Hydrogen peroxide and ADP-ribose induce TRPM2-mediated calcium influx and cation currents in microglia

Interaction of surfactant protein A with the intermediate filaments desmin and vimentin

Surfactant protein A (SP-A), a member of the collectin family that modulates innate immunity, has recently been involved in the physiology of reproduction. Consistent with the activation of ERK-1/2 and COX-2 induced by SP-A in myometrial cells, we reported previously the presence of two major proteins recognized by SP-A in these cells. Here we identify by mass spectrometry one of these SP-A targets as the intermediate filament (IF) desmin. In myometrial preparations derived from desmin-deficient mice, the absence of binding of SP-A to any 50 kDa protein confirmed the identity of this SP-A-binding site as desmin. Our data based on partial chymotrypsin digestion of pure desmin suggested that SP-A recognizes especially its rod domain, which is known to play an important role during the assembly of desmin into filaments. In line with that, electron microscopy experiments showed that SP-A inhibits in vitro the polymerization of desmin filaments. SP-A also recognized in vitro polymerized filaments in a calcium-dependent manner at a physiological ionic strength but not the C1q receptor gC1qR. Furthermore, Texas Red-labeled SP-A colocalized with desmin filaments in myometrial cells. Interestingly, vimentin, the IF characteristic of leukocytes, is one of the major proteins recognized by SP-A in protein extracts of U937 cells after PMA-induced differentiation of this monocytic cell line. Interaction of SP-A with vimentin was further confirmed using recombinant vimentin in solid-phase binding assays. The ability of SP-A to interact with desmin and vimentin, and to prevent polymerization of desmin monomers, shed light on unexpected and wider biological roles of this collectin.

The encephalopathy in sepsis

Brain dysfunction is a severe complication of sepsis with an incidence ranging from 9% to 71% that is associated with increased morbidity and mortality. Its diagnosis relies mainly on neurologic examination with clinical manifestations ranging from confusion to coma. An electroencephalogram, somatosensory evoked potentials, and measurement of plasma S-100b protein and neuron-specific enolase can be useful for the detection of brain dysfunction. Brain MRI can identify brain lesions such as cerebral infarction, posterior reversible encephalopathy syndrome, and leukoencephalopathy. The mechanism of sepsis-associated encephalopathy involves inflammatory and non-inflammatory processes that affect endothelial cells, glial cells, and neurons and induce blood-brain barrier breakdown, derangements of intracellular metabolism, and cell death. Specific treatments for sepsis-associated encephalopathy need to be developed. Currently, treatment is mainly the management of sepsis.

A calcium influx pathway regulated separately by oxidative stress and ADP-Ribose in TRPM2 channels: single channel events

A melastatin-like transient receptor potential 2 (TRPM2) channel is activated in concert with Ca2+ by intracellular adenosine diphosphoribose (ADPR) binding to the channel's enzyme Nudix domain. Channel activity is also seen with nicotinamide dinucleotide (NAD+) and hydrogen peroxide (H2O2) although the mechanisms remain unknown. Hence, we tested the effects of ADPR, NAD+ and H2O2 on the activation of TRPM2 currents in transfected Chinese hamster ovary (CHO) cells. The CHO cells were transfected with cDNA coding for TRPM2. The intracellular solution used EDTA (10 mM) as a chelator for Ca2+ and heavy metal ions. Moreover, we balanced the intracellular Ca2+ concentration at 1 microM. H2O2 (10 mM) in the bath chamber was extracellularly added although ADPR (0.3 mM) and NAD+ (1 mM) in pipette solution were intracellularly added. Using these conditions, the channel currents were evoked by the three stimulators. The time course of ADPR, NAD+ and H2O2 effects was characterized by a delay of 0.6, 3.0 min and 2-5 min, respectively and a slow current induction reached a clear plateau with ADPR and NAD+ although H2O2 currents continued to gain in amplitude over at least 15 min and it did not reach a clear plateau in many experiments. Furthermore, H2O2-induced a single-channel conductance in the current study; the first time that this has been resolved in CHO. The conductance of ADPR and H2O2 was 48.80 pS and 39.14 pS, respectively and the cells seem to be separately activated by ADPR and H2O2. In conclusion, we observed further support for a calcium influx pathway regulated separately by oxidative stress and ADPR in TRPM2 channels in transfected cells. A second novel result of the present study was that the TRPM2 channels were constitutionally activated by H2O2.

Reactive oxygen species produced up- or downstream of calcium influx regulate proinflammatory mediator release from mast cells: role of NADPH oxidase and mitochondria

Earlier studies have demonstrated that mast cells produce reactive oxygen species (ROS), which play a role in regulating Ca(2+) influx, while in other cell types ROS are produced in a Ca(2+)-dependent manner. We sought to determine whether ROS are produced downstream of the extracellular Ca(2+) entry in mast cells. Thapsigargin (TG), a receptor-independent agonist, could evoke a robust burst of intracellular ROS. However, this response was distinct from the antigen-induced burst of ROS with respect to time course and dependence on Ca(2+) and phosphatidylinositol-3-kinase (PI3K). The antigen-induced ROS generation occurred immediately, while the TG-induced ROS generation occurred with a significant lag time (~2 min). Antigen but not TG caused extracellular release of superoxide (O(2)(*-))/hydrogen peroxide (H(2)O(2)), which was blocked by diphenyleneiodonium, apocynin, and wortmannin. A capacitative Ca(2+) entry resulted in the generation of O(2)(*-) in the mitochondria in a PI3K-independent manner. Blockade of ROS generation inhibited TG-induced mediator release. Finally, when used together, antigen and TG evoked the release of leukotriene C(4), tumor necrosis factor-alpha, and interleukin-13 as well as ROS generation synergistically. These results suggest that ROS produced upstream of Ca(2+) influx by NADPH oxidase and downstream of Ca(2+) influx by the mitochondria regulate the proinflammatory response of mast cells.

Development and validation of a cell-based high-throughput screening assay for TRPM2 channel modulators

TRPM2 is a member of the transient receptor potential melastatin (TRPM)-related ion channel family. The activation of TRPM2 induced by oxidative/nitrosative stress leads to an increase in intracellular free Ca(2+). Although further progress in understanding TRPM2's role in cell and organism physiology would be facilitated by isolation of compounds able to specifically modulate its function in primary cells or animal models, no cell-based assays for TRPM2 function well suited for high-throughput screening have yet been described. Here, a novel suspension B lymphocyte cell line stably expressing TRPM2 was used to develop a cell-based assay. The assay uses the Ca(2+)-sensitive fluorescence dye, Fluo-4 NW (no wash), to measure TRPM2-dependent Ca(2+) transients induced by H(2)O(2) and N-methyl-N'-nitrosoguanidine in a 96-well plate format. Assay performance was evaluated by statistical analysis of the Z' factor value and was consistently greater than 0.5 under optimal conditions, suggesting that the assay is very robust. For assay validation, the effects of known inhibitors of TRPM2 and TRPM2 gating secondary messenger production were determined. Overall, the authors have developed a cell-based assay that may be used to identify TRPM2 ion channel modulators from large compound libraries.

Role of TRPM2 channel in mediating H2O2-induced Ca2+ entry and endothelial hyperpermeability

Oxidative stress through the production of oxygen metabolites such as hydrogen peroxide (H2O2) increases vascular endothelial permeability. H2O2 stimulates ADP-ribose formation, which in turn opens transient receptor potential melastatin (TRPM)2 channels. Here, in endothelial cells, we demonstrate transcript and protein expression of TRPM2, a Ca2+-permeable, nonselective cation channel. We further show the importance of TRPM2 expression in signaling of increased endothelial permeability by oxidative stress. Exposure of endothelial cell monolayers to sublytic concentrations of H2O2 induced a cationic current measured by patch-clamp recording and Ca2+ entry detected by intracellular fura-2 fluorescence. H2O2 in a concentration-dependent manner also decreased trans-monolayer transendothelial electrical resistance for 3 hours (with maximal effect seen at 300 micromol/L H2O2), indicating opening of interendothelial junctions. The cationic current, Ca2+ entry, and transendothelial electrical resistance decrease elicited by H2O2 were inhibited by siRNA depleting TRPM2 or antibody blocking of TRPM2. H2O2 responses were attenuated by overexpression of the dominant-negative splice variant of TRPM2 or inhibition of ADP-ribose formation. Overexpression of the full-length TRPM2 enhanced H2O2-mediated Ca2+ entry, cationic current, and the transendothelial electrical resistance decrease. Thus, TRPM2 mediates H2O2-induced increase in endothelial permeability through the activation of Ca2+ entry via TRPM2. TRPM2 represents a novel therapeutic target directed against oxidant-induced endothelial barrier disruption.

A helix-breaking mutation in TRPML3 leads to constitutive activity underlying deafness in the varitint-waddler mouse

Homozygote varitint-waddler (Va) mice, expressing a mutant isoform (A419P) of TRPML3 (mucolipin 3), are profoundly deaf and display vestibular and pigmentation deficiencies, sterility, and perinatal lethality. Here we show that the varitint-waddler isoform of TRPML3 carrying an A419P mutation represents a constitutively active cation channel that can also be identified in native varitint-waddler hair cells as a distinct inwardly rectifying current. We hypothesize that the constitutive activation of TRPML3 occurs as a result of a helix-breaking proline substitution in transmembrane-spanning domain 5 (TM5). A proline substitution scan demonstrated that the inner third of TRPML3's TM5 is highly susceptible to proline-based kinks. Proline substitutions in TM5 of other TRP channels revealed that TRPML1, TRPML2, TRPV5, and TRPV6 display a similar susceptibility at comparable positions, whereas other TRP channels were not affected. We conclude that the molecular basis for deafness in the varitint-waddler mouse is the result of hair cell death caused by constitutive TRPML3 activity. To our knowledge, our study provides the first direct mechanistic link of a mutation in a TRP ion channel with mammalian hearing loss.

Hydrogen peroxide activates calcium influx in human neutrophils

The correlation between an increased production of reactive oxygen species (ROS) and an enhanced calcium entry in primed neutrophils stimulated with fMLP suggests that endogenous ROS could serve as an agonist to reinforce calcium signaling by positive feedback. This work shows that exogenous H2O2 produced a rapid influx of Mn2+ and an increase of intracellular calcium. The H2O2 was insufficient to produce significant changes in the absence of extracellular calcium but addition of Ca2+ to H2O2-treated cells suspended in a free Ca2+/EGTA buffer resulted in a great increase in [Ca2+]i reflecting influx of Ca2+ across the cell membrane. The increase of intracellular calcium was inhibited by Ni2+, La3+, and hyperosmotic solutions of mannitol and other osmolytes. This raises the possibility that the secretion of H2O2 by activated neutrophils could act as an autocrine regulator of neutrophil function through the activation of calcium entry.

Surfactant protein A activation of atypical protein kinase C zeta in IkappaB-alpha-dependent anti-inflammatory immune regulation

The pulmonary collectin surfactant protein (SP)-A has a pivotal role in anti-inflammatory modulation of lung immunity. The mechanisms underlying SP-A-mediated inhibition of LPS-induced NF-kappaB activation in vivo and in vitro are only partially understood. We previously demonstrated that SP-A stabilizes IkappaB-alpha, the primary regulator of NF-kappaB, in alveolar macrophages (AM) both constitutively and in the presence of LPS. In this study, we show that in AM and PBMC from IkappaB-alpha knockout/IkappaB-beta knockin mice, SP-A fails to inhibit LPS-induced TNF-alpha production and p65 nuclear translocation, confirming a critical role for IkappaB-alpha in SP-A-mediated LPS inhibition. We identify atypical (a) protein kinase C (PKC) zeta as a pivotal upstream regulator of SP-A-mediated IkappaB-alpha/NF-kappaB pathway modulation deduced from blocking experiments and confirmed by using AM from PKCzeta-/- mice. SP-A transiently triggers aPKCThr(410/403) phosphorylation, aPKC kinase activity, and translocation in primary rat AM. Coimmunoprecipitation experiments reveal that SP-A induces aPKC/p65 binding under constitutive conditions. Together the data indicate that anti-inflammatory macrophage activation via IkappaB-alpha by SP-A critically depends on PKCzeta activity, and thus attribute a novel, stimulus-specific signaling function to PKCzeta in SP-A-modulated pulmonary immune response.

Three-dimensional reconstruction using transmission electron microscopy reveals a swollen, bell-shaped structure of transient receptor potential melastatin type 2 cation channel

Transient receptor potential melastatin type 2 (TRPM2) is a redox-sensitive, calcium-permeable cation channel activated by various signals, such as adenosine diphosphate ribose (ADPR) acting on the ADPR pyrophosphatase (ADPRase) domain, and cyclic ADPR. Here, we purified the FLAG-tagged tetrameric TRPM2 channel, analyzed it using negatively stained electron microscopy, and reconstructed the three-dimensional structure at 2.8-nm resolution. This multimodal sensor molecule has a bell-like shape of 18 nm in width and 25 nm in height. The overall structure is similar to another multimodal sensor channel, TRP canonical type 3 (TRPC3). In both structures, the small extracellular domain is a dense half-dome, whereas the large cytoplasmic domain has a sparse, double-layered structure with multiple internal cavities. However, a unique square prism protuberance was observed under the cytoplasmic domain of TRPM2. The FLAG epitope, fused at the C terminus of the ADPRase domain, was assigned by the antibody to a position close to the protuberance. This indicates that the agonist-binding ADPRase domain and the ion gate in the transmembrane region are separately located in the molecule.

What Cardiologists Should Know About Calcium Ion Channels and Their Regulation by Reactive Oxygen Species

Ion channels underlie the electrical activity of cells. Calcium channels have a unique functional role, because not only do they participate in this activity, they form the means by which electrical signals are converted to responses within the cell. Calcium channels play an integral role in excitation in the heart and shaping the cardiac action potential. In addition, calcium influx through calcium channels is responsible for initiating contraction. Abnormalities in calcium homeostasis underlie cardiac arrhythmia, contractile dysfunction and cardiac remodelling. Reactive oxygen species participate in the development of pathology by altering the redox state of regulatory proteins. There is now good evidence that reactive oxygen species regulate the function of calcium channels. In this mini-review, the evidence for regulation of calcium channels by reactive oxygen species and implications with respect to pathology are presented. Calcium channels may represent a target for intervention during hypoxic trigger of arrhythmia or chronic pathological remodelling.

Pulmonary stromal-derived factor-1 expression and effect on neutrophil recruitment during acute lung injury

The severe and protracted inflammation that characterizes acute lung injury (ALI) is driven by the ongoing recruitment of neutrophils to the lung. Although much of the cytokine signaling responsible for the initial phase of ALI has been elaborated, relatively little is known about the mechanisms governing the recruitment of neutrophils from the bone marrow to the lung in the later period of this disease. Given its previously described chemoattractant effects on marrow neutrophils, we investigated whether stromal-derived factor-1 (SDF-1) (CXCL12) might participate in this later phase of recruitment. Using immunohistochemistry to examine both banked human lung specimens from patients with ALI and lungs from mice with LPS-induced pneumonitis, we found that pulmonary SDF-1 expression increases during ALI. We further determined that both lung SDF-1 protein expression and mRNA expression rise in a delayed but sustained pattern in this mouse model and that the major source of the increase in expression appears to be the lung epithelium. Lastly, we found that expression of the SDF-1 receptor CXCR4 rises in a similar temporal pattern on neutrophils in both the blood and airspace of LPS-injured mice and that Ab-mediated SDF-1 blockade significantly attenuates late but not early pulmonary neutrophilia in this model. These results implicate SDF-1 in neutrophil recruitment to the lung in the later period of acute lung injury and suggest a novel role for this cytokine in coordinating the transition from the inflammatory response to the initiation of tissue repair.

Multiple cascade effects of oxidative stress on astroglia

Many neurodegenerative diseases share common underlying features, most prominent of which are dysregulation of calcium homeostasis and reactive astrogliosis, ultimately triggered by oxidative stress. Interestingly, an additional feature of the early response to stress conditions is the upregulation and release of acetylcholinesterase (AChE). This study therefore investigates the link between oxidative stress, calcium influx, gene expression, protein synthesis, and AChE release. We report that, in astroglia and in an immortalized cell line, GH4-halpha7, acute oxidative stress causes influx of extracellular calcium through L-type voltage-gated calcium channels (L-VGCC), followed by increased release of AChE into the extracellular medium. Moreover, rapid and sustained changes in mRNA expression of AChE, L-VGCC, and melastatin-like transient receptor potential 2 (TRPM2) accompany profound suppression of global protein synthesis. Application of L-VGCC blockers selectively reduces stress-induced calcium influx and AChE release, mitigates changes in gene expression, and facilitates recovery from protein synthesis suppression. Although glia exhibit greater sensitivity in their responses, the results are comparable in astroglia and GH4-halpha7 cells, and suggest a generalized and integrated cellular response to stress conditions that characterizes changes observed in neurodegeneration.

Reactive oxygen species as a signal in glucose-stimulated insulin secretion

One of the unique features of beta-cells is their relatively low expression of many antioxidant enzymes. This could render beta-cells susceptible to oxidative damage but may also provide a system that is sensitive to reactive oxygen species as signals. In isolated mouse islets and INS-1(832/13) cells, glucose increases intracellular accumulation of H2O2. In both models, insulin secretion could be stimulated by provision of either exogenous H2O2 or diethyl maleate, which raises intracellular H2O2 levels. Provision of exogenous H2O2 scavengers, including cell permeable catalase and N-acetyl-L-cysteine, inhibited glucose-stimulated H2O2 accumulation and insulin secretion (GSIS). In contrast, cell permeable superoxide dismutase, which metabolizes superoxide into H2O2, had no effect on GSIS. Because oxidative stress is an important risk factor for beta-cell dysfunction in diabetes, the relationship between glucose-induced H2O2 generation and GSIS was investigated under various oxidative stress conditions. Acute exposure of isolated mouse islets or INS-1(832/13) cells to oxidative stressors, including arsenite, 4-hydroxynonenal, and methylglyoxal, led to decreased GSIS. This impaired GSIS was associated with increases in a battery of endogenous antioxidant enzymes. Taken together, these findings suggest that H2O2 derived from glucose metabolism is one of the metabolic signals for insulin secretion, whereas oxidative stress may disturb its signaling function.

The transmembrane segment S6 determines cation versus anion selectivity of TRPM2 and TRPM8

TRPM2 and TRPM8, closely related members of the transient receptor potential (TRP) family, are cation channels activated by quite different mechanisms. Their transmembrane segments S5 and S6 are highly conserved. To identify common structures in S5 and S6 that govern interaction with the pore, we created a chimera in which the S5-pore-S6 region of TRPM8 was inserted into TRPM2, along with a lysine at each transition site. Currents through this chimera were induced by ADP-ribose (ADPR) in cooperation with Ca(2+). In contrast to wild-type TRPM2 channels, currents through the chimera were carried by Cl(-), as demonstrated in ion substitution experiments using the cation N-methyl-D-glucamine (NMDG) and the anion glutamate. Extracellular NMDG had no effects. The substitution of either intracellular or extracellular Cl(-) with glutamate shifted the reversal potential, decreased the current amplitude and induced a voltage-dependent block relieved by depolarization. The lysine in S6 was responsible for the anion selectivity; insertion of a lysine into corresponding sites within S6 of either TRPM2 or TRPM8 created anion channels that were activated by ADPR (TRPM2 I1045K) or by cold temperatures (TRPM8 V976K). The positive charge of the lysine was decisive for the glutamate block because the mutant TRPM2 I1045H displayed cation currents that were blocked at acidic but not alkaline intracellular pH values. We conclude that the distal part of S6 is crucial for the discrimination of charge. Because of the high homology of S6 in the whole TRP family, this new role of S6 may apply to further TRP channels.

Cyclic ADP-ribose as a universal calcium signal molecule in the nervous system

beta-NAD(+) is as abundant as ATP in neuronal cells. beta-NAD(+) functions not only as a coenzyme but also as a substrate. beta-NAD(+)-utilizing enzymes are involved in signal transduction. We focus on ADP-ribosyl cyclase/CD38 which synthesizes cyclic ADP-ribose (cADPR), a universal Ca(2+) mobilizer from intracellular stores, from beta-NAD(+). cADPR acts through activation/modulation of ryanodine receptor Ca(2+) releasing Ca(2+) channels. cADPR synthesis in neuronal cells is stimulated or modulated via different pathways and various factors. Subtype-specific coupling of various neurotransmitter receptors with ADP-ribosyl cyclase confirms the involvement of the enzyme in signal transduction in neurons and glial cells. Moreover, cADPR/CD38 is critical in oxytocin release from the hypothalamic cell dendrites and nerve terminals in the posterior pituitary. Therefore, it is possible that pharmacological manipulation of intracellular cADPR levels through ADP-ribosyl cyclase activity or synthetic cADPR analogues may provide new therapeutic opportunities for treatment of neurodevelopmental disorders.

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.

Hydrogen peroxide-induced Ca2+ mobilization in pulmonary arterial smooth muscle cells

Reactive oxygen species (ROS) generated from NADPH oxidases and mitochondria have been implicated as key messengers for pulmonary vasoconstriction and vascular remodeling induced by agonists and hypoxia. Since Ca2+ mobilization is essential for vasoconstriction and cell proliferation, we sought to characterize the Ca2+ response and to delineate the Ca2+ pathways activated by hydrogen peroxide (H2O2) in rat intralobar pulmonary arterial smooth muscle cells (PASMCs). Exogenous application of 10 μM to 1 mM H2O2 elicited concentration-dependent increase in intracellular Ca2+ concentration in PASMCs, with an initial rise followed by a plateau or slow secondary increase. The initial phase was related to intracellular release. It was attenuated by the inositol trisphosphate (IP3) receptor antagonist 2-aminoethyl diphenylborate, ryanodine, or thapsigargin, but was unaffected by the removal of Ca2+ in external solution. The secondary phase was dependent on extracellular Ca2+ influx. It was unaffected by the voltage-gated Ca2+ channel blocker nifedipine or the nonselective cation channel blockers SKF-96365 and La3+, but inhibited concentration dependently by millimolar Ni2+, and potentiated by the Na+/Ca2+ exchange inhibitor KB-R 7943. H2O2 did not alter the rate of Mn2+ quenching of fura 2, suggesting store- and receptor-operated Ca2+ channels were not involved. By contrast, H2O2 elicited a sustained inward current carried by Na+ at −70 mV, and the current was inhibited by Ni2+. These results suggest that H2O2 mobilizes intracellular Ca2+ through multiple pathways, including the IP3- and ryanodine receptor-gated Ca2+ stores, and Ni2+-sensitive cation channels. Activation of these Ca2+ pathways may play important roles in ROS signaling in PASMCs.

N-(p-Amylcinnamoyl)anthranilic Acid (ACA): A Phospholipase A2 Inhibitor and TRP Channel Blocker

Phospholipase A(2) enzymes display a superfamily of structurally different enzymes classified in at least nine subfamilies by biochemical and structural properties. N-(p-amylcinnamoyl)anthranilic acid commonly referred to as ACA is often used as a broad-spectrum inhibitor for the characterization of phospholipase A(2)-mediated pathways. Compounds like ACA and ACA-like structures have been described to block the receptor-induced release of arachidonic acid and subsequent signaling cascades in the pancreas and the cardiovascular system. We showed that ACA directly blocks several transient receptor potential (TRP) channels (TRPC6, TRPM2, TRP and TRPM8). With respect to the published data of ACA in the phospholipase A(2) field, the finding that ACA blocks diacylglycerol-activated TRP channels is of specific interest as it offers the opportunity to interfere with receptor-induced calcium-dependent signaling processes in platelets and vascular smooth muscle cells. Overall, N-phenylcinnamides, as a new pharmaceutical lead structure, form the first class of synthetic TRP channel blockers and represent a promising start for the development of small organic TRP channel-specific blockers.

Redox control of calcium channels: from mechanisms to therapeutic opportunities

Calcium plays an integral role in cellular function. It is a well-recognized second messenger necessary for signaling cellular responses, but in excessive amounts can be deleterious to function, causing cell death. The main route by which calcium enters the cytoplasm is either from the extracellular compartment or internal addistores via calcium channels. There is good evidence that calcium channels can respond to pharmacological compounds that reduce or oxidize thiol groups on the channel protein. In addition, reactive oxygen species such as hydrogen peroxide and superoxide that can mediate oxidative pathology also mediate changes in channel function via alterations of thiol groups. This review looks at the structure and function of calcium channels, the evidence that changes in cellular redox state mediate changes in channel function, and the role of redox modification of channels in disease processes. Understanding how redox modification of the channel protein alters channel structure and function is providing leads for the design of therapeutic interventions that target oxidative stress responses.

Transient receptor potential channels in Alzheimer's disease

Cognitive impairment and emotional disturbances in Alzheimer's disease (AD) result from the degeneration of synapses and neuronal death in the limbic system and associated regions of the cerebral cortex. An alteration in the proteolytic processing of the amyloid precursor protein (APP) results in increased production and accumulation of amyloid beta-peptide (Abeta) in the brain. Abeta can render neurons vulnerable to excitotoxicity and apoptosis by disruption of cellular Ca(2+) homeostasis and neurotoxic factors including reactive oxygen species (ROS), nitric oxide (NO), and cytokines. Many lines of evidence have suggested that transient receptor potential (TRP) channels consisting of six main subfamilies termed the TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin), TRPML (mucolipin), and TRPA (ankyrin) are involved in Ca(2+) homeostasis disruption. Thus, emerging evidence of the pathophysiological role of TRP channels has yielded promising candidates for molecular entities mediating Ca(2+) homeostasis disruption in AD. In this review, we focus on the TRP channels in AD and highlight some TRP "suspects" for which a role in AD can be anticipated. An understanding of the involvement of TRP channels in AD may lead to the development of new target therapies.

TRPM7 and TRPM2-Candidate susceptibility genes for Western Pacific ALS and PD?

Recent findings implicating TRPM7 and TRPM2 in oxidative stress-induced neuronal death thrust these channels into the spotlight as possible therapeutic targets for neurodegenerative diseases. In this review, we describe how the functional properties of TRPM7 and TRPM2 are interconnected with calcium (Ca(2+)) and magnesium (Mg(2+)) homeostasis, oxidative stress, mitochondrial dysfunction, and immune mechanisms, all principal suspects in neurodegeneration. We focus our discussion on Western Pacific Amyotrophic Lateral Sclerosis (ALS) and Parkinsonism Dementia (PD) because extensive studies conducted over the years strongly suggest that these diseases are ideal candidates for a gene-environment model of etiology. The unique mineral environment identified in connection with Western Pacific ALS and PD, low Mg(2+) and Ca(2+), yet high in transition metals, creates a condition that could affect the proper function of these two channels.

The human corneal endothelium: new insights into electrophysiology and ion channels

The corneal endothelium is a monolayer that mediates the flux of solutes and water across the posterior corneal surface. Thereby, it plays an essential role to maintain the transparency of the cornea. Unlike the epithelium, the human endothelium is an amitotic cell layer with a critical cell density and the risk of corneal decompensation. The number of endothelial cells subsequently decreases with age. Moreover, the endothelial cell loss is accelerated after various impairments such as surgical trauma (e.g. cataract extraction) and following corneal transplantation. This cell loss is associated with programmed cell death (apoptosis) and changed ion channel activity. However, little is known about the electrophysiology and ion channel expression (in particular Ca2+ channels) in corneal endothelial cells. This article reviews our current knowledge about the electrophysiology of the corneal endothelium. It highlights ion channel expression, which may have a major role in corneal cell physiology and pathological events. A better understanding of the (electro)physiological function of the cornea may lead to the development of clinical relevant new therapeutic and preventive measures.

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.

Intracellular coiled-coil domain engaged in subunit interaction and assembly of melastatin-related transient receptor potential channel 2

TRPM2 channels, activated by adenosine diphosphoribose and related molecules, are assembled as oligomers and most likely tetramers. However, the molecular determinants driving the subunit interaction and assembly of the TRPM2 channels are not well defined. Here we examined, using site-directed mutagenesis in conjunction with co-immunoprecipitation and patch clamp recording, the role of a coiled-coil domain in the intracellular C terminus of TRPM2 subunit in subunit interaction and channel assembly. Deletion of the coiled-coil domain resulted in severe disruption of the subunit interaction and substantial loss of the adenosine diphosphoribose-evoked channel currents. Individual or combined mutations to glutamine of the hydrophobic residues at positions a and d of the abcdef heptad repeat, key residues for protein-protein interaction, significantly reduced the subunit interaction and channel currents; the mutational effects on the subunit interaction and channel currents were clearly correlated. Furthermore, deletion of the coiled-coil domain in a pore mutant subunit abolished its dominant negative phenotypic functional suppression. These results provide strong evidence that the coiled-coil domain is critically engaged in the TRPM2 subunit interaction and such interaction is required for assembly of functional TRPM2 channel. The coiled-coil domain, which is highly conserved within the TRPM subfamily, may serve as a general structural element governing the assembly of TRPM channels.

Inhibition of TRPM2 cation channels by N-(p-amylcinnamoyl)anthranilic acid

1. TRPM2 is a Ca2+ -permeable nonselective cation channel activated by intracellular ADP-ribose (ADPR) and by hydrogen peroxide (H2O2). We investigated the modulation of TRPM2 activity by N-(p-amylcinnamoyl)anthranilic acid (ACA). ACA has previously been reported to inhibit phospholipase A2 (PLA2). 2. Using patch-clamp and calcium-imaging techniques, we show that extracellular application of 20 microM ACA completely blocked ADPR-induced whole-cell currents and H2O2-induced Ca2+ signals (IC50 = 1.7 microM) in HEK293 cells transfected with human TRPM2. Two other PLA2 inhibitors, p-bromophenacyl bromide (BPB; 100 microM) and arachidonyl trifluoromethyl ketone (20 microM), had no significant effect on ADPR-stimulated TRPM2 activity. 3. Inhibition of TRPM2 whole-cell currents by ACA was voltage independent and accelerated at decreased pH. ACA was ineffective when applied intracellularly. The single-channel conductance was not changed during ACA treatment, suggesting a reduction of TRPM2 open probability by modulating channel gating. 4. ACA (20 microM) also blocked currents through human TRPM8 and TRPC6 expressed in HEK293 cells, while BPB (100 microM) was ineffective. TRPC6-mediated currents (IC50 = 2.3 microM) and TRPM8-induced Ca2+ signals (IC50 = 3.9 microM) were blocked in a concentration-dependent manner. 5. ADPR-induced currents in human U937 cells, endogeneously expressing TRPM2 protein, were fully suppressed by 20 microM ACA. 6. Our data indicate that ACA modulates the activity of different TRP channels independent of PLA2 inhibition. Owing to its high potency and efficacy ACA can serve, in combination with other blockers, as a useful tool for studying the unknown function of TRPM2 in native cells.

Functional role of calcium signals for microglial function

In this review we summarize mechanisms of Ca(2+) signaling in microglial cells and the impact of Ca(2+) signaling and Ca(2+) levels on microglial function. So far, Ca(2+) signaling has been only characterized in cultured microglia and thus these data refer rather to activated microglia as observed in pathology when compared with the resting form found under physiological conditions. Purinergic receptors are the most prominently expressed ligand-gated Ca(2+)-permeable channels in microglia and control several microglial functions such as cytokine release in a Ca(2+)-dependent fashion. A large variety of metabotropic receptors are linked to Ca(2+) release from intracellular stores. Depletion of these intracellular stores triggers a capacitative Ca(2+) entry. While microglia are already in an activated state in culture, they can be further activated, for example, by exposure to bacterial endotoxin. This activation leads to a chronic increase of [Ca(2+)](i) and this Ca(2+) increase is a prerequisite for the release of nitric oxide and cytokines. Moreover, several factors (TNFalpha, IL-1beta, and IFN-gamma) regulate resting [Ca(2+)](i) levels.

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.

Cyclic ADP-ribose is a second messenger in the lipopolysaccharide-stimulated activation of murine N9 microglial cell line

Lipopolysaccharide, the main component of the cell wall of Gram-negative bacteria, is known to activate microglial cells following its interaction with the CD14/Toll-like receptor complex (TLR-4). The activation pathway triggered by lipopolysaccharide in microglia involves enhanced basal levels of intracellular calcium ([Ca2+]i) and terminates with increased generation of cytokines/chemokines and nitric oxide. Here we demonstrate that in lipopolysaccharide-stimulated murine N9 microglial cells, cyclic ADP-ribose, a universal and potent Ca2+ mobiliser generated from NAD+ by ADP-ribosyl cyclases (ADPRC), behaves as a second messenger in the cell activation pathway. Lipopolysaccharide induced phosphorylation, mediated by multiple protein kinases, of the mammalian ADPRC CD38, which resulted in significantly enhanced ADPRC activity and in a 1.7-fold increase in the concentration of intracellular cyclic ADP-ribose. This event was paralleled by doubling of the basal [Ca2+]i levels, which was largely prevented by the cyclic ADP-ribose antagonists 8-Br-cyclic ADP-ribose and ryanodine (by 75% and 88%, respectively). Both antagonists inhibited, although incompletely, functional events downstream of the lipopolysaccharide-induced microglia-activating pathway, i.e. expression of inducible nitric oxide synthase, overproduction and release of nitric oxide and of tumor necrosis factor alpha. The identification of cyclic ADP-ribose as a key signal metabolite in the complex cascade of events triggered by lipopolysaccharide and eventually leading to enhanced generation of pro-inflammatory molecules may suggest a new therapeutic target for treatment of neurodegenerative diseases related to microglia activation.

The dynamics of the LPS triggered inflammatory response of murine microglia under different culture and in vivo conditions

Overall, the inflammatory potential of lipopolysaccharide (LPS) in vitro and in vivo was investigated using different omics technologies. We investigated the hippocampal response to intracerebroventricular (i.c.v) LPS in vivo, at both the transcriptional and protein level. Here, a time course analysis of interleukin-6 (IL-6) and monocyte chemotactic protein-1 (MCP-1) showed a sharp peak at 4 h and a return to baseline at 16 h. The expression of inflammatory mediators was not temporally correlated with expression of the microglia marker F4/80, which did not peak until 2 days after LPS injection. Of 480 inflammation-related genes present on a microarray, 29 transcripts were robustly up-regulated and 90% of them were also detected in LPS stimulated primary microglia (PM) cultures. Further in vitro to in vivo comparison showed that the counter regulation response observed in vivo was less evident in vitro, as transcript levels in PM decreased relatively little over 16 h. This apparent deficiency of homeostatic control of the innate immune response in cultures may also explain why a group of genes comprising tnf receptor associated factor-1, endothelin-1 and schlafen-1 were regulated strongly in vitro, but not in vivo. When the overall LPS-induced transcriptional response of PM was examined on a large Affymetrix chip, chemokines and cytokines constituted the most strongly regulated and largest groups. Interesting new microglia markers included interferon-induced protein with tetratricopeptide repeat (ifit), immune responsive gene-1 (irg-1) and thymidylate kinase family LPS-inducible member (tyki). The regulation of the former two was confirmed on the protein level in a proteomics study. Furthermore, conspicuous regulation of several gene clusters was identified, for instance that of genes pertaining to the extra-cellular matrix and enzymatic regulation thereof. Although most inflammatory genes induced in vitro were transferable to our in vivo model, the observed discrepancy for some genes potentially represents regulatory factors present in the central nervous system (CNS) but not in vitro.

CCL5 evokes calcium signals in microglia through a kinase-, phosphoinositide-, and nucleotide-dependent mechanism

Microglia, the resident macrophages of the CNS, are responsible for the innate immune response in the brain and participate in the pathogenesis of certain neurodegenerative disorders. Chemokines initiate activation and migration of microglia. The beta-chemokine CCL5 induces an elevation in intracellular calcium concentration ([Ca(2+)](i)) in human microglia. Here, we examined the signal transduction pathway linking activation of chemokine receptor CCR5 to an elevation in [Ca(2+)](i) in cultured microglia by using pharmacological approaches in combination with Fura-2-based digital imaging. The CCL5-induced response required Janus kinase (Jak) activity and the stimulation of an inhibitory G protein. Multiple downstream signaling pathways were involved, including phosphatidylinositol 3-kinase (PI3K), Bruton's tyrosine kinase (Btk), and phospholipase C (PLC)-mediated release of Ca(2+) from inositol 1,4,5-trisphosphate (IP(3))-sensitive stores. Activation of both the kinase and the lipase pathways was required for eliciting the Ca(2+) response. However, the majority of the [Ca(2+)](i) increase was derived from sources activated by NAD metabolites. Cyclic ADP-ribose (cADPR) evoked Ca(2+) release from intracellular stores, and ADPR evoked Ca(2+) influx via a nimodipine-sensitive channel. Thus, a multistep cascade couples CCR5 activation to Ca(2+) increases in human microglia. Because changes in [Ca(2+)](i) affect chemotaxis, secretion, and gene expression, pharmacologic modulation of this pathway may alter inflammatory and degenerative processes in the CNS.

Surfactant Protein A Directly Interacts with TLR4 and MD-2 and Regulates Inflammatory Cellular Response

The purpose of the current study was to examine the binding of pulmonary surfactant protein A (SP-A) to TLR4 and MD-2, which are critical signaling receptors for lipopolysaccharides (LPSs). The direct binding of SP-A to the recombinant soluble form of extracellular TLR4 domain (sTLR4) and MD-2 was detected using solid-phase binding, immunoprecipitation, and BIAcore. SP-A bound to sTLR4 and MD-2 in a Ca2+-dependent manner, and an anti-SP-A monoclonal antibody whose epitope lies in the region Thr184-Gly194 blocked the SP-A binding to sTLR4 and MD-2, indicating the involvement of the carbohydrate recognition domain (CRD) in the binding. SP-A avidly bound to the deglycosylated forms of sTLR4 and MD-2, suggesting a protein/protein interaction. In addition, SP-A attenuated cell surface binding of smooth LPS and smooth LPS-induced NF-kappaB activation in TLR4/MD-2-expressing cells. To know the role of oligomerization in the interaction of SP-A with TLR4 and MD-2, the collagenase-resistant fragment (CRF), which consisted of CRD plus neck domain of SP-A, was isolated. CRF assembled as a trimer, whereas SP-A assembled as a higher order oligomer. Although CRD was suggested to be involved in the binding, CRF exhibited approximately 600- and 155-fold higher KD for the binding to TLR4 and MD-2, respectively, when compared with SP-A. Consistently significantly higher molar concentrations of CRF were required to inhibit smooth LPS-induced NF-kappaB activation and tumor necrosis factor-alpha secretion. These results demonstrate for the first time the direct interaction between SP-A and TLR4/MD-2 and suggest the importance of supratrimeric oligomerization in the immunomodulatory function of SP-A.

Conserved cysteine residues in the pore region are obligatory for human TRPM2 channel function

TRPM2 proteins belong to the melastatin-related transient receptor potential or TRPM subfamily and form Ca(2+)-permeable cationic channels activated by intracellular adenosine diphosphoribose (ADPR). The TRPM2 channel subunit, like all its close relatives, is structurally homologous to the well-characterized voltage-gated potassium channel subunits, each containing six transmembrane segments and a putative pore loop between the fifth and sixth segments. Nevertheless, the structural elements determining the TRPM2 channel functions are still not well understood. In this study, we investigated the functional role of two conserved cysteine residues (at positions 996 and 1008) in the putative pore region of the human TRPM2 by site-directed mutagenesis, combined with electrophysiological and biochemical approaches. Expression of wild-type hTRPM2 channels in human embryonic kidney (HEK-293) cells resulted in robust ADPR-evoked currents. Substitution of cysteine with alanine or serine generated mutant channels that failed to be activated by ADPR. Furthermore, experiments done by Western blot analysis, immunocytochemistry, biotin labeling, and coimmunoprecipitation techniques showed no obvious changes in protein expression, trafficking or membrane localization, and the ability to interact with neighboring subunits that is required for channel assembly. Coexpression of wild-type and mutant subunits significantly reduced the ADPR-evoked currents; for the combination of wild-type and C996S mutant subunits, the reduction was approximately 95%, indicating that incorporation of one or more nonfunctional C996S subunits leads to the loss of channel function. These results taken together suggest that the cysteine residues in the pore region are obligatory for TRPM2 channel function.

The role of TRP channels in oxidative stress-induced cell death

The transient receptor potential (TRP) protein superfamily is a diverse group of voltage-independent calcium-permeable cation channels expressed in mammalian cells. These channels have been divided into six subfamilies, and two of them, TRPC and TRPM, have members that are widely expressed and activated by oxidative stress. TRPC3 and TRPC4 are activated by oxidants, which induce Na(+) and Ca(2+) entry into cells through mechanisms that are dependent on phospholipase C. TRPM2 is activated by oxidative stress or TNFalpha, and the mechanism involves production of ADP-ribose, which binds to an ADP-ribose binding cleft in the TRPM2 C-terminus. Treatment of HEK 293T cells expressing TRPM2 with H(2)O(2) resulted in Ca(2+) influx and increased susceptibility to cell death, whereas coexpression of the dominant negative isoform TRPM2-S suppressed H(2)O(2)-induced Ca(2+) influx, the increase in [Ca(2+)](i), and onset of apoptosis. U937-ecoR monocytic cells expressing increased levels of TRPM2 also exhibited significantly increased [Ca(2+)](i) and increased apoptosis after treatment with H(2)O(2) or TNFalpha. A dramatic increase in caspase 8, 9, 3, 7, and PARP cleavage was observed in TRPM2-expressing cells, demonstrating a downstream mechanism through which cell death is mediated. Inhibition of endogenous TRPM2 function through three approaches, depletion of TRPM2 by RNA interference, blockade of the increase in [Ca(2+)](i) through TRPM2 by calcium chelation, or expression of the dominant negative splice variant TRPM2-S protected cell viability. H(2)O(2) and amyloid beta-peptide also induced cell death in primary cultures of rat striatal cells, which endogenously express TRPM2. TRPM7 is activated by reactive oxygen species/nitrogen species, resulting in cation conductance and anoxic neuronal cell death, which is rescued by suppression of TRPM7 expression. TRPM2 and TRPM7 channels are physiologically important in oxidative stress-induced cell death.

TRPM2 activation by cyclic ADP-ribose at body temperature is involved in insulin secretion

There are eight thermosensitive TRP (transient receptor potential) channels in mammals, and there might be other TRP channels sensitive to temperature stimuli. Here, we demonstrate that TRPM2 can be activated by exposure to warm temperatures (>35 degrees C) apparently via direct heat-evoked channel gating. beta-NAD(+)- or ADP-ribose-evoked TRPM2 activity is robustly potentiated at elevated temperatures. We also show that, even though cyclic ADP-ribose (cADPR) does not activate TRPM2 at 25 degrees C, co-application of heat and intracellular cADPR dramatically potentiates TRPM2 activity. Heat and cADPR evoke similar responses in rat insulinoma RIN-5F cells, which express TRPM2 endogenously. In pancreatic islets, TRPM2 is coexpressed with insulin, and mild heating of these cells evokes increases in both cytosolic Ca(2+) and insulin release, which is K(ATP) channel-independent and protein kinase A-mediated. Heat-evoked responses in both RIN-5F cells and pancreatic islets are significantly diminished by treatment with TRPM2-specific siRNA. These results identify TRPM2 as a potential molecular target for cADPR, and suggest that TRPM2 regulates Ca(2+) entry into pancreatic beta-cells at body temperature depending on the production of cADPR-related molecules, thereby regulating insulin secretion.

TRPM2 activation by cyclic ADP-ribose at body temperature is involved in insulin secretion

There are eight thermosensitive TRP (transient receptor potential) channels in mammals, and there might be other TRP channels sensitive to temperature stimuli. Here, we demonstrate that TRPM2 can be activated by exposure to warm temperatures (>35 degrees C) apparently via direct heat-evoked channel gating. beta-NAD(+)- or ADP-ribose-evoked TRPM2 activity is robustly potentiated at elevated temperatures. We also show that, even though cyclic ADP-ribose (cADPR) does not activate TRPM2 at 25 degrees C, co-application of heat and intracellular cADPR dramatically potentiates TRPM2 activity. Heat and cADPR evoke similar responses in rat insulinoma RIN-5F cells, which express TRPM2 endogenously. In pancreatic islets, TRPM2 is coexpressed with insulin, and mild heating of these cells evokes increases in both cytosolic Ca(2+) and insulin release, which is K(ATP) channel-independent and protein kinase A-mediated. Heat-evoked responses in both RIN-5F cells and pancreatic islets are significantly diminished by treatment with TRPM2-specific siRNA. These results identify TRPM2 as a potential molecular target for cADPR, and suggest that TRPM2 regulates Ca(2+) entry into pancreatic beta-cells at body temperature depending on the production of cADPR-related molecules, thereby regulating insulin secretion.

Nicotinic acid adenine dinucleotide phosphate and cyclic ADP-ribose regulate TRPM2 channels in T lymphocytes

TRPM2 (previously designated TRPC7 or LTRPC2) is a Ca2+-permeable nonselective cation channel that contains a C-terminal enzymatic domain with pyrophosphatase activity, which specifically binds ADP-ribose. Cyclic ADP-ribose (cADPR) and hydrogen peroxide (H2O2) can facilitate ADPR-mediated activation of heterologously expressed TRPM2. Here, we show that the two Ca2+-mobilizing second messengers cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) strongly activate natively expressed TRPM2 channels in Jurkat T cells. TRPM2 activation by both agonists can be partially suppressed by the ADPR antagonist adenosine monophosphate (AMP), which suggests that cADPR and NAADP lead to mobilization of endogenous ADPR presumably via metabolic conversion. The remaining channel activity is due to direct gating of TRPM2 by the two agonists and can be completely suppressed by 8-Br-cADPR, which suggests that cADPR and NAADP share a common binding site on TRPM2 that can regulate TRPM2 activity in synergy with ADPR. We conclude that cADPR and NAADP, in combination with ADPR, represent physiological co-activators of TRPM2 that contribute to Ca2+ influx in T lymphocytes and presumably other cell types that express this channel.

Metabolite of SIR2 reaction modulates TRPM2 ion channel

The transient receptor potential melastatin-related channel 2 (TRPM2) is a nonselective cation channel, whose prolonged activation by oxidative and nitrative agents leads to cell death. Here, we show that the drug puromycin selectively targets TRPM2-expressing cells, leading to cell death. Our data suggest that the silent information regulator 2 (Sir2 or sirtuin) family of enzymes mediates this susceptibility to cell death. Sirtuins are protein deacetylases that regulate gene expression, apoptosis, metabolism, and aging. These NAD+-dependent enzymes catalyze a reaction in which the acetyl group from substrate is transferred to the ADP-ribose portion of NAD+ to form deacetylated product, nicotinamide, and the metabolite OAADPr, whose functions remain elusive. Using cell-based assays and RNA interference, we show that puromycin-induced cell death is greatly diminished by nicotinamide (a potent sirtuin inhibitor), and by decreased expression of sirtuins SIRT2 and SIRT3. Furthermore, we demonstrate using channel current recordings and binding assays that OAADPr directly binds to the cytoplasmic domain of TRPM2 and activates the TRPM2 channel. ADP-ribose binds TRPM2 with similarly affinity, whereas NAD+ displays almost negligible binding. These studies provide the first evidence for the potential role of sirtuin-generated OAADPr in TRPM2 channel gating.

Critical role of phospholipase Cgamma1 in the generation of H2O2-evoked [Ca2+]i oscillations in cultured rat cortical astrocytes

Reactive oxygen species, such as the superoxide anion, H2O2, and the hydroxyl radical, have been considered as cytotoxic by-products of cellular metabolism. However, recent studies have provided evidence that H2O2 serves as a signaling molecule modulating various physiological functions. Here we investigated the effect of H2O2 on the regulation of intracellular Ca2+ signaling in rat cortical astrocytes. H2O2 triggered the generation of oscillations of intracellular Ca2+ concentration ([Ca2+]i) in a concentration-dependent manner over the range 10-100 microM. The H2O2-induced [Ca2+]i oscillations persisted in the absence of extracellular Ca2+ and were prevented by depletion of intracellular Ca2+ stores with thapsigargin. The H2O2-induced [Ca2+]i oscillations were not inhibited by pretreatment with ryanodine but were prevented by 2-aminoethoxydiphenyl borate and caffeine, known antagonists of inositol 1,4,5-trisphosphate receptors. H2O2 activated phospholipase C (PLC) gamma1 in a dose-dependent manner, and U73122, an inhibitor of PLC, completely abolished the H2O2-induced [Ca2+]i oscillations. In addition, RNA interference against PLCgamma1 and the expression of the inositol 1,4,5-trisphosphate-sequestering "sponge" prevented the generation of [Ca2+]i oscillations. H2O2-induced [Ca2+]i oscillations and PLC1 phosphorylation were inhibited by pretreatment with dithiothreitol, a sulfhydryl-reducing agent. Finally, epidermal growth factor induced H2O2 production, PLCgamma1 activation, and [Ca2+]i increases, which were attenuated by N-acetylcysteine and diphenyleneiodonium and by the overexpression of peroxiredoxin type II. Therefore, we conclude that low concentrations of exogenously applied H2O2 generate [Ca2+]i oscillations by activating PLCgamma1 through sulfhydryl oxidation-dependent mechanisms. Furthermore, we show that this mechanism underlies the modulatory effect of endogenously produced H2O2 on epidermal growth factor-induced Ca2+ signaling in rat cortical astrocytes.

Pulmonary Lipopolysaccharide (LPS)-Binding Protein Inhibits the LPS-Induced Lung Inflammation In Vivo

LPS-binding protein (LBP) facilitates the interaction of the Gram-negative cell wall component LPS with CD14, thereby enhancing the immune response to LPS. Although lung epithelial cells have been reported to produce LBP in vitro, knowledge of the in vivo role of pulmonary LBP is limited. Therefore, in the present study we sought to determine the function of pulmonary LBP in lung inflammation induced by intranasal administration of LPS in vivo. Using LBP-deficient (LBP-/-) and normal wild-type mice, we show that the contribution of LBP to pulmonary LPS responsiveness depended entirely on the LPS dose. Although the inflammatory response to low dose (1 ng) LPS was attenuated in LBP-/- mice, neutrophil influx and cytokine/chemokine concentrations in the bronchoalveolar compartment were enhanced in LBP-/- mice treated with higher (>10 ng) LPS doses. This finding was specific for LBP, because the exogenous administration of LBP to LBP-/- mice reversed this phenotype and reduced the local inflammatory response to higher LPS doses. Our results indicate that pulmonary LBP acts as an important modulator of the LPS response in the respiratory tract in vivo. This newly identified function of pulmonary LBP might prove beneficial by enabling a protective immune response to low LPS doses while preventing an overwhelming, potentially harmful immune response to higher doses of LPS.

Regulation of the transient receptor potential channel TRPM2 by the Ca2+ sensor calmodulin

TRPM2, a member of the transient receptor potential (TRP) superfamily, is a Ca(2+)-permeable channel activated by oxidative stress or tumor necrosis factoralpha involved in susceptibility to cell death. TRPM2 activation is dependent on the level of intracellular Ca(2+). We explored whether calmodulin (CaM) is the Ca(2+) sensor for TRPM2. HEK 293T cells were transfected with TRPM2 and wild type CaM or mutant CaM (CaM(MUT)) with substitutions of all four EF hands. Treatment of cells expressing TRPM2 with H(2)O(2) or tumor necrosis factor alpha resulted in a significant increase in intracellular calcium ([Ca(2+)](i)). This was not affected by coexpression of CaM, suggesting that endogenous CaM levels are sufficient for maximal response. Cotransfection of CaM(MUT) with TRPM2 dramatically inhibited the increase in [Ca(2+)](i), demonstrating the requirement for CaM in TRPM2 activation. Immunoprecipitation confirmed direct interaction of CaM and CaM(MUT) with TRPM2, and the Ca(2+) dependence of this association. CaM bound strongly to the TRPM2 N terminus (amino acids 1-730), but weakly to the C terminus (amino acids 1060-1503). CaM binding to an IQ-like motif (amino acids 406-416) in the TRPM2 N terminus was demonstrated utilizing gel shift, immunoprecipitation, biotinylated CaM overlay, and pull-down assays. A substitution mutant of the IQ-like motif of TRPM2 (TRPM2-IQ(MUT1)) reduced but did not eliminate CaM binding to TRPM2, suggesting the presence of at least one other CaM binding site. The functional importance of the TRPM2 IQ-like motif was demonstrated by treatment of TRPM2-IQ(MUT1)-expressing cells with H(2)O(2). The increase in [Ca(2+)](i) observed with wild type TRPM2 was absent and cell viability was preserved. These data demonstrate the requirement for CaM in TRPM2 activation. They suggest that Ca(2+) entering through TRPM2 enhances interaction of CaM with TRPM2 at the IQ-like motif in the N terminus, providing crucial positive feedback for channel activation.

Calcium, mitochondria and oxidative stress in neuronal pathology. Novel aspects of an enduring theme

The interplay among reactive oxygen species (ROS) formation, elevated intracellular calcium concentration and mitochondrial demise is a recurring theme in research focusing on brain pathology, both for acute and chronic neurodegenerative states. However, causality, extent of contribution or the sequence of these events prior to cell death is not yet firmly established. Here we review the role of the alpha-ketoglutarate dehydrogenase complex as a newly identified source of mitochondrial ROS production. Furthermore, based on contemporary reports we examine novel concepts as potential mediators of neuronal injury connecting mitochondria, increased [Ca2+]c and ROS/reactive nitrogen species (RNS) formation; specifically: (a) the possibility that plasmalemmal nonselective cationic channels contribute to the latent [Ca2+]c rise in the context of glutamate-induced delayed calcium deregulation; (b) the likelihood of the involvement of the channels in the phenomenon of 'Ca2+ paradox' that might be implicated in ischemia/reperfusion injury; and (c) how ROS/RNS and mitochondrial status could influence the activity of these channels leading to loss of ionic homeostasis and cell death.

A Critical Role of TRPM2 in Neuronal Cell Death by Hydrogen Peroxide

A brief exposure to hydrogen peroxide (H2O2) induces severe deterioration of primary cultured neurons in vitro. We have investigated a link between the H2O2-induced neuronal death and Ca2+-permeable TRPM2 channels regulated by ADP-ribose (ADPR). In cultured cerebral cortical neurons from fetal rat, TRPM2 proteins were detected at cell bodies and neurite extensions. Application of H2O2 to the cultured neurons elicited an increase in intracellular Ca2+ concentration ([Ca2+]i) caused by Ca2+ influx and the Ca2+-dependent neuronal death in a similar concentration range. Molecular cloning of TRPM2 cDNA from rat brain revealed several differences in amino acid sequences within the Nudix box region as compared with those of human and mouse TRPM2. ADPR-induced current responses, H2O2-induced Ca2+ influx, and H2O2-induced cell death were induced in human embryonic kidney cells heterologously expressing rat TRPM2. Treatment of cultured neurons with small interfering RNA against rat TRPM2,which efficiently suppressed immunoreactive TRPM2 content and the H2O2-induced Ca2+ influx,significantly inhibited H2O2-induced neuronal death. These results suggest that TRPM2 plays a pivotal role in H2O2-induced neuronal death as redox-sensitive Ca2+-permeable channels expressed in neurons.

Zinc and the cytoskeleton in the neuronal modulation of transcription factor NFAT

Transcription factor NFAT is crucial in the development of the nervous system due to its role in neuronal plasticity and survival. In this study we characterized the role of zinc and the cytoskeleton in the modulation of NFAT in neuronal cells. The incubation of cells in zinc deficient media led to NFAT activation that was inhibited by the calcium chelator BAPTA and the antioxidants (+/-)-alpha-lipoic acid and N-acetyl cysteine, suggesting the involvement of calcium and oxidants in the initial steps of NFAT activation associated with zinc deficiency. At a second step of regulation, a decrease in cellular zinc led to an impaired transport of the active NFAT from the cytosol into the nucleus due to alterations in tubulin polymerization secondary to a decrease in neuronal zinc. Furthermore, disruption of the cytoskeleton structure by cold and chemical agents (colchicine (Col), vinblastine (VB), cytochalasin D (Cyt)) also inhibited NFAT transport into the nucleus. The altered nuclear transport caused a decrease in NFAT-dependent gene expression. This study demonstrates for the first time that zinc can modulate transcription factor NFAT in neuronal cells, and that microtubules are involved in NFAT nuclear translocation, crucial event in the regulation of NFAT transcriptional activity.

TRPM2 is an ion channel that modulates hematopoietic cell death through activation of caspases and PARP cleavage

TRPM2 is a Ca(2+)-permeable channel activated by oxidative stress or TNF-alpha, and TRPM2 activation confers susceptibility to cell death. The mechanisms were examined here in human monocytic U937-ecoR cells. This cell line expresses full-length TRPM2 (TRPM2-L) and several isoforms including a short splice variant lacking the Ca(2+)-permeable pore region (TRPM2-S), which functions as a dominant negative. Treatment with H(2)O(2), a model of oxidative stress, or TNF-alpha results in reduced cell viability. Expression of TRPM2-L and TRPM2-S was modulated by retroviral infection. U937-ecoR cells expressing increased levels of TRPM2-L were treated with H(2)O(2) or TNF-alpha, and these cells exhibited significantly increased intracellular calcium concentration ([Ca(2+)](i)), decreased viability, and increased apoptosis. A dramatic increase in cleavage of caspases-8, -9, -3, and -7 and poly(ADP-ribose)polymerase (PARP) was observed, demonstrating a downstream mechanism through which cell death is mediated. Bcl-2 levels were unchanged. Inhibition of the [Ca(2+)](i) rise with the intracellular Ca(2+) chelator BAPTA blocked caspase/PARP cleavage and cell death induced after activation of TRPM2-L, demonstrating the critical role of [Ca(2+)](i) in mediating these effects. Downregulation of endogenous TRPM2 by RNA interference or increased expression of TRPM2-S inhibited the rise in [Ca(2+)](i), enhanced cell viability, and reduced numbers of apoptotic cells after exposure to oxidative stress or TNF-alpha, demonstrating the physiological importance of TRPM2. Our data show that one mechanism through which oxidative stress or TNF-alpha mediates cell death is activation of TRPM2, resulting in increased [Ca(2+)](i), followed by caspase activation and PARP cleavage. Inhibition of TRPM2-L function by reduction in TRPM2 levels, interaction with TRPM2-S, or Ca(2+) chelation antagonizes this important cell death pathway.

Regulation of microglial behavior by ion channel activity

Microglia play an important role in the central nervous system, where these cells, it is believed, have both neuroprotective and neurotoxic effects. In response to acute brain injury or during neurodegenerative and neuroinflammatory diseases, activated microglial cells undergo shape changes, migrate to the affected sites of neuronal damage, proliferate, and release a variety of substances, such as cytokines and reactive oxygen species (ROS). This review summarizes the physiological mechanisms underlying microglial activation and deactivation processes, with particular focus on the involvement of microglial ion channels. Microglial ion channels have been shown to be capable, by regulating membrane potential, cell volume, and intracellular ion concentrations, of modulating or facilitating proliferation, migration, cytokine secretion, shape changes, and the respiratory burst of microglial cells.

Characterisation of recombinant rat TRPM2 and a TRPM2-like conductance in cultured rat striatal neurones

TRPM2, a member of the TRP ion channel family, is expressed both in the brain and immune cells of the monocyte lineage. Functionally, it is unique in its activation by intracellular ADP-ribose and both oxidative and nitrosative stress. To date studies of this channel have concentrated on human recombinant channels and rodent native preparations. This provides the potential for cross-species complications in the interpretation of native tissue observations based on recombinant channel phenotype. Consequently, we have cloned and heterologously expressed rat TRPM2 (rTRPM2) in HEK293 cells. We find that, like hTRPM2, it responds to intracellular ADP-ribose in a manner dependent on extracellular Ca(2+). At the single channel level rTRPM2 is a slow gating, large conductance (84pS) channel that rapidly runs down in isolated membrane patches. Pharmacologically, rTRPM2 is rapidly and irreversibly blocked by clotrimazole (10muM), thus resembling hTRPM2 but not the TRPM2-like current of the rat-derived insulinoma CRI-G1, which exhibits reversible inhibition by this agent. We show that cultured rat striatal neurones exhibit an ADP-ribose-activated conductance at both the whole cell and single channel level. Pharmacologically this neuronal current can be irreversibly inhibited by clotrimazole. It is also sensitive to removal of extracellular Ca(2+), suggesting that it is mediated by TRPM2-containing channels. These data provide a functional characterisation of heterologously expressed rTRPM2 and demonstrate that, in addition to the previous descriptions in immune cells, microglia and insulinomas, a TRPM2-like conductance can be found in neurones derived from the rodent CNS.

Identification of the surfactant protein A receptor 210 as the unconventional myosin 18A

Mass spectrometric characterization of the surfactant protein A (SP-A) receptor 210 (SP-R210) led to the identification of myosin (Myo) XVIIIA and nonmuscle myosin IIA. Antibodies generated against the unique C-terminal tail of MyoXVIIIA revealed that MyoXVIIIA, MyoIIA, and SP-R210 have overlapping tissue distribution, all being highly expressed in myeloid cells, bone marrow, spleen, lymph nodes, and lung. Western blot analysis of COS-1 cells stably transfected with either MyoXVIIIA or MyoIIA indicated that SP-R210 antibodies recognize MyoXVIIIA. Furthermore, MyoXVIIIA but not MyoIIA localized to the surface of COS-1 cells, and most importantly, expression of MyoXVIIIA in COS-1 cells conferred SP-A binding. Western analysis of recombinant MyoXVIIIA domains expressed in bacteria mapped the epitopes of previously derived SP-R210 antibodies to the neck region of MyoXVIIIA. Antibodies raised against the neck domain of MyoXVIIIA blocked the binding of SP-A to macrophages. Together, these findings indicate that MyoXVIIIA constitutes a novel receptor for SP-A.

Oxidative and antioxidative potential of brain microglial cells

Microglial cells are the resident immune cells of the central nervous system. These cells defend the central nervous system against invading microorganisms and clear the debris from damaged cells. Upon activation, microglial cells produce a large number of neuroactive substances that include cytokines, proteases, and prostanoids. In addition, activated microglial cells release radicals, such as superoxide and nitric oxide, that are products of the enzymes NADPH oxidase and inducible nitric oxide synthase, respectively. Microglia-derived radicals, as well as their reactive reaction products hydrogen peroxide and peroxynitrite, have the potential to harm cells and have been implicated in contributing to oxidative damage and neuronal cell death in neurological diseases. For self-protection against oxidative damage, microglial cells are equipped with efficient antioxidative defense mechanisms. These cells contain glutathione in high concentrations, substantial activities of the antioxidative enzymes superoxide dismutase, catalase, glutathione peroxidase, and glutathione reductase, as well as NADPH-regenerating enzymes. Their good antioxidative potential protects microglial cells against oxidative damage that could impair important functions of these cells in defense and repair of the brain.

Amyloid beta-peptide(1-42) and hydrogen peroxide-induced toxicity are mediated by TRPM2 in rat primary striatal cultures

Amyloid beta-peptide (Abeta) is the main component of senile plaques which characterize Alzheimer's disease and may induce neuronal death through mechanisms which include oxidative stress. To date, the signalling pathways linking oxidant stress, a component of several neurodegenerative diseases, to cell death in the CNS are poorly understood. Melastatin-like transient receptor potential 2 (TRPM2) is a Ca(2+)-permeant non-selective cation channel, which responds to increases in oxidative stress levels in the cell and is activated by oxidants such as hydrogen peroxide. We demonstrate here that Abeta and hydrogen peroxide both induce death in cultured rat striatal cells which express TRPM2 endogenously. Transfection with a splice variant that acts as a dominant negative blocker of TRPM2 function (TRPM2-S) inhibited both hydrogen peroxide- and Abeta-induced increases in intracellular-free Ca(2+) and cell death. Functional inhibition of TRPM2 activation by the poly(ADP-ribose)polymerase inhibitor SB-750139, a modulator of intracellular pathways activating TRPM2, attenuated hydrogen peroxide- and Abeta-induced cell death. Furthermore, a small interfering RNA which targets TRPM2, reduced TRPM2 mRNA levels and the toxicity induced by hydrogen peroxide and Abeta. These data demonstrate that activation of TRPM2, functionally expressed in primary cultures of rat striatum, contributes to Abeta- and oxidative stress-induced striatal cell death.