Functional characterization of homo- and heteromeric channel kinases TRPM6 and TRPM7

How the kidney regulates magnesium: a modelling study

The kidneys are crucial for maintaining Mg2+ homeostasis. Along the proximal tubule and thick ascending limb, Mg2+ is reabsorbed paracellularly, while along the distal convoluted tubule (DCT), Mg2+ is reabsorbed transcellularly via transient receptor potential melastatin 6 (TRPM6). TRPM6 and other renal transporter expressions are regulated by sex hormones. To investigate renal Mg2 handling, we have developed sex-specific computational models of electrolyte transport along rat superficial nephron. Model simulations indicated that along the proximal tubule and thick ascending limb, Mg2+ and Na+ transport occur parallelly, but they are dissociated along the DCT. In addition, our models predicted higher paracellular Mg2+ permeability in females to attain similar cortical thick ascending limb fractional Mg2+ reabsorption in both sexes. Furthermore, DCT fractional Mg2+ reabsorption is higher in females than in males, allowing females to better fine-tune Mg2+ excretion. We validated our models by simulating the administration of three classes of diuretics. The model predicted significantly increased, marginally increased and significantly decreased Mg2+ excretions for loop, thiazide and K-sparing diuretics, respectively, aligning with experimental findings. The models can be used to conduct in silico studies on kidney adaptations to Mg2+ homeostasis alterations during conditions such as pregnancy, diabetes and chronic kidney disease.

Cannabigerolic Acid (CBGA) Inhibits the TRPM7 Ion Channel Through its Kinase Domain

Cannabinoids are a major class of compounds produced by the plant Cannabis sativa. Previous work has demonstrated that the main cannabinoids cannabidiol (CBD) and tetrahydrocannabinol (THC) can have some beneficial effects on pain, inflammation, epilepsy, and chemotherapy-induced nausea and vomiting. While CBD and THC represent the two major plant cannabinoids, some hemp varieties with enzymatic deficiencies produce mainly cannabigerolic acid (CBGA). We recently reported that CBGA has a potent inhibitory effect on both Store-Operated Calcium Entry (SOCE) via inhibition of Calcium Release-Activated Calcium (CRAC) channels as well as currents carried by the channel-kinase TRPM7. Importantly, CBGA prevented kidney damage and suppressed mRNA expression of inflammatory cytokines through inhibition of these mechanisms in an acute nephropathic mouse model. In the present study, we investigate the most common major and minor cannabinoids to determine their potential efficacy on TRPM7 channel function. We find that approximately half of the tested cannabinoids suppress TRPM7 currents to some degree, with CBGA having the strongest inhibitory effect on TRPM7. We determined that the CBGA-mediated inhibition of TRPM7 requires a functional kinase domain, is sensitized by both intracellular Mg⋅ATP and free Mg2+ and reduced by increases in intracellular Ca2+. Finally, we demonstrate that CBGA inhibits native TRPM7 channels in a B lymphocyte cell line. In conclusion, we demonstrate that CBGA is the most potent cannabinoid in suppressing TRPM7 activity and possesses therapeutic potential for diseases in which TRPM7 is known to play an important role such as cancer, stroke, and kidney disease.

Calcium's Role and Signaling in Aging Muscle, Cellular Senescence, and Mineral Interactions

Calcium research, since its pivotal discovery in the early 1800s through the heating of limestone, has led to the identification of its multi-functional roles. These include its functions as a reducing agent in chemical processes, structural properties in shells and bones, and significant role in cells relating to this review: cellular signaling. Calcium signaling involves the movement of calcium ions within or between cells, which can affect the electrochemical gradients between intra- and extracellular membranes, ligand binding, enzyme activity, and other mechanisms that determine cell fate. Calcium signaling in muscle, as elucidated by the sliding filament model, plays a significant role in muscle contraction. However, as organisms age, alterations occur within muscle tissue. These changes include sarcopenia, loss of neuromuscular junctions, and changes in mineral concentration, all of which have implications for calcium's role. Additionally, a field of study that has gained recent attention, cellular senescence, is associated with aging and disturbed calcium homeostasis, and is thought to affect sarcopenia progression. Changes seen in calcium upon aging may also be influenced by its crosstalk with other minerals such as iron and zinc. This review investigates the role of calcium signaling in aging muscle and cellular senescence. We also aim to elucidate the interactions among calcium, iron, and zinc across various cells and conditions, ultimately deepening our understanding of calcium signaling in muscle aging.

TRP Channels in Stroke

Ischemic stroke is a devastating disease that affects millions of patients worldwide. Unfortunately, there are no effective medications for mitigating brain injury after ischemic stroke. TRP channels are evolutionally ancient biosensors that detect external stimuli as well as tissue or cellular injury. To date, many members of the TRP superfamily have been reported to contribute to ischemic brain injury, including the TRPC subfamily (1, 3, 4, 5, 6, 7), TRPV subfamily (1, 2, 3, 4) and TRPM subfamily (2, 4, 7). These TRP channels share structural similarities but have distinct channel functions and properties. Their activation during ischemic stroke can be beneficial, detrimental, or even both. In this review, we focus on discussing the interesting features of stroke-related TRP channels and summarizing the underlying cellular and molecular mechanisms responsible for their involvement in ischemic brain injury.

Essential role of Mg2+ in mouse preimplantation embryo development revealed by TRPM7 chanzyme-deficient gametes

TRPM7 (transient receptor potential cation channel subfamily M member 7) is a chanzyme with channel and kinase domains essential for embryo development. Using gamete-specific Trpm7-null lines, we report that TRPM7-mediated Mg2+ influx is indispensable for reaching the blastocyst stage. TRPM7 is expressed dynamically from gametes to blastocysts; displays stage-specific localization on the plasma membrane, cytoplasm, and nucleus; and undergoes cleavage that produces C-terminal kinase fragments. TRPM7 underpins Mg2+ homeostasis, and excess Mg2+ but not Zn2+ or Ca2+ overcomes the arrest of Trpm7-null embryos; expressing Trpm7 mRNA restores development, but mutant versions fail or are partially rescued. Transcriptomic analyses of Trpm7-null embryos reveal an abundance of oxidative stress-pathway genes, confirmed by mitochondrial dysfunction, and a reduction in transcription factor networks essential for proliferation; Mg2+ supplementation corrects these defects. Hence, TRPM7 underpins Mg2+ homeostasis in preimplantation embryos, prevents oxidative stress, and promotes gene expression patterns necessary for developmental progression and cell-lineage specification.

Ratiometric Fluorescent Sensors Illuminate Cellular Magnesium Imbalance in a Model of Acetaminophen-Induced Liver Injury

Magnesium(II) plays catalytic, structural, regulatory, and signaling roles in living organisms. Abnormal levels of this metal have been associated with numerous pathologies, including cardiovascular disease, diabetes, metabolic syndrome, immunodeficiency, cancer, and, most recently, liver pathologies affecting humans. The role of Mg2+ in the pathophysiology of liver disease, however, has been occluded by concomitant changes in concentration of interfering divalent cations, such as Ca2+, which complicates the interpretation of experiments conducted with existing molecular Mg2+ indicators. Herein, we introduce a new quinoline-based fluorescent sensor, MagZet1, that displays a shift in its excitation and emission wavelengths, affording ratiometric detection of cellular Mg2+ by both fluorescence microscopy and flow cytometry. The new sensor binds the target metal with a submillimolar dissociation constant─well suited for detection of changes in free Mg2+ in cells─and displays a 10-fold selectivity against Ca2+. Furthermore, the fluorescence ratio is insensitive to changes in pH in the physiological range, providing an overall superior performance over existing indicators. We provide insights into the metal selectivity profile of the new sensor based on computational modeling, and we apply it to shed light on a decrease in cytosolic free Mg2+ and altered expression of metal transporters in cellular models of drug-induced liver injury caused by acetaminophen overdose.

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.

TRPV3 Ion Channel: From Gene to Pharmacology

Transient receptor potential vanilloid subtype 3 (TRPV3) is an ion channel with a sensory function that is most abundantly expressed in keratinocytes and peripheral neurons. TRPV3 plays a role in Ca²⁺ homeostasis due to non-selective ionic conductivity and participates in signaling pathways associated with itch, dermatitis, hair growth, and skin regeneration. TRPV3 is a marker of pathological dysfunctions, and its expression is increased in conditions of injury and inflammation. There are also pathogenic mutant forms of the channel associated with genetic diseases. TRPV3 is considered as a potential therapeutic target of pain and itch, but there is a rather limited range of natural and synthetic ligands for this channel, most of which do not have high affinity and selectivity. In this review, we discuss the progress in the understanding of the evolution, structure, and pharmacology of TRPV3 in the context of the channel's function in normal and pathological states.

ABT-333 (Dasabuvir) Increases Action Potential Duration and Provokes Early Afterdepolarizations in Canine Left Ventricular Cells via Inhibition of IKr

ABT-333 (dasabuvir) is an antiviral agent used in hepatitis C treatment. The molecule, similarly to some inhibitors of hERG channels, responsible for the delayed rectifier potassium current (IKr), contains the methanesulfonamide group. Reduced IKr current leads to long QT syndrome and early afterdepolarizations (EADs), therefore potentially causing life-threatening arrhythmias and sudden cardiac death. Our goal was to investigate the acute effects of ABT-333 in enzymatically isolated canine left ventricular myocardial cells. Action potentials (APs) and ion currents were recorded with a sharp microelectrode technique and whole-cell patch clamp, respectively. Application of 1 μM ABT-333 prolonged the AP in a reversible manner. The maximal rates of phases 0 and 1 were irreversibly decreased. Higher ABT-333 concentrations caused larger AP prolongation, elevation of the early plateau potential, and reduction of maximal rates of phases 0, 1, and 3. EADs occurred in some cells in 3–30 μM ABT-333 concentrations. The 10 μM ABT-333-sensitive current, recorded with AP voltage clamp, contained a late outward component corresponding to IKr and an early outward one corresponding to transient outward potassium current (Ito). ABT-333 reduced hERG-channel-mediated ion current in a concentration-dependent, partially reversible manner with a half-inhibitory concentration of 3.2 μM. As the therapeutic plasma concentration of ABT-333 is 1 nM, the arrhythmic risk of ABT-333 is very low, even in the case of drug overdose.

On the modulation of TRPM channels: Current perspectives and anticancer therapeutic implications

The transient melastatin receptor potential (TRPM) ion channel subfamily functions as cellular sensors and transducers of critical biological signal pathways by regulating ion homeostasis. Some members of TRPM have been cloned from cancerous tissues, and their abnormal expressions in various solid malignancies have been correlated with cancer cell growth, survival, or death. Recent evidence also highlights the mechanisms underlying the role of TRPMs in tumor epithelial-mesenchymal transition (EMT), autophagy, and cancer metabolic reprogramming. These implications support TRPM channels as potential molecular targets and their modulation as an innovative therapeutic approach against cancer. Here, we discuss the general characteristics of the different TRPMs, focusing on current knowledge about the connection between TRPM channels and critical features of cancer. We also cover TRPM modulators used as pharmaceutical tools in biological trials and an indication of the only clinical trial with a TRPM modulator about cancer. To conclude, the authors describe the prospects for TRPM channels in oncology.

ORAI Calcium Channels: Regulation, Function, Pharmacology, and Therapeutic Targets

The changes in intracellular free calcium (Ca2+) levels are one of the most widely regulators of cell function; therefore, calcium as a universal intracellular mediator is involved in very important human diseases and disorders. In many cells, Ca2+ inflow is mediated by store-operated calcium channels, and it is recognized that the store-operated calcium entry (SOCE) is mediated by the two partners: the pore-forming proteins Orai (Orai1-3) and the calcium store sensor, stromal interaction molecule (STIM1-2). Importantly, the Orai/STIM channels are involved in crucial cell signalling processes such as growth factors, neurotransmitters, and cytokines via interaction with protein tyrosine kinase coupled receptors and G protein-coupled receptors. Therefore, in recent years, the issue of Orai/STIM channels as a drug target in human diseases has received considerable attention. This review summarizes and highlights our current knowledge of the Orai/STIM channels in human diseases and disorders, including immunodeficiency, myopathy, tubular aggregate, Stormorken syndrome, York platelet syndrome, cardiovascular and metabolic disorders, and cancers, as well as suggesting these channels as drug targets for pharmacological therapeutic intervention. Moreover, this work will also focus on the pharmacological modulators of Orai/STIM channel complexes. Together, our thoughtful of the biology and physiology of the Orai/STIM channels have grown remarkably during the past three decades, and the next important milestone in the field of store-operated calcium entry will be to identify potent and selective small molecules as a therapeutic agent with the purpose to target human diseases and disorders for patient benefit.

TRPM7 channels regulate breathing during sleep in obesity by acting peripherally in the carotid bodies

Sleep-disordered breathing (SDB) affects over 50% of obese individuals. Exaggerated hypoxic chemoreflex is a cardinal trait of SDB in obesity. We have shown that leptin acts in the carotid bodies (CB) to augment chemoreflex and that leptin activates the transient receptor potential melastatin 7 (TRPM7) channel. However, the effect of leptin-TRPM7 signalling in CB on breathing and SDB has not been characterized in diet-induced obesity (DIO). We hypothesized that leptin acts via TRPM7 in the CB to increase chemoreflex leading to SDB in obesity. DIO mice were implanted with EEG/EMG electrodes and transfected with Leprb short hairpin RNA (shRNA) or Trpm7 shRNA vs. control shRNA in the CB area bilaterally. Mice underwent a full-polysomnography and metabolic studies at baseline and after transfection. Ventilatory responses to hypoxia and hypercapnia were assessed during wakefulness. Leprb and Trpm7 were upregulated and their promoters were demethylated in the CB of DIO mice. Leprb knockdown in the CB did not significantly affect ventilation. Trpm7 knockdown in the CB stimulated breathing during sleep in normoxia. These effects were not driven by changes in CB chemosensitivity or metabolism. Under sustained hypoxia, Trpm7 shRNA in the CB augmented ventilation during sleep, but decreased oxyhaemoglobin saturation. We conclude that the suppression of TRPM7 in the CB improved sleep-related hypoventilation and that the respiratory effects of CB TRPM7 channels in obesity are independent of leptin. TRPM7 signalling in the CB could be a therapeutic target for the treatment of obesity-related SDB. KEY POINTS: The leptin-TRPM7 axis in the carotid bodies may play an important role in the pathogenesis of sleep-disordered breathing. TRPM7 channels regulate breathing during sleep by acting peripherally in the carotid bodies. Suppression of TRPM7 signalling in the carotid bodies improves the obesity-induced hypoventilation in mice. Pharmacological blockade of TRPM7 channels in the carotid bodies could be a therapy for sleep-disordered breathing in obesity.

Detection of TRPM6 and TRPM7 Proteins in Normal and Diseased Cardiac Atrial Tissue and Isolated Cardiomyocytes

Magnesium-sensitive transient receptor potential melastatin (TRPM) ion channels, TRPM6 and TRPM7, are present in several organs, but their roles in the heart remain unclear. Therefore, here, we studied the expression patterns of TRPM6 and TRPM7 in normal and diseased myocardium. Cardiac atrial tissue and cardiomyocytes were obtained from healthy pigs and undiseased human hearts as well as from hearts of patients with ischemic heart disease (IHD) or atrial fibrillation (AF). Immunofluorescence and ELISA were used to detect TRP proteins. TRPM6 and TRPM7 immunofluorescence signals, localized at/near the cell surface or intracellularly, were detected in pig and human atrial tissues. The TRP channel modulators carvacrol (CAR, 100 µM) or 2-aminoethoxydiphenyl borate (2-APB, 500 µM) decreased the TRPM7 signal, but enhanced that of TRPM6. At a higher concentration (2 mM), 2-APB enhanced the signals of both proteins. TRPM6 and TRPM7 immunofluorescence signals and protein concentrations were increased in atrial cells and tissues from IHD or AF patients. TRPM6 and TRPM7 proteins were both detected in cardiac atrial tissue, with relatively similar subcellular localization, but distinctive drug sensitivity profiles. Their upregulated expression in IHD and AF suggests a possible role of the channels in cardiac atrial disease.

SLC41A1 knockout mice display normal magnesium homeostasis

Transcellular Mg²⁺ reabsorption in the distal convoluted tubule (DCT) of the kidneys plays an important role in maintaining systemic Mg²⁺ homeostasis. SLC41A1 is a Na⁺/Mg²⁺ exchanger that mediates Mg²⁺ efflux from cells and is hypothesized to facilitate basolateral extrusion of Mg²⁺ in the DCT. In this study, we generated a SLC41A1 knockout mouse model to examine the role of SLC41A1 in Mg²⁺ homeostasis. Slc41a1^-/- mice exhibited similar serum and urine Mg²⁺ levels as their wild-type littermates. Dietary restriction of Mg²⁺ resulted in reduced serum Mg²⁺ concentration and urinary Mg²⁺ excretion, which was similar in the wild-type and knockout groups. Expression of genes encoding Mg²⁺ channels and transporters such as transient receptor potential melastatin 6 (Trpm6), transient receptor potential melastatin 7 (Trpm7), cyclin and CBS domain divalent metal cation transport mediator 2 (Cnnm2), and Slc41a3 were unchanged based on genotype. We investigated the potential redundancy of SLC41A1 and its homolog SLC41A3 by generating a double knockout mouse. Although Slc41a3^-/- knockout mice showed significantly reduced serum Mg²⁺ compared with wild-type and Slc41a1^-/- knockout groups, double knockout mice displayed similar serum Mg²⁺ levels as Slc41a3^-/- knockout mice. In conclusion, our data show that SLC41A1 is not involved in the regulation of systemic Mg²⁺ homeostasis in mice. Our data also demonstrate that SLC41A1 does not compensate for the loss of SLC41A3, suggesting different functions of these SLC41 proteins in vivo.NEW & NOTEWORTHY SLC41A1 has been hypothesized to mediate Mg²⁺ extrusion in the distal convoluted tubule and thus regulate Mg²⁺ homeostasis. This study investigated the role of SLC41A1 in Mg²⁺ homeostasis in vivo using a transgenic mouse model. Our results demonstrate that SLC41A1 is not required to maintain normal Mg²⁺ balance in mice. We also show that SLC41A3 is more important than SLC41A1 in regulating systemic Mg²⁺ levels.

HBXIP blocks myosin-IIA assembly by phosphorylating and interacting with NMHC-IIA in breast cancer metastasis

Tumor metastasis depends on the dynamic balance of the actomyosin cytoskeleton. As a key component of actomyosin filaments, non-muscle myosin-IIA disassembly contributes to tumor cell spreading and migration. However, its regulatory mechanism in tumor migration and invasion is poorly understood. Here, we found that oncoprotein hepatitis B X-interacting protein (HBXIP) blocked the myosin-IIA assemble state promoting breast cancer cell migration. Mechanistically, mass spectrometry analysis, co-immunoprecipitation assay and GST-pull down assay proved that HBXIP directly interacted with the assembly-competent domain (ACD) of non-muscle heavy chain myosin-IIA (NMHC-IIA). The interaction was enhanced by NMHC-IIA S1916 phosphorylation via HBXIP-recruited protein kinase PKCβII. Moreover, HBXIP induced the transcription of PRKCB, encoding PKCβII, by coactivating Sp1, and triggered PKCβII kinase activity. Interestingly, RNA sequencing and mouse metastasis model indicated that the anti-hyperlipidemic drug bezafibrate (BZF) suppressed breast cancer metastasis via inhibiting PKCβII-mediated NMHC-IIA phosphorylation in vitro and in vivo. We reveal a novel mechanism by which HBXIP promotes myosin-IIA disassembly via interacting and phosphorylating NMHC-IIA, and BZF can serve as an effective anti-metastatic drug in breast cancer.

Potential of the TRPM7 channel as a novel therapeutic target for pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is an intractable vascular disease characterized by a progressive increase in pulmonary vascular resistance caused by pulmonary vascular remodeling, which ultimately leads to right-sided heart failure. PAH remains incurable, despite the development of PAH-targeted therapeutics centered on pulmonary artery relaxants. It is necessary to identify the target molecules that contribute to pulmonary artery remodeling. Transient receptor potential (TRP) channels have been suggested to modulate pulmonary artery remodeling. Our study focused on the transient receptor potential ion channel subfamily M, member 7, or the TRPM7 channel, which modulates endothelial-to-mesenchymal transition and smooth muscle proliferation in the pulmonary artery. In this review, we summarize the role and expression profile of TRPM7 channels in PAH progression and discuss TRPM7 channels as possible therapeutic targets. In addition, we discuss the therapeutic effect of a Chinese herbal medicine, Ophiocordyceps sinensis (OCS), on PAH progression, which partly involves TRPM7 inhibition.

Recent Advances in the Structural Biology of Mg2+ Channels and Transporters

Magnesium ions (Mg²⁺) are the most abundant divalent cations in living organisms and are essential for various physiological processes, including ATP utilization and the catalytic activity of numerous enzymes. Therefore, the homeostatic mechanisms associated with cellular Mg²⁺ are crucial for both eukaryotic and prokaryotic organisms and are thus strictly controlled by Mg²⁺ channels and transporters. Technological advances in structural biology, such as the expression screening of membrane proteins, in meso phase crystallization, and recent cryo-EM techniques, have enabled the structure determination of numerous Mg²⁺ channels and transporters. In this review article, we provide an overview of the families of Mg²⁺ channels and transporters (MgtE/SLC41, TRPM6/7, CorA/Mrs2, CorC/CNNM), and discuss the structural biology prospects based on the known structures of MgtE, TRPM7, CorA and CorC.

Structural and functional comparison of magnesium transporters throughout evolution

Magnesium (Mg²⁺) is the most prevalent divalent intracellular cation. As co-factor in many enzymatic reactions, Mg²⁺ is essential for protein synthesis, energy production, and DNA stability. Disturbances in intracellular Mg²⁺ concentrations, therefore, unequivocally result in delayed cell growth and metabolic defects. To maintain physiological Mg²⁺ levels, all organisms rely on balanced Mg²⁺ influx and efflux via Mg²⁺ channels and transporters. This review compares the structure and the function of prokaryotic Mg²⁺ transporters and their eukaryotic counterparts. In prokaryotes, cellular Mg²⁺ homeostasis is orchestrated via the CorA, MgtA/B, MgtE, and CorB/C Mg²⁺ transporters. For CorA, MgtE, and CorB/C, the motifs that form the selectivity pore are conserved during evolution. These findings suggest that CNNM proteins, the vertebrate orthologues of CorB/C, also have Mg²⁺ transport capacity. Whereas CorA and CorB/C proteins share the gross quaternary structure and functional properties with their respective orthologues, the MgtE channel only shares the selectivity pore with SLC41 Na⁺/Mg²⁺ transporters. In eukaryotes, TRPM6 and TRPM7 Mg²⁺ channels provide an additional Mg²⁺ transport mechanism, consisting of a fusion of channel with a kinase. The unique features these TRP channels allow the integration of hormonal, cellular, and transcriptional regulatory pathways that determine their Mg²⁺ transport capacity. Our review demonstrates that understanding the structure and function of prokaryotic magnesiotropic proteins aids in our basic understanding of Mg²⁺ transport.

Ca2+ Signalling and Hypoxia/Acidic Tumour Microenvironment Interplay in Tumour Progression

Solid tumours are characterised by an altered microenvironment (TME) from the physicochemical point of view, displaying a highly hypoxic and acidic interstitial fluid. Hypoxia results from uncontrolled proliferation, aberrant vascularization and altered cancer cell metabolism. Tumour cellular apparatus adapts to hypoxia by altering its metabolism and behaviour, increasing its migratory and metastatic abilities by the acquisition of a mesenchymal phenotype and selection of aggressive tumour cell clones. Extracellular acidosis is considered a cancer hallmark, acting as a driver of cancer aggressiveness by promoting tumour metastasis and chemoresistance via the selection of more aggressive cell phenotypes, although the underlying mechanism is still not clear. In this context, Ca²⁺ channels represent good target candidates due to their ability to integrate signals from the TME. Ca²⁺ channels are pH and hypoxia sensors and alterations in Ca²⁺ homeostasis in cancer progression and vascularization have been extensively reported. In the present review, we present an up-to-date and critical view on Ca²⁺ permeable ion channels, with a major focus on TRPs, SOCs and PIEZO channels, which are modulated by tumour hypoxia and acidosis, as well as the consequent role of the altered Ca²⁺ signals on cancer progression hallmarks. We believe that a deeper comprehension of the Ca²⁺ signalling and acidic pH/hypoxia interplay will break new ground for the discovery of alternative and attractive therapeutic targets.

Mechanisms of proton pump inhibitor-induced hypomagnesemia

Proton pump inhibitors (PPIs) reliably suppress gastric acid secretion and are therefore the first-line treatment for gastric acid-related disorders. Hypomagnesemia (serum magnesium [Mg²⁺ ] <0.7 mmol/L) is a commonly reported side effect of PPIs. Clinical reports demonstrate that urinary Mg²⁺ excretion is low in PPI users with hypomagnesemia, suggesting a compensatory mechanism by the kidney for malabsorption of Mg²⁺ in the intestines. However, the exact mechanism by which PPIs cause impaired Mg²⁺ absorption is still unknown. In this review, we show that current experimental evidence points toward reduced Mg²⁺ solubility in the intestinal lumen. Moreover, the absorption pathways in both the small intestine and the colon may be reduced by changes in the expression and activity of key transporter proteins. Additionally, the gut microbiome may contribute to the development of PPI-induced hypomagnesemia, as PPI use affects the composition of the gut microbiome. In this review, we argue that the increase of the luminal pH during PPI treatment may contribute to several of these mechanisms. Considering the fact that bacterial fermentation of dietary fibers results in luminal acidification, we propose that targeting the gut microbiome using dietary intervention might be a promising treatment strategy to restore hypomagnesemia in PPI users.

Possible role for rare TRPM7 variants in patients with hypomagnesaemia with secondary hypocalcaemia

BACKGROUND

Hypomagnesaemia with secondary hypocal-caemia (HSH) is a rare autosomal recessive disorder caused by pathogenic variants in TRPM6, encoding the channel-kinase transient receptor potential melastatin type 6. Patients have very low serum magnesium (Mg2+) levels and suffer from muscle cramps and seizures. Despite genetic testing, a subgroup of HSH patients remains without a diagnosis.

METHODS

In this study, two families with an HSH phenotype but negative for TRPM6 pathogenic variants were subjected to whole exome sequencing. Using a complementary combination of biochemical and functional analyses in overexpression systems and patient-derived fibroblasts, the effect of the TRPM7-identified variants on Mg2+ transport was examined.

RESULTS

For the first time, variants in TRPM7 were identified in two families as a potential cause for hereditary HSH. Patients suffer from seizures and muscle cramps due to magnesium deficiency and episodes of hypocalcaemia. In the first family, a splice site variant caused the incorporation of intron 1 sequences into the TRPM7 messenger RNA and generated a premature stop codon. As a consequence, patient-derived fibroblasts exhibit decreased cell growth. In the second family, a heterozygous missense variant in the pore domain resulted in decreased TRPM7 channel activity.

CONCLUSIONS

We establish TRPM7 as a prime candidate gene for autosomal dominant hypomagnesaemia and secondary hypocalcaemia. Screening of unresolved patients with hypocalcaemia and secondary hypocalcaemia may further establish TRPM7 pathogenic variants as a novel Mendelian disorder.

Transcriptional Control of Trpm6 by the Nuclear Receptor FXR

Farnesoid x receptor (FXR) is a nuclear bile acid receptor that belongs to the nuclear receptor superfamily. It plays an essential role in bile acid biosynthesis, lipid and glucose metabolism, liver regeneration, and vertical sleeve gastrectomy. A loss of the FXR gene or dysregulations of FXR-mediated gene expression are associated with the development of progressive familial intrahepatic cholestasis, tumorigenesis, inflammation, and diabetes mellitus. Magnesium ion (Mg²⁺) is essential for mammalian physiology. Over 600 enzymes are dependent on Mg²⁺ for their activity. Here, we show that the Trpm6 gene encoding a Mg²⁺ channel is a direct FXR target gene in the intestinal epithelial cells of mice. FXR expressed in the intestinal epithelial cells is absolutely required for sustaining a basal expression of intestinal Trpm6 that can be robustly induced by the treatment of GW4064, a synthetic FXR agonist. Analysis of FXR ChIP-seq data revealed that intron regions of Trpm6 contain two prominent FXR binding peaks. Among them, the proximal peak from the transcription start site contains a functional inverted repeat 1 (IR1) response element that directly binds to the FXR-RXRα heterodimer. Based on these results, we proposed that an intestinal FXR-TRPM6 axis may link a bile acid signaling to Mg²⁺ homeostasis.

Severe Hypomagnesemia Associated with the Long-term Use of the Potassium-competitive Acid Blocker Vonoprazan

Hypomagnesemia caused by a proton pump inhibitor (PPI) was first reported in 2006. We herein report a case of hypomagnesemia due to the long-term use of vonoprazan, a potassium-competitive acid blocker (P-CAB). A 66-year-old man was admitted to our hospital complaining of disturbance of consciousness with evidence of hypomagnesemia noted on blood testing. The long-term use of vonoprazan was considered the cause of his hypomagnesemia, so it was discontinued, after which the hypomagnesemia improved. Hypomagnesemia can be induced not only by PPI treatment but also following the long-term use of P-CAB.

Potential Implications of Mammalian Transient Receptor Potential Melastatin 7 in the Pathophysiology of Myalgic Encephalomyelitis/Chronic Fatigue Syndrome: A Review

The transient receptor potential (TRP) superfamily of ion channels is involved in the molecular mechanisms that mediate neuroimmune interactions and activities. Recent advancements in neuroimmunology have identified a role for TRP cation channels in several neuroimmune disorders including amyotropic lateral sclerosis, multiple sclerosis, and myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). ME/CFS is a debilitating disorder with an obscure aetiology, hence considerable examination of its pathobiology is warranted. Dysregulation of TRP melastatin (TRPM) subfamily members and calcium signalling processes are implicated in the neurological, immunological, cardiovascular, and metabolic impairments inherent in ME/CFS. In this review, we present TRPM7 as a potential candidate in the pathomechanism of ME/CFS, as TRPM7 is increasingly recognized as a key mediator of physiological and pathophysiological mechanisms affecting neurological, immunological, cardiovascular, and metabolic processes. A focused examination of the biochemistry of TRPM7, the role of this protein in the aforementioned systems, and the potential of TRPM7 as a molecular mechanism in the pathophysiology of ME/CFS will be discussed in this review. TRPM7 is a compelling candidate to examine in the pathobiology of ME/CFS as TRPM7 fulfils several key roles in multiple organ systems, and there is a paucity of literature reporting on its role in ME/CFS.

Electrophysiological and Molecular Mechanisms of Sinoatrial Node Mechanosensitivity

The understanding of the electrophysiological mechanisms that underlie mechanosensitivity of the sinoatrial node (SAN), the primary pacemaker of the heart, has been evolving over the past century. The heart is constantly exposed to a dynamic mechanical environment; as such, the SAN has numerous canonical and emerging mechanosensitive ion channels and signaling pathways that govern its ability to respond to both fast (within second or on beat-to-beat manner) and slow (minutes) timescales. This review summarizes the effects of mechanical loading on the SAN activity and reviews putative candidates, including fast mechanoactivated channels (Piezo, TREK, and BK) and slow mechanoresponsive ion channels [including volume-regulated chloride channels and transient receptor potential (TRP)], as well as the components of mechanochemical signal transduction, which may contribute to SAN mechanosensitivity. Furthermore, we examine the structural foundation for both mechano-electrical and mechanochemical signal transduction and discuss the role of specialized membrane nanodomains, namely, caveolae, in mechanical regulation of both membrane and calcium clock components of the so-called coupled-clock pacemaker system responsible for SAN automaticity. Finally, we emphasize how these mechanically activated changes contribute to the pathophysiology of SAN dysfunction and discuss controversial areas necessitating future investigations. Though the exact mechanisms of SAN mechanosensitivity are currently unknown, identification of such components, their impact into SAN pacemaking, and pathological remodeling may provide new therapeutic targets for the treatment of SAN dysfunction and associated rhythm abnormalities.

Evidence for the expression of TRPM6 and TRPM7 in cardiomyocytes from all four chamber walls of the human heart

The expression of the channels-enzymes TRPM6 and TRPM7 in the human heart remains poorly defined, and TRPM6 is generally considered not to be expressed in cardiomyocytes. We examined their expression at protein and mRNA levels using right atrial samples resected from patients (n = 72) with or without ischemic heart disease (IHD) and samples from all chamber walls of explanted human hearts (n = 9). TRPM6 and TRPM7 proteins were detected using immunofluorescence on isolated cardiomyocytes, ELISA on tissue homogenates, and immunostaining of cardiac tissue, whereas their mRNAs were detected by RT-qPCR. Both TRPM6 and TRPM7 were present in all chamber walls, with TRPM7 being more abundant. TRPM6 was co-expressed with TRPM7. The expression levels were dependent on cell incubation conditions (presence or absence of divalent cations, pH of the extracellular milieu, presence of TRP channel inhibitors 2-aminoethoxydiphenyl-borate and carvacrol). These drugs reduced TRPM7 immunofluorescence but increased that of TRPM6. TRPM6 and TRPM7 expression was increased in tissues from IHD patients. This is the first demonstration of the presence and co-expression of TRPM6 and TRPM7 in cardiomyocytes from all chamber walls of the human heart. The increased TRPM6 and TRPM7 expression in IHD suggests that the chanzymes are involved in the pathophysiology of the disease.

Deletion of TRPV3 and CaV3.2 T-type channels in mice undermines fertility and Ca2+ homeostasis in oocytes and eggs

Ca2+ influx during oocyte maturation and after sperm entry is necessary to fill the internal Ca2+ stores and for complete egg activation. We knocked out the transient receptor potential vanilloid member 3 (TRPV3) and the T-type channel, CaV3.2, to determine their necessity for maintaining these functions in mammalian oocytes/eggs. Double-knockout (dKO) females were subfertile, their oocytes and eggs showed reduced internal Ca2+ stores, and, following sperm entry or Plcz (also known as Plcz1) cRNA injection, fewer dKO eggs displayed Ca2+ responses compared to wild-type eggs, which were also of lower frequency. These parameters were rescued and/or enhanced by removing extracellular Mg2+, suggesting that the residual Ca2+ influx could be mediated by the TRPM7 channel, consistent with the termination of divalent-cation oscillations in dKO eggs by a TRPM7 inhibitor. In total, we demonstrated that TRPV3 and CaV3.2 mediate the complete filling of the Ca2+ stores in mouse oocytes and eggs. We also showed that they are required for initiating and maintaining regularly spaced-out oscillations, suggesting that Ca2+ influx through PM ion channels dictates the periodicity and persistence of Ca2+ oscillations during mammalian fertilization.

Cell death modulation by transient receptor potential melastatin channels TRPM2 and TRPM7 and their underlying molecular mechanisms

Transient receptor potential melastatin (TRPM) channels are members of the transient receptor potential (TRP) channels, a family of evolutionarily conserved integral membrane proteins. TRPM channels are nonselective cation channels, mediating the influx of various ions including Ca²⁺, Na⁺ and Zn²⁺. The function of TRPM channels is vital for cell proliferation, cell development and cell death. Cell death is a key procedure during embryonic development, organism homeostasis, aging and disease. The category of cell death modalities, beyond the traditionally defined concepts of necrosis, autophagy, and apoptosis, were extended with the discovery of pyroptosis, necroptosis and ferroptosis. As upstream signaling regulators of cell death, TRPM channels have been involved inrelevant pathologies. In this review, we introduced several cell death modalities, then summarized the contribution of TRPM channels (especially TRPM2 and TRPM7) to different cell death modalities and discussed the underlying regulatory mechanisms. Our work highlighted the possibility of TRPM channels as potential therapeutic targets in cell death-related diseases.

Development of Store-Operated Calcium Entry-Targeted Compounds in Cancer

Store-operated Ca²⁺ entry (SOCE) is the major pathway of Ca²⁺ entry in mammalian cells, and regulates a variety of cellular functions including proliferation, motility, apoptosis, and death. Accumulating evidence has indicated that augmented SOCE is related to the generation and development of cancer, including tumor formation, proliferation, angiogenesis, metastasis, and antitumor immunity. Therefore, the development of compounds targeting SOCE has been proposed as a potential and effective strategy for use in cancer therapy. In this review, we summarize the current research on SOCE inhibitors and blockers, discuss their effects and possible mechanisms of action in cancer therapy, and induce a new perspective on the treatment of cancer.

Genetic and drug-induced hypomagnesemia: different cause, same mechanism

Magnesium (Mg2+) plays an essential role in many biological processes. Mg2+ deficiency is therefore associated with a wide range of clinical effects including muscle cramps, fatigue, seizures and arrhythmias. To maintain sufficient Mg2+ levels, (re)absorption of Mg2+ in the intestine and kidney is tightly regulated. Genetic defects that disturb Mg2+ uptake pathways, as well as drugs interfering with Mg2+ (re)absorption cause hypomagnesemia. The aim of this review is to provide an overview of the molecular mechanisms underlying genetic and drug-induced Mg2+ deficiencies. This leads to the identification of four main mechanisms that are affected by hypomagnesemia-causing mutations or drugs: luminal transient receptor potential melastatin type 6/7-mediated Mg2+ uptake, paracellular Mg2+ reabsorption in the thick ascending limb of Henle's loop, structural integrity of the distal convoluted tubule and Na+-dependent Mg2+ extrusion driven by the Na+/K+-ATPase. Our analysis demonstrates that genetic and drug-induced causes of hypomagnesemia share common molecular mechanisms. Targeting these shared pathways can lead to novel treatment options for patients with hypomagnesemia.

Cyclin M2 (CNNM2) knockout mice show mild hypomagnesaemia and developmental defects

Patients with mutations in Cyclin M2 (CNNM2) suffer from hypomagnesaemia, seizures, and intellectual disability. Although the molecular function of CNNM2 is under debate, the protein is considered essential for renal Mg²⁺ reabsorption. Here, we used a Cnnm2 knock out mouse model, generated by CRISPR/Cas9 technology, to assess the role of CNNM2 in Mg²⁺ homeostasis. Breeding Cnnm2^+/- mice resulted in a Mendelian distribution at embryonic day 18. Nevertheless, only four Cnnm2^-/- pups were born alive. The Cnnm2^-/- pups had a significantly lower serum Mg²⁺ concentration compared to wildtype littermates. Subsequently, adult Cnnm2^+/- mice were fed with low, control, or high Mg²⁺ diets for two weeks. Adult Cnnm2^+/- mice showed mild hypomagnesaemia compared to Cnnm2^+/+ mice and increased serum Ca²⁺ levels, independent of dietary Mg²⁺ intake. Faecal analysis displayed increased Mg²⁺ and Ca²⁺ excretion in the Cnnm2^+/- mice. Transcriptional profiling of Trpm6, Trpm7, and Slc41a1 in kidneys and colon did not reveal effects based on genotype. Microcomputed tomography analysis of the femurs demonstrated equal bone morphology and density. In conclusion, CNNM2 is vital for embryonic development and Mg²⁺ homeostasis. Our data suggest a previously undescribed role of CNNM2 in the intestine, which may contribute to the Mg²⁺ deficiency in mice and patients.

ORAI1 Ca2+ Channel as a Therapeutic Target in Pathological Vascular Remodelling

In the adult, vascular smooth muscle cells (VSMC) are normally physiologically quiescent, arranged circumferentially in one or more layers within blood vessel walls. Remodelling of native VSMC to a proliferative state for vascular development, adaptation or repair is driven by platelet-derived growth factor (PDGF). A key effector downstream of PDGF receptors is store-operated calcium entry (SOCE) mediated through the plasma membrane calcium ion channel, ORAI1, which is activated by the endoplasmic reticulum (ER) calcium store sensor, stromal interaction molecule-1 (STIM1). This SOCE was shown to play fundamental roles in the pathological remodelling of VSMC. Exciting transgenic lineage-tracing studies have revealed that the contribution of the phenotypically-modulated VSMC in atherosclerotic plaque formation is more significant than previously appreciated, and growing evidence supports the relevance of ORAI1 signalling in this pathologic remodelling. ORAI1 has also emerged as an attractive potential therapeutic target as it is accessible to extracellular compound inhibition. This is further supported by the progression of several ORAI1 inhibitors into clinical trials. Here we discuss the current knowledge of ORAI1-mediated signalling in pathologic vascular remodelling, particularly in the settings of atherosclerotic cardiovascular diseases (CVDs) and neointimal hyperplasia, and the recent developments in our understanding of the mechanisms by which ORAI1 coordinates VSMC phenotypic remodelling, through the activation of key transcription factor, nuclear factor of activated T-cell (NFAT). In addition, we discuss advances in therapeutic strategies aimed at the ORAI1 target.

The Underlying Mechanism of Modulation of Transient Receptor Potential Melastatin 3 by protons

Transient receptor potential melastatin 3 channel (TRPM3) is a calcium-permeable nonselective cation channel that plays an important role in modulating glucose homeostasis in the pancreatic beta cells. However, how TRPM3 is regulated under physiological and pathological conditions is poorly understood. In this study, we found that both intracellular and extracellular protons block TRPM3 through its binding sites in the pore region. We demonstrated that external protons block TRPM3 with an inhibitory pH₅₀ of 5.5. whereas internal protons inhibit TRPM3 with an inhibitory pH₅₀ of 6.9. We identified three titratable residues, D1059, D1062, and D1073, at the vestibule of the channel pore that contributes to pH sensitivity. The mutation of D1073Q reduced TRPM3 current by low external pH 5.5 from 62 ± 3% in wildtype to 25 ± 6.0% in D1073Q mutant. These results indicate that D1073 is essential for pH sensitivity. In addition, we found that a single mutation of D1059 or D1062 enhanced pH sensitivity. In summary, our findings identify molecular determinants respionsible for the pH regulation of TRPM3. The inhibition of TRPM3 by protons may indicate an endogenous mechanism governing TRPM3 gating and its physiological/pathological functions.

Mechanisms coupling sodium and magnesium reabsorption in the distal convoluted tubule of the kidney

Hypomagnesaemia is a common feature of renal Na⁺ wasting disorders such as Gitelman and EAST/SeSAME syndrome. These genetic defects specifically affect Na⁺ reabsorption in the distal convoluted tubule, where Mg²⁺ reabsorption is tightly regulated. Apical uptake via TRPM6 Mg²⁺ channels and basolateral Mg²⁺ extrusion via a putative Na⁺ -Mg²⁺ exchanger determines Mg²⁺ reabsorption in the distal convoluted tubule. However, the mechanisms that explain the high incidence of hypomagnesaemia in patients with Na⁺ wasting disorders of the distal convoluted tubule are largely unknown. In this review, we describe three potential mechanisms by which Mg²⁺ reabsorption in the distal convoluted tubule is linked to Na⁺ reabsorption. First, decreased activity of the thiazide-sensitive Na⁺ /Cl^- cotransporter (NCC) results in shortening of the segment, reducing the Mg²⁺ reabsorption capacity. Second, the activity of TRPM6 and NCC are determined by common regulatory pathways. Secondary effects of NCC dysregulation such as hormonal imbalance, therefore, might disturb TRPM6 expression. Third, the basolateral membrane potential, maintained by the K⁺ permeability and Na⁺ -K⁺ -ATPase activity, provides the driving force for Na⁺ and Mg²⁺ extrusion. Depolarisation of the basolateral membrane potential in Na⁺ wasting disorders of the distal convoluted tubule may therefore lead to reduced activity of the putative Na⁺ -Mg²⁺ exchanger SLC41A1. Elucidating the interconnections between Mg²⁺ and Na⁺ transport in the distal convoluted tubule is hampered by the currently available models. Our analysis indicates that the coupling of Na⁺ and Mg²⁺ reabsorption may be multifactorial and that advanced experimental models are required to study the molecular mechanisms.

Transient Receptor Potential Melastatin 7 Promotes Vascular Adventitial Fibroblasts Phenotypic Transformation and Inflammatory Reaction Induced by Mechanical Stretching Stress via p38 MAPK/JNK Pathway

Remodeling of the arteries is one of the pathological bases of hypertension. We have previously shown that transient receptor potential melastatin 7 (TRPM7) aggravates the vascular adventitial remodeling caused by pressure overload in the transverse aortic constriction (TAC) model. In this study, we sought to explore the functional expression and downstream signaling of TRPM7 in vascular adventitial fibroblasts (AFs) stimulated by mechanical stretching stress (MSS). The expression of TRPM7 was upregulated with a concomitant translocation to the cytoplasm in the AFs stimulated with 20% MSS. Meanwhile, the expression of α-smooth muscle actin (α-SMA), a marker of transformation from AFs to myofibroblasts (MFs) was also increased. Moreover, AF-conditioned medium caused a significant migration of macrophages after treatment with MSS and contained high levels of monocyte chemotactic protein-1 (MCP-1), interleukin-6 (IL-6), interleukin-8 (IL-8), and tumor necrosis factor-α (TNF-α). Pharmacological and RNA interference approaches using the TRPM7 inhibitor 2-aminoethoxydiphenyl borate (2-APB) and specific anti-TRPM7 small interfering RNA (si-RNA-TRPM7) abrogated these changes significantly. Further exploration uncloaked that inhibition of TRPM7 reduced the phosphorylation of p38 MAP kinase (p38MAPK) and c-Jun N-terminal kinase (JNK) in the AFs stimulated with MSS. Furthermore, inhibition of the phosphorylation of p38MAPK or JNK could also alleviate the MSS-induced expression of α-SMA and secretion of inflammatory factors. These observations indicate that activated TRPM7 participates in the phenotypic transformation and inflammatory action of AFs in response to MSS through the p38MAPK/JNK pathway and suggest that TRPM7 may be a potential therapeutic target for vascular remodeling caused by hemodynamic changes in hypertension.

Role of TRPM7 in cardiac fibrosis: A potential therapeutic target (Review)

Cardiac fibrosis is a hallmark of cardiac remodeling associated with nearly all forms of heart disease. Clinically, no effective therapeutic drugs aim to inhibit cardiac fibrosis, owing to the complex etiological heterogeneity and pathogenesis of this disease. A two-in-one protein structure, a ubiquitous expression profile and unique biophysical characteristics enable the involvement of transient receptor potential melastatin-subfamily member 7 (TRPM7) in the pathogenesis and development of fibrosis-related cardiac diseases, such as heart failure (HF), cardiomyopathies, arrhythmia and hyperaldosteronism. In response to a variety of stimuli, multiple bioactive molecules can activate TRPM7 and related signaling pathways, leading to fibroblast proliferation, differentiation and extracellular matrix production in cardiac fibroblasts. TRPM7-mediated Ca²⁺ signaling and TGF-β1 signaling pathways are critical for the formation of fibrosis. Accumulating evidence has demonstrated that TRPM7 is a potential pharmacological target for halting the development of fibrotic cardiac diseases. Reliable drug-like molecules for further development of high-affinity in vivo drugs targeting TRPM7 are urgently needed. The present review discusses the widespread and significant role of TRPM7 in cardiac fibrosis and focuses on its potential as a therapeutic target for alleviating heart fibrogenesis.

Altered Ca2+ Homeostasis in Immune Cells during Aging: Role of Ion Channels

Aging is an unstoppable process and begins shortly after birth. Each cell of the organism is affected by the irreversible process, not only with equal density but also at varying ages and with different speed. Therefore, aging can also be understood as an adaptation to a continually changing cellular environment. One of these very prominent changes in age affects Ca²⁺ signaling. Especially immune cells highly rely on Ca²⁺-dependent processes and a strictly regulated Ca²⁺ homeostasis. The intricate patterns of impaired immune cell function may represent a deficit or compensatory mechanisms. Besides, altered immune function through Ca²⁺ signaling can profoundly affect the development of age-related disease. This review attempts to summarize changes in Ca²⁺ signaling due to channels and receptors in T cells and beyond in the context of aging.

TRPM Channels in Human Diseases

The transient receptor potential melastatin (TRPM) subfamily belongs to the TRP cation channels family. Since the first cloning of TRPM1 in 1989, tremendous progress has been made in identifying novel members of the TRPM subfamily and their functions. The TRPM subfamily is composed of eight members consisting of four six-transmembrane domain subunits, resulting in homomeric or heteromeric channels. From a structural point of view, based on the homology sequence of the coiled-coil in the C-terminus, the eight TRPM members are clustered into four groups: TRPM1/M3, M2/M8, M4/M5 and M6/M7. TRPM subfamily members have been involved in several physiological functions. However, they are also linked to diverse pathophysiological human processes. Alterations in the expression and function of TRPM subfamily ion channels might generate several human diseases including cardiovascular and neurodegenerative alterations, organ dysfunction, cancer and many other channelopathies. These effects position them as remarkable putative targets for novel diagnostic strategies, drug design and therapeutic approaches. Here, we review the current knowledge about the main characteristics of all members of the TRPM family, focusing on their actions in human diseases.

Emerging role of transient receptor potential (TRP) channels in cancer progression

Transient receptor potential (TRP) channels comprise a diverse family of ion channels, the majority of which are calcium permeable and show sophisticated regulatory patterns in response to various environmental cues. Early studies led to the recognition of TRP channels as environmental and chemical sensors. Later studies revealed that TRP channels mediated the regulation of intracellular calcium. Mutations in TRP channel genes result in abnormal regulation of TRP channel function or expression, and interfere with normal spatial and temporal patterns of intracellular local Ca2+ distribution. The resulting dysregulation of multiple downstream effectors, depending on Ca2+ homeostasis, is associated with hallmarks of cancer pathophysiology, including enhanced proliferation, survival and invasion of cancer cells. These findings indicate that TRP channels affect multiple events that control cellular fate and play a key role in cancer progression. This review discusses the accumulating evidence supporting the role of TRP channels in tumorigenesis, with emphasis on prostate cancer. [BMB Reports 2020; 53(3): 125-132].

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.

Synthesis and Pharmacological Characterization of 2-Aminoethyl Diphenylborinate (2-APB) Derivatives for Inhibition of Store-Operated Calcium Entry (SOCE) in MDA-MB-231 Breast Cancer Cells

Calcium ions regulate a wide array of physiological functions including cell differentiation, proliferation, muscle contraction, neurotransmission, and fertilization. The endoplasmic reticulum (ER) is the major intracellular Ca²⁺ store and cellular events that induce ER store depletion (e.g., activation of inositol 1,4,5-triphosphate (IP₃) receptors) trigger a refilling process known as store-operated calcium entry (SOCE). It requires the intricate interaction between the Ca²⁺ sensing stromal interaction molecules (STIM) located in the ER membrane and the channel forming Orai proteins in the plasma membrane (PM). The resulting active STIM/Orai complexes form highly selective Ca²⁺ channels that facilitate a measurable Ca²⁺ influx into the cytosol followed by successive refilling of the ER by the sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA). STIM and Orai have attracted significant therapeutic interest, as enhanced SOCE has been associated with several cancers, and mutations in STIM and Orai have been linked to immunodeficiency, autoimmune, and muscular diseases. 2-Aminoethyl diphenylborinate (2-APB) is a known modulator and depending on its concentration can inhibit or enhance SOCE. We have synthesized several novel derivatives of 2-APB, introducing halogen and other small substituents systematically on each position of one of the phenyl rings. Using a fluorometric imaging plate reader (FLIPR) Tetra-based calcium imaging assay we have studied how these structural changes of 2-APB affect the SOCE modulation activity at different compound concentrations in MDA-MB-231 breast cancer cells. We have discovered 2-APB derivatives that block SOCE at low concentrations, at which 2-APB usually enhances SOCE.

Gating of human TRPV3 in a lipid bilayer

The TRPV3 channel plays a critical role in skin physiology, and mutations in TRPV3 result in the development of a congenital skin disorder, Olmsted syndrome. Here we describe multiple cryo-electron microscopy structures of human TRPV3 reconstituted into lipid nanodiscs, representing distinct functional states during the gating cycle. The ligand-free, closed conformation reveals well-ordered lipids interacting with the channel and two physical constrictions along the ion conduction pore involving both the extracellular selectivity filter and intracellular helix bundle crossing. Both the selectivity filter and bundle crossing expand upon activation, accompanied by substantial structural rearrangements at the cytoplasmic inter-subunit interface. Transition to the inactivated state involves a secondary structure change of the pore-lining helix, which contains a π-helical segment in the closed and open conformations but becomes entirely α-helical upon inactivation. Together with electrophysiological characterization, structures of TRPV3 in a lipid membrane environment provide unique insights into channel activation and inactivation mechanisms.

Steroids and TRP Channels: A Close Relationship

Transient receptor potential (TRP) channels are remarkable transmembrane protein complexes that are essential for the physiology of the tissues in which they are expressed. They function as non-selective cation channels allowing for the signal transduction of several chemical, physical and thermal stimuli and modifying cell function. These channels play pivotal roles in the nervous and reproductive systems, kidney, pancreas, lung, bone, intestine, among others. TRP channels are finely modulated by different mechanisms: regulation of their function and/or by control of their expression or cellular/subcellular localization. These mechanisms are subject to being affected by several endogenously-produced compounds, some of which are of a lipidic nature such as steroids. Fascinatingly, steroids and TRP channels closely interplay to modulate several physiological events. Certain TRP channels are affected by the typical genomic long-term effects of steroids but others are also targets for non-genomic actions of some steroids that act as direct ligands of these receptors, as will be reviewed here.

TRPM6 and TRPM7: Novel players in cell intercalation during vertebrate embryonic development

A common theme in organogenesis is how the final structure of organs emerge from epithelial tube structures, with the formation of the neural tube being one of the best examples. Two types of cell movements co-occur during neural tube closure involving the migration of cells toward the midline of the embryo (mediolateral intercalation or convergent extension) as well as the deep movement of cells from inside the embryo to the outside of the lateral side of the neural plate (radial intercalation). Failure of either type of cell movement will prevent neural tube closure, which can produce a range of neural tube defects (NTDs), a common congenital disease in humans. Numerous studies have identified signaling pathways that regulate mediolateral intercalation during neural tube closure. Less understood are the pathways that govern radial intercalation. Using the Xenopus laevis system, our group reported the identification of transient receptor potential (TRP) channels, TRPM6 and TRPM7, and the Mg²⁺ ion they conduct, as novel and key factors regulating both mediolateral and radial intercalation during neural tube closure. Here we broadly discuss tubulogenesis and cell intercalation from the perspective of neural tube closure and the respective roles of TRPM7 and TRPM6 in this critical embryonic process.

RAC1-Dependent ORAI1 Translocation to the Leading Edge Supports Lamellipodia Formation and Directional Persistence

Tumor invasion requires efficient cell migration, which is achieved by the generation of persistent and polarized lamellipodia. The generation of lamellipodia is supported by actin dynamics at the leading edge where a complex of proteins known as the WAVE regulatory complex (WRC) promotes the required assembly of actin filaments to push the front of the cell ahead. By using an U2OS osteosarcoma cell line with high metastatic potential, proven by a xenotransplant in zebrafish larvae, we have studied the role of the plasma membrane Ca²⁺ channel ORAI1 in this process. We have found that epidermal growth factor (EGF) triggered an enrichment of ORAI1 at the leading edge, where colocalized with cortactin (CTTN) and other members of the WRC, such as CYFIP1 and ARP2/3. ORAI1-CTTN co-precipitation was sensitive to the inhibition of the small GTPase RAC1, an upstream activator of the WRC. RAC1 potentiated ORAI1 translocation to the leading edge, increasing the availability of surface ORAI1 and increasing the plasma membrane ruffling. The role of ORAI1 at the leading edge was studied in genetically engineered U2OS cells lacking ORAI1 expression that helped us to prove the key role of this Ca²⁺ channel on lamellipodia formation, lamellipodial persistence, and cell directness, which are required for tumor cell invasiveness in vivo.

Structure and function of the calcium-selective TRP channel TRPV6

Epithelial calcium channel TRPV6 is a member of the vanilloid subfamily of TRP channels that is permeable to cations and highly selective to Ca²⁺ ; it shows constitutive activity regulated negatively by Ca²⁺ and positively by phosphoinositol and cholesterol lipids. In this review, we describe the molecular structure of TRPV6 and discuss how its structural elements define its unique functional properties. High Ca²⁺ selectivity of TRPV6 originates from the narrow selectivity filter, where Ca²⁺ ions are directly coordinated by a ring of anionic aspartate side chains. Divalent cations Ca²⁺ and Ba²⁺ permeate TRPV6 pore according to the knock-off mechanism, while tight binding of Gd³⁺ to the aspartate ring blocks the channel and prevents Na⁺ from permeating the pore. The iris-like channel opening is accompanied by an α-to-π helical transition in the pore-lining transmembrane helix S6. As a result of this transition, the intracellular halves of the S6 helices bend and rotate by about 100 deg, exposing different residues to the channel pore in the open and closed states. Channel opening is also associated with changes in occupancy of the transmembrane domain lipid binding sites. The inhibitor 2-aminoethoxydiphenyl borate (2-APB) binds to TRPV6 in a pocket formed by the cytoplasmic half of the S1-S4 transmembrane helical bundle and shifts open-closed channel equilibrium towards the closed state by outcompeting lipids critical for activation. Ca²⁺ inhibits TRPV6 via binding to calmodulin (CaM), which mediates Ca²⁺ -dependent inactivation. The TRPV6-CaM complex exhibits 1:1 stoichiometry; one TRPV6 tetramer binds both CaM lobes, which adopt a distinct head-to-tail arrangement. The CaM C-terminal lobe plugs the channel through a unique cation-π interaction by inserting the side chain of lysine K115 into a tetra-tryptophan cage at the ion channel pore intracellular entrance. Recent studies of TRPV6 structure and function described in this review advance our understanding of the role of this channel in physiology and pathophysiology and inform new therapeutic design.

Study of Magnesium Formulations on Intestinal Cells to Influence Myometrium Cell Relaxation

Background: Magnesium is involved in a wide variety of physiological processes including direct relaxation of smooth muscle. A magnesium imbalance can be considered the primary cause or consequence of many pathophysiological conditions. The smooth muscle tissue of the uterus, i.e., the myometrium, undergoes numerous physiological changes during life, fundamental for uterine activities, and it receives proven benefits from magnesium supplementation. However, magnesium supplements have poor absorption and bioavailability. Furthermore, no data are available on the direct interaction between intestinal absorption of magnesium and relaxation of the myometrium. Methods: Permeability in human intestinal cells (Caco-2 cells) and direct effects on myometrial cells (PHM1-41 cells) of two different forms of magnesium, i.e., sucrosomial and bisglycinate, were studied in order to verify the magnesium capacity of modulate contractility. Cell viability, reactive oxygen species (ROS) and nitric oxide (NO) production, magnesium concentration, contractility, and pathways involved were analyzed. Results: Data showed a better influence of buffered chelate bisglycinate on intestinal permeability and myometrial relaxation over time with a maximum effect at 3 h and greater availability compared to the sucrosomial form. Conclusions: Magnesium-buffered bisglycinate chelate showed better intestinal absorption and myometrial contraction, indicating a better chance of effectiveness in human applications.

Divalent cation influx and calcium homeostasis in germinal vesicle mouse oocytes

Prior to maturation, mouse oocytes are arrested at the germinal vesicle (GV) stage during which they experience constitutive calcium (Ca²⁺) influx and spontaneous Ca²⁺ oscillations. The oscillations cease during maturation but Ca²⁺ influx continues, as the oocytes' internal stores attain maximal content at the culmination of maturation, the metaphase II stage. The identity of the channel(s) that underlie this Ca²⁺ influx has not been completely determined. GV and matured oocytes are known to express three Ca²⁺ channels, Ca_V3.2, TRPV3 and TRPM7, but females null for each of these channels are fertile and their oocytes display minor modifications in Ca²⁺ homeostasis, suggesting a complex regulation of Ca²⁺ influx. To define the contribution of these channels at the GV stage, we used different divalent cations, pharmacological inhibitors and genetic models. We found that the three channels are active at this stage. Ca_V3.2 and TRPM7 channels contributed the majority of Ca²⁺ influx, as inhibitors and oocytes from homologous knockout (KO) lines showed severely reduced Ca²⁺ entry. Sr²⁺ influx was promoted by Ca_V3.2 channels, as Sr²⁺ oscillations were negligible in Ca_V3.2-KO oocytes but robust in control and Trpv3-KO GV oocytes. Mn²⁺ entry relied on expression of Ca_V3.2 and TRPM7 channels, but Ni²⁺ entry depended on the latter. Ca_V3.2 and TRPV3 channels combined to fill the Ca²⁺ stores, although Ca_V3.2 was the most impactful. Studies with pharmacological inhibitors effectively blocked the influx of divalent cations, but displayed off-target effects, and occasionally agonist-like properties. In conclusion, GV oocytes express channels mediating Ca²⁺ and other divalent cation influx that are pivotal for fertilization and early development. These channels may serve as targets for intervention to improve the success of assisted reproductive technologies.