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.

The Mg2+ and Mg2+-Nucleotide-Regulated Channel-Kinase TRPM7

“…The unusual permeation profile of TRPM7 is further evident from ion substitution experiments in which other divalent cations were tested for permeation relative to Ca 2+ with the sequence Zn 2+ ≈ Ni 2+ ≫Ba 2+ >Co 2+ >Mg 2+ ≥Mn 2+ ≥≥Sr 2+ ≥Cd 2+ ≥Ca 2+ (Monteilh-Zoller et al 2003). Interestingly, it has recently been proposed that the selectivity of TRPM7 may be regulated by extracellular pH (Jiang et al 2005).…”

“…The asymmetrical permeation properties of TRPM7, with exclusive divalent ion influx at negative voltages and exclusive efflux of monovalent cations at high positive voltages, as well as the pronounced block of monovalent cation permeation by divalent cations, renders classical analyses of selectivity based on constant field equations inaccurate, since these rely on the assumption that the permeating ion species move through the channel independently of each other. Therefore, TRPM7 cannot be classified as a nonselective cation channel, and the proposed permeability of cations relative to that of Cs + of 1.1, 0.97, and 0.34 for K + , Na + , and Ca 2+ , respectively (Runnels et al 2001), does not reflect the channel's true selectivity (Monteilh-Zoller et al 2003).…”

“…On the other hand, in retinoblastoma cells, the role of Ca 2+ influx through TRPM7 may be more significant, since molecular suppression of TRPM7 in these cells by small interfering RNA (siRNA) silencing markedly reduces TRPM7 currents and the magnitude of resting Ca 2+ influx, which is paralleled by decreased TRPM7 immunoreactivity, decelerated cell proliferation, and retarded G1/S cell cycle progression (Hanano et al 2004). In addition to Ca 2+ and Mg 2+ , TRPM7 is also capable of transporting other divalent cations, exhibiting a strong preference for the essential trace metals Zn 2+ , Mn 2+ , and Co 2+ (Monteilh-Zoller et al 2003). While undeniably crucial for cellular function, if they accumulate above trace levels, these ions are highly toxic.…”

“…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 (Maynard et al 2005). Furthermore, TRPM7 also allows significant entry of toxic metals such as Ni 2+ and Cd 2+ (Monteilh-Zoller et al 2003). This permeation profile coupled with the constitutive activity of TRPM7 indicates that it may represent a ubiquitous basal influx pathway for metal ions with the potential of significant physiological and toxicological repercussions that remain to be studied in more detail.…”

“…TRPM7 is a bi-functional protein that contains both ion channel and protein kinase domains (Nadler et al 2001;Runnels et al 2001;Ryazanova et al 2001) and provides a pathway for the transport of Ca 2+ and Mg 2+ as well trace metal ions (Nadler et al 2001;Monteilh-Zoller et al 2003). The protein was originally discovered in three independent studies that used very different approaches to clone it: Ryazanova et al identified TRPM7 by screening data bases for homologs of human eukaryotic elongation factor 2 kinase (Ryazanova et al 2001), Runnels et al found TRPM7 in a yeast two-hybrid screen using the C2 domain of phospholipase C as a bait (Runnels et al 2001), and Nadler et al obtained TRPM7 using a bioinformatics approach aimed at identifying novel ion channels expressed in immune cells (Nadler et al 2001).…”