Regulation of TRPM2 channels in neutrophil granulocytes by ADP-ribose: a promising pharmacological target

Update on Calcium Signaling in Cystic Fibrosis Lung Disease

Cystic fibrosis (CF) is an autosomal recessive disorder characterized by mutations in the cystic fibrosis transmembrane conductance regulator gene, which causes multifunctional defects that preferentially affect the airways. Abnormal viscosity of mucus secretions, persistent pathogen infections, hyperinflammation, and lung tissue damage compose the classical pathological manifestation referred to as CF lung disease. Among the multifunctional defects associated with defective CFTR, increasing evidence supports the relevant role of perturbed calcium (Ca²⁺) signaling in the pathophysiology of CF lung disease. The Ca²⁺ ion is a critical player in cell functioning and survival. Its intracellular homeostasis is maintained by a fine balance between channels, transporters, and exchangers, mediating the influx and efflux of the ion across the plasma membrane and the intracellular organelles. An abnormal Ca²⁺ profile has been observed in CF cells, including airway epithelial and immune cells, with heavy repercussions on cell function, viability, and susceptibility to pathogens, contributing to proinflammatory overstimulation, organelle dysfunction, oxidative stress, and excessive cytokines release in CF lung. This review discusses the role of Ca²⁺ signaling in CF and how its dysregulation in airway epithelial and immune cells contributes to hyperinflammation in the CF lung. Finally, we provide an outlook on the therapeutic options that target the Ca²⁺ signaling to treat the CF lung disease.

CD38, CD157, and RAGE as Molecular Determinants for Social Behavior

Recent studies provide evidence to support that cluster of differentiation 38 (CD38) and CD157 meaningfully act in the brain as neuroregulators. They primarily affect social behaviors. Social behaviors are impaired in Cd38 and Cd157 knockout mice. Single-nucleotide polymorphisms of the CD38 and CD157/BST1 genes are associated with multiple neurological and psychiatric conditions, including autism spectrum disorder, Parkinson’s disease, and schizophrenia. In addition, both antigens are related to infectious and immunoregulational processes. The most important clues to demonstrate how these molecules play a role in the brain are oxytocin (OT) and the OT system. OT is axo-dendritically secreted into the brain from OT-containing neurons and causes activation of OT receptors mainly on hypothalamic neurons. Here, we overview the CD38/CD157-dependent OT release mechanism as the initiation step for social behavior. The receptor for advanced glycation end-products (RAGE) is a newly identified molecule as an OT binding protein and serves as a transporter of OT to the brain, crossing over the blood–brain barrier, resulting in the regulation of brain OT levels. We point out new roles of CD38 and CD157 during neuronal development and aging in relation to nicotinamide adenine dinucleotide⁺ levels in embryonic and adult nervous systems. Finally, we discuss how CD38, CD157, and RAGE are crucial for social recognition and behavior in daily life.

Adenine nucleotides as paracrine mediators and intracellular second messengers in immunity and inflammation

Adenine nucleotides (AdNs) play important roles in immunity and inflammation. Extracellular AdNs, such as adenosine triphosphate (ATP) or nicotinamide adenine dinucleotide (NAD) and their metabolites, act as paracrine messengers by fine-tuning both pro- and anti-inflammatory processes. Moreover, intracellular AdNs derived from ATP or NAD play important roles in many cells of the immune system, including T lymphocytes, macrophages, neutrophils and others. These intracellular AdNs are signaling molecules that transduce incoming signals into meaningful cellular responses, e.g. activation of immune responses against pathogens.

Dysregulated Calcium Homeostasis in Cystic Fibrosis Neutrophils Leads to Deficient Antimicrobial Responses

Cystic fibrosis (CF), one of the most common human genetic diseases worldwide, is caused by a defect in the CF transmembrane conductance regulator (CFTR). Patients with CF are highly susceptible to infections caused by opportunistic pathogens (including Burkholderia cenocepacia), which induce excessive lung inflammation and lead to the eventual loss of pulmonary function. Abundant neutrophil recruitment into the lung is a key characteristic of bacterial infections in CF patients. In response to infection, inflammatory neutrophils release reactive oxygen species and toxic proteins, leading to aggravated lung tissue damage in patients with CF. The present study shows a defect in reactive oxygen species production by mouse Cftr^-/- , human F508del-CFTR, and CF neutrophils; this results in reduced antimicrobial activity against B. cenocepacia Furthermore, dysregulated Ca²⁺ homeostasis led to increased intracellular concentrations of Ca²⁺ that correlated with significantly diminished NADPH oxidase response and impaired secretion of neutrophil extracellular traps in human CF neutrophils. Functionally deficient human CF neutrophils recovered their antimicrobial killing capacity following treatment with pharmacological inhibitors of Ca²⁺ channels and CFTR channel potentiators. Our findings suggest that regulation of neutrophil Ca²⁺ homeostasis (via CFTR potentiation or by the regulation of Ca²⁺ channels) can be used as a new therapeutic approach for reestablishing immune function in patients with CF.

TRPM Family Channels in Cancer

Members of the TRPM ("Melastatin") family fall into the subclass of the TRP channels having varying permeability to Ca²⁺ and Mg²⁺, with three members of the TRPM family being chanzymes, which contain C-terminal enzyme domains. The role of different TRPM members has been shown in various cancers such as prostate cancer for mostly TRPM8 and TRPM2, breast cancer for mostly TRPM2 and TRPM7, and pancreatic cancer for TRPM2/7/8 channels. The role of TRPM5 channels has been shown in lung cancer, TRPM1 in melanoma, and TRPM4 channel in prostate cancer as well. Thus, the TRPM family of channels may represent an appealing target for the anticancer therapy.

What is the evidence for the role of TRP channels in inflammatory and immune cells?

A complex network of many interacting mechanisms orchestrates immune and inflammatory responses. Among these, the cation channels of the transient receptor potential (TRP) family expressed by resident tissue cells, inflammatory and immune cells and distinct subsets of primary sensory neurons, have emerged as a novel and interrelated system to detect and respond to harmful agents. TRP channels, by means of their direct effect on the intracellular levels of cations and/or through the indirect modulation of a large series of intracellular pathways, orchestrate a range of cellular processes, such as cytokine production, cell differentiation and cytotoxicity. The contribution of TRP channels to the transition of inflammation and immune responses from a defensive early response to a chronic and pathological condition is also emerging as a possible underlying mechanism in various diseases. This review discusses the roles of TRP channels in inflammatory and immune cell function and provides an overview of the effects of inflammatory and immune TRP channels on the pathogenesis of human diseases.

Understanding autoimmunity: The ion channel perspective

Ion channels are integral membrane proteins that orchestrate the passage of ions across the cell membrane and thus regulate various key physiological processes of the living system. The stringently regulated expression and function of these channels hold a pivotal role in the development and execution of various cellular functions. Malfunction of these channels results in debilitating diseases collectively termed channelopathies. In this review, we highlight the role of these proteins in the immune system with special emphasis on the development of autoimmunity. The role of ion channels in various autoimmune diseases is also listed out. This comprehensive review summarizes the ion channels that could be used as molecular targets in the development of new therapeutics against autoimmune disorders.

The ion channel transient receptor potential melastatin-2 does not play a role in inflammatory mouse models of chronic obstructive pulmonary diseases

Background

There is strong evidence that oxidative stress is associated with the pathogenesis of chronic obstructive pulmonary disease (COPD). The transient receptor potential melastatin-2 (TRPM2) is an oxidative stress sensing channel that is expressed in a number of inflammatory cells and therefore it has been suggested that inhibition of TRPM2 could lead to a beneficial effect in COPD patients. In this study, we have investigated the role of TRPM2 in a variety of mouse models of oxidative stress and COPD using TRPM2-deficent mice.

Methods

Mice were exposed to ozone (3 ppm for 4 h) or lipopolysaccharide (LPS, 0.3 mg/kg, intranasaly). In another model, mice were exposed to tobacco smoke (750 μg/l total wet particulate matter) for 30 min twice a day on three consecutive days. For the exacerbation model, the smoke exposure on the morning of day 3 animals was replaced with intranasal administration of LPS (0.3 mg/kg). Animals were killed 3 and 24 h after the challenge (ozone and LPS model) or 18 h after the last tobacco smoke exposure. In vitro neutrophil chemotaxis and monocyte activation were also studied using cells isolated from wild type and TRPM2-deficient animals. Statistical significance for the in vivo data (P < 0.05) was determined using analysis of variance with Kruskal-Wallis and Dunns multiple comparison test.

Results

In all models studied, no difference in the bronchoalveolar lavage inflammation could be evidenced when comparing wild type and TRPM2-deficient mice. In addition, no difference could be seen in the lung inflammation as assessed by the measurement of various cytokines/chemokines. Similarly in various in vitro cellular activation assays using isolated neutrophils and monocytes no significant differences could be observed when comparing wild type and TRPM2-deficient mice.

Discussion

We have shown, in all the models tested, no difference in the development of airway inflammation or cell activation between TRPM2-deficient mice and their wild type counterparts. These results would suggest that inhibiting TRPM2 activity in COPD would have no anti-inflammatory effect.

The ΔC splice-variant of TRPM2 is the hypertonicity-induced cation channel in HeLa cells, and the ecto-enzyme CD38 mediates its activation

Hypertonicity-induced cation channels (HICCs) are key-players in proliferation and apoptosis but their molecular correlate remains obscure. Furthermore, the activation profile of HICCs is not well defined yet. We report here that, in HeLa cells, intracellular adenosine diphosphate ribose (ADPr) and cyclic ADPr (cADPr), as supposed activators of TRPM2, elicited cation currents that were virtually identical to the osmotic activation of HICCs. Silencing of the expression of TRPM2 and of the ecto-enzyme CD38 (as a likely source of ADPr and cADPr) inhibited HICC as well as nucleotide-induced currents and, in parallel, the hypertonic volume response of cells (the regulatory volume increase, RVI) was attenuated. Quantification of intracellular cADPr levels and the systematic application of extra- vs. intracellular nucleotides indicate that the outwardly directed gradient rather than the cellular activity of ADPr and cADPr triggers TRPM2 activation, probably along with a simultaneous biotransformation of nucleotides.Cloning of TRPM2 identified the ΔC-splice variant as the molecular correlate of the HICC, which could be strongly supported by a direct comparison of the respective Ca²⁺ selectivity. Finally, immunoprecipitation and high-resolution FRET/FLIM imaging revealed the interaction of TRPM2 and CD38 in the native as well as in a heterologous (HEK293T) expression system. We propose transport-related nucleotide export via CD38 as a novel mechanism of TRPM2/HICC activation. With the biotransformation of nucleotides running in parallel, continuous zero trans-conditions are achieved which will render the system infinitely sensitive.

Calcium influx, a new potential therapeutic target in the control of neutrophil-dependent inflammatory diseases in bovines

Neutrophils are the first line of defense against pathogens in bovines; however, they are also one of the most aggressive cells during the inflammatory process, causing injury in surrounding tissues. At present, anti-inflammatory drugs are limited in acute diseases, such as pneumonia, mastitis and endometritis, because neutrophils are mostly insensitive. One of the earliest events during neutrophil activation is the increase in intracellular calcium concentration. The calcium movement is attributed to the release from intracellular stores and influx through the calcium channels in the plasma membrane, a process called store operated calcium entry (SOCE). Recently, several calcium influx blockers have been shown to have strong effects on bovine neutrophils, and this suggests that the manipulation of this pathway can be useful in the control of neutrophil functions during acute inflammatory processes. In this paper, we will review the role of calcium influx as a potential anti-inflammatory target and summarize the most recent evidences for this in bovine neutrophils.

TRP channels: emerging targets for respiratory disease

The mammalian transient receptor potential (TRP) superfamily of cation channels is divided into six subfamilies based on sequence homology TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPA (ankyrin), TRPP (polycystin) and TRPML (mucolipin). The expression of these channels is especially abundant in sensory nerves, and there is increasing evidence demonstrating their existence in a broad range of cell types which are thought to play a key role in respiratory diseases such as asthma and chronic obstructive pulmonary disease (COPD). These ion channels can be activated by a diverse range of chemical and physical stimuli. Physical stimuli include temperature, membrane potential changes and osmotic stress, and some of the more well known chemical stimuli include capsaicin (TRPV1), menthol (TRPM8) and acrolein (TRPA1). There is increasing evidence in this rapidly moving field to suggest that selective blockers of these channels may represent attractive novel strategies to treat characteristic features of respiratory diseases such as asthma and COPD. This review focuses on summarising the evidence that modulation of selected TRP channels may have beneficial effects at targeting key features of these respiratory diseases including airways inflammation, airways hyper-reactivity, mucus secretion and cough.

Review: NAD +: a modulator of immune functions

Latterly, nicotinamide adenine dinucleotide (NAD+) has emerged as a molecule with versatile functions and of enormous impact on the maintenance of cell integrity. Besides playing key roles in almost all major aspects of energy metabolism, there is mounting evidence that NAD+ and its degradation products affect various biological activities including calcium homeostasis, gene transcription, DNA repair, and intercellular communication. This review is aimed at giving a brief insight into the life cycle of NAD+ in the cell, referring to synthesis, action and degradation aspects. With respect to their immunological relevance, the importance and function of the major NAD+ metabolizing enzymes, namely CD38/CD157, ADP-ribosyltransferases (ARTs), poly-ADP-ribose-polymerases (PARPs), and sirtuins are summarized and roles of NAD+ and its main degradation product adenosine 5'-diphosphoribose (ADPR) in cell signaling are discussed. In addition, an outline of the variety of immunological processes depending on the activity of nicotinamide phosphoribosyltransferase (Nampt), the key enzyme of the salvage pathway of NAD+ synthesis, is presented. Taken together, an efficient supply of NAD+ seems to be a crucial need for a multitude of cell functions, underlining the yet only partly revealed potency of this small molecule to influence cell fate.

Transient receptor potential melastatin 2 is required for lipopolysaccharide-induced cytokine production in human monocytes

Transient receptor potential melastatin 2 (TRPM2) is a Ca(2+)-permeable nonselective cation channel that is stimulated by oxidative stress and specifically activated by intracellular ADP-ribose. Because TRPM2 is highly expressed in immunocytes, a role of this channel in inflammation processes has been proposed. The aim of the current study was to determine the function of TRPM2 in LPS-induced cytokine production of human monocytes. Incubation of human primary monocytes with LPS resulted in an upregulation of TRPM2 mRNA, protein, and of ADP-ribose-induced membrane currents. By using short hairpin RNA to downregulate TRPM2 expression in THP-1 monocytes, we demonstrate that TRPM2 is required for the LPS-induced production of IL-6, IL-8, IL-10, and TNF-alpha. Application of LPS led to a time-dependent increase in intracellular Ca(2+) concentrations in THP-1 cells that was clearly reduced by downregulation of TRPM2. Omission of extracellular Ca(2+) strongly decreased TNF-alpha production in TRPM2-expressing cells. Thus, TRPM2-mediated Ca(2+) entry is a central mechanism for LPS-induced cytokine production in monocytic cells. The identification of TRPM2 as a major player in this LPS-dependent process makes it a promising tool in modulating monocyte functions.

Modulation of TRPM2 by acidic pH and the underlying mechanisms for pH sensitivity

TRPM2 is a Ca²⁺-permeable nonselective cation channel that plays important roles in oxidative stress–mediated cell death and inflammation processes. However, how TRPM2 is regulated under physiological and pathological conditions is not fully understood. Here, we report that both intracellular and extracellular protons block TRPM2 by inhibiting channel gating. We demonstrate that external protons block TRPM2 with an IC₅₀ of pH_o = 5.3, whereas internal protons inhibit TRPM2 with an IC₅₀ of pH_i = 6.7. Extracellular protons inhibit TRPM2 by decreasing single-channel conductance. We identify three titratable residues, H958, D964, and E994, at the outer vestibule of the channel pore that are responsible for pH_o sensitivity. Mutations of these residues reduce single-channel conductance, decrease external Ca²⁺ ([Ca²⁺]_o) affinity, and inhibit [Ca²⁺]_o-mediated TRPM2 gating. These results support the following model: titration of H958, D964, and E994 by external protons inhibits TRPM2 gating by causing conformation change of the channel, and/or by decreasing local Ca²⁺ concentration at the outer vestibule, therefore reducing [Ca²⁺]_o permeation and inhibiting [Ca²⁺]_o-mediated TRPM2 gating. We find that intracellular protons inhibit TRPM2 by inducing channel closure without changing channel conductance. We identify that D933 located at the C terminus of the S4-S5 linker is responsible for intracellular pH sensitivity. Replacement of Asp⁹³³ by Asn⁹³³ changes the IC₅₀ from pH_i = 6.7 to pH_i = 5.5. Moreover, substitution of Asp⁹³³ with various residues produces marked changes in proton sensitivity, intracellular ADP ribose/Ca²⁺ sensitivity, and gating profiles of TRPM2. These results indicate that D933 is not only essential for intracellular pH sensitivity, but it is also crucial for TRPM2 channel gating. Collectively, our findings provide a novel mechanism for TRPM2 modulation as well as molecular determinants for pH regulation of TRPM2. Inhibition of TRPM2 by acidic pH may represent an endogenous mechanism governing TRPM2 gating and its physiological/pathological functions.

Inhibitors of TRP channels reveal stimulus-dependent differential activation of Ca2+ influx pathways in human neutrophil granulocytes

A pharmacological characterization of Ca(2+) influx pathways in neutrophil granulocytes is problematic because of the lack of specific inhibitors. The activation of transient receptor potential cation channel, subfamily M, member 2 (TRPM2) channels by intracellular adenosine diphosphate ribose (ADPR), well characterized in neutrophils, is reportedly inhibited by 8-bromo-ADPR (8Br-ADPR). TRPM2 is blocked by N-(p-amylcinnamoyl)anthranilic acid (ACA) interfering with the pore, but ACA is as well effective on other transient receptor potential channels, especially transient receptor potential canonical (TRPC) channels. We wished to analyze whether ACA and 8Br-ADPR were suitable probes to demonstrate that different Ca(2+) entry pathways are activated in human neutrophil granulocytes by the receptor-dependent stimuli N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP) and platelet-activating factor (PAF) and the receptor-independent thapsigargin. Ca(2+)-influx-related increases in [Ca(2+)](i) were calculated by comparing aliquots of fluo-3-loaded neutrophils in the presence and absence of extracellular Ca(2+). Moreover, Mn(2+) quenching was used in fura-2-loaded cells. We compared 8Br-ADPR with ACA. 8Br-ADPR was exclusively effective when Ca(2+) influx (or Mn(2+) quenching) was induced by fMLP; it did not affect influx when PAF or thapsigargin was the stimulus. ACA inhibited Ca(2+) influx significantly more strongly when this was induced by PAF than by fMLP. Moreover, it reduced thapsigargin-induced Ca(2+) influx. The contribution of TRPM2 to Ca(2+) influx in neutrophils strongly depends on the stimulus; it is sizeable in the case of fMLP and minimal in the case of PAF. PAF induces Ca(2+) entry pathways different from TRPM2; the inhibition by ACA suggests the contribution of channels of the TRPC family.

Regulation of blood-brain barrier permeability by transient receptor potential type C and type v calcium-permeable channels

OBJECTIVE

To identify plasma membrane ion channels mediating calcium influx at the blood-brain barrier in response to disrupting stimuli.

METHODS

We examined the expression and function of candidate transient receptor potential channels using reverse transcriptase polymerase chain recation, Fura-2 calcium imaging, and permeability assays.

RESULTS

Immortalized mouse brain microvessel endothelial cells expressed multiple transient receptor potential isoforms: transient receptor potential C1, C2, C4, and C7, M2, M3, M4, and M7, and V2 and V4. Similar profiles were observed in freshly isolated cerebral microvessels and primary cultured rat brain endothelial cells. Thrombin-stimulated calcium influx in brain endothelial cells was blocked by transient receptor potential C inhibitors. Transient receptor potential V activating stimuli also increased intracellular calcium. This increase was inhibited by a transient receptor potential V blocker or by removal of extracellular calcium. Barrier integrity was compromised by thrombin, hypo-osmolar stress, and PMA treatment. The increase in barrier permeability induced by transient receptor potential V activators was blocked by transient receptor potential V inhibition, while thrombin effects were inhibited by transient receptor potential C inhibitors.

CONCLUSIONS

These results demonstrate that transient receptor potential C and transient receptor potential V channels mediate calcium influx at the blood-brain barrier, and as a consequence, may modulate barrier integrity.

The Poly(ADP-ribose) polymerase PARP-1 is required for oxidative stress-induced TRPM2 activation in lymphocytes

TRPM2 cation channels are widely expressed in the immune system and are thought to play a role in immune cell responses to oxidative stress. Patch clamp analyses suggest that TRPM2 channel activation can occur through a direct action of oxidants on TRPM2 channels or indirectly through the actions of a related group of adenine nucleotide 2nd messengers. However, the contribution of each gating mechanism to oxidative stress-induced TRPM2 activation in lymphocytes remains undefined. To better understand the molecular events leading to TRPM2 activation in lymphocytes, we analyzed oxidative stress-induced turnover of intracellular NAD, the metabolic precursor of adenine nucleotide 2nd messengers implicated in TRPM2 gating, and oxidative stress-induced TRPM2-mediated currents and Ca2+ transients in DT40 B cells. TRPM2-dependent Ca2+ entry did not influence the extent or time course of oxidative stress-induced turnover of NAD. Furthermore, expression of oxidative stress-activated poly(ADP-ribose) polymerases (PARPs) was required for oxidative stress-induced NAD turnover, TRPM2 currents, and TRPM2-dependent Ca2+ transients; no oxidant-induced activation of TRPM2 channels could be detected in PARP-deficient cells. Together, our results suggest that during conditions of oxidative stress in lymphocytes, TRPM2 acts as a downstream effector of the PARP/poly(ADP-ribose) glycohydrolase pathway through PARP-dependent formation of ADP-ribose.

Regulation of superoxide production in neutrophils: role of calcium influx

Upon stimulation, activation of NADPH oxidase complexes in neutrophils produces a burst of superoxide anions contributing to oxidative stress and the development of inflammatory process. Store-operated calcium entry (SOCE), whereby the depletion of intracellular stores induces extracellular calcium influx, is known to be a crucial element of NADPH oxidase regulation. However, the mechanistic basis mediating SOCE is still only partially understood, as is the signal-coupling pathway leading to modulation of store-operated channels. This review emphasizes the role of calcium influx in the control of the NADPH oxidase and summarizes the current knowledge of pathways mediating this extracellular calcium entry in neutrophils. Such investigations into the cross-talk between NADPH oxidase and calcium might allow the identification of novel pharmacological targets with clinical use, particularly in inflammatory diseases.

Cyclic ADP-ribose mediates formyl methionyl leucyl phenylalanine (fMLP)-induced intracellular Ca(2+) rise and migration of human neutrophils

Although cyclic ADP-ribose (cADPR), a novel Ca(2+)-mobilizing mediator, is suggested to be involved in the functions of neutrophils in rodents, its role in human neutrophils remains unclear. The present study examined the ability of cADPR to mobilize Ca(2+) and mediate formyl methionyl leucyl phenylalanine (fMLP)-stimulated increase in cytosolic free Ca(2+) concentration ([Ca(2+)](i)) and migration in human neutrophils. cADPR induced Ca(2+) release from digitonin-permeabilized neutrophils, and the release was blocked by 8Br-cADPR, an antagonist of cADPR. Immunophilin ligands, FK506 and rapamycin, but not cyclosporine A, inhibited cADPR-induced Ca(2+) release. 8Br-cADPR partially reduced fMLP-induced [Ca(2+)](i) rise and abolished the rise in combination with 2APB, an IP(3)-receptor antagonist. Anti-CD38Ab and NADase that interfere with cADPR formation, reduced the fMLP-induced [Ca(2+)](i) rise. When beta-NAD(+), a substrate of ADP-ribosyl cyclase, and cADPR were added to the medium, the former gradually increased [Ca(2+)](i) and the latter potentiated the fMLP-induced [Ca(2+)](i) rise. The beta-NAD(+)-induced [Ca(2+)](i) rise in Ca(2+)-free medium was inhibited by anti-CD38Ab, 8Br-cADPR, FK506, ruthenium red, and thapsigargin. mRNAs of nucleoside transporter (NT), ENT1, ENT2, CNT, and CNT3 were expressed in neutrophils; and their inhibitors, inosine, uridine, and s-(4-nitrobenzyl)-6-thioinosine, reduced the [Ca(2+)](i) rise induced by beta-NAD(+) and fMLP. fMLP-timulated migration was inhibited by the removal of Ca(2+) from the medium or by the addition of 8Br-cADPR, anti-CD38Ab, NADase, and NT inhibitors. These results suggest that cADPR was synthesized extracellularly by CD38, transported into the cells through NTs, and then Ca(2+) was mobilized by FK506-binding protein-dependent process. This process may be involved in fMLP-induced intracellular Ca(2+) signaling and migration in human neutrophils.

Store-operated calcium entry mediates intracellular alkalinization, ERK1/2, and Akt/PKB phosphorylation in bovine neutrophils

Neutrophil's responses to G protein-coupled chemoattractants are highly dependent on store-operated calcium (Ca(2+)) entry (SOCE). Platelet-activating factor (PAF), a primary chemoattractant, simultaneously increases cytosolic-free Ca(2+), intracellular pH (pH(i)), ERK1/2, and Akt/protein kinase B (PKB) phosphorylation. In this study, we looked at the efficacy of several putative SOCE inhibitors and whether SOCE mediates intracellular alkalinization, ERK1/2, and Akt/PKB phosphorylation in bovine neutrophils. We demonstrated that the absence of external Ca(2+) and the presence of EGTA reduced the intracellular alkalinization and ERK1/2 phosphorylation induced by PAF, apparently via SOCE influx inhibition. Next, we tested the efficacy of several putative SOCE inhibitors such as 2-aminoethoxydiphenyl borate (2-APB), capsaicin, flufenamic acid, 1-{beta-[3-(4-methoxy-phenyl)propoxy]-4-methoxyphenethyl}-1H-imidazole hydrochloride (SK&F 96365), and N-(4-[3,5-bis(trifluoromethyl)-1H-pyrazol-1-yl]phenyl)-4-methyl-1,2,3-thiadiazole-5-carboxamide (BTP2) on Ca(2+) entry induced by PAF or thapsigargin. 2-APB was the most potent SOCE inhibitor, followed by capsaicin and flufenamic acid. Conversely, SK&F 96365 reduced an intracellular calcium ([Ca(2+)](i)) peak but SOCE partially. BTP2 did not show an inhibitory effect on [Ca(2+)](i) following PAF stimuli. 2-APB strongly reduced the pH(i) recovery, whereas the effect of flufenamic acid and SK&F 96365 was partial. Capsaicin and BTP2 did not affect the pH(i) changes induced by PAF. Finally, we observed that 2-APB reduced the ERK1/2 and Akt phosphorylation completely, whereas the inhibition with flufenamic acid was partial. The results suggest that 2-APB is the most potent SOCE inhibitor and support a key role of SOCE in pH alkalinization and PI-3K-ERK1/2 pathway control. Finally, 2-APB could be an important tool to characterize Ca(2+) signaling in neutrophils.

New molecular mechanisms on the activation of TRPM2 channels by oxidative stress and ADP-ribose

The Na(+) and Ca(2+)-permeable melastatin related transient receptor potential (TRPM2) cation channels can be gated either by ADP-ribose (ADPR) in concert with Ca(2+) or by hydrogen peroxide (H(2)O(2)), an experimental model for oxidative stress, and binding to the channel's enzymatic Nudix domain. Since the mechanisms that lead to TRPM2 inhibiting in response to ADPR and H(2)O(2) are not understood, I reviewed the effects of various inhibitors such as flufenamic acid and PARP inhibitors on ADPR, NAD(+) and H(2)O(2)-induced TRPM2 currents. In our experimental study, TRPM2 cation channels in chinese hamster ovary transected cells were gated both by ADPR and NAD(+). In addition, H(2)O(2) seems to activate TRPM2 by changing to the hydroxyl radical in the intracellular space after passing the plasma membrane. Experimental studies with respect to patch-clamp and Ca(2+) imaging, inhibitor roles of antioxidants are also summarized in the review.

TRPM2

TRPM2 is a cation channel enabling influx of Na+ and Ca2+, leading to depolarization and increases in the cytosolic Ca2+ concentration ([Ca2+]i). It is widely expressed, e.g. in many neurons, blood cells and the endocrine pancreas. Channel gating is induced by ADP-ribose (ADPR) that binds to a Nudix box motif in the cytosolic C-terminus of the channel. Endogenous ADPR concentrations in leucocytes are sufficiently high to activate TRPM2 in the presence of an increased [Ca2+]i but probably not at resting [Ca2+]i. Another channel activator is oxidative stress, especially hydrogen peroxide (H2O2) that may act through ADPR after ADPR polymers have been formed by poly(ADP-ribose) polymerases (PARPs) and hydolysed by glycohydrolases. H2O2-stimulated TRPM2 channels essentially contribute to insulin secretion in pancreatic beta-cells and alloxan-induced diabetes mellitus. Inhibition of TRPM2 channels may be achieved by channel blockers such as flufenamic acid or the anti-fungal agents clotrimazole or econazole. Selective blockers of TRPM2 are not yet available; those would be valuable for a characterization of biological roles of TRPM2 in various tissues and as potential drugs directed against oxidative cell damage, reperfusion injury or leucocyte activation. Activation of TRPM2 may be prevented by anti-oxidants, PARP inhibitors and glycohydrolase inhibitors. In future, binding of ADPR to the Nudix box may be targeted. In light of the wide-spread expression and growing list of cellular functions of TRPM2, useful therapeutic applications are expected for future drugs that block TRPM2 channels or inhibit their activation.

Overexpression of human CD38/ADP-ribosyl cyclase enhances acetylcholine-induced Ca2+ signalling in rodent NG108-15 neuroblastoma cells

The role of cyclic ADP-ribose (cADPR) and its synthetic enzyme, CD38, as a downstream signal of muscarinic acetylcholine receptors (mAChRs) was examined in neuroblastoma cells expressing M1 mAChRs (NGM1). NGM1 cells were further transformed with both wild-type and mutant (C119K/C201E) human CD38. The dual transformed cells exhibited higher cADPR formation than ADPR production and elevated intracellular free Ca(2+) concentrations ([Ca(2+)](i)) in response to ACh. These phenotypes were analyzed in detail in a representative CD38 clone. The intracellular cADPR concentration by ACh application was significantly increased by CD38 overexpression. Digital image analysis by a confocal microscopy revealed that topographical distribution of the sites of Ca(2+) release was unchanged between control and overexpressed cells. These results indicate that cADPR is an intracellular messenger of Ca(2+) signalling, suggesting that CD38 can contribute to mAChR-cADPR signalling.

TRPM channels, calcium and redox sensors during innate immune responses

Melastatin-related TRPM ion channels have emerged as novel therapeutic targets due to their potential ability to modulate the function and fate of immune cells during inflammation, innate, and adaptive immunity. Four family members, TRPM1, TRPM2, TRPM4 and TRPM7 have a strong presence in the immune system. TRPM channels regulate ion-homeostasis by sensing cellular redox status and cytoplasmic calcium levels. TRPM2 for example, is highly expressed in phagocytes. This channel is activated by intracellular ADP-ribose upon exposure to oxidative stress and induces cell death. Here we will review the functional links between TRPM-mediated ion conductance, chemotaxis, apoptosis, and innate immunity.

Cells move when ions and water flow

Cell migration is a process that plays an important role throughout the entire life span. It starts early on during embryogenesis and contributes to shaping our body. Migrating cells are involved in maintaining the integrity of our body, for instance, by defending it against invading pathogens. On the other side, migration of tumor cells may have lethal consequences when tumors spread metastatically. Thus, there is a strong interest in unraveling the cellular mechanisms underlying cell migration. The purpose of this review is to illustrate the functional importance of ion and water channels as part of the cellular migration machinery. Ion and water flow is required for optimal migration, and the inhibition or genetic ablation of channels leads to a marked impairment of migration. We briefly touch cytoskeletal mechanisms of migration as well as cell-matrix interactions. We then present some general principles by which channels can affect cell migration before we discuss each channel group separately.

Cyclic ADP-ribose requires FK506-binding protein to regulate intracellular Ca2+ dynamics and catecholamine release in acetylcholine-stimulated bovine adrenal chromaffin cells

The present study was undertaken to elucidate whether cyclic ADP-ribose (cADPR) mediates the amplification of Ca2+ signaling and catecholamine release via the involvement of FK506-binding proteins (FKBPs)/ryanodine receptor (RyR) in bovine adrenal chromaffin cells. cADPR induced Ca2+ release in digitonin-permeabilized chromaffin cells and this was blocked by FK506 and rapamycin, ligands for FKBPs; 8Br-cADPR, a competitive antagonist for cADPR; and antibody for FKBP12/12.6, while it was enhanced by cyclosporin A. Ryanodine-induced Ca2+ release was not affected by 8Br-cADPR and was remarkably enhanced by FK506, rapamycin, cyclosporin A, and cADPR. FK506 binds to FKBP12.6 and removes it from RyRs, but cADPR did not affect the binding between FKBP12.6 and RyR. In intact chromaffin cells, 8Br-cADPR, FK506, and rapamycin, but not cyclosporin A attenuated the sustained intracellular free Ca2+ concentration ([Ca2+]i) rise induced by acetylcholine (ACh). 8Br-cADPR, FK506, and SK&F 96365 reduced the Mn2+ entry stimulated with ACh only when Ca2+ was present in the extracellular medium. 8Br-cADPR, FK506, and rapamycin concentration-dependently inhibited the ACh-induced catecholamine (CA) release. Here, we present evidence that FKBP12.6 associated with RyR may be required for Ca2+ release induced by cADPR in bovine adrenal chromaffin cells. cADPR-mediated Ca2+ release from endoplasmic reticulum in ACh-stimulated chromaffin cells is coupled with Ca2+ influx through the plasma membrane which is essential for ACh-stimulated CA release.

Transcriptional regulation and processing increase the functional variability of TRPM channels

Mammalian TRP channels display heterogenous biophysical properties and are involved in a variety of signal transduction pathways. To carry out their diverse biological functions and to adapt these functions to changes of the environment, mechanisms to regulate their molecular structure are required. Transcriptional regulation and post-transcriptional RNA processing represent essential instruments to generate TRP channel variants with modified properties. TRP variants are expressed depending on the tissue and developmental state. They can show distinct biophysical properties and mechanisms of activation, and thereby determine channel function and malfunction in certain human diseases. In this review, we give an overview of the variants of a given TRP gene, with the focus on the TRPM subfamily, and discuss their relevance with respect to their function under physiological and pathological conditions.

Function and pharmacology of TRPM cation channels

The physiological function and cellular role of some members of the TRPM family are poorly understood and still mysterious. Melastatin, the founding member of the TRPM group, is the most prominent example of the mysteries involved in understanding TRP channel function. Melastatin or TRPM1 was first cloned in 1998 and since then it has been suggested that it functions as a tumor suppressor protein in melanocytes. On the other hand, TRPM8 and TRPA1 have been described as cold receptors, TRPM4 and TRPM5 as calcium-activated nonselective cation channels, TRPM6 and TRPM7 as magnesium-permeable and magnesium-modulated cation channels, TRPM2 as an ADP-ribose-activated channel of macrophages, and TRPM3 as a hypo-osmolarity- and sphingosine-activated channel. There are many unsolved questions and many studies have to be performed to understand the overall function of the TRPM family. In addition to electrophysiological recordings and biochemical characterization, the use of compounds modulating TRPM channel function has often been helpful to study TRPM channels in a cellular context. Therefore, the review will summarize the known functions, activation mechanisms, and pharmacological modulations of the TRPM channels.