Activation and inhibition of transient receptor potential TRPM3-induced gene transcription

A Cataract-Causing Mutation in the TRPM3 Cation Channel Disrupts Calcium Dynamics in the Lens

TRPM3 belongs to the melastatin sub-family of transient receptor potential (TRPM) cation channels and has been shown to function as a steroid-activated, heat-sensitive calcium ion (Ca2+) channel. A missense substitution (p.I65M) in the TRPM3 gene of humans (TRPM3) and mice (Trpm3) has been shown to underlie an inherited form of early-onset, progressive cataract. Here, we model the pathogenetic effects of this cataract-causing mutation using 'knock-in' mutant mice and human cell lines. Trpm3 and its intron-hosted micro-RNA gene (Mir204) were strongly co-expressed in the lens epithelium and other non-pigmented and pigmented ocular epithelia. Homozygous Trpm3-mutant lenses displayed elevated cytosolic Ca2+ levels and an imbalance of sodium (Na+) and potassium (K+) ions coupled with increased water content. Homozygous TRPM3-mutant human lens epithelial (HLE-B3) cell lines and Trpm3-mutant lenses exhibited increased levels of phosphorylated mitogen-activated protein kinase 1/extracellular signal-regulated kinase 2 (MAPK1/ERK2/p42) and MAPK3/ERK1/p44. Mutant TRPM3-M65 channels displayed an increased sensitivity to external Ca2+ concentration and an altered dose response to pregnenolone sulfate (PS) activation. Trpm3-mutant lenses shared the downregulation of genes involved in insulin/peptide secretion and the upregulation of genes involved in Ca2+ dynamics. By contrast, Trpm3-deficient lenses did not replicate the pathophysiological changes observed in Trpm3-mutant lenses. Collectively, our data suggest that a cataract-causing substitution in the TRPM3 cation channel elicits a deleterious gain-of-function rather than a loss-of-function mechanism in the lens.

Calmodulin Regulates Transient Receptor Potential TRPM3 and TRPM8-Induced Gene Transcription

Calmodulin is a small protein that binds Ca²⁺ ions via four EF-hand motifs. The Ca²⁺/calmodulin complex as well as Ca²⁺-free calmodulin regulate the activities of numerous enzymes and ion channels. Here, we used genetic and pharmacological tools to study the functional role of calmodulin in regulating signal transduction of TRPM3 and TRPM8 channels. Both TRPM3 and TRPM8 are important regulators of thermosensation. Gene transcription triggered by stimulation of TRPM3 or TRPM8 channels was significantly impaired in cells expressing a calmodulin mutant with mutations in all four EF-hand Ca²⁺ binding motifs. Similarly, incubation of cells with the calmodulin inhibitor ophiobolin A reduced TRPM3 and TRPM8-induced signaling. The Ca²⁺/calmodulin-dependent protein phosphatase calcineurin was shown to negatively regulate TRPM3-induced gene transcription. Here, we show that TRPM8-induced transcription is also regulated by calcineurin. We propose that calmodulin plays a dual role in regulating TRPM3 and TRPM8 functions. Calmodulin is required for the activation of TRPM3 and TRPM8-induced intracellular signaling, most likely through a direct interaction with the channels. Ca²⁺ influx through TRPM3 and TRPM8 feeds back to TRPM3 and TRPM8-induced signaling by activation of the calmodulin-regulated enzyme calcineurin, which acts as a negative feedback loop for both TRPM3 and TRPM8 channel signaling.

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.

Effects of Progesterone and Other Gonadal Hormones on Glutamatergic Circuits in the Retina

BACKGROUND

Gonadal hormones function in the retina; however, their targets have not yet been identified. Therefore, the present study examined the effects of progesterone and other gonadal hormones on glutamatergic circuits in the retina.

METHODS

Extracellular glutamate concentrations, which correspond to the amount of glutamate released, were examined using an enzyme-linked fluorescent assay system. The activity of glutamatergic synapses between bipolar cells and ganglion cells was investigated using a patch clamp technique. Changes in retinal thickness during pregnancy were assessed using optical coherence tomography (OCT) images.

RESULTS

Progesterone and pregnenolone sulfate increased extracellular glutamate concentrations, whereas estrogen and testosterone did not. Progesterone increased the activity of glutamatergic synapses between bipolar cells and ganglion cells. A temporal decrease in the thickness of the peripheral retina was observed in the 1st trimester.

CONCLUSIONS

Progesterone, but not estrogen or testosterone, activated glutamate release in the mouse retina. Increases in the concentration of progesterone during pregnancy did not induce any detectable change in retinal thickness.

Expression of the C-Terminal Domain of Phospholipase Cβ3 Inhibits Signaling via Gαq-Coupled Receptors and Transient Receptor Potential Channels

Transient receptor potential (TRP) channels are cation channels that play a regulatory role in pain and thermosensation, insulin secretion, and neurotransmission. It has been proposed that activation of TRP channels requires phosphatidylinositol 4,5-bisphosphate, the major substrate for phospholipase C (PLC). We investigated whether inhibition of PLCβ has an impact on TRP channel signaling. A genetic approach was used to avoid off-target effects observed when using a pharmacological PLCβ inhibitor. In this study, we show that expression of PLCβ1ct and PLCβ3ct, truncated forms of PLCβ1 or PLCβ3 that contain the C-terminal membrane binding domains, almost completely blocked the signal transduction of a Gαq-coupled designer receptor, including the phosphorylation of ERK1/2. In contrast, expression of the helix-turn-helix motif (Hα1—Hα2) of the proximal C-terminal domain of PLCβ3 did not affect Gαq-coupled receptor signaling. PLCβ3ct expression impaired signaling of the TRP channels TRPM3 and TRPM8, stimulated with either prognenolone sulfate or icilin. Thus, the C-terminal domain of PLCβ3 interacts with plasma membrane targets, most likely phosphatidylinositol 4,5-bisphosphate, and in this way blocks the biological activation of TRPM3 and TRPM8, which require interaction with this phospholipid. PLCβ thus regulates TRPM3 and TRPM8 channels by masking phosphatidylinositol 4,5-bisphosphate with its C-terminal domain.

TRPM3-Induced Gene Transcription Is under Epigenetic Control

Transient receptor potential M3 (TRPM3) cation channels regulate numerous biological functions, including gene transcription. Stimulation of TRPM3 channels with pregnenolone sulfate activates stimulus-responsive transcription factors, which bind to short cognate sequences in the promoters of their target genes. In addition, coregulator proteins are involved that convert the chromatin into a configuration that is permissive for gene transcription. In this study, we determined whether TRPM3-induced gene transcription requires coactivators that change the acetylation pattern of histones. We used compound A485, a specific inhibitor of the histone acetyltransferases CBP and p300. In addition, the role of bromodomain proteins that bind to acetylated lysine residues of histones was analyzed. We used JQ1, an inhibitor of bromodomain and extra terminal domain (BET) family proteins. The results show that both compounds attenuated the activation of AP-1 and CREB-regulated gene transcription following stimulation of TRPM3 channels. Inhibition of CBP/p300 and BET proteins additionally reduced the transcriptional activation potential of the transcription factors c-Fos and Elk-1. Transcriptional upregulation of the interleukin-8 gene was attenuated by A485 and JQ1, indicating that proinflammatory cytokine expression is controlled by CBP/p300 and bromodomain proteins. We conclude that TRPM3-induced signaling involves transcriptional coactivators and acetyl-lysine-bound bromodomain proteins for activating gene transcription.

Steroids, steroid associated substances and gestational diabetes mellitus

As gestational diabetes mellitus (GDM) is both a frequent and serious complication, steroid levels in pregnancy are extremely elevated and their role in pregnancy is crucial, this review focuses on the role of steroids and related substances in the GDM pathophysiology. Low SHBG levels are associated with insulin resistance and hyperinsulinemia, while also predicting a predisposition to GDM. Other relevant agents are placental hormones such as kisspeptin and CRH, playing also an important role beyond pregnancy, but which are synthesized here in smaller amounts in the hypothalamus. These hormones affect both the course of pregnancy as well as the synthesis of pregnancy steroids and may also be involved in the GDM pathophysiology. Steroids, whose biosynthesis is mainly provided by the fetal adrenal glands, placenta, maternal adrenal glands, and both maternal and fetal livers, are also synthesized in limited amounts directly in the pancreas and may influence the development of GDM. These substances involve the sulfated Δ5 steroids primarily acting via modulating different ion channels and influencing the development of GDM in different directions, mostly diabetogenic progesterone and predominantly anti-diabetic estradiol acting both in genomic and non-genomic way, androgens associated with IR and hyperinsulinemia, neuroactive steroids affecting the pituitary functioning, and cortisol whose production is stimulated by CRH but which suppresses its pro-inflammatory effects. Due to the complex actions of steroids, studies assessing their predominant effect and studies assessing their predictive values for estimating predisposition to GDM are needed.

Altered Steroidome in Women with Gestational Diabetes Mellitus: Focus on Neuroactive and Immunomodulatory Steroids from the 24th Week of Pregnancy to Labor

Gestational diabetes mellitus (GDM) is a complication in pregnancy, but studies focused on the steroidome in patients with GDM are not available in the public domain. This article evaluates the steroidome in GDM+ and GDM- women and its changes from 24 weeks (± of gestation) to labor. The study included GDM+ (n = 44) and GDM- women (n = 33), in weeks 24-28, 30-36 of gestation and at labor and mixed umbilical blood after delivery. Steroidomic data (101 steroids quantified by GC-MS/MS) support the concept that the increasing diabetogenic effects with the approaching term are associated with mounting progesterone levels. The GDM+ group showed lower levels of testosterone (due to reduced AKR1C3 activity), estradiol (due to a shift from the HSD17B1 towards HSD17B2 activity), 7-oxygenated androgens (competing with cortisone for HSD11B1 and shifting the balance from diabetogenic cortisol towards the inactive cortisone), reduced activities of SRD5As, and CYP17A1 in the hydroxylase but higher CYP17A1 activity in the lyase step. With the approaching term, the authors found rising activities of CYP3A7, AKR1C1, CYP17A1 in its hydroxylase step, but a decline in its lyase step, rising conjugation of neuroinhibitory and pregnancy-stabilizing steroids and weakening AKR1D1 activity.

Regulation and function of AP-1 in insulinoma cells and pancreatic β-cells

Ca_v1.2 L-type voltage-gated Ca²⁺ channels play a central role in pancreatic β-cells by integrating extracellular signals with intracellular signaling events leading to insulin secretion and altered gene transcription. Here, we investigated the intracellular signaling pathway following stimulation of Ca_v1.2 Ca²⁺ channels and addressed the function of the transcription factor activator protein-1 (AP-1) in pancreatic β-cells of transgenic mice. Stimulation of Ca_v1.2 Ca²⁺ channels activates AP-1 in insulinoma cells. Pharmacological and genetic experiments identified c-Jun N-terminal protein kinase as a signal transducer connecting Ca_v1.2 Ca²⁺ channel activation with gene transcription. Moreover, the basic region-leucine zipper proteins ATF2 and c-Jun or c-Jun-related proteins were involved in stimulus-transcription coupling. We addressed the functions of AP-1 in pancreatic β-cells analyzing a newly generated transgenic mouse model. These transgenic mice expressed A-Fos, a mutant of c-Fos that attenuates DNA binding of c-Fos dimerization partners. In insulinoma cells, A-Fos completely blocked AP-1 activation following stimulation of Ca_v1.2 Ca²⁺ channels. The analysis of transgenic A-Fos-expressing mice revealed that the animals displayed impaired glucose tolerance. Thus, we show here for the first time that AP-1 controls an important function of pancreatic β-cells in vivo, the regulation of glucose homeostasis.

Immediate-early transcriptional response to insulin receptor stimulation

Insulin binding to the insulin receptor triggers intracellular signaling cascades involving the activation of protein and lipid kinases. As a result, multiple biological functions of the cells are changed. Here, we analyzed the regulation and signaling cascades leading to insulin-induced activation of the stimulus-responsive transcription factors. For the analyses, we used chromatin-embedded reporter genes having a cellular nucleosomal organisation, and fibroblasts expressing human insulin receptors (HIRcB cells). The results show that stimulation of the insulin receptor induced the expression of the transcription factor Egr-1. Attenuation of Egr-1 promoter activation was observed following expression of a dominant-negative mutant of the ternary complex factor Elk-1. These data were corroborated by experiments showing that insulin receptor stimulation increased the transcriptional activation potential of Elk-1. In addition, the transcriptional activity of AP-1 was significantly elevated in insulin-stimulated HIRcB cells. Expression of the dominant-negative mutant of Elk-1 reduced insulin-induced activation of AP-1, indicating that Elk-1 controls both serum response element and AP-1-regulated transcription. Moreover, we show that stimulation of the insulin receptor activates cyclic AMP response element (CRE)-controlled transcription, involving the transcription factor CREB. Insulin-induced transcription of Elk-1 and CREB-controlled reporter genes was attenuated by overexpression of MAP kinase phosphatase-1 or a constitutively active mutant of calcineurin A, indicating that both phosphatases are part of a negative feedback loop for reducing insulin-mediated gene transcription. Finally, we show that expression of the adenoviral protein E1A selectively reduced CRE-mediated transcription following stimulation of the insulin receptor. These data indicate that insulin-regulated transcription of CRE-containing genes is under epigenetic control.

Ca2+ Microdomains, Calcineurin and the Regulation of Gene Transcription

Ca²⁺ ions function as second messengers regulating many intracellular events, including neurotransmitter release, exocytosis, muscle contraction, metabolism and gene transcription. Cells of a multicellular organism express a variety of cell-surface receptors and channels that trigger an increase of the intracellular Ca²⁺ concentration upon stimulation. The elevated Ca²⁺ concentration is not uniformly distributed within the cytoplasm but is organized in subcellular microdomains with high and low concentrations of Ca²⁺ at different locations in the cell. Ca²⁺ ions are stored and released by intracellular organelles that change the concentration and distribution of Ca²⁺ ions. A major function of the rise in intracellular Ca²⁺ is the change of the genetic expression pattern of the cell via the activation of Ca²⁺-responsive transcription factors. It has been proposed that Ca²⁺-responsive transcription factors are differently affected by a rise in cytoplasmic versus nuclear Ca²⁺. Moreover, it has been suggested that the mode of entry determines whether an influx of Ca²⁺ leads to the stimulation of gene transcription. A rise in cytoplasmic Ca²⁺ induces an intracellular signaling cascade, involving the activation of the Ca²⁺/calmodulin-dependent protein phosphatase calcineurin and various protein kinases (protein kinase C, extracellular signal-regulated protein kinase, Ca²⁺/calmodulin-dependent protein kinases). In this review article, we discuss the concept of gene regulation via elevated Ca²⁺ concentration in the cytoplasm and the nucleus, the role of Ca²⁺ entry and the role of enzymes as signal transducers. We give particular emphasis to the regulation of gene transcription by calcineurin, linking protein dephosphorylation with Ca²⁺ signaling and gene expression.

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.

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.

Pharmacological and genetic inhibition of TRPC6-induced gene transcription

Transient receptor potential canonical-6 (TRPC6) channels are non-selective cation channels that can be activated by hyperforin, a constituent of Hypericum perforatum. TRPC6 activation has been linked to a variety of biological functions and pathologies, including focal segmental glomerulosclerosis and the development of various tumor entities. Thus, TRPC6 is an interesting drug target, and a specific pharmacological inhibitor would be very valuable for both basic research and therapy of TRPC6-mediated human pathologies. Here, we assessed the biological activity of various TRP channel inhibitors on hyperforin-stimulated TRPC6 channel signaling. Hyperforin stimulates the activity of the transcription factor AP-1 via TRPC6. Expression experiments involving a TRPC6-specific small hairpin RNA confirmed that hyperforin-induced gene transcription requires TRPC6. Cellular AP-1 activity was measured to assess which compound interrupted the TRPC6-induced intracellular signaling cascade. The results show that the compounds 2-APB, clotrimazole, BCTC, TC-I 2014, SAR 7334, and larixyl acetate blocked TRPC6-mediated activation of AP-1. In contrast, the TRPM8-specific inhibitor RQ-00203078 did not inhibit TRPC6-mediated signaling. 2-APB, clotrimazole, BCTC, and TC-I 2014 are broad-spectrum Ca²⁺ channel inhibitors, while SAR 7334 and larixyl acetate have been proposed to function as rather TRPC6-specific inhibitors. In this study it is shown that both compounds, in addition to inhibiting TRPC6-induced signaling, completely abolished pregnenolone sulfate-mediated signaling via TRPM3 channels. Thus, SAR 7334 and larixyl acetate are not TRPC6-specific inhibitors.

Zn2+ ions inhibit gene transcription following stimulation of the Ca2+ channels Cav1.2 and TRPM3

Zinc, a trace element, is necessary for the correct structure and function of many proteins. Therefore, Zn²⁺ has to be taken up by the cells, using specific Zn²⁺ transporters or Ca²⁺ channels. In this study, we have focused on two Ca²⁺ channels, the L-type voltage-gated Ca_v1.2 channel and the transient receptor potential channel TRPM3. Stimulation of either channel induces an intracellular signaling cascade leading to the activation of the transcription factor AP-1. The influx of Ca²⁺ ions into the cytoplasm is essential for this activity. We asked whether extracellular Zn²⁺ ions affect Ca_v1.2 or TRPM3-induced gene transcription following stimulation of the channels. The results show that extracellular Zn²⁺ ions reduced the activation of AP-1 by more than 80% following stimulation of either voltage-gated Ca_v1.2 channels or TRPM3 channels. Experiments performed with cells maintained in Ca²⁺-free medium revealed that Zn²⁺ ions cannot replace Ca²⁺ ions in inducing gene transcription via stimulation of Ca_v1.2 and TRPM3 channels. Re-addition of Ca²⁺ ions to the cell culture medium, however, restored the ability of these Ca²⁺ channels to induce a signaling cascade leading to the activation of AP-1. Secretory cells, including neurons and pancreatic β-cells, release Zn²⁺ ions during exocytosis. We propose that the released Zn²⁺ ions function as a negative feedback loop for stimulus-induced exocytosis by inhibiting Ca²⁺ channel signaling.

Developmental change in the gene expression of transient receptor potential melastatin channel 3 (TRPM3) in murine lacrimal gland

Transient receptor potential (TRP) channels are cation channels with ubiquitous expression. Various TRP channels are functionally active at the ocular surface and are involved in tear secretion and multiple inflammatory processes. So far, the impact of TRP channels regarding the development of the lacrimal gland (LG) is unclear. While investigating TRP channels in the LG, the TRPM3 channel presented itself as a promising candidate to play a role in the development and functioning of the LG. Therefore, Trpm3 expression was analyzed in different embryonic and postembryonic LGs. Thus, gene expression of TRPM channels including Trpm2, Trpm3, Trpm4 and Trpm6 was analyzed by quantitative RT-PCR in murine LGs at different developmental stages. Localization of TRPM3 in LGs was examined by immunohistochemistry. Primary LG epithelial cells (LGEC) and mesenchymal cells (MC) from newborn mice were cultured (either separately or collectively) for three days, and Trpm3 expression was analyzed in LGEC and MC. As a result, gene expression of Trpm2, Trpm4 and Trpm6 showed no significant difference in LGs in the different stages of development. However, Trpm3 gene expression was significantly higher in the embryonic stage than in the postnatal stage with the peak at E18. Postnatal, Trpm3 expression significantly decreased up to 28-fold until two years of age. Immunohistochemistry for TRPM3 revealed apical membranous expression in the excretory ducts, as well as in the acini of up to P7 old mice. Trpm3 expression in LGEC were significantly higher than that of MC. Our results indicate that Trpm3 expression in murine LG is age-dependent and peaks at age E18. Its expression is localized in the apical membrane of the glandular epithelium. However, its functional role still requires additional study in the LG.

TRPM3_miR-204: a complex locus for eye development and disease

First discovered in a light-sensitive retinal mutant of Drosophila, the transient receptor potential (TRP) superfamily of non-selective cation channels serve as polymodal cellular sensors that participate in diverse physiological processes across the animal kingdom including the perception of light, temperature, pressure, and pain. TRPM3 belongs to the melastatin sub-family of TRP channels and has been shown to function as a spontaneous calcium channel, with permeability to other cations influenced by alternative splicing and/or non-canonical channel activity. Activators of TRPM3 channels include the neurosteroid pregnenolone sulfate, calmodulin, phosphoinositides, and heat, whereas inhibitors include certain drugs, plant-derived metabolites, and G-protein subunits. Activation of TRPM3 channels at the cell membrane elicits a signal transduction cascade of mitogen-activated kinases and stimulus response transcription factors. The mammalian TRPM3 gene hosts a non-coding microRNA gene specifying miR-204 that serves as both a tumor suppressor and a negative regulator of post-transcriptional gene expression during eye development in vertebrates. Ocular co-expression of TRPM3 and miR-204 is upregulated by the paired box 6 transcription factor (PAX6) and mutations in all three corresponding genes underlie inherited forms of eye disease in humans including early-onset cataract, retinal dystrophy, and coloboma. This review outlines the genomic and functional complexity of the TRPM3_miR-204 locus in mammalian eye development and disease.

Pharmacological inhibition of TRPM8-induced gene transcription

Transient receptor potential melastatin-8 (TRPM8) channels are activated by cold temperature, menthol and icilin, leading to cold sensation. TRPM8 activation is connected with various diseases, indicating that a specific pharmacological antagonist, allowing nongenetic channel suppression, would be a valuable tool for therapy and basic research. Here, we assessed the biological activity and specificity of various TRPM8 inhibitors following stimulation of TRPM8 channels with either icilin or menthol. Recently, we showed that icilin strikingly upregulates the transcriptional activity of AP-1. By measuring AP-1 activity, we assessed which compound interrupted the TRPM8-induced intracellular signaling cascade from the plasma membrane to the nucleus. We tested the specificity of various TRPM8 inhibitors by analyzing AP-1 activation following stimulation of TRPM3 and TRPV1 channels, L-type voltage-gated Ca²⁺ channels, and Gαq-coupled receptors, either in the presence or absence of a particular TRPM8 inhibitor. The results show that the TRPM8 inhibitors BCTC, RQ-00203078, TC-1 2014, 2-APB, and clotrimazole blocked TRPM8-mediated activation of AP-1. However, only the compound RQ-00203078 showed TRPM8-specificity, while the other compounds function as broad-spectrum Ca²⁺ channel inhibitors. In addition, we show that progesterone interfered with TRPM8-induced activation of AP-1, as previously shown for TRPM3 and TRPC6 channels. TRPM8-induced transcriptional activation of AP-1 was additionally blocked by the compound PD98059, indicating that extracellular signal-regulated protein kinase-1/2 is essential to couple TRPM8 stimulation with transcriptional activation of AP-1. Moreover, an influx of Ca²⁺-ions is essential to induce the intracellular signaling cascade leading to the activation of AP-1.

Validation of impaired Transient Receptor Potential Melastatin 3 ion channel activity in natural killer cells from Chronic Fatigue Syndrome/ Myalgic Encephalomyelitis patients

Background

Chronic Fatigue Syndrome/ Myalgic Encephalomyelitis (CFS/ME) is a complex multifactorial disorder of unknown cause having multi-system manifestations. Although the aetiology of CFS/ME remains elusive, immunological dysfunction and more particularly reduced cytotoxic activity in natural killer (NK) cells is the most consistent laboratory finding. The Transient Receptor Potential (TRP) superfamily of cation channels play a pivotal role in the pathophysiology of immune diseases and are therefore potential therapeutic targets. We have previously identified single nucleotide polymorphisms in TRP genes in peripheral NK cells from CFS/ME patients. We have also described biochemical pathway changes and calcium signaling perturbations in NK cells from CFS/ME patients. Notably, we have previously reported a decrease of TRP cation channel subfamily melastatin member 3 (TRPM3) function in NK cells isolated from CFS/ME patients compared with healthy controls after modulation with pregnenolone sulfate and ononetin using a patch-clamp technique. In the present study, we aim to confirm the previous results describing an impaired TRPM3 activity in a new cohort of CFS/ME patients using a whole cell patch-clamp technique after modulation with reversible TRPM3 agonists, pregnenolone sulfate and nifedipine, and an effective TRPM3 antagonist, ononetin. Indeed, no formal research has commented on using pregnenolone sulfate or nifedipine to treat CFS/ME patients while there is evidence that clinicians prescribe calcium channel blockers to improve different symptoms.

Methods

Whole-cell patch-clamp technique was used to measure TRPM3 activity in isolated NK cells from twelve age- and sex-matched healthy controls and CFS/ME patients, after activation with pregnenolone sulfate and nifedipine and inhibition with ononetin.

Results

We confirmed a significant reduction in amplitude of TRPM3 currents after pregnenolone sulfate stimulation in isolated NK cells from another cohort of CFS/ME patients compared with healthy controls. The pregnenolone sulfate-evoked ionic currents through TRPM3 channels were again significantly modulated by ononetin in isolated NK cells from healthy controls compared with CFS/ME patients. In addition, we used nifedipine, another reversible TRPM3 agonist to support the previous findings and found similar results confirming a significant loss of the TRPM3 channel activity in CFS/ME patients.

Conclusions

Impaired TRPM3 activity was validated in NK cells isolated from CFS/ME patients using different pharmacological tools and whole-cell patch-clamp technique as the gold standard for ion channel research. This investigation further helps to establish TRPM3 channels as a prognostic marker and/ or a potential therapeutic target for CFS/ME.

Stimulation of TRPM3 channels increases the transcriptional activation potential of Elk-1 involving cytosolic Ca2+, extracellular signal-regulated protein kinase, and calcineurin

Stimulation of transient receptor potential M3 (TRPM3) channels with the steroid pregnenolone sulfate increases the transcriptional activation potential of Elk-1, a transcription factor that regulates serum response element-mediated transcription. Here, we show that an influx of Ca²⁺ ions into the cells is essential for the activation of Elk-1 following stimulation of TRPM3. Using genetically encoded Ca²⁺ buffers, we show that a rise in cytoplasmic Ca²⁺ is required for the upregulation of the transcriptional activation potential of Elk-1, while buffering of Ca²⁺ in the nucleus had no inhibitory effect on the transcriptional activity of Elk-1. Pharmacological and genetic experiments showed that extracellular signal-regulated protein kinase (ERK1/2) functions as signal transducer connecting TRPM3 channels with the Elk-1 transcription factor. Accordingly, dephosphorylation of ERK1/2 in the nucleus by MAP kinase phosphatase attenuated TRPM3-mediated Elk-1 activation. Moreover, we show that the Ca²⁺/calmodulin-dependent protein phosphatase calcineurin is part of a shut-off-device for the signaling cascade connecting TRPM3 channels with the activation of Elk-1. The fact that TRPM3 channel stimulation activates Elk-1 connects TRPM3 with the biological functions of Elk-1, including the regulation of proliferation, differentiation, survival, transcription, and cell migration.

Stimulation of B-Raf increases c-Jun and c-Fos expression and upregulates AP-1-regulated gene transcription in insulinoma cells

Stimulation of pancreatic β-cells with glucose activates the protein kinases B-Raf and extracellular signal-regulated protein kinase that participate in glucose sensing. Inhibition of both kinases results in impairment of glucose-regulated gene transcription. To analyze the signaling pathway controlled by B-Raf, we expressed a conditionally active form of B-Raf in INS-1 insulinoma cells. Here, we show that stimulation of B-Raf strongly activated the transcription factor AP-1 which is accompanied by increased c-Jun and c-Fos promoter activities, an upregulation of c-Jun and c-Fos biosynthesis, and elevated transcriptional activation potentials of c-Jun and c-Fos. Mutational analysis identified the AP-1 sites within the c-Jun promoter and the serum response element (SRE) within the c-Fos promoter as the essential genetic elements connecting B-Raf stimulation with AP-1 activation. In line with this, the transcriptional activation potential of the SRE-binding protein Elk-1 was increased following B-Raf activation. The signal pathway from B-Raf to AP-1 required the activation of c-Jun. We identified the cyclin D1 gene as a delayed response gene for AP-1 following stimulation of B-Raf in insulinoma cells. Moreover, MAP kinase phosphatase-1 and the Ca²⁺/calmodulin-dependent protein phosphatase calcineurin were identified to function as shut-off-devices for the signaling cascade connecting B-Raf stimulation with the activation of AP-1. The fact that stimulation with glucose, activation of L-type voltage-gated Ca²⁺ channels, and stimulation of B-Raf all trigger an activation of AP-1 indicates that AP-1 is a point of convergence of signaling pathways in β-cell.

Stimulation of TRPV1 channels activates the AP-1 transcription factor

Transient receptor potential vanilloid 1 (TRPV1) channels were originally described as the receptors of capsaicin, the main constituent of hot chili pepper. The biological functions of TRPV1 channels include pain sensation and inflammatory thermal hyperalgesia. Here, we show that stimulation of HEK293 cells expressing TRPV1 channels (H2C1 cells) with capsaicin or the TRPV1 ligand resiniferatoxin activated transcription mediated by the transcription factor AP-1. No cell death was occurring under these experimental conditions. The AP-1 activity was not altered in capsaicin or resiniferatoxin-stimulated HEK293 cells lacking TRPV1. We identified the AP-1 DNA binding site as the capsaicin/resiniferatoxin-responsive element. Stimulation with the TRPV1 ligand N-arachidonoyldopamine increased AP-1 activity in a TRPV1-dependent and TRPV1-independent manner. Stimulation of TRPV1 channels induced an influx of Ca²⁺ into the cells and this rise in intracellular Ca²⁺ was essential for activating AP-1 in capsaicin or resiniferatoxin-stimulated cells. N-arachidonoyldopamine stimulation induced a rise in intracellular Ca²⁺ in a TRPV-1 dependent and independent manner. AP-1 is a dimeric transcription factor, composed of proteins of the c-Jun, c-Fos and ATF families. Stimulation of TRPV1 channels with capsaicin increased c-Jun and c-Fos biosynthesis in H2C1 cells. The signal transduction of capsaicin, leading to enhanced AP-1-mediated transcription, required extracellular signal-regulated protein kinase ERK1/2 as a signal transducer and the activation of the transcription factors c-Jun and ternary complex factor. Together, these data suggest that the intracellular functions of TRPV1 stimulation may rely on the activation of a stimulus-regulated protein kinase and stimulus-responsive transcription factors.

Stimulation of transient receptor potential M3 (TRPM3) channels increases interleukin-8 gene promoter activity involving AP-1 and extracellular signal-regulated protein kinase

Stimulation of Ca²⁺ permeable TRPM3 (transient receptor potential melastatin-3) channels with the steroid ligand pregnenolone sulfate activates stimulus-responsive transcription factors, including the transcription factor AP-1 (activator protein-1). As part of a search for AP-1-regulated target genes we analyzed the gene encoding interleukin-8 (IL-8) in HEK293 cells expressing TRPM3 channels. Here, we show that stimulation of TRPM3 channels activated transcription of an IL-8 promoter-controlled reporter gene that was embedded into the chromatin of the cells. Mutational analysis of the IL-8 promoter revealed that the AP-1 binding site of the IL-8 promoter was essential to connect TRPM3 stimulation with the transcription of the IL-8 gene. Genetic experiments revealed that the basic region leucine zipper proteins c-Jun and ATF2 and the ternary complex factor Elk-1 are essential to couple TRPM3 channel stimulation with the IL-8 gene. Moreover, we identified extracellular signal-regulated protein kinase (ERK1/2) as signal transducer connecting TRPM3 stimulation with enhanced transcription of the IL-8 gene. Furthermore, we show that stimulation of TRPC6 (transient receptor potential canonical-6) channels with its ligand hyperforin also increased IL-8 promoter activity, involving the AP-1 binding site within the IL-8 gene, suggesting that activation of IL-8 gene transcription may be a common theme following TRP channel stimulation.

Regulation of Gene Transcription Following Stimulation of Transient Receptor Potential (TRP) Channels

Transient receptor potential (TRP) channels belong to a heterogeneous superfamily of cation channels that are involved in the regulation of numerous biological functions, including regulation of Ca²⁺ and glucose homeostasis, tumorigenesis, temperature, and pain sensation. To understand the functions of TRP channels, their associated intracellular signaling pathways and molecular targets have to be identified on the cellular level. Stimulation of TRP channels frequently induces an influx of Ca²⁺ ions into the cells and the subsequent activation of protein kinases. These intracellular signal transduction pathways ultimately induce changes in the gene expression pattern of the cells. Here, we review the effects of TRPC6, TRPM3, and TRPV1 channel stimulation on the activation of the stimulus-responsive transcription factors AP-1, CREB, Egr-1, Elk-1, and NFAT. Following activation, these transcription factors induce the transcription of delayed response genes. We propose that many biological functions of TRP channels can be explained by the activation of stimulus-responsive transcription factors and their delayed response genes. The proteins encoded by those delayed response genes may be responsible for the biochemical and physiological changes following TRP channel activation.

Extracellular Signal-Regulated Protein Kinase, c-Jun N-Terminal Protein Kinase, and Calcineurin Regulate Transient Receptor Potential M3 (TRPM3) Induced Activation of AP-1

Stimulation of transient receptor potential M3 (TRPM3) cation channels with pregnenolone sulfate induces an influx of Ca²⁺ ions into the cells and a rise in the intracellular Ca²⁺ concentration, leading to the activation of the activator protein-1 (AP-1) transcription factor. Here, we show that expression of a constitutively active mutant of the Ca²⁺ /calmodulin-dependent protein phosphatase calcineurin attenuated pregnenolone sulfate-induced AP-1 activation in TRPM3-expressing cells. Likewise, expression of the regulatory B subunit of calcineurin reduced AP-1 activity in the cells following stimulation of TRPM3 channels. MAP kinase phosphatase-1 has been shown to attenuate TRPM3-mediated AP-1 activation. Here, we show that pregnenolone sulfate-induced stimulation of TRPM3 triggers the phosphorylation and activation of the MAP kinase extracellular signal-regulated protein kinase (ERK1/2). Pharmacological and genetic experiments revealed that stimulation of ERK1/2 is essential for the activation of AP-1 in cells expressing stimulated TRPM3 channels. ERK1/2 is required for the activation of the transcription factor c-Jun, a key component of the AP-1 transcription factor, and regulates c-Fos promoter activity. In addition, we identified c-Jun N-terminal protein kinase (JNK1/2) as a second signal transducer of activated TRPM3 channels. Together, the data show that calcineurin and the protein kinases ERK1/2 and JNK1/2 are important regulators within the signaling cascade connecting TRPM3 channel stimulation with increased AP-1-regulated transcription. J. Cell. Biochem. 118: 2409-2419, 2017. © 2017 Wiley Periodicals, Inc.

Activation of gene transcription via CIM0216, a synthetic ligand of transient receptor potential melastatin-3 (TRPM3) channels

Several compounds have been proposed to stimulate TRPM3 Ca²⁺ channels. We recently showed that stimulation of TRPM3 channels with pregnenolone sulfate activated the transcription factor AP-1, while other proposed TRPM3 ligands (nifedipine, D-erythro-sphingosine) exhibited either no or TRPM3-independent effects on gene transcription. Here, we have analyzed the transcriptional activity of CIM0216, a synthetic TRPM3 ligand proposed to have a higher potency and affinity for TRPM3 than pregnenolone sulfate. The results show that CIM0216 treatment of HEK293 cells expressing TRPM3 channels activated AP-1 and stimulated the transcriptional activation potential of c-Jun and c-Fos, 2 basic region leucine zipper transcription factors that constitute AP-1. CIM0216-induced gene transcription was attenuated by knock-down of TRPM3 or treatment with mefenamic acid, a TRPM3 inhibitor. CIM0216 was similarly or less capable in activating TRPM3-mediated gene transcription, suggesting that pregnenolone sulfate is still the ligand of choice for changing the gene expression pattern via TRPM3.

Specificity of Stress-Responsive Transcription Factors Nrf2, ATF4, and AP-1

Cellular stress leads to an upregulation of gene transcription. We asked if there is a specificity in the activation of the stress-responsive transcription factors Nrf2, ATF4, and AP-1/c-Jun, or if activation of these proteins is a redundant cellular answer toward extracellular stressors. Here, we show that oxidative stress, induced by stimulation of the cells with the oxidant arsenite, strongly activated gene transcription via the stress-responsive element (StRE), while phorbol ester or tunicamycin, activators of AP-1/c-Jun or ATF4, respectively, activated AP-1 or nutrient-sensing response element-mediated transcription. Preincubation of the cells with N-acetyl-cysteine or overexpression of thioredoxin selectively attenuated arsenite-induced upregulation of StRE-regulated transcription. Expression of either dominant-negative or constitutively active mutants of Nrf2, ATF4, or c-Jun confirmed that distinct transcription units are regulated by these transcription factors. Physiological stimuli involving the activation of either Gαq-coupled designer receptors or the protein kinases c-Jun N-terminal protein kinase or p38 strongly stimulated transcription via AP-1/c-Jun, with minimal effects on Nrf2 or ATF4-responsive promoters. Thus, activation of transcription by extracellular signaling molecules shows specificity at the level of the chemical nature of the signaling molecule, at the level of the intracellular transduction process, and at the level of signal-responsive transcription factors. J. Cell. Biochem. 118: 127-140, 2017. © 2016 Wiley Periodicals, Inc.

Sphingosine inhibits the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) activity

The increase in the intracellular Ca(2+) concentration ([Ca(2+)]i) is the key variable for many different processes, ranging from regulation of cell proliferation to apoptosis. In this work we demonstrated that the sphingolipid sphingosine (Sph) increases the [Ca(2+)]i by inhibiting the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA), in a similar manner to thapsigargin (Tg), a specific inhibitor of this Ca(2+) pump. The results showed that addition of sphingosine produced a release of Ca(2+) from the endoplasmic reticulum followed by a Ca(2+) entrance from the outside mileu. The results presented in this work support that this sphingolipid could control the activity of the SERCA, and hence sphingosine may participate in the regulation of [Ca(2+)]I in mammalian cells.

Transient Receptor Potential Melastatin-3 (TRPM3) Mediates Nociceptive-Like Responses in Hydra vulgaris

The ability of mammals to feel noxious stimuli lies in a heterogeneous group of primary somatosensory neurons termed nociceptors, which express specific membrane receptors, such as the Transient Receptor Potential (TRP) family. Here, we show that one of the most important nociceptive-like pathways is conserved in the freshwater coelenterate Hydra vulgaris, the most primitive organism possessing a nervous system. In particular, we found that H. vulgaris expresses TRPM3, a nociceptor calcium channel involved in the detection of noxious heat in mammals. Furthermore, we detected that both heat shock and TRPM3 specific agonist (i.e., pregnenolone sulfate) induce the modulation of the heat shock protein 70 (HSP70) and the nitric oxide synthase (NOS), two genes activated by TRP-mediated heat painful stimuli in mammals. As expected, these effects are inhibited by a TRPM3 antagonist (i.e., mefenamic acid). Interestingly, the TRPM3 agonist and heat shock also induce the expression of nuclear transcription erythroid 2-related factor (Nrf2) and superoxide dismutase (SOD), known markers of oxidative stress; noteworthy gene expression was also inhibited by the TRPM3 antagonist. As a whole, our results demonstrate the presence of conserved molecular oxidative/nociceptive-like pathways at the primordial level of the animal kingdom.

CREB, AP-1, ternary complex factors and MAP kinases connect transient receptor potential melastatin-3 (TRPM3) channel stimulation with increased c-Fos expression

BACKGROUND AND PURPOSE

The rise in intracellular Ca(2+) stimulates the expression of the transcription factor c-Fos. Depending on the mode of entry of Ca(2+) into the cytosol, distinct signal transducers and transcription factors are required. Here, we have analysed the signalling pathway connecting a Ca(2+) influx via activation of transient receptor potential melastatin-3 (TRPM3) channels with enhanced c-Fos expression.

EXPERIMENTAL APPROACH

Transcription of c-Fos promoter/reporter genes that were integrated into the chromatin via lentiviral gene transfer was analysed in HEK293 cells overexpressing TRPM3. The transcriptional activation potential of c-Fos was measured using a GAL4-c-Fos fusion protein.

KEY RESULTS

The signalling pathway connecting TRPM3 stimulation with enhanced c-Fos expression requires the activation of MAP kinases. On the transcriptional level, three Ca(2+) -responsive elements, the cAMP-response element and the binding sites for the serum response factor (SRF) and AP-1, are essential for the TRPM3-mediated stimulation of the c-Fos promoter. Ternary complex factors are additionally involved in connecting TRPM3 stimulation with the up-regulation of c-Fos expression. Stimulation of TRPM3 channels also increases the transcriptional activation potential of c-Fos.

CONCLUSIONS AND IMPLICATIONS

Signalling molecules involved in connecting TRPM3 with the c-Fos gene are MAP kinases and the transcription factors CREB, SRF, AP-1 and ternary complex factors. As c-Fos constitutes, together with other basic region leucine zipper transcription factors, the AP-1 transcription factor complex, the results of this study explain TRPM3-induced activation of AP-1 and connects TRPM3 with the biological functions regulated by AP-1.

Daily isoflurane exposure increases barbiturate insensitivity in medullary respiratory and cortical neurons via expression of ε-subunit containing GABA ARs

The parameters governing GABA_A receptor subtype expression patterns are not well understood, although significant shifts in subunit expression may support key physiological events. For example, the respiratory control network in pregnant rats becomes relatively insensitive to barbiturates due to increased expression of ε-subunit-containing GABA_ARs in the ventral respiratory column. We hypothesized that this plasticity may be a compensatory response to a chronic increase in inhibitory tone caused by increased central neurosteroid levels. Thus, we tested whether increased inhibitory tone was sufficient to induce ε-subunit upregulation on respiratory and cortical neurons in adult rats. Chronic intermittent increases in inhibitory tone in male and female rats was induced via daily 5-min exposures to 3% isoflurane. After 7d of treatment, phrenic burst frequency was less sensitive to barbiturate in isoflurane-treated male and female rats in vivo. Neurons in the ventral respiratory group and cortex were less sensitive to pentobarbital in vitro following 7d and 30d of intermittent isoflurane-exposure in both male and female rats. The pentobarbital insensitivity in 7d isoflurane-treated rats was reversible after another 7d. We hypothesize that increased inhibitory tone in the respiratory control network and cortex causes a compensatory increase in ε-subunit-containing GABA_ARs.

The pharmacology of TRP channels

This themed issue of the British Journal of Pharmacology contains review and research articles on recent advances in transient receptor potential (TRP) channel pharmacology. The review articles, written by a panel of distinguished experts, address the rapid progress in TRP channel research in fields as diverse as oncology, urology, dermatology, migraine, inflammation and pain. These reviews are complemented by original research reports focusing, among others, on the emerging roles of TRPV1 in osteoporosis and cystitis and on evodiamine as a lead structure for the development of potent TRPV1 agonists/desensitizers. Other papers highlight the differences in TRPV3 pharmacology between recombinant and native systems, the mechanisms of TRPM3 activation/inhibition and TRPP2 as a target of naringenin, a dietary flavonoid with anticancer actions. New therapeutic opportunities in pain may arise from the strategy to combine TRP channel and cell membrane impermeant sodium channel blockers to inhibit sensory nerve activity.