E_rev. Therefore, even small changes in the activity of other ion currents may have an effect on E_rev. In particular, we have noticed that the current magnitude as well as E_rev of TRPM7 is influenced by intracellular Ca²⁺ concentrations. In experiments where intracellular Ca²⁺ remained largely unbuffered (e.g., Fig. 1 B), there is a slight shift of the apparent E_rev to more hyperpolarized potentials (−33 mV) compared with standard conditions in which [Ca²⁺]i is buffered to zero using 10 mM Cs-BAPTA (here, E_rev values are around −7.2 mV, Fig. 2 C). We have not assessed the reasons for this shift, but we suspect that elevated [Ca²⁺]i may cause small increases in Ca²⁺-activated K⁺ or Cl⁻ channels, which may account for the observed changes in E_rev.

TRPM7-mediated Trace Metal Entry Occurs at Physiological Levels of Ca²⁺ and Mg²⁺

Our present work extends our previous observation that TRPM7 sustains Ca²⁺ and Mg²⁺ currents under isotonic conditions (Nadler et al., 2001) by demonstrating other divalent ions to do so. An important question is whether these divalent ions permeate TRPM7 in the presence of physiological levels of extracellular Ca²⁺ and Mg²⁺? Our results presented in Fig. 4 demonstrate that physiological concentrations of Ca²⁺ and Mg²⁺ do not impede the permeation of Mn²⁺, Ni²⁺, or Co²⁺, providing strong evidence that neither Ca²⁺ nor Mg²⁺ interfere with the permeation of trace metal ions and that divalent ions permeate independently. Thus, the probability of a trace metal ion to pass through the channel would depend on its relative permeability and relative concentration levels. For example, the data in Fig. 2 D show that Zn²⁺ has a fourfold higher chance over Ca²⁺ to permeate TRPM7. Thus, one can presume that under physiological divalent concentrations (i.e., 1 mM Ca²⁺, 1 mM Mg²⁺, and 10 μM Zn²⁺ plasma levels; Perveen et al., 2002), ∼1 in every 50 ions passing through the channel will be a Zn²⁺ ion.

Permeation of Zn²⁺and Trivalent Ion Species

Zn²⁺ plays an important physiological role in many cellular functions. Our results under experimental conditions where Zn²⁺ is the only permeable ion species available to support inward currents through TRPM7 channels demonstrate that Zn²⁺ permeates TRPM7 at least twice as well as Ca²⁺, indicating unequivocally that Zn²⁺ is indeed a highly permeable ion species of TRPM7. Moreover, this finding could be confirmed in native TRPM7-like MagNuM currents. In the brain, Zn²⁺ levels can reach 100–500 μM during ischemia, making TRPM7 an attractive candidate for mediating at least part of the severe neurotoxic effects observed with this ion (Choi and Koh, 1998; Koh, 2001), particularly in view of the fact that Mg·ATP levels during ischemia are reduced and would favor activation of MagNuM.

Although TRPM7 clearly allows passage of divalent ions, the situation is less clear for trivalent ion influx. Generally, trivalent ions such as Gd³⁺ or La³⁺ are known to efficiently inhibit voltage- or store-operated Ca²⁺ channels at μM concentrations. However, when testing these two ion species, we found that 10 μM of either Gd³⁺ or La³⁺ were rather ineffective in suppressing inward or outward currents carried by TRPM7. At 10 mM concentrations, both ions completely inhibited inward currents with no sign of significant permeation. Interestingly, Gd³⁺ caused a complete suppression of outward currents. It remains to be determined whether this is due to some limited Gd³⁺ entry that causes block of outward currents from the cytosolic side. By contrast, the same concentration of La³⁺, while potently and persistently inhibiting inward currents, was far less effective in suppressing outward currents, which might be interpreted such that its action is limited to the extracellular side of the plasma membrane.

Conclusions

The present study establishes that the TRPM7 ion channel possesses the so far unique ability to efficiently conduct many nutritionally essential trace metal ions, along with several toxic metal ions as well. This ability in conjunction with its widespread expression and constitutive activity suggests that TRPM7 underlies a ubiquitous mechanism for entry of metal ions into cells. A remarkable finding is that trace divalent metal ions are significantly more permeable through TRPM7 than the dominant physiological divalent metal ions Ca²⁺ and Mg²⁺. Such a property would help ensure that trace metals are able to effectively compete with these ions and enter into cells despite their concentration disadvantage relative to the millimolar amounts of Ca²⁺ and Mg²⁺ typically present in extracellular fluids. Furthermore, the discovery of the general nature of metal ion permeation via TRPM7 has important implications for our understanding of pathophysiological situations in which toxicity of TRPM7-permeant ions is thought to play a role. Due to its transport capacity of this and other metal ions, TRPM7 may represent an intriguing target pathway for therapeutic intervention in these situations.