Disruption of Atrial Rhythmicity by the Air Pollutant 1,2-Naphthoquinone: Role of Beta-Adrenergic and Sensory Receptors

The combustion of fossil fuels contributes to air pollution (AP), which was linked to about 8.79 million global deaths in 2018, mainly due to respiratory and cardiovascular-related effects. Among these, particulate air pollution (PM2.5) stands out as a major risk factor for heart health, especially during vulnerable phases. Our prior study showed that premature exposure to 1,2-naphthoquinone (1,2-NQ), a chemical found in diesel exhaust particles (DEP), exacerbated asthma in adulthood. Moreover, increased concentration of 1,2-NQ contributed to airway inflammation triggered by PM2.5, employing neurogenic pathways related to the up-regulation of transient receptor potential vanilloid 1 (TRPV1). However, the potential impact of early-life exposure to 1,2-naphthoquinone (1,2-NQ) on atrial fibrillation (AF) has not yet been investigated. This study aims to investigate how inhaling 1,2-NQ in early life affects the autonomic adrenergic system and the role played by TRPV1 in these heart disturbances. C57Bl/6 neonate male mice were exposed to 1,2-NQ (100 nM) or its vehicle at 6, 8, and 10 days of life. Early exposure to 1,2-NQ impairs adrenergic responses in the right atria without markedly affecting cholinergic responses. ECG analysis revealed altered rhythmicity in young mice, suggesting increased sympathetic nervous system activity. Furthermore, 1,2-NQ affected β1-adrenergic receptor agonist-mediated positive chronotropism, which was prevented by metoprolol, a β1 receptor blocker. Capsazepine, a TRPV1 blocker but not a TRPC5 blocker, reversed 1,2-NQ-induced cardiac changes. In conclusion, neonate mice exposure to AP 1,2-NQ results in an elevated risk of developing cardiac adrenergic dysfunction, potentially leading to atrial arrhythmia at a young age.

Ca2+ Influx through TRPC Channels Is Regulated by Homocysteine-Copper Complexes

An elevated level of circulating homocysteine (Hcy) has been regarded as an independent risk factor for cardiovascular disease; however, the clinical benefit of Hcy lowering-therapy is not satisfying. To explore potential unrevealed mechanisms, we investigated the roles of Ca²⁺ influx through TRPC channels and regulation by Hcy-copper complexes. Using primary cultured human aortic endothelial cells and HEK-293 T-REx cells with inducible TRPC gene expression, we found that Hcy increased the Ca²⁺ influx in vascular endothelial cells through the activation of TRPC4 and TRPC5. The activity of TRPC4 and TRPC5 was regulated by extracellular divalent copper (Cu²⁺) and Hcy. Hcy prevented channel activation by divalent copper, but monovalent copper (Cu⁺) had no effect on the TRPC channels. The glutamic acids (E542/E543) and the cysteine residue (C554) in the extracellular pore region of the TRPC4 channel mediated the effect of Hcy-copper complexes. The interaction of Hcy-copper significantly regulated endothelial proliferation, migration, and angiogenesis. Our results suggest that Hcy-copper complexes function as a new pair of endogenous regulators for TRPC channel activity. This finding gives a new understanding of the pathogenesis of hyperhomocysteinemia and may explain the unsatisfying clinical outcome of Hcy-lowering therapy and the potential benefit of copper-chelating therapy.

Protective effects of dapagliflozin against oxidative stress-induced cell injury in human proximal tubular cells

Elevated reactive oxygen species (ROS) in type 2 diabetes cause cellular damage in many organs. Recently, the new class of glucose-lowering agents, SGLT-2 inhibitors, have been shown to reduce the risk of developing diabetic complications; however, the mechanisms of such beneficial effect are largely unknown. Here we aimed to investigate the effects of dapagliflozin on cell proliferation and cell death under oxidative stress conditions and explore its underlying mechanisms. Human proximal tubular cells (HK-2) were used. Cell growth and death were monitored by cell counting, water-soluble tetrazolium-1 (WST-1) and lactate dehydrogenase (LDH) assays, and flow cytometry. The cytosolic and mitochondrial (ROS) production was measured using fluorescent probes (H2DCFDA and MitoSOX) under normal and oxidative stress conditions mimicked by addition of H₂O₂. Intracellular Ca²⁺ dynamics was monitored by FlexStation 3 using cell-permeable Ca²⁺ dye Fura-PE3/AM. Dapagliflozin (0.1–10 μM) had no effect on HK-2 cell proliferation under normal conditions, but an inhibitory effect was seen at an extreme high concentration (100 μM). However, dapagliflozin at 0.1 to 5 μM showed remarkable protective effects against H₂O₂-induced cell injury via increasing the viable cell number at phase G0/G1. The elevated cytosolic and mitochondrial ROS under oxidative stress was significantly decreased by dapagliflozin. Dapagliflozin increased the basal intracellular [Ca²⁺]_i in proximal tubular cells, but did not affect calcium release from endoplasmic reticulum and store-operated Ca²⁺ entry. The H₂O₂-sensitive TRPM2 channel seemed to be involved in the Ca²⁺ dynamics regulated by dapagliflozin. However, dapagliflozin had no direct effects on ORAI1, ORAI3, TRPC4 and TRPC5 channels. Our results suggest that dapagliflozin shows anti-oxidative properties by reducing cytosolic and mitochondrial ROS production and altering Ca²⁺ dynamics, and thus exerts its protective effects against cell damage under oxidative stress environment.

Curcumin ameliorates mercuric chloride-induced liver injury via modulating cytochrome P450 signaling and Nrf2/HO-1 pathway

Environmental mercury is a concern for coastal ecosystem health, and exerts adverse effects on human health. Despite the growing body of evidence showing the hepatoprotective roles of curcumin on mercury, the knowledge between the macroscopic descriptions and the actual mechanism(s) underlying these processes is getting larger remains elusive. Herein, mice received single injection of mercuric chloride (HgCl₂) (5 mg/kg body weight) and/or curcumin (50 mg/kg, body weight, p.o.). Firstly, the results showed curcumin could decline HgCl₂-induced up-regulated the levels of alanine aminotransferase (ALT) and aspartate aminotransferase (AST). Additionally, we also found that curcumin could suppress inflammatory damage, unbalance of trace elements (including sodium, magnesium, kalium, calcium overload), oxidative burst induced by HgCl₂, which could be associated with cytochrome P450 (CYP450) signaling. Secondly, we found that curcumin could prevent HgCl₂-induced cell death both in vivo and in vitro. Furthermore, curcumin significantly increased the nuclear translocation of nuclear factor E2-related factor 2 (Nrf2) and consequently upregulated the expression of heme oxygenase 1 (HO-1) under HgCl₂ treatment. Meanwhile, inhibition of HO-1 by zinc protoporphyria could abolish the cytoprotective effects of curcumin in HgCl₂-treated L02 hepatocytes. In conclusion, our data identify that curcumin could enhance Nrf2-mediated HO-1 to upregulate antioxidant ability, which might be associate with CYP450 signaling to suppress liver damage induced by HgCl₂. The present study further enriches and perfects the mechanism theory of HgCl₂ toxicity and suggest that the CYP450 signaling and Nrf2/HO-1 pathway is important in shedding light on curcumin's hepatoprotective effects in HgCl₂ toxicity.

Toxicity of mercury: Molecular evidence

Minamata disease in Japan and the large-scale poisoning by methylmercury (MeHg) in Iraq caused wide public concerns about the risk emanating from mercury for human health. Nowadays, it is widely known that all forms of mercury induce toxic effects in mammals, and increasing evidence supports the concern that environmentally relevant levels of MeHg could impact normal biological functions in wildlife. The information of mechanism involved in mercurial toxicity is growing but knowledge gaps still exist between the adverse effects and mechanisms of action, especially at the molecular level. A body of data obtained from experimental studies on mechanisms of mercurial toxicity in vivo and in vitro points to that disruption of the antioxidant system may play an important role in the mercurial toxic effects. Moreover, the accumulating evidence indicates that signaling transduction, protein or/and enzyme activity, and gene regulation are involving in mediating toxic and adaptive response to mercury exposure. We conducted here a comprehensive review of mercurial toxic effects on wildlife and human, in particular synthesized key findings of molecular pathways involved in mercurial toxicity from the cells to human. We discuss the molecular evidence related mercurial toxicity to the adverse effects, with particular emphasis on the gene regulation. The further studies relying on Omic analysis connected to adverse effects and modes of action of mercury will aid in the evaluation and validation of causative relationship between health outcomes and gene expression.

Mibefradil, a T-type Ca2+ channel blocker also blocks Orai channels by action at the extracellular surface

BACKGROUND AND PURPOSE

Mibefradil, a T-type Ca²⁺ channel blocker, has been investigated for treating solid tumours. However, its underlying mechanisms are still unclear. Here, we have investigated the pharmacological actions of mibefradil on Orai store-operated Ca²⁺ channels.

EXPERIMENTAL APPROACH

Human Orai1-3 cDNAs in tetracycline-regulated pcDNA4/TO vectors were transfected into HEK293 T-REx cells with stromal interaction molecule 1 (STIM1) stable expression. The Orai currents were recorded by whole-cell and excised-membrane patch clamp. Ca²⁺ influx or release was measured by Fura-PE3/AM. Cell growth and death were monitored by WST-1, LDH assays and flow cytometry.

KEY RESULTS

Mibefradil inhibited Orai1, Orai2, and Orai3 currents dose-dependently. The IC₅₀ for Orai1, Orai2, and Orai3 channels was 52.6, 14.1, and 3.8 μM respectively. Outside-out patch demonstrated that perfusion of 10-μM mibefradil to the extracellular surface completely blocked Orai3 currents and single channel activity evoked by 2-APB. Intracellular application of mibefradil did not alter Orai3 channel activity. Mibefradil at higher concentrations (>50 μM) inhibited Ca²⁺ release but had no effect on cytosolic STIM1 translocation evoked by thapsigargin. Inhibition on Orai channels by mibefradil was structure-related, as other T-type Ca²⁺ channel blockers with different structures, such as ethosuximide and ML218, had no or minimal effects on Orai channels. Moreover, mibefradil inhibited cell proliferation, induced apoptosis, and arrested cell cycle progression.

CONCLUSIONS AND IMPLICATIONS

Mibefradil is a potent cell surface blocker of Orai channels, demonstrating a new pharmacological action of this compound in regulating cell growth and death, which could be relevant to its anti-cancer activity.

Prediabetes and diabetes in a cohort of Qatari women screened for polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is associated with an increased risk of type 2 diabetes mellitus (T2DM) but its association with prediabetes and T2DM is unknown in Qatar. A cross sectional analysis of 3,017 Qatari subjects from the Qatar Biobank, identified 749 women aged 18–40 years, 720 of whom were assessed by the National Institute for Health (NIH) Guidelines for PCOS. Prediabetes (HbA1c 5.7–6.4% and/or impaired fasting glucose (IFG): fasting plasma glucose (FPG) 100–125 mg/dL (5.6–6.9 mmol/L)), and T2DM (fasting plasma glucose > 125 mg/dL (≥7 mmol/L), and/or HbA1c ≥ 6.5%) were determined. The prevalence of prediabetes was 10.6% and the prevalence of undiagnosed diabetes was found to be 4.0% in the total population. Overall, 12.1% of 720 women had PCOS, of whom FPG and HbA1c were available in 62 women with PCOS: 19.4% had prediabetes and 9.7% had diabetes. An adverse cardiovascular risk profile for IFG women compared to normal women was found. Women with PCOS alone had a similar adverse cardiovascular profile as those with IFG alone and T2DM. Thus, the risk of prediabetes and diabetes is increased in Qatari women with PCOS, with an adverse cardiovascular risk profile similar to that seen in prediabetes and T2DM.

Protective role of FKBP51 in calcium entry-induced endothelial barrier disruption

Pulmonary artery endothelial cells (PAECs) express a cation current, I_SOC (store-operated calcium entry current), which when activated permits calcium entry leading to inter-endothelial cell gap formation. The large molecular weight immunophilin FKBP51 inhibits I_SOC but not other calcium entry pathways in PAECs. However, it is unknown whether FKBP51-mediated inhibition of I_SOC is sufficient to protect the endothelial barrier from calcium entry-induced disruption. The major objective of this study was to determine whether FKBP51-mediated inhibition of I_SOC leads to decreased calcium entry-induced inter-endothelial gap formation and thus preservation of the endothelial barrier. Here, we measured the effects of thapsigargin-induced I_SOC on the endothelial barrier in control and FKBP51 overexpressing PAECs. FKBP51 overexpression decreased actin stress fiber and inter-endothelial cell gap formation in addition to attenuating the decrease in resistance observed with control cells using electric cell-substrate impedance sensing. Finally, the thapsigargin-induced increase in dextran flux was abolished in FKBP51 overexpressing PAECs. We then measured endothelial permeability in perfused lungs of FKBP51 knockout (FKBP51^–/–) mice and observed increased calcium entry-induced permeability compared to wild-type mice. To begin to dissect the mechanism underlying the FKBP51-mediated inhibition of I_SOC, a second goal of this study was to determine the role of the microtubule network. We observed that FKBP51 overexpressing PAECs exhibited increased microtubule polymerization that is critical for inhibition of I_SOC by FKBP51. Overall, we have identified FKBP51 as a novel regulator of endothelial barrier integrity, and these findings are significant as they reveal a protective mechanism for endothelium against calcium entry-induced disruption.

Defining potential roles of Pb(2+) in neurotoxicity from a calciomics approach

Metal ions play crucial roles in numerous biological processes, facilitating biochemical reactions by binding to various proteins. An increasing body of evidence suggests that neurotoxicity associated with exposure to nonessential metals (e.g., Pb(2+)) involves disruption of synaptic activity, and these observed effects are associated with the ability of Pb(2+) to interfere with Zn(2+) and Ca(2+)-dependent functions. However, the molecular mechanism behind Pb(2+) toxicity remains a topic of debate. In this review, we first discuss potential neuronal Ca(2+) binding protein (CaBP) targets for Pb(2+) such as calmodulin (CaM), synaptotagmin, neuronal calcium sensor-1 (NCS-1), N-methyl-d-aspartate receptor (NMDAR) and family C of G-protein coupled receptors (cGPCRs), and their involvement in Ca(2+)-signalling pathways. We then compare metal binding properties between Ca(2+) and Pb(2+) to understand the structural implications of Pb(2+) binding to CaBPs. Statistical and biophysical studies (e.g., NMR and fluorescence spectroscopy) of Pb(2+) binding are discussed to investigate the molecular mechanism behind Pb(2+) toxicity. These studies identify an opportunistic, allosteric binding of Pb(2+) to CaM, which is distinct from ionic displacement. Together, these data suggest three potential modes of Pb(2+) activity related to molecular and/or neural toxicity: (i) Pb(2+) can occupy Ca(2+)-binding sites, inhibiting the activity of the protein by structural modulation, (ii) Pb(2+) can mimic Ca(2+) in the binding sites, falsely activating the protein and perturbing downstream activities, or (iii) Pb(2+) can bind outside of the Ca(2+)-binding sites, resulting in the allosteric modulation of the protein activity. Moreover, the data further suggest that even low concentrations of Pb(2+) can interfere at multiple points within the neuronal Ca(2+) signalling pathways to cause neurotoxicity.

Effects of methylmercury on spinal cord afferents and efferents-A review

Methylmercury (MeHg) is an environmental neurotoxicant of public health concern. It readily accumulates in exposed humans, primarily in neuronal tissue. Exposure to MeHg, either acutely or chronically, causes severe neuronal dysfunction in the central nervous system and spinal neurons; dysfunction of susceptible neuronal populations results in neurodegeneration, at least in part through Ca2+-mediated pathways. Biochemical and morphologic changes in peripheral neurons precede those in central brain regions, despite the fact that MeHg readily crosses the blood-brain barrier. Consequently, it is suggested that unique characteristics of spinal cord afferents and efferents could heighten their susceptibility to MeHg toxicity. Transient receptor potential (TRP) ion channels are a class of Ca2+-permeable cation channels that are highly expressed in spinal afferents, among other sensory and visceral organs. These channels can be activated in numerous ways, including directly via chemical irritants or indirectly via Ca2+ release from intracellular storage organelles. Early studies demonstrated that MeHg interacts with heterologous TRP channels, though definitive mechanisms of MeHg toxicity on sensory neurons may involve more complex interaction with, and among, differentially-expressed TRP populations. In spinal efferents, glutamate receptors of the N-methyl-D-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and possibly kainic acid (KA) classes are thought to play a major role in MeHg-induced neurotoxicity. Specifically, the Ca2+-permeable AMPA receptors, which are abundant in motor neurons, have been identified as being involved in MeHg-induced neurotoxicity. In this review, we will describe the mechanisms that could contribute to MeHg-induced spinal cord afferent and efferent neuronal degeneration, including the possible mediators, such as uniquely expressed Ca2+-permeable ion channels.

Copper-Induced Membrane Depolarizations Involve the Induction of Mosaic TRP Channels, Which Activate VDCC Leading to Calcium Increases in Ulva compressa

The marine macroalga Ulva compressa (Chlorophyceae) is a cosmopolitan species, tolerant to heavy metals, in particular to copper. U. compressa was cultivated with 10 μM copper for 12 h and membrane depolarization events were detected. First, seven depolarization events occurred at 4, 8, 12-13, 80, and 86 min, and at 5 and 9 h of copper exposure. Second, bathocuproine sulphonate, a specific copper-chelating compound, was added before incorporating copper to the culture medium. Copper-induced depolarizations were inhibited by bathocuproine at 4, 8, 12-13, 80, and 86 min, but not at 5 and 9 h, indicating that initial events are due to copper ions entry. Third, specific inhibitors of human TRPA1, C4, C5, M8, and V1corresponding to HC030031, ML204, SKF96363, M8B, and capsazepin, respectively, were used to analyze whether copper-induced depolarizations were due to activation of transient receptor potentials (TRPs). Inhibitor effects indicate that the seven depolarizations involved the activation of functional mosaic TRPs that displayed properties similar to human TRPA, C, M, and/or V. Finally, inhibition of copper-induced depolarizations using specific TRP inhibitors suppressed calcium increases at 2, 3, and 12 h due to activation of voltage-dependent calcium channels (VDCCs). Thus, copper induces seven depolarization events that involve activation of mosaic TRPs which, in turn, activates VDCC leading to calcium increases at 2, 3, and 12 h in U. compressa.

Lipopolysaccharide potentiates endothelin-1-induced proliferation of pulmonary arterial smooth muscle cells by upregulating TRPC channels

Lipopolysaccharide (LPS) and endothelin-1 (ET-1) are critical pathogenic factors in sepsis-induced pulmonary hypertension; however it is unknown whether they have a coordinated action in the pathogenesis of this disease. Here we found that although LPS did not change the contractility of rat pulmonary arterial smooth muscle cells (PASMCs) in response to ET-1, it significantly promoted ET-1-induced PASMC proliferation. Measurement of ET-1-evoked Ca(2+) transients in PASMCs showed that LPS dramatically enhanced Ca(2+) influx mediated by transient receptor potential canonical (TRPC) channels. LPS did not directly activate TRPC channels, instead it selectively upregulated the expression of TRPC3 and TRPC4 in pulmonary arteries. Small interfering RNA (siRNA) and chemical blockers against TRPC channels abolished LPS-induced PASMC proliferation. LPS-induced cell proliferation and TRPC expression was mediated by the Ca(2+)-dependent calcineurin/NFAT signaling pathway. We suggest that blocking TRPC channels could be an effective strategy in controlling pulmonary arterial remodeling after endotoxin exposure.

Heavy metal chelator TPEN attenuates fura-2 fluorescence changes induced by cadmium, mercury and methylmercury

Stimulation with heavy metals is known to induce calcium (Ca²⁺) mobilization in many cell types. Interference with the measurement of intracellular Ca²⁺ concentration by the heavy metals in cells loaded with Ca²⁺ indicator fura-2 is an ongoing problem. In this study, we analyzed the effect of heavy metals on the fura-2 fluorescence ratio in human SH-SY5Y neuroblastoma cells by using TPEN, a specific cell-permeable heavy metal chelator. Manganese chloride (30–300 µM) did not cause significant changes in the fura-2 fluorescence ratio. A high concentration (300 µM) of lead acetate induced a slight elevation in the fura-2 fluorescence ratio. In contrast, stimulation with cadmium chloride, mercury chloride or MeHg (3–30 µM) elicited an apparent elevation of the fura-2 fluorescence ratio in a dose-dependent manner. In cells stimulated with 10 or 30 µM cadmium chloride, the addition of TPEN decreased the elevated fura-2 fluorescence ratio to basal levels. In cells stimulated with mercury or MeHg, the addition of TPEN significantly decreased the elevation of the fura-2 fluorescence ratio induced by lower concentrations (10 µM) of mercury or MeHg, but not by higher concentrations (30 µM). Pretreatment with Ca²⁺ channel blockers, such as verapamil, 2-APB or lanthanum chloride, resulted in different effects on the fura-2 fluorescence ratio. Our study provides a characterization of the effects of several heavy metals on the mobilization of divalent cations and the toxicity of heavy metals to neuronal cells.

Copper-induced activation of TRP channels promotes extracellular calcium entry, activation of CaMs and CDPKs, copper entry and membrane depolarization in Ulva compressa

In order to identify channels involved in membrane depolarization, Ulva compressa was incubated with agonists of TRP channels C5, A1 and V1, and the level of intracellular calcium was detected. Agonists of TRPC5, A1 and V1 induced increases in intracellular calcium at 4, 9, and 11 min of exposure, respectively, and antagonists of TRPC5, A1, and V1 corresponding to SKF-96365 (SKF), HC-030031 (HC), and capsazepin (CPZ), respectively, inhibited calcium increases indicating that functional TRPs exist in U. compressa. In addition, copper excess induced increases in intracellular calcium at 4, 9, and 12 min which were inhibited by SKF, HC, and CPZ, respectively, indicating that copper activate TRPC5, A1, and V1 channels. Moreover, copper-induced calcium increases were inhibited by EGTA, a non-permeable calcium chelating agent, but not by thapsigargin, an inhibitor of endoplasmic reticulum (ER) calcium ATPase, indicating that activation of TRPs leads to extracellular calcium entry. Furthermore, copper-induced calcium increases were not inhibited by W-7, an inhibitor of CaMs, and staurosporine, an inhibitor of CDPKs, indicating that extracellular calcium entry did not require activation of CaMs and CDPKs. In addition, copper induced membrane depolarization events at 4, 8, and 11 min and these events were inhibited by SKF, HC, CPZ, and bathocuproine, a specific copper chelating agent, indicating that copper entry through TRP channels leads to membrane depolarization. Moreover, membrane depolarization events were inhibited by W-7 and staurosporine, indicating that activation of CaMs and CDPKs is required to allow copper entry through TRPs. Interestingly, copper-induced calcium increases and depolarization events were light-dependent and were inhibited by DCMU, an inhibitor of photosystem II, and ATP-γ-S, a non-hydrolizable analog of ATP, suggesting that ATP derived from photosynthesis is required to activate TRPs. Thus, light-dependent copper-induced activation TRPC5, A1 and V1 promotes extracellular calcium entry leading to activation of CaMs and CDPKs which, in turn, promotes copper entry through TRP channels and membrane depolarization.

High glucose enhances store-operated calcium entry by upregulating ORAI/STIM via calcineurin-NFAT signalling

ORAI and stromal interaction molecule (STIM) are store-operated channel molecules that play essential roles in human physiology through a coupling mechanism of internal Ca(2+) store to Ca(2+) influx. However, the roles of ORAI and STIM in vascular endothelial cells under diabetic conditions remain unknown. Here, we investigated expression and signalling pathways of ORAI and STIM regulated by high glucose or hyperglycaemia using in vitro cell models, in vivo diabetic mice and tissues from patients. We found that ORAI1-3 and STIM1-2 were ubiquitously expressed in human vasculatures. Their expression was upregulated by chronic treatment with high glucose (HG, 25 mM D-glucose), which was accompanied by enhanced store-operated Ca(2+) influx in vascular endothelial cells. The increased expression was also observed in the aortae from genetically modified Akita diabetic mice (C57BL/6-Ins2(Akita)/J) and streptozocin-induced diabetic mice, and aortae from diabetic patients. HG-induced upregulation of ORAI and STIM genes was prevented by the calcineurin inhibitor cyclosporin A and NFATc3 siRNA. Additionally, in vivo treatment with the nuclear factor of activated T cells (NFAT) inhibitor A-285222 prevented the gene upregulation in Akita mice. However, HG had no direct effects on ORAI1-3 currents and the channel activation process through cytosolic STIM1 movement in the cells co-expressing STIM1-EYFP/ORAIs. We concluded that upregulation of STIM/ORAI through Ca(2+)-calcineurin-NFAT pathway is a novel mechanism causing abnormal Ca(2+) homeostasis and endothelial dysfunction under hyperglycaemia.

KEY MESSAGE

ORAI1-3 and STIM1-2 are ubiquitously expressed in vasculatures and upregulated by high glucose. Increased expression is confirmed in Akita (Ins2(Akita)/J) and STZ diabetic mice and patients. Upregulation mechanism is mediated by Ca(2+)/calcineurin/NFATc3 signalling. High glucose has no direct effects on ORAI1-3 channel activity and channel activation process.

Canonical transient receptor potential 4 and its small molecule modulators

Canonical transient receptor potential 4 (TRPC4) forms non-selective cation channels that contribute to phospholipase C-dependent Ca(2+) entry into cells following stimulation of G protein coupled receptors and receptor tyrosine kinases. Moreover, the channels are regulated by pertussis toxin-sensitive Gi/o proteins, lipids, and various other signaling mechanisms. TRPC4-containing channels participate in the regulation of a variety of physiological functions, including excitability of both gastrointestinal smooth muscles and brain neurons. This review is to present recent advances in the understanding of physiology and development of small molecular modulators of TRPC4 channels.

Permeation, regulation and control of expression of TRP channels by trace metal ions

Transient receptor potential (TRP) channels form a diverse family of cation channels comprising 28 members in mammals. Although some TRP proteins can only be found on intracellular membranes, most of the TRP protein isoforms reach the plasma membrane where they form ion channels and control a wide number of biological processes. There, their involvement in the transport of cations such as calcium and sodium has been well documented. However, a growing number of studies have started to expand our understanding of these proteins by showing that they also transport other biologically relevant metal ions like zinc, magnesium, manganese and cobalt. In addition to this newly recognized property, the activity and expression of TRP channels can be regulated by metal ions like magnesium, gadolinium, lanthanum or cisplatin. The aim of this review is to highlight the complex relationship between metal ions and TRP channels.

Alteration in MARCKS phosphorylation and expression by methylmercury in SH-SY5Y cells and rat brain

The molecular mechanisms mediating methylmercury (MeHg)-induced neurotoxicity are not completely understood. Because myristoylated alanine-rich C kinase substrate (MARCKS) plays an essential role in the differentiation and development of neuronal cells, we studied the alteration of MARCKS expression and phosphorylation in MeHg-induced neurotoxicity of neuroblastoma SH-SY5Y cells and in the rat brain. Exposure to MeHg induced a decrease in cell viability of SH-SY5Y cells, which was accompanied by a significant increase in phosphorylation and a reduction in MARCKS expression. Pretreatment of cells with a protein kinase C inhibitor or an extracellular Ca(2+) chelator suppressed MeHg-induced MARCKS phosphorylation. In MARCKS knock-down cells, MeHg-induced cell death was significantly augmented in comparison to control siRNA. In brain tissue from MeHg-treated rats, MARCKS phosphorylation was enhanced in the olfactory bulb in comparison to control rats. The present study may indicate that alteration in MARCKS expression or phosphorylation has consequences for MeHg-induced neurotoxicity.

TRPs: modulation by drug-like compounds

Drug-like compounds that exert biological activity towards TRP channels are either being used as cell biological tools or further developed into pharmacological lead structures aiming at therapeutic use in diseased states. Although drug-likeliness is not easy to predict, common rules include a relatively low molecular weight, physicochemical constraints, and the absence of known reactive or otherwise toxic groups. Small molecules that exert a biological activity to block, activate, or modulate TRP channels are intensely sought. Such tool compounds may be useful to assign native currents to a certain TRP channel and to validate the channel as a candidate target for future pharmacological intervention. Depending on the TRP channel isotype, these activities have reached different levels, with only few TRP channels modulators already being clinically tested in humans, whereas other compounds only underwent a preliminary validation. For some TRP channels, reliable low molecular weight inhibitors are not yet available. Hence, further efforts need to be undertaken in order to explore the physiological impact and possible therapeutic potential of TRP channel targeting with drug-like compounds.

TRPC4- and TRPC4-containing channels

TRPC4 proteins comprise six transmembrane domains, a putative pore-forming region, and an intracellularly located amino- and carboxy-terminus. Among eleven splice variants identified so far, TRPC4α and TRPC4β are the most abundantly expressed and functionally characterized. TRPC4 is expressed in various organs and cell types including the soma and dendrites of numerous types of neurons; the cardiovascular system including endothelial, smooth muscle, and cardiac cells; myometrial and skeletal muscle cells; kidney; and immune cells such as mast cells. Both recombinant and native TRPC4-containing channels differ tremendously in their permeability and other biophysical properties, pharmacological modulation, and mode of activation depending on the cellular environment. They vary from inwardly rectifying store-operated channels with a high Ca(2+) selectivity to non-store-operated channels predominantly carrying Na(+) and activated by Gαq- and/or Gαi-coupled receptors with a complex U-shaped current-voltage relationship. Thus, individual TRPC4-containing channels contribute to agonist-induced Ca(2+) entry directly or indirectly via depolarization and activation of voltage-gated Ca(2+) channels. The differences in channel properties may arise from variations in the composition of the channel complexes, in the specific regulatory pathways in the corresponding cell system, and/or in the expression pattern of interaction partners which comprise other TRPC proteins to form heteromultimeric channels. Additional interaction partners of TRPC4 that can mediate the activity of TRPC4-containing channels include (1) scaffolding proteins (e.g., NHERF) that may mediate interactions with signaling molecules in or in close vicinity to the plasma membrane such as Gα proteins or phospholipase C and with the cytoskeleton, (2) proteins in specific membrane microdomains (e.g., caveolin-1), or (3) proteins on cellular organelles (e.g., Stim1). The diversity of TRPC4-containing channels hampers the development of specific agonists or antagonists, but recently, ML204 was identified as a blocker of both recombinant and endogenous TRPC4-containing channels with an IC50 in the lower micromolar range that lacks activity on most voltage-gated channels and other TRPs except TRPC5 and TRPC3. Lanthanides are specific activators of heterologously expressed TRPC4- and TRPC5-containing channels but can block individual native TRPC4-containing channels. The biological relevance of TRPC4-containing channels was demonstrated by knockdown of TRPC4 expression in numerous native systems including gene expression, cell differentiation and proliferation, formation of myotubes, and axonal regeneration. Studies of TRPC4 single and TRPC compound knockout mice uncovered their role for the regulation of vascular tone, endothelial permeability, gastrointestinal contractility and motility, neurotransmitter release, and social exploratory behavior as well as for excitotoxicity and epileptogenesis. Recently, a single-nucleotide polymorphism (SNP) in the Trpc4 gene was associated with a reduced risk for experience of myocardial infarction.

Involvement of TRPC channels in lung cancer cell differentiation and the correlation analysis in human non-small cell lung cancer

The canonical transient receptor potential (TRPC) channels are Ca(2+)-permeable cationic channels controlling the Ca(2+) influx evoked by G protein-coupled receptor activation and/or by Ca(2+) store depletion. Here we investigate the involvement of TRPCs in the cell differentiation of lung cancer. The expression of TRPCs and the correlation to cancer differentiation grade in non-small cell lung cancer (NSCLC) were analyzed by real-time PCR and immunostaining using tissue microarrays from 28 patient lung cancer samples. The association of TRPCs with cell differentiation was also investigated in the lung cancer cell line A549 by PCR and Western blotting. The channel activity was monitored by Ca(2+) imaging and patch recording after treatment with all-trans-retinoic acid (ATRA). The expression of TRPC1, 3, 4 and 6 was correlated to the differentiation grade of NSCLC in patients, but there was no correlation to age, sex, smoking history and lung cancer cell type. ATRA upregulated TRPC3, TRPC4 and TRPC6 expression and enhanced Ca(2+) influx in A549 cells, however, ATRA showed no direct effect on TRPC channels. Inhibition of TRPC channels by pore-blocking antibodies decreased the cell mitosis, which was counteracted by chronic treatment with ATRA. Blockade of TRPC channels inhibited A549 cell proliferation, while overexpression of TRPCs increased the proliferation. We conclude that TRPC expression correlates to lung cancer differentiation. TRPCs mediate the pharmacological effect of ATRA and play important roles in regulating lung cancer cell differentiation and proliferation, which gives a new understanding of lung cancer biology and potential anti-cancer therapy.

Pharmacological comparison of novel synthetic fenamate analogues with econazole and 2-APB on the inhibition of TRPM2 channels

BACKGROUND AND PURPOSE

Fenamate analogues, econazole and 2-aminoethoxydiphenyl borate (2-APB) are inhibitors of transient receptor potential melastatin 2 (TRPM2) channels and are used as research tools. However, these compounds have different chemical structures and therapeutic applications. Here we have investigated the pharmacological profile of TRPM2 channels by application of newly synthesized fenamate analogues and the existing channel blockers.

EXPERIMENTAL APPROACH

Human TRPM2 channels in tetracycline-regulated pcDNA4/TO vectors were transfected into HEK293 T-REx cells and the expression was induced by tetracycline. Whole cell currents were recorded by patch-clamp techniques. Ca(2+) influx or release was monitored by fluorometry.

KEY RESULTS

Flufenamic acid (FFA), mefenamic acid (MFA) and niflumic acid (NFA) concentration-dependently inhibited TRPM2 current with potency order FFA > MFA = NFA. Modification of the 2-phenylamino ring by substitution of the trifluoromethyl group in FFA with -CH(3), -F, -CF(3), -OCH(3), -OCH(2)CH(3), -COOH, and -NO(2) at various positions, reduced channel blocking potency. The conservative substitution of 3-CF(3) in FFA by -CH(3) (3-MFA), however, gave the most potent fenamate analogue with an IC(50) of 76 µM, comparable to that of FFA, but unlike FFA, had no effect on Ca(2+) release. 3-MFA and FFA inhibited the channel intracellularly. Econazole and 2-APB showed non-selectivity by altering cytosolic Ca(2+) movement. Econazole also evoked a non-selective current.

CONCLUSION AND IMPLICATIONS

The fenamate analogue 3-MFA was more selective than other TRPM2 channel blockers. FFA, 2-APB and econazole should be used with caution as TRPM2 channel blockers, as these compounds can interfere with intracellular Ca(2+) movement.

The effects of para-chloromercuribenzoic acid and different oxidative and sulfhydryl agents on a novel, non-AT1, non-AT2 angiotensin binding site identified as neurolysin

A novel, non-AT1, non-AT2 brain binding site for angiotensin peptides that is unmasked by p-chloromercuribenzoate (PCMB) has been identified as a membrane associated variant of neurolysin. The ability of different organic and inorganic oxidative and sulfhydryl reactive agents to unmask or inhibit 125I-Sar1Ile8 angiotensin II (SI-Ang II) binding to this site was presently examined. In tissue membranes from homogenates of rat brain and testis incubated in assay buffer containing losartan (10 μM) and PD123319 (10 μM) plus 100 μM PCMB, 5 of the 39 compounds tested inhibited 125I-SI Ang II binding in brain and testis. Mersalyl acid, mercuric chloride (HgCl2) and silver nitrate (AgNO3) most potently inhibited 125I-SI Ang II binding with IC50s ~1-20 μM. This HgCl2 inhibition was independent of any interaction of HgCl2 with angiotensin II (Ang II) based on the lack of effect of HgCl2 on the dipsogenic effects of intracerebroventricularly administered Ang II and 125I-SI Ang II binding to AT1 receptors in the liver. Among sulfhydryl reagents, cysteamine and reduced glutathione (GSH), but not oxidized glutathione (GSSG) up to 1mM, inhibited PCMB-unmasked 125I-SI Ang II binding in brain and testis. Thimerosal and 4-hydroxymercuribenzoate moderately inhibited PCMB-unmasked 125I-SI Ang II binding in brain and testis at 100 μM; however, they also unmasked non-AT1, non-AT2 binding independent of PCMB. 4-Hydroxybenzoic acid did not promote 125 I-SI Ang II binding to this binding site indicating that only specific organomercurial compounds can unmask the binding site. The common denominator for all of these interacting substances is the ability to bind to protein cysteine sulfur. Comparison of cysteines between neurolysin and the closely related enzyme thimet oligopeptidase revealed an unconserved cysteine (cys650, based on the full length variant) in the proposed ligand binding channel (Brown et al., 2001) [45] near the active site of neurolysin. It is proposed that the mercuric ion in PCMB and closely related organomercurial compounds binds to cys650, while the acidic anion forms an ionic bond with a nearby arginine or lysine along the channel to effect a conformational change in neurolysin that promotes Ang II binding.

The roles of G proteins in the activation of TRPC4 and TRPC5 transient receptor potential channels

TRPC4 and TRPC5 channels are important regulators of electrical excitability in both gastrointestinal myocytes and neurons. Much is known regarding the assembly and function of these channels including TRPC1 as a homotetramer or a heteromultimer and the roles that their interacting proteins play in controlling these events. Further, they are one of the best-studied targets of G protein-coupled receptors and growth factors in general and Gαq protein coupled receptor or epidermal growth factor in particular. However, our understanding of the roles of Gαi/o proteins on TRPC4/5 channels is still rudimentary. We discuss potential roles for Gαi/o proteins in channel activation in addition to their known role in cellular signaling.

Store-independent pathways for cytosolic STIM1 clustering in the regulation of store-operated Ca(2+) influx

STIM1 is a Ca(2+) sensing molecule. Once the Ca(2+) stores are depleted, STIM1 moves towards the plasma membrane (PM) (translocation), forms puncta (clustering), and triggers store-operated Ca(2+) entry (SOCE). Although this process has been regarded as a main mechanism for store-operated Ca(2+) channel activation, the STIM1 clustering is still unclear. Here we discovered a new phenomenon of STIM1 clustering, which is not triggered by endoplasmic reticulum (ER) Ca(2+) depletion. STIM1 subplasmalemmal translocation and clustering can be induced by ER Ca(2+) store depletion with thapsigargin (TG), G-protein-coupled receptor activator trypsin and ryanodine receptor (RyR) agonists caffeine and 4-chloro-3-ethylphenol (4-CEP) in the HEK293 cells stably transfected with STIM1-EYFP. The STIM1 clustering induced by TG was more sustained than that induced by trypsin and RyR agonists. Interestingly, 4-CEP-induced STIM1 clustering also happened in the cytosol without ER Ca(2+) store depletion. Application of some pharmacological regulators including flufenamic acid, 2-APB, and carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP) at concentrations without affecting ER Ca(2+) store also evoked cytosolic STIM1 clustering. However, the direct store-operated ORAI channel blockers (SKF-96365, Gd(3+) and diethylstilbestrol) or the signaling pathway inhibitors (genistein, wortmannin, Y-27632, forskolin and GF109203X) did not change the STIM1 movement. Disruption of cytoskeleton by colchicine and cytochalasin D also showed no effect on STIM1 movement. We concluded that STIM1 clustering and translocation are two dynamic processes that can be pharmacologically dissociated. The ER Ca(2+) store-independent mechanism for STIM1 clustering is a new alternative mechanism for regulating store-operated channel activity, which could act as a new pharmacological target.

Role of calcium and mitochondria in MeHg-mediated cytotoxicity

Methylmercury (MeHg) mediated cytotoxicity is associated with loss of intracellular calcium (Ca²⁺) homeostasis. The imbalance in Ca²⁺ physiology is believed to be associated with dysregulation of Ca²⁺ intracellular stores and/or increased permeability of the biomembranes to this ion. In this paper we summarize the contribution of glutamate dyshomeostasis in intracellular Ca²⁺ overload and highlight the mitochondrial dysfunctions induced by MeHg via Ca²⁺ overload. Mitochondrial disturbances elicited by Ca²⁺ may involve several molecular events (i.e., alterations in the activity of the mitochondrial electron transport chain complexes, mitochondrial proton gradient dissipation, mitochondrial permeability transition pore (MPTP) opening, thiol depletion, failure of energy metabolism, reactive oxygen species overproduction) that could culminate in cell death. Here we will focus on the role of oxidative stress in these phenomena. Additionally, possible antioxidant therapies that could be effective in the treatment of MeHg intoxication are briefly discussed.

Divalent copper is a potent extracellular blocker for TRPM2 channel

Transient receptor potential melastatin 2 (TRPM2) is a Ca(2+)-permeable cationic channel in the TRP channel family. The channel activity can be regulated by reactive oxygen species (ROS) and cellular acidification, which has been implicated to the pathogenesis of diabetes and some neuronal disorders. However, little is known about the effect of redox-active metal ions, such as copper, on TRPM2 channels. Here we investigated the effect of divalent copper on TRPM2. TRPM2 channel was over-expressed in HEK-293 cells and the whole-cell current was recorded by patch clamp. We found the whole-cell current evoked by intracellular ADP-ribose was potently inhibited by Cu(2+) with a half maximal inhibitory concentration (IC(50)) of 2.59 μM. The inhibitory effect was irreversible. The single channel activity was abolished in the outside-out patches, and intracellular application of Cu(2+) did not prevent the channel activation, suggesting that the action site of Cu(2+) is located in the extracellular domains of the channel. TRPM2 current was also blocked by Hg(2+), Pb(2+), Fe(2+) and Se(2+). We concluded that Cu(2+) is a potent TRPM2 channel blocker. The sensitivity of TRPM2 channel to heavy metal ions could be a new mechanism for the pathogenesis of some metal ion-related diseases.

Effect of non-steroidal anti-inflammatory drugs and new fenamate analogues on TRPC4 and TRPC5 channels

Non-steroidal anti-inflammatory drugs (NSAIDs) are widely used anti-inflammatory therapeutic agents, among which the fenamate analogues play important roles in regulating intracellular Ca²⁺ transient and ion channels. However, the effect of NSAIDs on TRPC4 and TRPC5 is still unknown. To understand the structure-activity of fenamate analogues on TRPC channels, we have synthesized a series of fenamate analogues and investigated their effects on TRPC4 and TRPC5 channels. Human TRPC4 and TRPC5 cDNAs in tetracycline-regulated vectors were transfected into HEK293 T-REx cells. The whole cell current and Ca²⁺ movement were recorded by patch clamp and calcium imaging, respectively. Flufenamic acid (FFA), mefenamic acid (MFA), niflumic acid (NFA) and diclofenac sodium (DFS) showed inhibition on TRPC4 and TRPC5 channels in a concentration-dependent manner. The potency was FFA>MFA>NFA>DFS. Modification of 2-phenylamino ring by substitution of the trifluoromethyl group in FFA with F, CH₃, OCH₃, OCH₂CH₃, COOH, and NO₂ led to the changes in their channel blocking activity. However, 2-(2'-methoxy-5'-methylphenyl)aminobenzoic acid stimulated TRPC4 and TRPC5 channels. Selective COX1-3 inhibitors (aspirin, celecoxib, acetaminophen, and indomethacin) had no effect on the channels. Longer perfusion (> 5 min) with FFA (100 μM) and MFA (100 μM) caused a potentiation of TRPC4 and TRPC5 currents after their initial blocking effects that appeared to be partially mediated by the mitochondrial Ca²⁺ release. Our results suggest that fenamate analogues are direct modulators of TRPC4 and TRPC5 channels. The substitution pattern and conformation of the 2-phenylamino ring could alter their blocking activity, which is important for understanding fenamate pharmacology and new drug development targeting the TRPC channels.