Transient receptor potential cation channel, subfamily M, member 7 (TRPM7) is a bi-functional protein comprising an ion channel moiety covalently linked to a protein kinase domain. Currently, the prevailing view is that a decrease in the cytosolic Mg(2+) concentration leads to activation of divalent cation-selective TRPM7 currents. TRPM7 plays a role in immune responses, hypotension, tissue fibrosis, and tumor progression and, therefore, represents a new promising therapeutic target. Because of the dearth of pharmacological tools, our mechanistic understanding of the role of TRPM7 in physiology and pathophysiology still lags behind. Therefore, we have recently carried out a high throughput screen for small-molecule activators of TRPM7. We have characterized the phenanthrene naltriben as a first stimulatory agonist of the TRPM7 channel. Surprisingly, the effect of naltriben on TRPM7 was found to be unaffected by the physiological levels of cytosolic Mg(2+). Here, we demonstrate that mibefradil and NNC 50-0396, two benzimidazole relatives of the TRPM7 inhibitor NS8593, are positive modulators of TRPM7. Using Ca(2+) imaging and the patch-clamp technique, we show that mibefradil activates TRPM7-mediated Ca(2+) entry and whole-cell currents. The response to mibefradil was fast, reversible, and reproducible. In contrast to naltriben, mibefradil efficiently activates TRPM7 currents only at physiological intracellular Mg(2+) concentrations, and its stimulatory effect was fully abrogated by high internal Mg(2+) levels. Consequently, a TRPM7 variant harboring a gain-of-function mutation was insensitive to further mibefradil activation. Finally, we observed that the effect of mibefradil was selective for TRPM7 when various TRP channels were tested. Taken together, mibefradil acts as a Mg(2+)-regulated agonist of the TRPM7 channel and, hence, uncovers a new class of TRPM7 agonists.
Keywords: Magnesium; Mibefradil; NNC 55–0396; NS8593; Naltriben; TRP channels; TRPM7; Voltage-dependent calcium channels.