Depletion of plasma membrane-associated phosphoinositides mimics inhibition of TRPM7 channels by cytosolic Mg2+, spermine, and pH

The role of ion channels in the regulation of dendritic cell function

Ion channels, membrane proteins that facilitate the transport of various inorganic ions across hydrophobic cellular lipid membranes, are ubiquitous in a wide variety of cell and tissue types. They are involved in establishing the cell membrane potential and play a role in various physiological activities by regulating ion concentrations within the cell. Dendritic cells (DCs) are specialised antigen-presenting cells found mainly on the surface of the body (skin and mucous membranes), in the mesenchyme of most organs, in the T-cell compartment of the spleen and in lymph nodes. DCs exert an important influence on the regulation of inflammation by activating T cells and producing cytokines. Studies have shown that ion channels expressed in DCs contribute to the regulation of the immune response, making them a key component of the immune system. This review summarises the major scientific advances in understanding the functional impact of ion channels (calcium channels, sodium channels and aquaporin) in DCs, including the regulation of inflammatory responses, antigen presentation, maturation, migration and cytokine production, to inform ongoing studies of ion channel function in DCs.

The PACT Network: PRL, ARL, CNNM, and TRPM Proteins in Magnesium Transport and Disease

Magnesium, the most abundant divalent metal within the cell, is essential for physiological function and critical in cellular signaling. To maintain cellular homeostasis, intracellular magnesium levels are tightly regulated, as dysregulation is linked to numerous diseases, including cancer, diabetes, cardiovascular disorders, and neurological conditions. Over the past two decades, extensive research on magnesium-regulating proteins has provided valuable insight into their pathogenic and therapeutic potential. This review explores an emerging mechanism of magnesium homeostasis involving proteins in the PRL (phosphatase of regenerating liver), ARL (ADP ribosylation factor-like GTPase family), CNNM (cyclin and cystathionine β-synthase domain magnesium transport mediator), and TRPM (transient receptor potential melastatin) families, collectively termed herein as the PACT network. While each PACT protein has been studied within its individual signaling and disease contexts, their interactions suggest a broader regulatory network with therapeutic potential. This review consolidates the current knowledge on the PACT proteins' structure, function, and interactions and identifies research gaps to encourage future investigation. As the field of magnesium homeostasis continues to advance, understanding PACT protein interactions offers new opportunities for basic research and therapeutic development targeting magnesium-related disorders.

Transient Receptor Potential (TRP) Channels in Cochlear Function: Looking Beyond Mechanotransduction

Transient receptor potential (TRP) channels play key roles in sensory biology as transducers of various stimuli. Although these ion channels are expressed in the cochlea, their functions remain poorly understood. Recent studies by Vélez-Ortega and colleagues indicate that their expression by non-sensory supporting cells helps limit damage from acoustic trauma.

Melastatin subfamily Transient Receptor Potential channels support spermatogenesis in planarian flatworms

The Transient Receptor Potential superfamily of proteins (TRPs) form cation channels that are abundant in animal sensory systems. Amongst TRPs, the Melastatin-related subfamily (TRPMs) is composed of members that respond to temperature, pH, sex hormones, and various other stimuli. Some TRPMs exhibit enriched expression in gonads of vertebrate and invertebrate species, but their contributions to germline development remain to be determined. We identified twenty-one potential TRPMs in the planarian flatworm Schmidtea mediterranea and analyzed their anatomical distribution of expression by whole-mount in situ hybridization. Enriched expression of two TRPMs (Smed-TRPM-c and Smed-TRPM-l) was detected in testis, whereas eight TRPM genes had detectable expression in patterns representative of neuronal and/or sensory cell types. Functional analysis of TRPM homologs by RNA-interference (RNAi) revealed that disruption of Smed-TRPM-c expression results in reduced sperm development, indicating a role for this receptor in supporting spermatogenesis. Smed-TRPM-l RNAi did not result in a detectable phenotype, but it increased sperm development deficiencies when combined with Smed-TRPM-c RNAi. Fluorescence in situ hybridization revealed expression of Smed-TRPM-c in early spermatogenic cells within testes, suggesting cell-autonomous regulatory functions in germ cells for this gene. In addition, Smed-TRPM-c RNAi resulted in reduced numbers of presumptive germline stem cell clusters in asexual planarians, suggesting that Smed-TRPM-c supports establishment, maintenance, and/or expansion of spermatogonial germline stem cells. While further research is needed to identify the factors that trigger Smed-TRPM-c activity, these findings reveal one of few known examples for TRPM function in direct regulation of sperm development.

Use of tetraethylammonium (TEA) and Tris loading for blocking TRPM7 channels in intact cells

Tetraethylammonium (TEA), a quaternary ammonium compound, is a well-known blocker of potassium channels belonging to various subfamilies, such as KV1-3, KCa1, 2 and prokaryotic KcsA. In many cases, TEA acts from the extracellular side by open pore blockade. TEA can also block transient receptor potential (TRP) cation channels, such as TRPM7, in a voltage-dependent manner. In human T lymphocytes, intracellular (cytosolic) TEA and its analog TMA (tetramethylammonium) inhibit TRPM7 channel currents in the outward but not inward direction. By contrast, intracellular Mg2+, protons and polyamines inhibit both outward and inward current components equally. Likewise, the majority of available pharmacological tools inhibit TRPM7 channels in a voltage-independent manner. Since TRPM7 is a steeply outwardly rectifying conductance, voltage-dependent blockers can be useful for studying the cellular functions of this channel. TRPM7 protein is endogenously expressed in diverse cell lines, including HEK, HeLa, CHO, RBL and Jurkat. Using patch-clamp electrophysiology, we found that incubating HEK293 and Jurkat T cells overnight in the presence of 20 mM TEA-Cl, resulted in the nearly complete blockade of whole-cell TRPM7 outward current, measured at break-in. By contrast, the inward current was unchanged in TEA-loaded cells. The blockade was fully reversible after washout of intracellular solution in whole-cell but not in perforated-patch recording configurations. Overnight incubation with 20 mM TMA-Cl resulted in a more modest blockade of the outward TRPM7 current. Internal 129 mM TMA and TEA eliminated most of the outward current. TEA uptake in transfected HEK293 cells led to blockade of recombinant murine TRPM7 and the Mg2+ and pH insensitive Ser1107Arg variant. Unexpectedly, Tris-HCl, a widely used pH buffer, could similarly be loaded into Jurkat and HEK cells, and preferentially blocked outward TRPM7 currents. 20 mM and 129 mM Tris in the internal solution blocked TRPM7 current in outward but not inward direction. Voltage-dependent channel blockade by TEA, TMA and Tris loading will be useful for studying the properties and functions of TRPM7-mediated ion transport in intact cells.

The role of transient receptor potential (TRP) channels in phagocytosis: A comprehensive review

When host cells are exposed to foreign particles, dead cells, or cell hazards, a sophisticated process called phagocytosis begins. During this process, macrophages, dendritic cells, and neutrophils engulf the target by expanding their membranes. Phagocytosis of apoptotic cells is called efferocytosis. This process is of significant importance as billions of cells are eliminated daily without provoking inflammation. Both phagocytosis and efferocytosis depend on Ca2+ signaling. A big family of Ca2+ permeable channels is transient receptor potentials (TRPs) divided into nine subfamilies. We aimed to review their roles in phagocytosis. The present review article shows that various TRP channels such as TRPV1, 2, 3, 4, TRPM2, 4, 7, 8, TRPML1, TRPA1, TRPC1, 3, 5, 6 have roles at various stages of phagocytosis. They are involved in the phagocytosis of amyloid β, α-synuclein, myelin debris, bacteria, and apoptotic cells. In particular, TRPC3 and TRPM7 contribute to efferocytosis. These effects are mediated by changing Ca2+ signaling or targeting intracellular enzymes such as Akt. In addition, they contribute to the chemotaxis of phagocytic cells towards targets. Although a limited number of studies have assessed the role of TRP channels in phagocytosis and efferocytosis, their findings indicate that they have critical roles in these processes. In some cases, their ablation completely abolished the phagocytic function of the cells. As a result, TRP channels are potential targets for developing new therapeutics that modulate phagocytosis.

Cell death induction and protection by activation of ubiquitously expressed anion/cation channels. Part 3: the roles and properties of TRPM2 and TRPM7

Cell volume regulation (CVR) is a prerequisite for animal cells to survive and fulfill their functions. CVR dysfunction is essentially involved in the induction of cell death. In fact, sustained normotonic cell swelling and shrinkage are associated with necrosis and apoptosis, and thus called the necrotic volume increase (NVI) and the apoptotic volume decrease (AVD), respectively. Since a number of ubiquitously expressed ion channels are involved in the CVR processes, these volume-regulatory ion channels are also implicated in the NVI and AVD events. In Part 1 and Part 2 of this series of review articles, we described the roles of swelling-activated anion channels called VSOR or VRAC and acid-activated anion channels called ASOR or PAC in CVR and cell death processes. Here, Part 3 focuses on therein roles of Ca2+-permeable non-selective TRPM2 and TRPM7 cation channels activated by stress. First, we summarize their phenotypic properties and molecular structure. Second, we describe their roles in CVR. Since cell death induction is tightly coupled to dysfunction of CVR, third, we focus on their participation in the induction of or protection against cell death under oxidative, acidotoxic, excitotoxic, and ischemic conditions. In this regard, we pay attention to the sensitivity of TRPM2 and TRPM7 to a variety of stress as well as to their capability to physicall and functionally interact with other volume-related channels and membrane enzymes. Also, we summarize a large number of reports hitherto published in which TRPM2 and TRPM7 channels are shown to be involved in cell death associated with a variety of diseases or disorders, in some cases as double-edged swords. Lastly, we attempt to describe how TRPM2 and TRPM7 are organized in the ionic mechanisms leading to cell death induction and protection.

Blockade of TRPV channels by intracellular spermine

The Vanilloid thermoTRP (TRPV1-4) subfamily of TRP channels are involved in thermoregulation, osmoregulation, itch and pain perception, (neuro)inflammation and immune response, and tight control of channel activity is required for perception of noxious stimuli and pain. Here we report voltage-dependent modulation of each of human TRPV1, 3, and 4 by the endogenous intracellular polyamine spermine. As in inward rectifier K channels, currents are blocked in a strongly voltage-dependent manner, but, as in cyclic nucleotide-gated channels, the blockade is substantially reduced at more positive voltages, with maximal blockade in the vicinity of zero voltage. A kinetic model of inhibition suggests two independent spermine binding sites with different affinities as well as different degrees of polyamine permeability in TRPV1, 3, and 4. Given that block and relief occur over the physiological voltage range of action potentials, voltage-dependent polyamine block may be a potent modulator of TRPV-dependent excitability in multiple cell types.

Diluent pH affects sperm motility via GSK3 α/β-hexokinase pathway for the efficient enrichment of X-sperm to increase the female kids rate of dairy goats

Dairy goats are the goats bred with the ability to produce large quantities of milk, and the increase of the female kid rate of breeding dairy goats is beneficial for milk production and economic benefits of dairy goat farms. Our previous study revealed that regulating the pH of dairy goat semen diluent to 6.2 or 7.4 respectively, the proportion of X chromosome bearing sperm (X-sperm) in the up and down layers of the tube after incubation was significantly higher than that of Y chromosome bearing sperm (Y-sperm) i.e. enriched X-sperm. In this study, fresh dairy goat semen collected in different seasons was diluted in different pH solutions to calculate the number and rate of X-sperm and to measure the functional parameters of enriched sperm. The artificial insemination experiments were performed with enriched X-sperm. The mechanisms of regulating the pH of diluent affecting sperm enrichment were further studied. The results showed that the proportion of enriched X-sperm in pH 6.2 and 7.4 diluents of sperm collected in different seasons showed no significantly different, but were significantly higher than that of the control group (pH 6.8). The in vitro functional parameters of X-sperm enriched in pH 6.2 and 7.4 diluent solution were not significantly different from those of the control group (P > 0.05). After artificial insemination with X-sperm enriched in pH7.4 diluent, the proportion of female offspring was significantly higher than that of the control group. It was found that the regulating pH of the diluent affected sperm mitochondrial activity and glucose uptake capacity via phosphorylating NF-κB and GSK3α/β proteins. The motility activity of X-sperm was enhanced under acidic conditions and weakened under alkaline conditions, which was conducive to the effective enrichment of X-sperm. This study demonstrated that the number and proportion of X-sperm enriched using pH 7.4 diluent were elevated, and the proportion of female kids was increased. This technology can be used for the reproduction and production of dairy goats in farms at large scales.

Alkaline Dilution Alters Sperm Motility in Dairy Goat by Affecting sAC/cAMP/PKA Pathway Activity

In dairy goat farming, increasing the female kid rate is beneficial to milk production and is, therefore, economically beneficial to farms. Our previous study demonstrated that alkaline incubation enriched the concentration of X-chromosome-bearing sperm; however, the mechanism by which pH affects the motility of X-chromosome-bearing sperm remains unclear. In this study, we explored this mechanism by incubating dairy goat sperm in alkaline dilutions, examining the pattern of changes in sperm internal pH and Ca2+ concentrations and investigating the role of the sAC/cAMP/PKA pathway in influencing sperm motility. The results showed that adding a calcium channel inhibitor during incubation resulted in a concentration-dependent decrease in the proportion of spermatozoa with forward motility, and the sperm sAC protein activity was positively correlated with the calcium ion concentration (r = 0.9972). The total motility activity, proportion of forward motility, and proportion of X-chromosome-bearing sperm decreased (p < 0.05) when cAMP/PKA protease activity was inhibited. Meanwhile, the enrichment of X-chromosome-bearing sperm by pH did not affect the sperm capacitation state. These results indicate that alkaline dilution incubation reduces Ca2+ entry into X-sperm and the motility was slowed down through the sAC/cAMP/PKA signaling pathway, providing a theoretical foundation for further optimization of the sex control method.

Butyrate reduces cellular magnesium absorption independently of metabolic regulation in Caco-2 human colon cells

Digestion of dietary fibers by gut bacteria has been shown to stimulate intestinal mineral absorption [e.g., calcium (Ca²⁺) and magnesium (Mg²⁺)]. Although it has been suggested that local pH and short-chain fatty acid (SCFA) concentrations determine divalent cation absorption, the exact molecular mechanisms are still unknown. Therefore, this study aimed to determine the effects of SCFAs on intestinal Mg²⁺ absorption. We show that the butyrate concentration in the colon negatively correlates with serum Mg²⁺ levels in wildtype mice. Moreover, Na-butyrate significantly inhibited Mg²⁺ uptake in Caco-2 cells, while Ca²⁺ uptake was unaffected. Although Na-butyrate significantly lowered total ATP production rate, and resulted in increased phosphorylation of AMP-activated protein kinase (AMPK), inhibition of Mg²⁺ uptake by butyrate preceded these consequences. Importantly, electrophysiological examinations demonstrated that intracellular butyrate directly reduced the activity of the heteromeric Mg²⁺ channel complex, transient receptor potential melastatin (TRPM)6/7. Blocking cellular butyrate uptake prevented its inhibitory effect on Mg²⁺ uptake, demonstrating that butyrate acts intracellularly. Our work identified butyrate as novel regulator of intestinal Mg²⁺ uptake that works independently from metabolic regulation. This finding further highlights the role of microbial fermentation in the regulation of mineral absorption.

Intrinsic mechanical sensitivity of mammalian auditory neurons as a contributor to sound-driven neural activity

Mechanosensation – by which mechanical stimuli are converted into a neuronal signal – is the basis for the sensory systems of hearing, balance, and touch. Mechanosensation is unmatched in speed and its diverse range of sensitivities, reaching its highest temporal limits with the sense of hearing; however, hair cells (HCs) and the auditory nerve (AN) serve as obligatory bottlenecks for sounds to engage the brain. Like other sensory neurons, auditory neurons use the canonical pathway for neurotransmission and millisecond-duration action potentials (APs). How the auditory system utilizes the relatively slow transmission mechanisms to achieve ultrafast speed, and high audio-frequency hearing remains an enigma. Here, we address this paradox and report that the mouse, and chinchilla, AN are mechanically sensitive, and minute mechanical displacement profoundly affects its response properties. Sound-mimicking sinusoidal mechanical and electrical current stimuli affect phase-locked responses. In a phase-dependent manner, the two stimuli can also evoke suppressive responses. We propose that mechanical sensitivity interacts with synaptic responses to shape responses in the AN, including frequency tuning and temporal phase locking. Combining neurotransmission and mechanical sensation to control spike patterns gives the mammalian AN a secondary receptor role, an emerging theme in primary neuronal functions.

Modulators of TRPM7 and its potential as a drug target for brain tumours

TRPM7 is a non-selective divalent cation channel with an alpha-kinase domain. Corresponding with its broad expression, TRPM7 has a role in a wide range of cell functions, including proliferation, migration, and survival. Growing evidence shows that TRPM7 is also aberrantly expressed in various cancers, including brain cancers. Because ion channels have widespread tissue distribution and result in extensive physiological consequences when dysfunctional, these proteins can be compelling drug targets. In fact, ion channels comprise the third-largest drug target type, following enzymes and receptors. Literature has shown that suppression of TRPM7 results in inhibition of migration, invasion, and proliferation in several human brain tumours. Therefore, TRPM7 presents a potential target for therapeutic brain tumour interventions. This article reviews current literature on TRPM7 as a potential drug target in the context of brain tumours and provides an overview of various selective and non-selective modulators of the channel relevant to pharmacology, oncology, and ion channel function.

NSAIDs Naproxen, Ibuprofen, Salicylate, and Aspirin Inhibit TRPM7 Channels by Cytosolic Acidification

Non-steroidal anti-inflammatory drugs (NSAIDs) are used for relieving pain and inflammation accompanying numerous disease states. The primary therapeutic mechanism of these widely used drugs is the inhibition of cyclooxygenase 1 and 2 (COX1, 2) enzymes that catalyze the conversion of arachidonic acid into prostaglandins. At higher doses, NSAIDs are used for prevention of certain types of cancer and as experimental treatments for Alzheimer’s disease. In the immune system, various NSAIDs have been reported to influence neutrophil function and lymphocyte proliferation, and affect ion channels and cellular calcium homeostasis. Transient receptor potential melastatin 7 (TRPM7) cation channels are highly expressed in T lymphocytes and are inhibited by Mg²⁺, acidic pH, and polyamines. Here, we report a novel effect of naproxen, ibuprofen, salicylate, and acetylsalicylate on TRPM7. At concentrations of 3–30mM, they reversibly inhibited TRPM7 channel currents. By measuring intracellular pH with the ratiometric indicator BCECF, we found that at 300μM to 30mM, these NSAIDs reversibly acidified the cytoplasm in a concentration-dependent manner, and propose that TRPM7 channel inhibition is a consequence of cytosolic acidification, rather than direct. NSAID inhibition of TRPM7 channels was slow, voltage-independent, and displayed use-dependence, increasing in potency upon repeated drug applications. The extent of channel inhibition by salicylate strongly depended on cellular PI(4,5)P₂ levels, as revealed when this phospholipid was depleted with voltage-sensitive lipid phosphatase (VSP). Salicylate inhibited heterologously expressed wildtype TRPM7 channels but not the S1107R variant, which is insensitive to cytosolic pH, Mg²⁺, and PI(4,5)P₂ depletion. NSAID-induced acidification was also observed in Schneider 2 cells from Drosophila, an organism that lacks orthologous COX genes, suggesting that this effect is unrelated to COX enzyme activity. A 24-h exposure to 300μM–10mM naproxen resulted in a concentration-dependent reduction in cell viability. In addition to TRPM7, the described NSAID effect would be expected to apply to other ion channels and transporters sensitive to intracellular pH.

Effects of Diluent pH on Enrichment and Performance of Dairy Goat X/Y Sperm

In this paper, on the basis of the differences in the hydrogen ion concentration (pH) of the diluent dairy goat semen on X/Y sperm motility, an X/Y sperm enrichment study was conducted to establish a simple and effective method for gender control in dairy goats. Dairy goat semen was diluted using different pH dilutions and was incubated. Then, the X/Y sperm ratio in the isolated upper sperm was determined using the double TaqMan qPCR method. The internal pH change pattern of sperm cells at different pH dilutions was measured using BCECF-AM probe, and the functional parameters of the isolated sperm were tested with the corresponding kit. Next, an in vitro fertilization test was conducted using isolated spermatozoa and oocytes to determine their fertilization rates, the percentages of female embryos, and the expression of genes related to developing potentially fertilized embryos. Results showed that the percentages of the X sperm cells in the upper sperm layer were 67.24% ± 2.61% at sperm dilution pH of 6.2 and 30.45% ± 1.03% at sperm dilution pH of 7.4, which was significantly different from 52.35% ± 1.72% of the control group (pH 6.8) (P < 0.01). Results also showed that there is a relationship between the external pHo and internal pHi of sperm cells. Furthermore, the percentages of female embryos after the in vitro fertilization of the isolated upper sperm with mature oocytes at pH 6.2 and 7.4 were 66.67% ± 0.05 and 29.73% ± 0.04%, respectively, compared with 48.57% ± 0.02% in the control group (pH 6.8). Highly significant differences occurred between groups (P < 0.01). Additionally, no significant difference was observed during the expression of genes related to embryonic development between the blastocysts formed from sperm isolated by changing the pH of the diluent and the control sperm (P > 0.05). Therefore, this study successfully established a simple and effective method for enriched X/Y sperms from dairy goats, which is important for regulating the desired sex progeny during dairy goat breeding and for guiding dairy goat production.

The zinc-binding motif of TRPM7 acts as an oxidative stress sensor to regulate its channel activity

The activity of the TRPM7 channel is negatively regulated by intracellular Mg²⁺. We previously reported that oxidative stress enhances the inhibition of TRPM7 by intracellular Mg²⁺. Here, we aimed to clarify the mechanism underlying TRPM7 inhibition by hydrogen peroxide (H₂O₂). Site-directed mutagenesis of full-length TRPM7 revealed that none of the cysteines other than C1809 and C1813 within the zinc-binding motif of the TRPM7 kinase domain were involved in the H₂O₂-induced TRPM7 inhibition. Mutation of C1809 or C1813 prevented expression of full-length TRPM7 on the plasma membrane. We therefore developed an assay to functionally reconstitute full-length TRPM7 by coexpressing the TRPM7 channel domain (M7cd) and the TRPM7 kinase domain (M7kd) as separate proteins in HEK293 cells. When M7cd was expressed alone, the current was inhibited by intracellular Mg²⁺ more strongly than that of full-length TRPM7 and was insensitive to oxidative stress. Coexpression of M7cd and M7kd attenuated the inhibition by intracellular Mg²⁺ and restored sensitivity to oxidative stress, indicating successful reconstitution of a full-length TRPM7-like current. We observed a similar effect when M7cd was coexpressed with the kinase-inactive mutant M7kd-K1645R, suggesting that the kinase activity is not essential for the reconstitution. However, coexpression of M7cd and M7kd carrying a mutation at either C1809 or C1813 failed to restore the full-length TRPM7-like current. No reconstitution was observed when using M7kd carrying a mutation at H1750 and H1807, which are involved in the zinc-binding motif formation with C1809 and C1813. These data suggest that the zinc-binding motif is essential for the intracellular Mg²⁺-dependent regulation of the TRPM7 channel activity by its kinase domain and that the cysteines in the zinc-binding motif play a role in the oxidative stress response of TRPM7.

Methods to study phosphoinositide regulation of ion channels

Ion channel are embedded in the lipid bilayers of biological membranes. Membrane phospholipids constitute a barrier to ion movement, and they have been considered for a long time as a passive environment for channel proteins. Membrane phospholipids, however, do not only serve as a passive amphipathic environment, but they also modulate channel activity by direct specific lipid-protein interactions. Phosphoinositides are quantitatively minor components of biological membranes, and they play roles in many cellular functions, including membrane traffic, cellular signaling and cytoskeletal organization. Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] is mainly found in the inner leaflet of the plasma membrane. Its role as a potential ion channel regulator was first appreciated over two decades ago and by now this lipid is a well-established cofactor or regulator of many different ion channels. The past two decades witnessed the steady development of techniques to study ion channel regulation by phosphoinositides with progress culminating in recent cryoEM structures that allowed visualization of how PI(4,5)P₂ opens some ion channels. This chapter will provide an overview of the methods to study regulation by phosphoinositides, focusing on plasma membrane ion channels and PI(4,5)P₂.

Phagocytic activity of splenic macrophages is enhanced and accompanied by cytosolic alkalinization in TRPM7 kinase-dead mice

Transient receptor potential melastatin 7 (TRPM7) is a unique protein functioning as a cation channel as well as a serine/threonine kinase and is highly expressed in immune cells such as lymphocytes and macrophages. TRPM7 kinase-dead (KD) mouse model has been used to investigate the role of this protein in immune cells; these animals display moderate splenomegaly and ectopic hemopoiesis. The basal TRPM7 current magnitudes in peritoneal macrophages isolated from KD mice were higher; however, the maximum currents, achieved after cytoplasmic Mg²⁺ washout, were not different. In the present study, we investigated the consequences of TRPM7 kinase inactivation in splenic and peritoneal macrophages. We measured the basal phagocytic activity of splenic macrophages using fluorescent latex beads, pHrodo zymosan bioparticles, and opsonized red blood cells. KD macrophages phagocytized more efficiently and had slightly higher baseline calcium levels compared to WT cells. We found no obvious differences in store-operated Ca²⁺ entry between WT and KD macrophages. By contrast, the resting cytosolic pH in KD macrophages was significantly more alkaline than in WT. Pharmacological blockade of sodium hydrogen exchanger 1 (NHE1) reversed the cytosolic alkalinization and reduced phagocytosis in KD macrophages. Basal TRPM7 channel activity in KD macrophages was also reduced after NHE1 blockade. Cytosolic Mg²⁺ sensitivity of TRPM7 channels measured in peritoneal macrophages was similar in WT and KD mice. The higher basal TRPM7 channel activity in KD macrophages is likely due to alkalinization. Our results identify a novel role for TRPM7 kinase as a suppressor of basal phagocytosis and a regulator of cellular pH.

TRPM7 channel activity in Jurkat T lymphocytes during magnesium depletion and loading: implications for divalent metal entry and cytotoxicity

TRPM7 is a cation channel-protein kinase highly expressed in T lymphocytes and other immune cells. It has been proposed to constitute a cellular entry pathway for Mg²⁺ and divalent metal cations such as Ca²⁺, Zn²⁺, Cd²⁺, Mn²⁺, and Ni²⁺. TRPM7 channels are inhibited by cytosolic Mg²⁺, rendering them largely inactive in intact cells. The dependence of channel activity on extracellular Mg²⁺ is less well studied. Here, we measured native TRPM7 channel activity in Jurkat T cells maintained in external Mg²⁺ concentrations varying between 400 nM and 1.4 mM for 1-3 days, obtaining an IC₅₀ value of 54 μM. Maintaining the cells in 400 nM or 8 μM [Mg²⁺]_o resulted in almost complete activation of TRPM7 in intact cells, due to cytosolic Mg²⁺ depletion. A total of 1.4 mM [Mg²⁺]_o was sufficient to fully eliminate the basal current. Submillimolar concentrations of amiloride prevented cellular Mg²⁺ depletion but not loading. We investigated whether the cytotoxicity of TRPM7 permeant metal ions Ni²⁺, Zn²⁺, Cd²⁺, Co²⁺, Mn²⁺, Sr²⁺, and Ba²⁺ requires TRPM7 channel activity. Mg²⁺ loading modestly reduced cytotoxicity of Zn²⁺, Co²⁺, Ni²⁺, and Mn²⁺ but not of Cd²⁺. Channel blocker NS8593 reduced Co²⁺ and Mn²⁺ but not Cd²⁺ or Zn²⁺ cytotoxicity and interfered with Mg²⁺ loading as evaluated by TRPM7 channel basal activity. Ba²⁺ and Sr²⁺ were neither detectably toxic nor permeant through the plasma membrane. These results indicate that in Jurkat T cells, entry of toxic divalent metal cations primarily occurs through pathways distinct from TRPM7. By contrast, we found evidence that Mg²⁺ entry requires TRPM7 channels.

Functional expression of TRPM7 as a Ca2+ influx pathway in adipocytes

In adipocytes, intracellular Ca²⁺ and Mg²⁺ modulates physiological functions, such as insulin action and the secretion of adipokines. TRPM7 is a Ca²⁺/Mg²⁺‐permeable non‐selective cation channel. TRPM7 mRNA is highly expressed in adipose tissue, however, its functional expression in adipocytes remains to be elucidated. In this study, we demonstrated for the first time that TRPM7 was functionally expressed in both freshly isolated white adipocytes and in 3T3‐L1 adipocytes differentiated from a 3T3‐L1 pre‐adipocyte cell line by whole‐cell patch‐clamp recordings. Consistent with known properties of TRPM7 current, the current in adipocytes was activated by the elimination of extracellular divalent cations and the reduction of intracellular free Mg²⁺ concentrations, and was inhibited by the TRPM7 inhibitors, 2‐aminoethyl diphenylborinate (2‐APB), hydrogen peroxide (H₂O₂), N‐methyl maleimide (NMM), NS8593, and 2‐amino‐2‐[2‐(4‐octylphenyl)ethyl]‐1,3‐propanediol (FTY720). Treatment with small‐interfering (si) RNA targeting TRPM7 resulted in a reduction in the current to 23 ± 7% of nontargeting siRNA‐treated adipocytes. Moreover a TRPM7 activator, naltriben, increased the TRPM7‐like current and [Ca²⁺]_i in 3T3‐L1 adipocytes but not in TRPM7‐knockdown adipocytes. These findings indicate that TRPM7 is functionally expressed, and plays a role as a Ca²⁺ influx pathway in adipocytes.

TRPM7 reflected in Cryo-EMirror

TRPM7 is an atypical type of ion channel because its pore-forming moiety is covalently linked to a protein kinase domain. The channel-kinase TRPM7 controls a wide range of biological processes such as mineral homeostasis, immune responses, cell motility, proliferation and differentiation. Earlier this year, Duan J & co-workers [1] published three TRPM7 structures resolved by cryo-electron microscopy (cryo-EM). This study tremendously advances our mechanistic understanding of TRPM7 channel function and forms the basis for informed structure-function assessment of this extraordinary protein.