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Transient receptor potential cation subfamily M7 (TRPM7) channels are ion channels permeable to divalent cations. They are abundantly expressed with particularly high expression in the brain. Previous studies have highlighted the importance of TRPM7 channels in brain diseases such as stroke and traumatic brain injury, yet evidence for a role in seizures and epilepsy is lacking. Here, we show that carvacrol, a food additive that inhibits TRPM7 channels, and waixenicin A, a novel selective and potent TRPM7 inhibitor, completely suppressed seizure‐like activity in rodent hippocampal‐entorhinal brain slices exposed to pentylenetetrazole or low magnesium. These findings support inhibition of TRPM7 channels as a novel target for antiseizure medications. [ABSTRACT FROM AUTHOR]

Key points Pharmacological and molecular inhibition of transient receptor potential melastatin 7 (TRPM7) reduces store-operated calcium entry (SOCE)., Overexpression of TRPM7 in TRPM7−/− cells restores SOCE., TRPM7 is not a store-operated calcium channel., TRPM7 kinase rather than channel modulates SOCE., TRPM7 channel activity contributes to the maintenance of store Ca2+ levels at rest., Abstract The transient receptor potential melastatin 7 (TRPM7) is a protein that combines an ion channel with an intrinsic kinase domain, enabling it to modulate cellular functions either by conducting ions through the pore or by phosphorylating downstream proteins via its kinase domain. In the present study, we report store-operated calcium entry (SOCE) as a novel target of TRPM7 kinase activity. TRPM7-deficient chicken DT40 B lymphocytes exhibit a strongly impaired SOCE compared to wild-type cells as a result of reduced calcium release activated calcium currents, and independently of potassium channel regulation, membrane potential changes or changes in cell-cycle distribution. Pharmacological blockade of TRPM7 with NS8593 or waixenicin A in wild-type B lymphocytes results in a significant decrease in SOCE, confirming that TRPM7 activity is acutely linked to SOCE, without TRPM7 representing a store-operated channel itself. Using kinase-deficient mutants, we find that TRPM7 regulates SOCE through its kinase domain. Furthermore, Ca2+ influx through TRPM7 is essential for the maintenance of endoplasmic reticulum Ca2+ concentration in resting cells, and for the refilling of Ca2+ stores after a Ca2+ signalling event. We conclude that the channel kinase TRPM7 and SOCE are synergistic mechanisms regulating intracellular Ca2+ homeostasis. [ABSTRACT FROM AUTHOR]

Key points The Mg2+ and Ca2+ conducting transient receptor potential melastatin 7 (TRPM7) channel-enzyme (chanzyme) has been implicated in immune cell function., Mice heterozygous for a TRPM7 kinase deletion are hyperallergic, while mice with a single point mutation at amino acid 1648, silencing kinase activity, are not., As mast cell mediators trigger allergic reactions, we here determine the function of TRPM7 in mast cell degranulation and histamine release., Our data establish that TRPM7 kinase activity regulates mast cell degranulation and release of histamine independently of TRPM7 channel function., Our findings suggest a regulatory role of TRPM7 kinase activity on intracellular Ca2+ and extracellular Mg2+ sensitivity of mast cell degranulation., Abstract Transient receptor potential melastatin 7 (TRPM7) is a divalent ion channel with a C-terminally located α-kinase. Mice heterozygous for a TRPM7 kinase deletion (TRPM7+/∆K) are hypomagnesaemic and hyperallergic. In contrast, mice carrying a single point mutation at amino acid 1648, which silences TRPM7 kinase activity (TRPM7KR), are not hyperallergic and are resistant to systemic magnesium (Mg2+) deprivation. Since allergic reactions are triggered by mast cell-mediated histamine release, we investigated the function of TRPM7 on mast cell degranulation and histamine release using wild-type (TRPM7+/+), TRPM7+/∆K and TRPM7KR mice. We found that degranulation and histamine release proceeded independently of TRPM7 channel function. Furthermore, extracellular Mg2+ assured unperturbed IgE-DNP-dependent exocytosis, independently of TRPM7. However, impairment of TRPM7 kinase function suppressed IgE-DNP-dependent exocytosis, slowed the cellular degranulation rate, and diminished the sensitivity to intracellular calcium (Ca2+) in G protein-induced exocytosis. In addition, G protein-coupled receptor (GPCR) stimulation revealed strong suppression of histamine release, whereas removal of extracellular Mg2+ caused the phenotype to revert. We conclude that the TRPM7 kinase activity regulates murine mast cell degranulation by changing its sensitivity to intracellular Ca2+ and affecting granular mobility and/or histamine contents. [ABSTRACT FROM AUTHOR]

Chemokines induce calcium (Ca2+) signaling and chemotaxis in dendritic cells (DCs), but the molecular players involved in shaping intracellular Ca2+ changes remain to be characterized. Using siRNA and knockout mice, we show that in addition to inositol 1,4,5-trisphosphate (IP3)-mediated Ca2+ release and store-operated Ca2+ entry (SOCE), the transient receptor potential melastatin 2 (TRPM2) channel contributes to Ca2+ release but not Ca2+ influx in mouse DCs. Consistent with these findings, TRPM2 expression in DCs is restricted to endolysosomal vesicles, whereas in neutrophils, the channel localizes to the plasma membrane. TRPM2-deficient DCs show impaired maturation and severely compromised chemokine-activated directional migration as well as bacterial-induced DC trafficking to the draining lymph nodes. Defective DC chemotaxis is due to perturbed chemokine-receptor-initiated Ca2+ signaling mechanisms, which include suppression of TRPM2-mediated Ca2+ release and secondary modification of SOCE. DCs deficient in both TRPM2 and IP3 receptor signaling lose their ability to perform chemotaxis entirely. These results highlight TRPM2 as a key player regulating DC chemotaxis through its function as Ca2+ release channel and confirm ADP-ribose as a novel second messenger for intracellular Ca2+ mobilization. [ABSTRACT FROM AUTHOR]

TRPM2 is a calcium-permeable non-selective cation channel expressed in the plasma membrane and in lysosomes that is critically involved in aggravating reactive oxygen species (ROS)-induced inflammatory processes and has been implicated in cell death. TRPM2 is gated by ADP-ribose (ADPR) and modulated by physiological processes that produce peroxide, cyclic ADP-ribose (cADPR), nicotinamide adenine dinucleotide phosphate (NAADP) and Ca2+. We investigated the role of extra- and intracellular acidification on heterologously expressed TRPM2 in HEK293 cells. Our results show that TRPM2 is inhibited by external acidification with an IC50 of pH 6.5 and is completely suppressed by internal pH of 6. Current inhibition requires channel opening and is strongly voltage dependent, being most effective at negative potentials. In addition, increased cytosolic pH buffering capacity or elevated [Ca2+]i reduces the rate of current inactivation elicited by extracellular acidification, and Na+ and Ca2+ influence the efficacy of proton-induced inactivation. Together, these results suggest that external protons permeate TRPM2 channels to gain access to an intracellular site that regulates channel activity. Consistent with this notion, single-channel measurements in HEK293 cells reveal that internal protons induce channel closure without affecting single-channel conductance, whereas external protons affect channel open probability as well as single-channel conductance of native TRPM2 in neutrophils. We conclude that protons compete with Na+ and Ca2+ for channel permeation and channel closure results from a competitive antagonism of protons at an intracellular Ca2+ binding site. [ABSTRACT FROM AUTHOR]

STIM1 and CRACM1 (or Orail) are essential molecular components mediating store-operated Ca2+ entry (SOCE) and Ca2+ release-activated Ca2+(CRAC) currents. Although STIM1 acts as a luminal Ca~+ sensor in the endoplasmic reticulum (ER), the function of STIM2 remains unclear. Here we reveal that STIM2 has two distinct modes of activating CRAC channels: a store-operated mode that is activated through depletion of ER Ca2+ stores by inositol 1,4,5-trisphosphate (InsP3) and store-independent activation that is mediated by cell dialysis during whole-cell perfusion. Both modes are regulated by calmodulin (CAM). The store-operated mode is transient in intact cells, possibly reflecting recruitment of CaM, whereas loss of CaM in perfused cells accounts for the persistence of the store-independent mode. The inhibition by CaM can be reversed by 2-aminoethoxydiphenyl borate (2-APB), resulting in rapid, store-independent activation of CRAC channels. The aminoglycoside antibiotic G418 is a highly specific and potent inhibitor of STIM2-dependent CRAC channel activation. The results reveal a novel bimodal control of CRAC channels by STIM2, the store dependence and CaM regulation, which indicates that the STIM2/CRACM1 complex may be under the control of both luminal and cytoplasmic Ca2+ levels. [ABSTRACT FROM AUTHOR]

Microglia are the main immunocompetent cells of the mammalian central nervous system (CNS). Activation of cultured microglial cells and subsequent release of nitric oxide and cytokines critically depends on intracellular calcium levels. Since microglia undergo dramatic morphological, biochemical and electrophysiological changes in response to pathological events in the CNS, we investigated temporal changes in expression levels of ion channels involved in cellular calcium homeostasis in mouse cortical microglial cells in culture. Specifically, we assessed the inward and delayed outward rectifier potassium currents ( IIRK and IDRK), calcium (Ca2+) release-activated Ca2+ currents ( ICRAC) and Ca2+-activated TRPM4-like currents ( ICAN) in non-activated microglia and cells that were activated by exposure to lipopolysaccharide (LPS) between 3 and 48 h. Unstimulated microglial cells, subcultured from an astrocyte coculture, typically exhibited a ramified, rod-shaped morphology. During the first 3 days of culture cell size and shape were maintained, but the percentage of cells showing prominent IIRK went up and those expressing IDRK went down. Cells retaining IDRK exhibited smaller amplitudes, whereas those of IIRK and ICRAC were not affected. However, after 24 h of exposure to 1 μg ml−1 LPS, most cells showed an amoeboid (‘fried egg’-shaped) morphology with a 62% increase in cell capacitance. At that point in time, only 14% of the cells revealed IIRK and 3% had IDRK exclusively, whereas the majority of cells expressed both currents. The amplitudes of ICRAC and IIRK progressively decreased after stimulation, whereas IDRK transiently reached a maximum after 6 h of LPS exposure and then returned to pre-stimulation expression levels. Cultured microglia also revealed TRPM4-like, Ca2+-activated non-selective currents ( ICAN) with an EC50 of 1.2 μm[Ca2+]i. The expression levels of this current did not change significantly during and after 24 h of LPS exposure. We propose that LPS-induced down-regulation of IIRK and ICRAC will reduce the cell's capacity to produce significant calcium influx upon receptor activation and result in decreased sensitivity to exogenous stimulation. In this scenario, ICAN expression would remain constant, although its activity would automatically be reduced due to the diminished calcium influx capacity of the cell. [ABSTRACT FROM AUTHOR]

TRPM7 (transient-receptor-potential melastatin 7) is an ion channel with α-kinase function. TRPM7 is divalent-selective and regulated by a range of receptor-stimulated second messenger pathways, intracellular Mg-nucleotides, divalent and polyvalent cations and pH. TRPM7 is ubiquitously found in mammalian cells, including kidney, the responsible organ for osmolyte regulation, posing the question whether the channel is osmosensitive. Recent reports investigated the sensitivity of native TRPM7-like currents to cell swelling with contradictory results. Here, we assess the sensitivity of TRPM7 to both hypo- and hyperosmotic conditions and explored the involvement of the channel's kinase domain. We find that hypotonicity facilitates TRPM7 at elevated intracellular magnesium and Mg·ATP (3–4 mm), but has no effect in the absence of these solutes. Hypertonic conditions, in contrast, inhibit TRPM7 with an IC50 of 430 mosmol l−1. This inhibitory effect is maintained in the complete absence of intra- and extracellular divalent ions, although shifted to higher osmolarities (IC50= 510 mosmol l−1). TRPM7 senses osmotic gradients rather than ionic strength and this is independent of cAMP or not affected by cytochalasin D treatment. Furthermore, the kinase-domain deletion mutant of TRPM7 shows a similar behaviour to osmolarity as the wild-type protein, both in the presence and absence of divalent ions. This indicates that at least part of the osmosensitivity resides in the channel domain. Physiologically, TRPM7 channels do not seem to play an active role in regulatory volume changes, but rather those volume changes modulate TRPM7 activity through changes in the cytosolic concentrations of free Mg, Mg-nucleotides and a further unidentified factor. We conclude that TRPM7 senses osmotically induced changes primarily through molecular crowding of solutes that affect channel activity. [ABSTRACT FROM AUTHOR]

Deals with a study which aimed to determine whether two Ca²+ -mobilizing second messengers, specifically cyclic adenosine diphosphate-ribose (cADPR) and hydrogen peroxide can facilitate ADPR-mediated activation of heterologously expressed TRPM2. Main difference between heterologously expressed TRPM2 and the native IADPR in T cells; Activation of TRPM2 in HEK-293 and Jurkat T cells by nicotinic acid adenine dinucleotide phosphate; Conclusions and significance.

An introduction is presented in which the editor discusses articles in the issue on topics including the meeting held in Honolulu, Hawaii sponsored by the Physiological Society on calium ion channel, the diffusion through endoplasmic reticulum (ER) lumen, and the ameoblast transport pathways.

Background: Despite extensive functional evidence for the existence of various regulated Mg2+ transport proteins, only two plasmalemal channels (TRPM6, TRPM7), and mitochondrial channel (Mrs2p) have been molecularly confirmed to be involved in Mg2+ transport in mammalian cells. The recent identification of putative Mg2+ transporter SLC41A1 therefore opens the possibility of linking this protein with physiologically known Mg2+ transport mechanisms. Objective: To investigate function of human SLC41A1 in human cell system. Methods: Tet-inducible cell line HEK-293-Trex/FLAG-SLC41A1 construction; L-CMS immuno-localization of SLC41A1, whole-cell mode patch-clamp and mag-fura 2 fast filter spectroscopy. Results: FLAG-hSLC41A1 co-localize in 59.28± 1.56 % (pixel analyse) together with fluorescently-labelled WGA. Despite extensive patch-clamp studies we were unable to identify a SLC41A1-specific Mg2+ current. Instead we have identified a DIDS-blockable Cl- current which is indirectly linked to hSLC41A1 over-expression. Mag-fura 2 FFS analyses showed capability of SLC41A1 to transport Mg2+. This transport was not influenced by DIDS (100 μM) or by the only known Mg2+ transport blocker cobalt(III)hexaammine (1mM). The transport capacity showed a strict temperature dependency. Conclusion: We conclude that hSLC41A1 is likely a plasma membrane Mg2+ carrier indirectly linked to X- transport. [ABSTRACT FROM AUTHOR]