Charge Screening by Internal pH and Polyvalent Cations as a Mechanism for Activation, Inhibition, and Rundown of TRPM7/MIC Channels

Authors

J. Ashot Kozak, Wright State University - Main CampusFollow
Masayuki Matsushita
Angus C. Nairn
Michael D. Cahalan

Document Type

Article

Publication Date

11-2005

Abstract

The Mg²⁺-inhibited cation (MIC) current, believed to represent activity of TRPM7 channels, is found in lymphocytes and mast cells, cardiac and smooth muscle, and several other eukaryotic cell types. MIC current is activated during whole-cell dialysis with divalent-free internal solutions. Millimolar concentrations of intracellular Mg²⁺ (or other divalent metal cations) inhibit the channels in a voltage-independent manner. The nature of divalent inhibition and the mechanism of channel activation in an intact cell remain unknown. We show that the polyamines (spermine, spermidine, and putrescine) inhibit the MIC current, also in a voltage-independent manner, with a potency that parallels the number of charges. Neomycin and poly-lysine also potently inhibited MIC current in the absence of Mg²⁺. These same positively charged ions inhibited IRK1 current in parallel with MIC current, suggesting that they probably act by screening the head group phosphates on PIP₂ and other membrane phospholipids. In agreement with this hypothesis, internal protons also inhibited MIC current. By contrast, tetramethylammonium, tetraethylammonium, and hexamethonium produced voltage-dependent block but no inhibition. We show that inhibition by internal polyvalent cations can be relieved by alkalinizing the cytosol using externally applied ammonium or by increasing pH in inside-out patches. Furthermore, in perforated-patch and cell-attached recordings, when intracellular Mg²⁺ is not depleted, endogenous MIC or recombinant TRPM7 currents are activated by cytosolic alkalinization and inhibited by acidification; and they can be reactivated by PIP₂ following rundown in inside-out patches. We propose that MIC (TRPM7) channels are regulated by a charge screening mechanism and may function as sensors of intracellular pH.

Comments

©Kozak, J. Ashot, et al., 2005. Originally published in The Journal of General Physiology. doi: 10.1085/jgp.200509324.

Repository Citation

Kozak, J. A., Matsushita, M., Nairn, A. C., & Cahalan, M. D. (2005). Charge Screening by Internal pH and Polyvalent Cations as a Mechanism for Activation, Inhibition, and Rundown of TRPM7/MIC Channels. The Journal of General Physiology, 126 (5), 499-514.
https://corescholar.libraries.wright.edu/ncbp/135

DOI

10.1085/jgp.200509324