Transient receptor potential (TRPM) is a family of non-selective cation channels that are widely expressed in mammalian cells. TRP channels are composed of six transmembrane domains and the family consists of eight different channels, TRPM1–TRPM8. TRPM7 is compromised of an ion channel moiety essential for the ion channel function, which serves to increase intracellular calcium levels and to help regulate magnesium ion homeostasis. The current hypothesis is that decreased cytosolic Mg2+ ion concentration activates the divalent cation-selective TRPM7 current 2.
Xiong and colleagues4 were the first to demonstrate TRPM7 mediated sensing of Ca2+ concentration. The permeability of TRPM7 channels to monovalent cations is decreased In the presence of divalent ions, promoting outward rectification in whole cell recording with a reversal potential of 0mV. When divalent ions were removed from the external buffer solution a large TRMP7 current was activated. The order of permeability of TRPM7 to divalent cations follows the order Zn2+> Ni2+> Ba2+> Co2+> Mg2+> Mn2+> Sr2+> Cd2+> Ca2+. Mutation of the mouse TRPM7 gene in vivo demonstrates that it is essential for early embryogenesis, organ development, cardiac automaticity, and systemic Mg2+ homeostasis. Deletion of the TRPM7 gene in cells in vitro showed that this bi-functional channel, not only regulates divalent cation homeostasis, but also cell motility, proliferation, mechanosensitivity and exocytosis. TRPM7 may also play a role in anoxic neuronal death, immune responses, hypertension, neurodegenerative disorders, tissue fibrosis and in tumour growth2,3, 4,5,. TPRM7 is clearly an important drug target.
Figure showing; Regulation of cellular Mg2+ homeostasis
Andrea M.P. Romani Arch Biochem Biophys. 2011 Aug 1; 512(1): 1–23.
Schafer & colleagues1 have recently reported on mibefradil and naltriben, two new classes of compound (fig 1a) that act on 5-HT7 receptors. In the calcium imaging assay, mibefradil potentiated TRPM7 current (fig 1b) in the absence of extracellular Mg2+ and blocked the TRPM7 current in it is presence (1C). Mibefradil also induced TRPM7 mediated Ca2+ entry in a concentration dependent manner with an EC50 of 53mM (fig 1d). The Influence of intracellular Mg2+ concentration was also investigated in a whole cell e-phys study which shown that in the absence of intracellular Mg2+ the addition of mibefradil failed to increase the current further (fig 2a). When cells were perfused with the physiological concentration of free Mg2+ (0.9mM) there was a pronounced increase in the whole cell current (fig 2b). These results suggest that mibefradil preferentially activates TRPM7 responses at physiological or low intracellular Mg2+. Extracellular divalent cations are known to block the TRPM7 channel, resulting in a small divalent cation selective inward current at the physiological membrane potential. Mibefradil elicited a moderate increase of the inward current for the TMRP7 (E1047Q) mutant compared to WT channel at physiological Mg2+ concentration. Application of mibefradil, stimulate both inward as well as outward monovalent currents in this mutant (fig 3a). The authors conclude that mibefradil acts as a specific agonist of the TRPM7 channel, and proposed that mibefradil specifically affects TRPM7 channel gating in a Mg2+ – dependent manner.
Sebastian Schafer et.al. (2016) Eur J Physiol 2016; 468; 623-634
Mibefradil is a calcium channel antagonist of both L and T type voltage-dependent calcium channels and has been used clinically for the treatment of angina and hypertension7. Unfortunately, side effects arising from inhibition of cytochrome p450 enzymes led to the withdrawal of mibefradil withdrawn from clinical use. Currently, mibefradil is only used experimentally as a tool compound to identify voltage gated calcium channels. Mibefradil shows moderate potency for TRPM7 activation when compared to P/L type voltage gated calcium channel, the EC50 53mM, compared to 2.7mM for T-type, and 18.6mM for L-type Ca2+ channels. Published data are consistent with mibefradil action at the TRPM7 channel (reference?). Specifically, mibefradil induces the TRPM7 –mediated influx of divalent cations such Ca2+, Zn2+ and Mg2+ which may alter cellular process.
Schafer & colleagues1 also identified naltriben as an activator of the TRPM7 channel at high-level intracellular Mg2+ levels. The authors have suggest that that two types of TRPM7 agonist act in different ways: type 1 naltribune, induces TRPM7 activity independently of Mg2+ concentration whereas type 2 Mibefradil acts in a Mg2+ -dependent manner2. Mg2+ is the most abundant intracellular cation (~15-18mM) and is mostly complexed with phosphormetabolites such that physiological levels of free Mg2+ are only in the range of 0.5-1mM. Under certain conditions, the free intracellular concentration may reduce further e.g. as a result of increasing phosphormetabolite synthesis during cell division. The authors propose that the two different types of TRPM7 activator could be directed to two different types of target; Type 1 to stimulate TRPM7 irrespective of Mg2+ concentration and Type 2 to act preferentially on cells with reduced Mg2+ levels.
Blog written by Shamim Choudhury
- Sebastian Schafer et.al. (2016) Mibefradil represents a new class of benzoimidazole TRPM7 channel agonists. Eur J Physiol 2016;468;623-634
- Andrea M.P. Romani (2011) Cellular Magnesium Homeostasis. Arch Biochem Biophys. 2011 august 1; 512(1): 1–23. Doi:10.1016/j.abb.2011.05.010.
- Valdimir Chubnav, silvia Ferioli, Thomas Gudermann (2017) Assessments of TRPM7 functions by drug like molecules. Call Calcium 2017;67;166-173
- Xiong Z, Zu W and Macdonald JF (1997) Extracellular calcium sensed by a novel cation channel in hippocampal neurons. Proc Natl Acad Sci USA 1997;94;7012-7017
- Yugang Sun et.al. (2015) TRPM7 and its role in neurodegenerative diseases Channels 2015; 9:5,253-261
- Yosuke Kaneko and Arpad Szallasi (2014) Transient receptor potential (TRP) channels : a clinical perspective British J Pharmacology 2014; 171 2474-2507
- S. Aczel, B. kurka, & S. Hering (1998) Mechanism of voltage and use dependent block of class Ca2+ channels by mibefradil. British J of Pharmacology 1998;125;447-45