Reduced glutathione inhibits beta-NAD+-activated non-selective cation currents in the CRI-G1 rat insulin-secreting cell line

Ophthalmic acid is a glutathione regulating tripeptide

Since its discovery in 1958 in the lens of cows, ophthalmic acid (OPH) has stood in the shadow of its anti-oxidant analog: glutathione (GSH). Lacking the thiol group that gives GSH many of its important properties, ophthalmic acid's function has remained elusive, and it has been widely presumed to be an accidental product of the same enzymes. In this review, we compile evidence demonstrating that OPH is a ubiquitous metabolite found in bacteria, plants, fungi, and animals, produced through several layers of metabolic regulation. We discuss the limitations of the oft-repeated suggestions that aberrations in OPH levels should solely indicate GSH deficiency or oxidative stress. Finally, we discuss the available literature and suggest OPH's role in metabolism as a GSH-regulating tripeptide; controlling both cellular and organelle influx and efflux of GSH, as well as modulating GSH-dependent reactions and signaling. Ultimately, we hope that this review reinvigorates and directs more research into this versatile metabolite.

Production of Ophthalmic Acid Using Engineered Escherichia coli

Ophthalmic acid (OA; l-γ-glutamyl-l-2-aminobutyryl-glycine) is an analog of glutathione (GSH; l-γ-glutamyl-l-cysteinyl-glycine) in which the cysteine moiety is replaced by l-2-aminobutyrate. OA is a useful peptide for the pharmaceutical and/or food industries. Herein, we report a method for the production of OA using engineered Escherichia coli cells. yggS-deficient E. coli, which lacks the highly conserved pyridoxal 5'-phosphate-binding protein YggS and naturally accumulates OA, was selected as the starting strain. To increase the production of OA, we overexpressed the OA biosynthetic enzymes glutamate-cysteine ligase (GshA) and glutathione synthase (GshB), desensitized the product inhibition of GshA, and eliminated the OA catabolic enzyme γ-glutamyltranspeptidase. The production of OA was further enhanced by the deletion of miaA and ridA with the aim of increasing the availability of ATP and attenuating the unwanted degradation of amino acids, respectively. The final strain developed in this study successfully produced 277 μmol/liter of OA in 24 h without the formation of by-products in a minimal synthetic medium containing 1 mM each glutamate, 2-aminobutyrate, and glycine.IMPORTANCE Ophthalmic acid (OA) is a peptide that has the potential for use in the pharmaceutical and/or food industries. An efficient method for the production of OA would allow us to expand our knowledge about its physiological functions and enable the industrial/pharmaceutical application of this compound. We demonstrated the production of OA using Escherichia coli cells in which OA biosynthetic enzymes and degradation enymes were engineered. We also showed that unique approaches, including the use of a ΔyggS mutant as a starting strain, the establishment of an S495F mutation in GshA, and the deletion of ridA or miaA, facilitated the efficient production of OA in E. coli.

TRPM2

TRPM2 is the second member of the transient receptor potential melastatin-related (TRPM) family of cation channels. The protein is widely expressed including in the brain, immune system, endocrine cells, and endothelia. It embodies both ion channel functionality and enzymatic ADP-ribose (ADPr) hydrolase activity. TRPM2 is a Ca(2+)-permeable nonselective cation channel embedded in the plasma membrane and/or lysosomal compartments that is primarily activated in a synergistic fashion by intracellular ADP-ribose (ADPr) and Ca(2+). It is also activated by reactive oxygen and nitrogen species (ROS/NOS) and enhanced by additional factors, such as cyclic ADPr and NAADP, while inhibited by permeating protons (acidic pH) and adenosine monophosphate (AMP). Activation of TRPM2 leads to increases in intracellular Ca(2+) levels, which can serve signaling roles in inflammatory and secretory cells through release of vesicular mediators (e.g., cytokines, neurotransmitters, insulin) and in extreme cases can induce apoptotic and necrotic cell death under oxidative stress.

Androgen and PARP-1 regulation of TRPM2 channels after ischemic injury

The calcium-permeable transient receptor potential M2 (TRPM2) ion channel was recently demonstrated to have a sexually dimorphic contribution to ischemic brain injury, with inhibition or knockdown of the channel protecting male brain preferentially. We tested the hypothesis that androgen signaling is required for this male-specific cell-death pathway. Additionally, we tested the hypothesis that differential activation of the enzyme poly (ADP-ribose) polymerase-1 (PARP-1) is responsible for male-specific TRPM2 channel activation and neuronal injury. We observed that administration of the TRPM2 inhibitor clotrimazole (CTZ) 2 hours after onset of ischemia reduced infarct volume in male mice and that protection from ischemic damage by CTZ was abolished by removal of testicular androgens (castration; CAST) and rescued by androgen replacement. Male PARP-1 knockout mice had reduced ischemic damage compared with WT mice and inhibition of TRPM2 with CTZ failed to reduce infarct size. Lastly, we observed that ischemia increased PARP activity in the peri-infarct region of male mice to a greater extent than female mice and the difference was abolished in CAST male mice. Data presented in the current study indicate that TRPM2-mediated neuronal death in the male brain requires intact androgen signaling and PARP-1 activity.

Loss of glutathione homeostasis associated with neuronal senescence facilitates TRPM2 channel activation in cultured hippocampal pyramidal neurons

Background

Glutathione (GSH) plays an important role in neuronal oxidant defence. Depletion of cellular GSH is observed in neurodegenerative diseases and thereby contributes to the associated oxidative stress and Ca²⁺ dysregulation. Whether depletion of cellular GSH, associated with neuronal senescence, directly influences Ca²⁺ permeation pathways is not known. Transient receptor potential melastatin type 2 (TRPM2) is a Ca²⁺ permeable non-selective cation channel expressed in several cell types including hippocampal pyramidal neurons. Moreover, activation of TRPM2 during oxidative stress has been linked to cell death. Importantly, GSH has been reported to inhibit TRPM2 channels, suggesting they may directly contribute to Ca²⁺ dysregulation associated with neuronal senescence. Herein, we explore the relation between cellular GSH and TRPM2 channel activity in long-term cultures of hippocampal neurons.

Results

In whole-cell voltage-clamp recordings, we observe that TRPM2 current density increases in cultured pyramidal neurons over time in vitro. The observed increase in current density was prevented by treatment with NAC, a precursor to GSH synthesis. Conversely, treatment of cultures maintained for 2 weeks in vitro with L-BSO, which depletes GSH by inhibiting its synthesis, augments TRPM2 currents. Additionally, we demonstrate that GSH inhibits TRPM2 currents through a thiol-independent mechanism, and produces a 3.5-fold shift in the dose-response curve generated by ADPR, the intracellular agonist for TRPM2.

Conclusion

These results indicate that GSH plays a physiologically relevant role in the regulation of TRPM2 currents in hippocampal pyramidal neurons. This interaction may play an important role in aging and neurological diseases associated with depletion of GSH.

TRPM2 channel properties, functions and therapeutic potentials

IMPORTANCE OF THE FIELD

Oxidative stress, through production of reactive oxygen species, triggers disturbance in intracellular calcium [Ca(2+)](i) homeostasis, which has been identified as an important causative factor in the pathogenesis of numerous inflammatory, cardiovascular and neurodegenerative diseases.

AREAS COVERED IN THIS REVIEW

Transient receptor potential melastatin 2 (TRPM2) protein forms a Ca(2+)-permeable cationic channel that is activated in response to oxidative stress and therefore acts as a cellular redox sensor. Research over the years has substantially advanced the knowledge of expression and functional properties of the TRPM2 channel, and particularly has accumulated compelling evidence for an important role for TRPM2 channel-mediated extracellular Ca(2+) influx in several physiological and pathophysiological functions exemplified by insulin release from pancreatic beta-cells, production of pro-inflammatory cytokines from immune cells, increased endothelial permeability, microglia activation and cell death. These findings suggest therapeutic potential of the TRPM2 channel as a drug target for combating oxidative-stress-related diseases.

WHAT THE READER WILL GAIN

The current state of knowledge with respect to the TRPM2 channel properties and the roles in oxidant stress signalling and functions.

TAKE HOME MESSAGE

TRPM2 may be a novel therapeutic target for oxidative stress-related diseases.

TRP channels of the pancreatic beta cell

Orchestrated ion fluctuations within pancreatic islets regulate hormone secretion and maybe essential to processes such as apoptosis. A diverse set of ion channels allows for islet cells to respond to a variety of signals and dynamically regulate hormone secretion and glucose homeostasis (reviewed by Houamed et al. 2004). This chapter focuses on transient receptor potential (TRP)-related channels found within the beta cells of the islet and reviews their roles in both insulin secretion and apoptosis.

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.

The role of TRPM channels in cell death

Transient receptor potential (TRP) channels of the melastatin-like family (TRPM) play critical roles in mediating cellular responses to a wide range of physiological stimuli that, under certain situations, can induce cell death. To date, two TRPM family members, TRPM2 and TRPM7, have been implicated directly as central components of cell death pathways. TRPM2, a Ca(2+)-permeant, non-selective cation channel, senses and responds to oxidative stress levels in the cell. TRPM7 is required for cell viability and has been proposed recently to mediate stress-induced cell death in the central nervous system. We review here the evidence for the involvement of these TRPM channels in cell death processes and discuss the mechanisms by which TRPM channel activation occurs. The ability to attenuate expression levels and functionality of these channels is necessary to understand the involvement of TRPM in cell death and we evaluate current approaches for modulation of TRPM channel function. Finally, we discuss the possibility that TRPM channels may provide therapeutic targets for degenerative diseases involving oxidative stress-related pathologies including diabetes and Alzheimer's disease.

Hydrogen-peroxide-induced toxicity of rat striatal neurones involves activation of a non-selective cation channel

Striatal neurones are particularly vulnerable to hypoxia/ischaemia-induced damage, and free radicals are thought to be prime mediators of this neuronal destruction. It has been shown that hydrogen peroxide (H2O2), through the production of free radicals, induces rat insulinoma cell death by activation of a non-selective cation channel, which leads to irreversible cell depolarization and unregulated Ca2+ entry into the cell. In the study presented here, we demonstrate that a subpopulation of striatal neurones (medium spiny neurones) is depolarized by H2O2 through the production of free radicals. Cell-attached recordings from rat cultured striatal neurones demonstrate that exposure to H2O2 opens a large-conductance channel that is characterized by extremely long open times (seconds). Inside-out recordings show that cytoplasmically applied beta-nicotinamide adenine dinucleotide activates a channel with little voltage dependence, a linear current-voltage relationship and a single-channel conductance of between 70 and 90 pS. This channel is permeable to Na+, K+ and Ca2+ ions. Fura-2 imaging from cultured striatal neurones reveals that H2O2 exposure induces a biphasic intracellular Ca2+ increase in a subpopulation of neurones, the second, later phase resulting in Ca2+ overload. This later component of the Ca2+ response is dependent on the presence of extracellular Ca2+ and is independent of synaptic activity or voltage-gated Ca2+ channel opening. Consequently, this channel may be an important contributor to free radical-induced selective striatal neurone destruction. These results are remarkably similar to those observed for insulinoma cells and suggest that this family of non-selective cation channels has a widespread distribution in mammalian tissues.

Effects of cyanide and deoxyglucose on Ca2+ signalling in macrovascular endothelial cells

1. We have studied the effects of the metabolic inhibitors cyanide (CN) and deoxyglucose (DG) on the intracellular Ca2+ concentration ([Ca2+]i) in macrovascular endothelial cells derived from human umbilical vein (EA cells). 2. CN- and DG increased [Ca2-]i in non-voltage clamped cells. This effect was dependent on extracellular Ca2+ concentration and membrane potential, indicating that CN- induced a Ca2+ entry. 3. During expose to CN- and/or DG, EA cells depolarise. This depolarisation is sometimes preceded by a small, but transient hyperpolarisation due to activation of a big - conductance K+ channels, BKCa, present in EA cells. However, in approximately 90% of the cells tested, the CN- and/or DG induced elevation of [Ca2+]i was insufficient to activate BKCa. 4. CN- and/or DG enhanced BKCa currents preactivated by an elevation of [Ca2+]i via cell dialysis with 0.5 and 1 microM, respectively. Thus, metabolic inhibition sensitises BKCa. 5. The CN- induced depolarisation of EA cells occurs by activating a current that reversed at positive membrane potentials. Substituting extracellular cations abolished the inward component of this current by NMDG, indicating that CN- activated a non-selective cation channel, NSC. This current was reduced by extracellular Ca2+ and Mg2'+ but is partially carried by Ca2+. 6. It is concluded that CN elevates [Ca2+]i by activating Ca2+ permeable NSC channels. The properties of these channels are similar to those of the recently described trp3 channels expressed in endothelium.

Nitric oxide and thiol reagent modulation of Ca2+-activated K+ (BKCa) channels in myocytes of the guinea-pig taenia caeci

The modulation of large conductance Ca2+-activated K+ (BKCa) channels by the nitric oxide (NO) donors S-nitroso-L-cysteine (NOCys) and sodium nitroprusside (SNP) and agents which oxidize or reduce reactive thiol groups were compared in excised inside-out membrane patches of the guinea-pig taenia caeci. When the cytosolic side of excised patches was bathed in a physiological salt solution (PSS) containing 130 mM K+ and 15 nM Ca2+, few BKCa channel openings were recorded at potentials negative to 0 mV. However, the current amplitude and open probability (NPo) of these BKCa channels increased with patch depolarization. A plot of ln(NPo) against the membrane potential (V) fitted with a straight line revealed a voltage at half-maximal activation (V0.5) of 9.4 mV and a slope (K) indicating an e-fold increase in NPo with 12.9 mV depolarization. As the cytosolic Ca2+ was raised to 150 nM, V0.5 shifted 11.5 mV in the negative direction, with little change in K (13.1 mV). NOCys (10 microM) and SNP (100 microM) transiently increased NPo 16- and 3. 7-fold, respectively, after a delay of 2-5 min. This increase in NPo was associated with an increase in the number of BKCa channel openings evoked at positive potentials by ramped depolarizations (between -60 and +60 mV). Moreover, this NOCys-induced increase in NPo was still evident in the presence of 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ; 10 microM), the specific blocker of soluble guanylyl cyclase. The sulfhydryl reducing agents dithiothreitol (DTT; 10 and 100 microM) and reduced glutathione (GSH; 1 mM) also significantly increased NPo (at 0 mV) 7- to 9-fold, as well as increasing the number of BKCa channel openings evoked during ramped depolarizations. Sulfhydryl oxidizing agents thimerosal (10 microM) and 4,4'-dithiodipyridine (4,4DTDP; 10 microM) and the thiol-specific alkylating agent N-ethylmaleimide (NEM; 1 mM) significantly decreased NPo (at 0 mV) to 40-50% of control values after 5-10 min. Ramped depolarizations to +100 mV evoked relatively few BKCa channel openings. The effects of thimerosal on NPo were readily reversed by DTT, while the effects of NOCys were prevented by NEM. It was concluded that both redox modulation and nitrothiosylation of cysteine groups on the cytosolic surface of the alpha subunit of the BKCa channel protein can alter channel gating.