L-cysteine, a bicarbonate-sensitive endogenous excitotoxin

Hypoxia-inducible factor induces cysteine dioxygenase and promotes cysteine homeostasis in Caenorhabditis elegans

Dedicated genetic pathways regulate cysteine homeostasis. For example, high levels of cysteine activate cysteine dioxygenase, a key enzyme in cysteine catabolism in most animal and many fungal species. The mechanism by which cysteine dioxygenase is regulated is largely unknown. In an unbiased genetic screen for mutations that activate cysteine dioxygenase (cdo-1) in the nematode Caenorhabditis elegans, we isolated loss-of-function mutations in rhy-1 and egl-9, which encode proteins that negatively regulate the stability or activity of the oxygen-sensing hypoxia inducible transcription factor (hif-1). EGL-9 and HIF-1 are core members of the conserved eukaryotic hypoxia response. However, we demonstrate that the mechanism of HIF-1-mediated induction of cdo-1 is largely independent of EGL-9 prolyl hydroxylase activity and the von Hippel-Lindau E3 ubiquitin ligase, the classical hypoxia signaling pathway components. We demonstrate that C. elegans cdo-1 is transcriptionally activated by high levels of cysteine and hif-1. hif-1-dependent activation of cdo-1 occurs downstream of an H2S-sensing pathway that includes rhy-1, cysl-1, and egl-9. cdo-1 transcription is primarily activated in the hypodermis where it is also sufficient to drive sulfur amino acid metabolism. Thus, the regulation of cdo-1 by hif-1 reveals a negative feedback loop that maintains cysteine homeostasis. High levels of cysteine stimulate the production of an H2S signal. H2S then acts through the rhy-1/cysl-1/egl-9 signaling pathway to increase HIF-1-mediated transcription of cdo-1, promoting degradation of cysteine via CDO-1.

Hypoxia-inducible factor induces cysteine dioxygenase and promotes cysteine homeostasis in Caenorhabditis elegans

Dedicated genetic pathways regulate cysteine homeostasis. For example, high levels of cysteine activate cysteine dioxygenase, a key enzyme in cysteine catabolism in most animal and many fungal species. The mechanism by which cysteine dioxygenase is regulated is largely unknown. In an unbiased genetic screen for mutations that activate cysteine dioxygenase (cdo-1) in the nematode C. elegans, we isolated loss-of-function mutations in rhy-1 and egl-9, which encode proteins that negatively regulate the stability or activity of the oxygen-sensing hypoxia inducible transcription factor (hif-1). EGL-9 and HIF-1 are core members of the conserved eukaryotic hypoxia response. However, we demonstrate that the mechanism of HIF-1-mediated induction of cdo-1 is largely independent of EGL-9 prolyl hydroxylase activity and the von Hippel-Lindau E3 ubiquitin ligase, the classical hypoxia signaling pathway components. We demonstrate that C. elegans cdo-1 is transcriptionally activated by high levels of cysteine and hif-1. hif-1-dependent activation of cdo-1 occurs downstream of an H2S-sensing pathway that includes rhy-1, cysl-1, and egl-9. cdo-1 transcription is primarily activated in the hypodermis where it is also sufficient to drive sulfur amino acid metabolism. Thus, the regulation of cdo-1 by hif-1 reveals a negative feedback loop that maintains cysteine homeostasis. High levels of cysteine stimulate the production of an H2S signal. H2S then acts through the rhy-1/cysl-1/egl-9 signaling pathway to increase HIF-1-mediated transcription of cdo-1, promoting degradation of cysteine via CDO-1.

Neurosteroids as stress modulators and neurotherapeutics: lessons from the retina

Neurosteroids are rapidly emerging as important new therapies in neuropsychiatry, with one such agent, brexanolone, already approved for treatment of postpartum depression, and others on the horizon. These steroids have unique properties, including neuroprotective effects that could benefit a wide range of brain illnesses including depression, anxiety, epilepsy, and neurodegeneration. Over the past 25 years, our group has developed ex vivo rodent models to examine factors contributing to several forms of neurodegeneration in the retina. In the course of this work, we have developed a model of acute closed angle glaucoma that involves incubation of ex vivo retinas under hyperbaric conditions and results in neuronal and axonal changes that mimic glaucoma. We have used this model to determine neuroprotective mechanisms that could have therapeutic implications. In particular, we have focused on the role of both endogenous and exogenous neurosteroids in modulating the effects of acute high pressure. Endogenous allopregnanolone, a major stress-activated neurosteroid in the brain and retina, helps to prevent severe pressure-induced retinal excitotoxicity but is unable to protect against degenerative changes in ganglion cells and their axons under hyperbaric conditions. However, exogenous allopregnanolone, at a pharmacological concentration, completely preserves retinal structure and does so by combined effects on gamma-aminobutyric acid type A receptors and stimulation of the cellular process of macroautophagy. Surprisingly, the enantiomer of allopregnanolone, which is inactive at gamma-aminobutyric acid type A receptors, is equally retinoprotective and acts primarily via autophagy. Both enantiomers are also equally effective in preserving retinal structure and function in an in vivo glaucoma model. These studies in the retina have important implications for the ongoing development of allopregnanolone and other neurosteroids as therapeutics for neuropsychiatric illnesses.

Non-Proteinogenic Amino Acid β-N-Methylamino-L-Alanine (BMAA): Bioactivity and Ecological Significance

Research interest in a non-protein amino acid β-N-methylamino-L-alanine (BMAA) arose due to the discovery of a connection between exposure to BMAA and the occurrence of neurodegenerative diseases. Previous reviews on this topic either considered BMAA as a risk factor for neurodegenerative diseases or focused on the problems of detecting BMAA in various environmental samples. Our review is devoted to a wide range of fundamental biological problems related to BMAA, including the molecular mechanisms of biological activity of BMAA and the complex relationships between producers of BMAA and the environment in various natural ecosystems. At the beginning, we briefly recall the most important facts about the producers of BMAA (cyanobacteria, microalgae, and bacteria), the pathways of BMAA biosynthesis, and reliable methods of identification of BMAA. The main distinctive feature of our review is a detailed examination of the molecular mechanisms underlying the toxicity of BMAA to living cells. A brand new aspect, not previously discussed in any reviews, is the effect of BMAA on cyanobacterial cells. These recent studies, conducted using transcriptomics and proteomics, revealed potent regulatory effects of BMAA on the basic metabolism and cell development of these ancient photoautotrophic prokaryotes. Exogenous BMAA strongly influences cell differentiation and primary metabolic processes in cyanobacteria, such as nitrogen fixation, photosynthesis, carbon fixation, and various biosynthetic processes involving 2-oxoglutarate and glutamate. Cyanobacteria were found to be more sensitive to exogenous BMAA under nitrogen-limited growth conditions. We suggest a hypothesis that this toxic diaminoacid can be used by phytoplankton organisms as a possible allelopathic tool for controlling the population of cyanobacterial cells during a period of intense competition for nitrogen and other resources in various ecosystems.

Assessment of Neurodegenerative Changes in Turkeys Fed Diets with Different Proportions of Arginine and Methionine Relative to Lysine

Simple Summary

It is important to take care of a properly balanced amino acid composition in the diet in order to inhibit or delay the occurrence of processes and changes related to the destruction of nervous tissue. Therefore, an attempt was made in this manuscript to evaluate the effect of different ratios of the key amino acids arginine and methionine, relative to lysine, in relation to two turkey feeding standards. The amino acid guidelines formulated by British United Turkeys (BUT) suggest higher levels of lysine (Lys) in turkey diets than those recommended by the National Research Council (NRC). In order to assess the impact of such supplementation, we analyzed the level of indicators informing the presence or degree of advancement of neurodegenerative processes in the nervous tissue (the level of acetylcholinesterase and amyloid-β; the concentration of AChE complexes with amyloid-β and Tau protein, called glycosylated acetylcholinesterase (GAChE), indicative of the destruction of neurons). The level of low-density lipoprotein receptor-related protein 1, or LRP-1, which facilitates the breakdown of toxic amyloid-β, was also assessed. In addition, the effect of different doses of these amino acids on neurodegenerative changes in DNA, especially the degree of methylation of histone proteins resulting from covalent modifications was compared between lysine and arginine residues.

Abstract

We postulated that the use of optimal levels and proportions of Arg and Met relative to a low or high concentration of Lys in diets for meat turkeys would reduce the occurrence of metabolic disturbances in the nervous tissue that can lead to neurodegenerative changes. The aim of the study was to determine the effect of various proportions of Lys, Arg, and Met in diets for turkeys, with a low content of Lys in accordance with NRC (Experiment 1) recommendations, and in diets with high Lys levels that are close to the recommendations of breeding companies (Experiment 2) on selected indicators of potential neurodegenerative effects in the brain and liver of turkeys. The Experiment 1 and Experiment 2 was conducted using 864 day-old turkey chicks randomly assigned to six groups, in eight replicates (6 groups × 18 birds × 8 replicates). A full description of the methodology can be found in previously published papers using the same experimental design. Indicators informing about the presence or advancement of neurodegenerative processes in the nervous tissue were determined in the brain and liver (level of: AChE, amyloid-β, GAChE, Tau protein, LRP1, and the degree of DNA methylation). It was established that in the case of both a low (National Research Council, NRC) and a high (British United Turkeys, BUT) level of Lys in the diet of turkeys, the Arg level can be reduced to 90% of the Lys level and Met to 30% of the Lys level, because this does not cause neurodegenerative changes in turkeys. Unfavorable neurodegenerative changes may appear if the Arg level is increased from 100 to 110% of the Lys level recommended by the NRC. However, due to the lack of such a relationship when Arg is increased from 100 to 110% of the Lys level recommended by BUT, at this stage of research no definitive conclusions can be drawn regarding the risk of neurodegenerative changes caused by increasing Arg in the diet of turkeys.

Physiological roles of hydrogen sulfide in mammalian cells, tissues and organs

H2S belongs to the class of molecules known as gasotransmitters, which also includes nitric oxide (NO) and carbon monoxide (CO). Three enzymes are recognized as endogenous sources of H2S in various cells and tissues: cystathionine g-lyase (CSE), cystathionine β-synthase (CBS) and 3-mercaptopyruvate sulfurtransferase (3-MST). The current article reviews the regulation of these enzymes as well as the pathways of their enzymatic and non-enzymatic degradation and elimination. The multiple interactions of H2S with other labile endogenous molecules (e.g. NO) and reactive oxygen species are also outlined. The various biological targets and signaling pathways are discussed, with special reference to H2S and oxidative posttranscriptional modification of proteins, the effect of H2S on channels and intracellular second messenger pathways, the regulation of gene transcription and translation and the regulation of cellular bioenergetics and metabolism. The pharmacological and molecular tools currently available to study H2S physiology are also reviewed, including their utility and limitations. In subsequent sections, the role of H2S in the regulation of various physiological and cellular functions is reviewed. The physiological role of H2S in various cell types and organ systems are overviewed. Finally, the role of H2S in the regulation of various organ functions is discussed as well as the characteristic bell-shaped biphasic effects of H2S. In addition, key pathophysiological aspects, debated areas, and future research and translational areas are identified A wide array of significant roles of H2S in the physiological regulation of all organ functions emerges from this review.

Long-Term Dynamic Changes of NMDA Receptors Following an Excitotoxic Challenge

Excitotoxicity is a form of neuronal death characterized by the sustained activation of N-methyl-D-aspartate receptors (NMDARs) triggered by the excitatory neurotransmitter glutamate. NADPH-diaphorase neurons (also known as nNOS (+) neurons) are a subpopulation of aspiny interneurons, largely spared following excitotoxic challenges. Unlike nNOS (−) cells, nNOS (+) neurons fail to generate reactive oxygen species in response to NMDAR activation, a critical divergent step in the excitotoxic cascade. However, additional mechanisms underlying the reduced vulnerability of nNOS (+) neurons to NMDAR-driven neuronal death have not been explored. Using functional, genetic, and molecular analysis in striatal cultures, we indicate that nNOS (+) neurons possess distinct NMDAR properties. These specific features are primarily driven by the peculiar redox milieu of this subpopulation. In addition, we found that nNOS (+) neurons exposed to a pharmacological maneuver set to mimic chronic excitotoxicity alter their responses to NMDAR-mediated challenges. These findings suggest the presence of mechanisms providing long-term dynamic regulation of NMDARs that can have critical implications in neurotoxic settings.

N-Acetylcysteine Induces Apoptotic, Oxidative and Excitotoxic Neuronal Death in Mouse Cortical Cultures

N-acetylcysteine (NAC) has been used as an antioxidant to prevent oxidative cell death. However, we found NAC itself to induce neuronal death in mouse cortical cultures. Therefore, the current study was performed to investigate the mechanism of neuronal death caused by NAC. Cell death was assessed by measuring lactate dehydrogenase efflux to bathing media after 24-48 h exposure to NAC. NAC (0.1-10 mM) induced neuronal death in a concentration- and exposure time-dependent manner. However, NAC did not injure astrocytes even at a concentration of 10 mM. Also, 10 mM NAC markedly attenuated oxidative astrocyte death induced by 0.5 mM diethyl maleate or 0.25 mM H₂O₂. The NMDA receptor antagonist MK-801 (10 µM) markedly attenuated the neuronal death caused by 10 mM NAC, while NBQX did not affect the neuronal death. Cycloheximide (a protein synthesis inhibitor, 0.1 µg/mL) and z-VAD-FMK (a caspase inhibitor, 100 µM) also significantly attenuated neuronal death. Apoptotic features such as chromatin condensation, nuclear fragmentation, and caspase 3 activation were observed 1 h after the NAC treatment. The neuronal death induced by 1 or 10 mM NAC was significantly attenuated by the treatment with 100 µM Trolox or 1 mM ascorbic acid. NAC induced the generation of intracellular reactive oxygen species (ROS), as measured by the fluorescent dye 2′,7′-dichlorofluorescein diacetate. The ROS generation was almost completely abolished by treatment with Trolox or ascorbic acid. These findings demonstrate that NAC can cause oxidative, apoptotic, and excitotoxic neuronal death in mouse neuronal cultures.

A sequentially activated bioluminescent probe for observation of cellular H2O2 production induced by cysteine

We report herein a caged luciferin probe Cy-Hy as a sequentially activated probe to selectively and sensitively sense L-Cys and H₂O₂. The probe displayed fluorescence and bioluminescence responses toward the two analytes. Utilizing the present probe, cellular excess L-Cys-induced H₂O₂ up-regulation was observed for the first time in living MDA-MB-231 cells.

Drug delivery platforms for neonatal brain injury

Hypoxic-ischemic encephalopathy (HIE), initiated by the interruption of oxygenated blood supply to the brain, is a leading cause of death and lifelong disability in newborns. The pathogenesis of HIE involves a complex interplay of excitotoxicity, inflammation, and oxidative stress that results in acute to long term brain damage and functional impairments. Therapeutic hypothermia is the only approved treatment for HIE but has limited effectiveness for moderate to severe brain damage; thus, pharmacological intervention is explored as an adjunct therapy to hypothermia to further promote recovery. However, the limited bioavailability and the side-effects of systemic administration are factors that hinder the use of the candidate pharmacological agents. To overcome these barriers, therapeutic molecules may be packaged into nanoscale constructs to enable their delivery. Yet, the application of nanotechnology in infants is not well examined, and the neonatal brain presents unique challenges. Novel drug delivery platforms have the potential to magnify therapeutic effects in the damaged brain, mitigate side-effects associated with high systemic doses, and evade mechanisms that remove the drugs from circulation. Encouraging pre-clinical data demonstrates an attenuation of brain damage and increased structural and functional recovery. This review surveys the current progress in drug delivery for treating neonatal brain injury.

The Role of Amino Acids in Neurotransmission and Fluorescent Tools for Their Detection

Neurotransmission between neurons, which can occur over the span of a few milliseconds, relies on the controlled release of small molecule neurotransmitters, many of which are amino acids. Fluorescence imaging provides the necessary speed to follow these events and has emerged as a powerful technique for investigating neurotransmission. In this review, we highlight some of the roles of the 20 canonical amino acids, GABA and β-alanine in neurotransmission. We also discuss available fluorescence-based probes for amino acids that have been shown to be compatible for live cell imaging, namely those based on synthetic dyes, nanostructures (quantum dots and nanotubes), and genetically encoded components. We aim to provide tool developers with information that may guide future engineering efforts and tool users with information regarding existing indicators to facilitate studies of amino acid dynamics.

Cysteine Toxicity Drives Age-Related Mitochondrial Decline by Altering Iron Homeostasis

Mitochondria and lysosomes are functionally linked, and their interdependent decline is a hallmark of aging and disease. Despite the long-standing connection between these organelles, the function(s) of lysosomes required to sustain mitochondrial health remains unclear. Here, working in yeast, we show that the lysosome-like vacuole maintains mitochondrial respiration by spatially compartmentalizing amino acids. Defects in vacuole function result in a breakdown in intracellular amino acid homeostasis, which drives age-related mitochondrial decline. Among amino acids, we find that cysteine is most toxic for mitochondria and show that elevated non-vacuolar cysteine impairs mitochondrial respiration by limiting intracellular iron availability through an oxidant-based mechanism. Cysteine depletion or iron supplementation restores mitochondrial health in vacuole-impaired cells and prevents mitochondrial decline during aging. These results demonstrate that cysteine toxicity is a major driver of age-related mitochondrial deterioration and identify vacuolar amino acid compartmentation as a cellular strategy to minimize amino acid toxicity.

Scalable synthesis and validation of PAMAM dendrimer-N-acetyl cysteine conjugate for potential translation

Dendrimer-N-acetyl cysteine (D-NAC) conjugate has shown significant promise in multiple preclinical models of brain injury and is undergoing clinical translation. D-NAC is a generation-4 hydroxyl-polyamidoamine dendrimer conjugate where N-acetyl cysteine (NAC) is covalently bound through disulfide linkages on the surface of the dendrimer. It has shown remarkable potential to selectively target and deliver NAC to activated microglia and astrocytes at the site of brain injury in several animal models, producing remarkable improvements in neurological outcomes at a fraction of the free drug dose. Here we present a highly efficient, scalable, greener, well-defined route to the synthesis of D-NAC, and validate the structure, stability and activity to define the benchmarks for this compound. This newly developed synthetic route has significantly reduced the synthesis time from three weeks to one week, uses industry-friendly solvents/reagents, and involves simple purification procedures, potentially enabling efficient scale up.

Could dietary glutamate be contributing to the symptoms of obsessive-compulsive disorder?

A 50-year-old man who had suffered from daily obsessive-compulsive disorder (OCD) symptoms for 39 years, in addition to fibromyalgia and irritable bowel syndrome symptoms, was enrolled in a randomized double-blind placebo-controlled clinical trial to test the effects of a low-glutamate diet on fibromyalgia/irritable bowel syndrome symptoms. After 1 month on the low-glutamate diet all of his symptoms remitted, including his OCD, which had previously been nonresponsive to pharmacological treatment. This case study is limited by self-report of symptoms; however, glutamatergic neurotransmission appears to be dysregulated in OCD, suggesting biological plausibility for this observation. Future research is needed.

An iron-oxygen intermediate formed during the catalytic cycle of cysteine dioxygenase

Combined spectroscopic, kinetic and computational studies provide first evidence of a short-lived intermediate in the catalytic cycle of cysteine dioxygenase.

Cysteine dioxygenase is a key enzyme in the breakdown of cysteine, but its mechanism remains controversial. A combination of spectroscopic and computational studies provides the first evidence of a short-lived intermediate in the catalytic cycle. The intermediate decays within 20 ms and has absorption maxima at 500 and 640 nm.

On the Dynamical Behavior of the Cysteine Dioxygenase-l-Cysteine Complex in the Presence of Free Dioxygen and l-Cysteine

In this work, viable models of cysteine dioxygenase (CDO) and its complex with l-cysteine dianion were built for the first time, under strict adherence to the crystal structure from X-ray diffraction studies, for all atom molecular dynamics (MD). Based on the CHARMM36 FF, the active site, featuring an octahedral dummy Fe(II) model, allowed us observing water exchange, which would have escaped attention with the more popular bonded models. Free dioxygen (O₂ ) and l-cysteine, added at the active site, could be observed being expelled toward the solvating medium under Random Accelerated Molecular Dynamics (RAMD) along major and minor pathways. Correspondingly, free dioxygen (O₂ ), added to the solvating medium, could be observed to follow the same above pathways in getting to the active site under unbiased MD. For the bulky l-cysteine, 600 ns of trajectory were insufficient for protein penetration, and the molecule was stuck at the protein borders. These models pave the way to free energy studies of ligand associations, devised to better clarify how this cardinal enzyme behaves in human metabolism.

Thiol trapping and metabolic redistribution of sulfur metabolites enable cells to overcome cysteine overload

Cysteine is an essential requirement in living organisms. However, due to its reactive thiol side chain, elevated levels of intracellular cysteine can be toxic and therefore need to be rapidly eliminated from the cellular milieu. In mammals and many other organisms, excess cysteine is believed to be primarily eliminated by the cysteine dioxygenase dependent oxidative degradation of cysteine, followed by the removal of the oxidative products. However, other mechanisms of tackling excess cysteine are also likely to exist, but have not thus far been explored. In this study, we use Saccharomyces cerevisiae, which naturally lacks a cysteine dioxygenase, to investigate mechanisms for tackling cysteine overload. Overexpressing the high affinity cysteine transporter, YCT1, enabled yeast cells to rapidly accumulate high levels of intracellular cysteine. Using targeted metabolite analysis, we observe that cysteine is initially rapidly interconverted to non-reactive cystine in vivo. A time course revealed that cells systematically convert excess cysteine to inert thiol forms; initially to cystine, and subsequently to cystathionine, S-Adenosyl-L-homocysteine (SAH) and S-Adenosyl L-methionine (SAM), in addition to eventually accumulating glutathione (GSH) and polyamines. Microarray based gene expression studies revealed the upregulation of arginine/ornithine biosynthesis a few hours after the cysteine overload, and suggest that the non-toxic, non-reactive thiol based metabolic products are eventually utilized for amino acid and polyamine biogenesis, thereby enabling cell growth. Thus, cells can handle potentially toxic amounts of cysteine by a combination of thiol trapping, metabolic redistribution to non-reactive thiols and subsequent consumption for anabolism.

Glutathione in the human brain: Review of its roles and measurement by magnetic resonance spectroscopy

We review the transport, synthesis and catabolism of glutathione in the brain as well as its compartmentation and biochemistry in different brain cells. The major reactions involving glutathione are reviewed and the factors limiting its availability in brain cells are discussed. We also describe and critique current methods for measuring glutathione in the human brain using magnetic resonance spectroscopy, and review the literature on glutathione measurements in healthy brains and in neurological, psychiatric, neurodegenerative and neurodevelopmental conditions In summary: Healthy human brain glutathione concentration is ∼1-2 mM, but it varies by brain region, with evidence of gender differences and age effects; in neurological disease glutathione appears reduced in multiple sclerosis, motor neurone disease and epilepsy, while being increased in meningiomas; in psychiatric disease the picture is complex and confounded by methodological differences, regional effects, length of disease and drug-treatment. Both increases and decreases in glutathione have been reported in depression and schizophrenia. In Alzheimer's disease and mild cognitive impairment there is evidence for a decrease in glutathione compared to age-matched healthy controls. Improved methods to measure glutathione in vivo will provide better precision in glutathione determination and help resolve the complex biochemistry of this molecule in health and disease.

Equilibrium Dynamics of β-N-Methylamino-L-Alanine (BMAA) and Its Carbamate Adducts at Physiological Conditions

Elevated incidences of Amyotrophic Lateral Sclerosis/Parkinsonism Dementia complex (ALS/PDC) is associated with β-methylamino-L-alanine (BMAA), a non-protein amino acid. In particular, the native Chamorro people living in the island of Guam were exposed to BMAA by consuming a diet based on the cycad seeds. Carbamylated forms of BMAA are glutamate analogues. The mechanism of neurotoxicity of the BMAA is not completely understood, and BMAA acting as a glutamate receptor agonist may lead to excitotoxicity that interferes with glutamate transport systems. Though the interaction of BMAA with bicarbonate is known to produce carbamate adducts, here we demonstrate that BMAA and its primary and secondary adducts coexist in solution and undergoes a chemical exchange among them. Furthermore, we determined the rates of formation/cleavage of the carbamate adducts under equilibrium conditions using two-dimensional proton exchange NMR spectroscopy (EXSY). The coexistence of the multiple forms of BMAA at physiological conditions adds to the complexity of the mechanisms by which BMAA functions as a neurotoxin.

Structure-Based Insights into the Role of the Cys-Tyr Crosslink and Inhibitor Recognition by Mammalian Cysteine Dioxygenase

In mammals, the non-heme iron enzyme cysteine dioxygenase (CDO) helps regulate Cys levels through converting Cys to Cys sulfinic acid. Its activity is in part modulated by the formation of a Cys93-Tyr157 crosslink that increases its catalytic efficiency over 10-fold. Here, 21 high-resolution mammalian CDO structures are used to gain insight into how the Cys-Tyr crosslink promotes activity and how select competitive inhibitors bind. Crystal structures of crosslink-deficient C93A and Y157F variants reveal similar ~1.0-Å shifts in the side chain of residue 157, and both variant structures have a new chloride ion coordinating the active site iron. Cys binding is also different from wild-type CDO, and no Cys-persulfenate forms in the C93A or Y157F active sites at pH6.2 or 8.0. We conclude that the crosslink enhances activity by positioning the Tyr157 hydroxyl to enable proper Cys binding, proper oxygen binding, and optimal chemistry. In addition, structures are presented for homocysteine (Hcy), D-Cys, thiosulfate, and azide bound as competitive inhibitors. The observed binding modes of Hcy and D-Cys clarify why they are not substrates, and the binding of azide shows that in contrast to what has been proposed, it does not bind in these crystals as a superoxide mimic.

A novel pathway for the production of H2 S by DAO in rat jejunum

BACKGROUND

Hydrogen sulfide (H2 S) is endogenously generated from L-cysteine (L-Cys) by the enzymes cystathionine-β-synthase (CBS) and cystathionine-γ-Lyase (CSE). Hydrogen sulfide is also produced from D-cysteine (D-Cys) by D-Amino acid oxidase (DAO).

METHODS

The H2 S production was measured by the methylene blue assay. The expression of DAO was investigated by Western blotting and immunohistochemistry. The short-circuit current (Isc) was recorded using the Ussing chamber technique.

KEY RESULTS

The epithelium in rat jejunum possesses DAO, and generates H2 S. D-cysteine, originally used as a negative control for L-Cys, significantly increases the H2 S release, which is inhibited by I2CA, an inhibitor of DAO. In vitro study by Ussing chamber technique reveals that D-Cys decreases the Isc across the epithelium of the rat jejunum and enhances the Na(+) -coupled L-alanine transport.

CONCLUSIONS & INFERENCES

A novel pathway for the production of H2 S by DAO exists in rat jejunum.

Excitotoxicity as a Common Mechanism for Fetal Neuronal Injury with Hypoxia and Intrauterine Inflammation

Excitotoxicity is a mechanism of neuronal injury, implicated in the pathogenesis of many acute and chronic neurologic disorders, including perinatal brain injury associated with hypoxia-ischemia and exposure to intrauterine inflammation. Glutamate, the primary excitatory neurotransmitter, signals through N-methyl-d-aspartic acid (NMDA)/α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors. Proper functioning of both of these receptors, in conjunction with glutamate signaling, is crucial for normal development. However, even a small imbalance can result in perinatal neuronal injury. Therefore, a mechanistic understanding of the role of excitotoxicity and the NMDA/AMPA receptor functions is critical to establishing the pathogenesis of hypoxic-ischemic encephalopathy (HIE) and perinatal brain injury due to exposure to intrauterine inflammation. Evidence from experimental animal models and clinical studies indicates that both oxygen and glucose deficiencies play a major role in fetal neuronal injury. However, the connection between these deficiencies, excitotoxicity, and HIE is not well established. The excitotoxic mechanisms in animal models and humans have many parallels, suggesting that detailed animal studies can elicit clinically relevant discoveries. While current therapies for HIE include hypothermia and other neuroprotective measures, emphasizing prevention of acute injuries, increase of therapeutic time window, and increased neural repair, there are no effective widely used treatment modalities for fetuses and neonates exposed to intrauterine inflammation. Further studies of HIE and intrauterine inflammation (as in cases of preterm birth and chorioamnionitis) will provide a better insight into development of effective therapeutic interventions for these conditions.

The cysteine dioxygenase homologue from Pseudomonas aeruginosa is a 3-mercaptopropionate dioxygenase

Thiol dioxygenation is the initial oxidation step that commits a thiol to important catabolic or biosynthetic pathways. The reaction is catalyzed by a family of specific non-heme mononuclear iron proteins each of which is reported to react efficiently with only one substrate. This family of enzymes includes cysteine dioxygenase, cysteamine dioxygenase, mercaptosuccinate dioxygenase, and 3-mercaptopropionate dioxygenase. Using sequence alignment to infer cysteine dioxygenase activity, a cysteine dioxygenase homologue from Pseudomonas aeruginosa (p3MDO) has been identified. Mass spectrometry of P. aeruginosa under standard growth conditions showed that p3MDO is expressed in low levels, suggesting that this metabolic pathway is available to the organism. Purified recombinant p3MDO is able to oxidize both cysteine and 3-mercaptopropionic acid in vitro, with a marked preference for 3-mercaptopropionic acid. We therefore describe this enzyme as a 3-mercaptopropionate dioxygenase. Mössbauer spectroscopy suggests that substrate binding to the ferrous iron is through the thiol but indicates that each substrate could adopt different coordination geometries. Crystallographic comparison with mammalian cysteine dioxygenase shows that the overall active site geometry is conserved but suggests that the different substrate specificity can be related to replacement of an arginine by a glutamine in the active site.

Systemic dendrimer-drug treatment of ischemia-induced neonatal white matter injury

Extreme prematurity is a major risk factor for perinatal and neonatal brain injury, and can lead to white matter injury that is a precursor for a number of neurological diseases, including cerebral palsy (CP) and autism. Neuroinflammation, mediated by activated microglia and astrocytes, is implicated in the pathogenesis of neonatal brain injury. Therefore, targeted drug delivery to attenuate neuroinflammation may greatly improve therapeutic outcomes in models of perinatal white matter injury. In this work, we use a mouse model of ischemia-induced neonatal white matter injury to study the biodistribution of generation 4, hydroxyl-functionalized polyamidoamine dendrimers. Following systemic administration of the Cy5-labeled dendrimer (D-Cy5), we demonstrate dendrimer uptake in cells involved in ischemic injury, and in ongoing inflammation, leading to secondary injury. The sub-acute response to injury is driven by astrocytes. Within five days of injury, microglial proliferation and migration occurs, along with limited differentiation of oligodendrocytes and oligodendrocyte death. From one day to five days after injury, a shift in dendrimer co-localization occurred. Initially, dendrimer predominantly co-localized with astrocytes, with a subsequent shift towards microglia. Co-localization with oligodendrocytes reduced over the same time period, demonstrating a region-specific uptake based on the progression of the injury. We further show that systemic administration of a single dose of dendrimer-N-acetyl cysteine conjugate (D-NAC) at either sub-acute or delayed time points after injury results in sustained attenuation of the 'detrimental' pro-inflammatory response up to 9days after injury, while not impacting the 'favorable' anti-inflammatory response. The D-NAC therapy also led to improvement in myelination, suggesting reduced white matter injury. Demonstration of treatment efficacy at later time points in the postnatal period provides a greater understanding of how microglial activation and chronic inflammation can be targeted to treat neonatal brain injury. Importantly, it may also provide a longer therapeutic window.

Signaling molecules: hydrogen sulfide and polysulfide

SIGNIFICANCE

Hydrogen sulfide (H2S) has been recognized as a signaling molecule as well as a cytoprotectant. It modulates neurotransmission, regulates vascular tone, and protects various tissues and organs, including neurons, the heart, and kidneys, from oxidative stress and ischemia-reperfusion injury. H2S is produced from l-cysteine by cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE), and 3-mercaptopyruvate sulfurtransferase (3MST) along with cysteine aminotransferase.

RECENT ADVANCES

In addition to these enzymes, we recently identified a novel pathway to produce H2S from d-cysteine, which involves d-amino acid oxidase (DAO) along with 3MST. These enzymes are localized in the cytoplasm, mitochondria, and peroxisomes. However, some enzymes translocate to organelles under specific conditions. Moreover, H2S-derived potential signaling molecules such as polysulfides and HSNO have been identified.

CRITICAL ISSUES

The physiological stimulations, which trigger the production of H2S and its derivatives and maintain their local levels, remain unclear.

FUTURE DIRECTIONS

Understanding the regulation of the H2S production and H2S-derived signaling molecules and the specific stimuli that induce their release will provide new insights into the biology of H2S and therapeutic development in diseases involving these substances.

Hemin/G-quadruplex-catalyzed aerobic oxidation of thiols to disulfides: application of the process for the development of sensors and aptasensors and for probing acetylcholine esterase activity

This study describes the novel hemin/G-quadruplex DNAzyme-catalyzed aerobic oxidation of thiols to disulfides and the respective mechanism. The mechanism of the reaction involves the DNAzyme-catalyzed oxidation of thiols to disulfides and the thiol-mediated autocatalytic generation of H2O2 from oxygen. The coupling of a concomitant H2O2-mediated hemin/G-quadruplex-catalyzed oxidation of Amplex Red to the fluorescent resorufin as a transduction module provides a fluorescent signal for probing the catalyzed oxidation of the thiol to disulfides and for probing sensing processes that yield the hemin/G-quadruplex as a functional label. Accordingly, a versatile sensing method for analyzing thiols (L-cysteine, glutathione) using the H2O2-mediated DNAzyme-catalyzed oxidation of Amplex Red to the resorufin was developed. Also, the L-cysteine and Amplex Red system was implemented as an auxiliary fluorescent transduction module for probing recognition events that form the catalytic hemin/G-quadruplex structures. This is exemplified with the development of thrombin aptasensor. The thrombin/thrombin binding aptamer recognition complex binds hemin, and the resulting catalytic complex activates the auxiliary transduction module, involving the aerobic oxidation of l-cysteine and the concomitant formation of the fluorescent resorufin. Finally, the hemin/G-quadruplex DNAzyme/Amplex Red system was used to follow the activity of acetylcholine esterase, AChE, and to probe its inhibition. The AChE-catalyzed hydrolysis of acetylthiocholine to the thiol-functionalized thiocholine enabled the probing of the enzymatic activity of AChE through the hemin/G-quadruplex-catalyzed aerobic oxidation of thiocholine to the respective disulfide and the concomitant generation of the fluorescent resorufin product.

Stereoisomers of cysteine and its analogs Potential effects on chemo- and radioprotection strategies

The thiol-containing amino acid, cysteine, and its analogs are useful for a variety of protective applications, including protecting normal tissues against the unwanted side effects of cancer chemotherapeutic agents and radiation treatment. The protection can result from both direct action of the amino acid and/or after its conversion to glutathione (GSH), sulfate, or other sulfur-based protective substances. Unfortunately, high GSH levels have been implicated in the problematic development of tumor cells' resistance to therapy. Due to numerous differences in the metabolic processing of the cysteine stereoisomers, chemo- and radioprotective strategies might be developed using the D-form of the amino acid, which can participate in protection directly, but which cannot be used to support GSH biosynthesis. In this way, protection of normal tissue may be achieved, while the potential development of resistance in tumor cells is minimized. Greatly enhanced therapeutic efficacy of cancer treatment regimens may be the result.

Impaired glutathione synthesis in neurodegeneration

Glutathione (GSH) was discovered in yeast cells in 1888. Studies of GSH in mammalian cells before the 1980s focused exclusively on its function for the detoxication of xenobiotics or for drug metabolism in the liver, in which GSH is present at its highest concentration in the body. Increasing evidence has demonstrated other important roles of GSH in the brain, not only for the detoxication of xenobiotics but also for antioxidant defense and the regulation of intracellular redox homeostasis. GSH also regulates cell signaling, protein function, gene expression, and cell differentiation/proliferation in the brain. Clinically, inborn errors in GSH-related enzymes are very rare, but disorders of GSH metabolism are common in major neurodegenerative diseases showing GSH depletion and increased levels of oxidative stress in the brain. GSH depletion would precipitate oxidative damage in the brain, leading to neurodegenerative diseases. This review focuses on the significance of GSH function, the synthesis of GSH and its metabolism, and clinical disorders of GSH metabolism. A potential approach to increase brain GSH levels against neurodegeneration is also discussed.

BMAA inhibits nitrogen fixation in the cyanobacterium Nostoc sp. PCC 7120

Cyanobacteria produce a range of secondary metabolites, one being the neurotoxic non-protein amino acid β-N-methylamino-L-alanine (BMAA), proposed to be a causative agent of human neurodegeneration. As for most cyanotoxins, the function of BMAA in cyanobacteria is unknown. Here, we examined the effects of BMAA on the physiology of the filamentous nitrogen-fixing cyanobacterium Nostoc sp. PCC 7120. Our data show that exogenously applied BMAA rapidly inhibits nitrogenase activity (acetylene reduction assay), even at micromolar concentrations, and that the inhibition was considerably more severe than that induced by combined nitrogen sources and most other amino acids. BMAA also caused growth arrest and massive cellular glycogen accumulation, as observed by electron microscopy. With nitrogen fixation being a process highly sensitive to oxygen species we propose that the BMAA effects found here may be related to the production of reactive oxygen species, as reported for other organisms.

HIV Associated Neurocognitive Disorders

Human immunodeficiency virus type 1 is associated with the development of neurocognitive disorders in many infected individuals, including a broad spectrum of motor impairments and cognitive deficits. Despite extensive research, the pathogenesis of HIV-associated neurocognitive disorders (HAND) is still not clear. This review provides a comprehensive view of HAND, including HIV neuroinvasion, HAND diagnosis and different level of disturbances, influence of highly-active antiretroviral therapy to HIV-associated dementia (HAD), possible pathogenesis of HAD, etc. Together, this review will give a thorough and clear understanding of HAND, especially HAD, which will be vital for future research, diagnosis and treatment.

Neuroprotective properties of the excitatory amino acid carrier 1 (EAAC1)

Extracellular glutamate should be maintained at low levels to conserve optimal neurotransmission and prevent glutamate neurotoxicity in the brain. Excitatory amino acid transporters (EAATs) play a pivotal role in removing extracellular glutamate in the central nervous system (CNS). Excitatory amino acid carrier 1 (EAAC1) is a high-affinity Na⁺-dependent neuronal EAAT that is ubiquitously expressed in the brain. However, most glutamate released in the synapses is cleared by glial EAATs, but not by EAAC1 in vivo. In the CNS, EAAC1 is widely distributed in somata and dendrites but not in synaptic terminals. The contribution of EAAC1 to the control of extracellular glutamate levels seems to be negligible in the brain. However, EAAC1 can transport not only extracellular glutamate but also cysteine into the neurons. Cysteine is an important substrate for glutathione (GSH) synthesis in the brain. GSH has a variety of neuroprotective functions, while its depletion induces neurodegeneration. Therefore, EAAC1 might exert a critical role for neuroprotection in neuronal GSH metabolism rather than glutamatergic neurotransmission, while EAAC1 dysfunction would cause neurodegeneration. Despite the potential importance of EAAC1 in the brain, previous studies have mainly focused on the glutamate neurotoxicity induced by glial EAAT dysfunction. In recent years, however, several studies have revealed regulatory mechanisms of EAAC1 functions in the brain. This review will summarize the latest information on the EAAC1-regulated neuroprotective functions in the CNS.

Production of hydrogen sulfide from d-cysteine and its therapeutic potential

Accumulating evidence shows that H2S has physiological functions in various tissues and organs. It includes regulation of neuronal activity, vascular tension, a release of insulin, and protection of the heart, kidney, and brain from ischemic insult. H2S is produced by enzymes from l-cysteine; cystathionine β-synthase, cystathionine γ-lyase, and 3-mercaptopyruvate sulfurtransferase (3MST) along with cysteine aminotransferase. We recently discovered an additional pathway for the production of H2S from d-cysteine. d-Amino acid oxidase provides 3-mercaptopyruvate for 3MST to produce H2S. d-Cysteine protects cerebellar neurons from oxidative stress and attenuates ischemia-reperfusion injury caused in the kidney more effectively than l-cysteine. This review focuses on a novel pathway for the production of H2S and its therapeutic application especially to the renal diseases.

Pressor response to L-cysteine injected into the cisterna magna of conscious rats involves recruitment of hypothalamic vasopressinergic neurons

The sulfur-containing non-essential amino acid L-cysteine injected into the cisterna magna of adult conscious rats produces an increase in blood pressure. The present study examined if the pressor response to L-cysteine is stereospecific and involves recruitment of hypothalamic vasopressinergic neurons and medullary noradrenergic A1 neurons. Intracisternally injected D-cysteine produced no cardiovascular changes, while L-cysteine produced hypertension and tachycardia in freely moving rats, indicating the stereospecific hemodynamic actions of L-cysteine via the brain. The double labeling immunohistochemistry combined with c-Fos detection as a marker of neuronal activation revealed significantly higher numbers of c-Fos-positive vasopressinergic neurons both in the supraoptic and paraventricular nuclei and tyrosine hydroxylase containing medullary A1 neurons, of L-cysteine-injected rats than those injected with D-cysteine as iso-osmotic control. The results indicate that the cardiovascular responses to intracisternal injection of L-cysteine in the conscious rat are stereospecific and include recruitment of hypothalamic vasopressinergic neurons both in the supraoptic and paraventricular nuclei, as well as of medullary A1 neurons. The findings may suggest a potential function of L-cysteine as an extracellular signal such as neuromodulators in central regulation of blood pressure.

Glutathione and glutathione analogues; therapeutic potentials

BACKGROUND

Glutathione (GSH) and related enzymes are critical to cell protection from toxins, both endogenous and environmental, including a number of anti-cancer cytotoxic agents.

SCOPE OF REVIEW

Enhancing GSH and associated enzymes represents a longtime and persistent aim in the search for cytoprotective strategies against cancer, neurologic degeneration, pulmonary and inflammatory conditions, as well as cardiovascular ailments. The challenge is to identify effective GSH analogues or precursors that generate mimic molecules with glutathione's cellular protective effects. This review will provide an update on these efforts. Much effort has also been directed at depleting cellular GSH and related cytoprotective effects, in order to sensitize established tumors to the cytotoxic effects of anti-cancer agents. Efforts to deplete GSH have been limited by the challenge of selectivity doing so in tumor and not in normal tissue so as to avoid enhancing the toxicity of anti-cancer drugs. This review will also provide an update of efforts at overcoming the challenge of targeting the desired GSH depletion to tumor cells.

MAJOR CONCLUSIONS

This chapter provides a brief background and update of progress in the development and use of GSH analogues in the therapeutic setting, including the pharmacological aspects of these compounds.

GENERAL SIGNIFICANCE

This is an area of enormous research activity, and major advances promise the advent of novel therapeutic opportunities in the near future. This article is part of a Special Issue entitled Cellular functions of glutathione.

Redox outside the box: linking extracellular redox remodeling with intracellular redox metabolism

Aerobic organisms generate reactive oxygen species as metabolic side products and must achieve a delicate balance between using them for signaling cellular functions and protecting against collateral damage. Small molecule (e.g. glutathione and cysteine)- and protein (e.g. thioredoxin)-based buffers regulate the ambient redox potentials in the various intracellular compartments, influence the status of redox-sensitive macromolecules, and protect against oxidative stress. Less well appreciated is the fact that the redox potential of the extracellular compartment is also carefully regulated and is dynamic. Changes in intracellular metabolism alter the redox poise in the extracellular compartment, and these are correlated with cellular processes such as proliferation, differentiation, and death. In this minireview, the mechanism of extracellular redox remodeling due to intracellular sulfur metabolism is discussed in the context of various cell-cell communication paradigms.

Excitotoxicity triggered by Neurobasal culture medium

Neurobasal defined culture medium has been optimized for survival of rat embryonic hippocampal neurons and is now widely used for many types of primary neuronal cell culture. Therefore, we were surprised that routine medium exchange with serum- and supplement-free Neurobasal killed as many as 50% of postnatal hippocampal neurons after a 4 h exposure at day in vitro 12–15. Minimal Essential Medium (MEM), in contrast, produced no significant toxicity. Detectable Neurobasal-induced neuronal death occurred with as little as 5 min exposure, measured 24 h later. D-2-Amino-5-phosphonovalerate (D-APV) completely prevented Neurobasal toxicity, implicating direct or indirect N-methyl-D-aspartate (NMDA) receptor-mediated neuronal excitotoxicity. Whole-cell recordings revealed that Neurobasal but not MEM directly activated D-APV-sensitive currents similar in amplitude to those gated by 1 µM glutamate. We hypothesized that L-cysteine likely mediates the excitotoxic effects of Neurobasal incubation. Although the original published formulation of Neurobasal contained only 10 µM L-cysteine, commercial recipes contain 260 µM, a concentration in the range reported to activate NMDA receptors. Consistent with our hypothesis, 260 µM L-cysteine in bicarbonate-buffered saline gated NMDA receptor currents and produced toxicity equivalent to Neurobasal. Although NMDA receptor-mediated depolarization and Ca²⁺ influx may support survival of young neurons, NMDA receptor agonist effects on development and survival should be considered when employing Neurobasal culture medium.

Correlating crosslink formation with enzymatic activity in cysteine dioxygenase

Cysteine dioxygenase (CDO) from rat and other mammals exhibits a covalent post-translational modification between the residues C93 and Y157 that is in close proximity to the active site, and whose presence enhances the enzyme's activity. Protein with and without C93-Y157 crosslink migrates as distinct bands in SDS-PAGE, allowing quantification of the relative ratios between the two forms by densitometry of the respective bands. Expression of recombinant rat wild type CDO in Escherichia coli typically produces 40-50% with the C93-Y157 crosslink. A strategy was developed to increase the ratio of the non-crosslinked form in an enzyme preparation of reasonable quantity and purity, allowing direct assessment of the activity of non-crosslinked CDO and mechanism of formation of the crosslink. The presence of ferrous iron and oxygen is a prerequisite for C93-Y157 crosslink formation. Absence of oxygen during protein expression increased the fraction of non-crosslinked CDO, while presence of the metal chelator EDTA had little effect. Metal affinity chromatography was used to enrich non-crosslinked content. Both the enzymatic rate of cysteine oxidation and the amount of cross-linking between C93 and Y157 increased significantly upon exposure of CDO to air/oxygen and substrate cysteine in the presence of iron in a hitherto unreported two-phase process. The instantaneous activity was proportional to the amount of crosslinked enzyme present, demonstrating that the non-crosslinked form has negligible enzymatic activity. The biphasic kinetics suggest the existence of an as yet uncharacterised intermediate in crosslink formation and enzyme activation.

S-glutathionylation: from molecular mechanisms to health outcomes

Redox homeostasis governs a number of critical cellular processes. In turn, imbalances in pathways that control oxidative and reductive conditions have been linked to a number of human disease pathologies, particularly those associated with aging. Reduced glutathione is the most prevalent biological thiol and plays a crucial role in maintaining a reduced intracellular environment. Exposure to reactive oxygen or nitrogen species is causatively linked to the disease pathologies associated with redox imbalance. In particular, reactive oxygen species can differentially oxidize certain cysteine residues in target proteins and the reversible process of S-glutathionylation may mitigate or mediate the damage. This post-translational modification adds a tripeptide and a net negative charge that can lead to distinct structural and functional changes in the target protein. Because it is reversible, S-glutathionylation has the potential to act as a biological switch and to be integral in a number of critical oxidative signaling events. The present review provides a comprehensive account of how the S-glutathionylation cycle influences protein structure/function and cellular regulatory events, and how these may impact on human diseases. By understanding the components of this cycle, there should be opportunities to intervene in stress- and aging-related pathologies, perhaps through prevention and diagnostic and therapeutic platforms.

Astrocytic redox remodeling by amyloid beta peptide

Astrocytes are critical for neuronal redox homeostasis providing them with cysteine needed for glutathione synthesis. In this study, we demonstrate that the astrocytic redox response signature provoked by amyloid beta (Aβ) is distinct from that of a general oxidant (tertiary-butylhydroperoxide [t-BuOOH]). Acute Aβ treatment increased cystathionine β-synthase (CBS) levels and enhanced transsulfuration flux in contrast to repeated Aβ exposure, which decreased CBS and catalase protein levels. Although t-BuOOH also increased transsulfuration flux, CBS levels were unaffected. The net effect of Aβ treatment was an oxidative shift in the intracellular glutathione/glutathione disulfide redox potential in contrast to a reductive shift in response to peroxide. In the extracellular compartment, Aβ, but not t-BuOOH, enhanced cystine uptake and cysteine accumulation, and resulted in remodeling of the extracellular cysteine/cystine redox potential in the reductive direction. The redox changes elicited by Aβ but not peroxide were associated with enhanced DNA synthesis. CBS activity and protein levels tended to be lower in cerebellum from patients with Alzheimer's disease than in age-matched controls. Our study suggests that the alterations in astrocytic redox status could compromise the neuroprotective potential of astrocytes and may be a potential new target for therapeutic intervention in Alzheimer's disease.