1,720,972 research outputs found
GLUTAMATE UPTAKE IS INHIBITED BY ARACHIDONIC ACID AND OXYGEN RADICALS VIA 2 DISTINCT AND ADDITIVE MECHANISMS
No full textReuptake of glutamate in astrocytes, a critical mechanism involved in the maintenance of physiological excitatory amino acid neurotransmission, is inhibited by both arachidonic acid (AA) and reactive oxygen species (ROS), via incompletely defined molecular mechanisms. Because ROS are generated during AA metabolism and AA can be released as a result of ROS-mediated phospholipase A(2) activation, it seems likely that their effects on uptake are mediated by a common mechanism. However, here we show that rapid (10-min) uptake inhibitions by AA or by ROS generated by the xanthine plus xanthine oxidase (XO) reaction are selectively abolished by distinct agents; bovine serum albumin (BSA) acts only on AA, whereas the scavenger enzymes superoxide dismutase (SOD) and catalase (CAT) and the disulfide-reducing agent dithiothreitol (DTT) act only on ROS. Moreover, when added together, xanthine/XO and AA decrease uptake in a fully additive manner. In particular, the effect of xanthine/XO is seen also in the presence of maximal AA inhibition. No major signs of cell damage or chemical reaction between AA and radicals accompany their cumulative effects on uptake. Finally, uptake inhibition elicited by AA and xanthine/XO together is attenuated but not blocked by either BSA, DTT, or SOD/CAT individually, whereas it is fully blocked and substantially reversed by a combination of SOD/CAT and BSA or SOD/CAT, DTT, and BSA. Together, these data indicate that AA and ROS act on glial glutamate transport via distinct noninteracting mechanisms. Therefore, they could independently and additively contribute to the impairment of reuptake function, a phenomenon observed in pathological conditions such as ischemia/reperfusion injury
GLUTAMATE UPTAKE INHIBITION BY OXYGEN-FREE RADICALS IN RAT CORTICAL ASTROCYTES
No full textFormation of reactive oxygen species and disfunction of the excitatory amino acid (EAA) system are thought to be key events in the development of neuronal injury in several acute and long-term neurodegenerative diseases. Recent evidence suggests that the two phenomena may be interdependent. The present study is aimed at exploring possible molecular mechanisms underlying oxygen radical-EAA interaction. Exposure of cortical astrocytic cultures to either xanthine + xanthine oxidase (X/XO), a free radical-generating system, or hydrogen peroxide (H2O2) results in a marked decrease of high-affinity glutamate transport. Within 10 min of X/XO application, uptake falls to approximate to 60% of its control value. In parallel no detectable release of lactate dehydrogenase occurs. X/XO effect is abolished in the presence of a mixture of scavenger enzymes (superoxide dismutase + catalase) or by the disulfide-reducing agents glutathione and dithiothreitol (DTT), but not by lipophilic antioxidants or ascorbate. The time course of inhibition shows an almost linear decline of glutamate transport during cell exposure to free radicals, while upon their inactivation the decline stops but established inhibition persists for at least 1 hr. In this situation, application of DTT significantly restores transport function. These data suggest that free radicals inhibit glutamate uptake primarily by long-lasting oxidation of protein sulfhydryl (SH) groups. Chemical modifiers of free SH groups, such as p-chloromercuribenzoate and N-ethylmaleimide, also induce uptake inhibition. Na+/K+ ATPase is a known target of oxygen radicals and may be involved in glutamate uptake inhibition. Indeed, ouabain, a blocker of the pump, reduces uptake in astrocytes. However, its effect is largely additive with that of radicals. Electrophysiological recording of astrocytic resting conductance shows, in some cells, a Ba2+-insensitive, inward current in response to H2O2. However, in the majority of the cells, the oxidant has no effect on membrane current or voltage. In the same cells, application of glutamate in the presence of inhibitors of ionotropic EAA receptors elicits a large inward current representing electrogenic uptake. In six of seven tested cells, H2O2 significantly inhibited such current. These results indicate that inactivation of Na+/K+ ATPase can be only part of the mechanism by which oxygen radicals inhibit glutamate uptake and that a direct action on glutamate transport is likely. In all, our data suggest that free radicals may induce extracellular accumulation of glutamate by reduction of glial uptake. In pathologies such as ischemia/reoxygenation or amyotrophic lateral sclerosis, where evidence for both oxidative stress and EAA uptake disfunction exists, this mechanism may link oxygen radical toxicity to excitotoxicity and represent an important step in the genesis of neurotoxic damage
IRON AND ZINC TRANSPORT : OXIDATIVE STRESS-MODULATION OF DCT1
No full textIn humans, iron plays a key role in a number of metabolic functions. Its deficiency as well as its overload leads to important pathologies. Excess of free iron can amplify the effect of oxidative stress, via Fenton's reaction, associated with a great number of diseases such as ischemia-reperfusion injury, inflammation, carcinogenesis and degenerative diseases of the SNC.
The functional properties and the localisation of the divalent cation transporter (DCT1/Nramp2/DMT1) in recycling endosomes and in plasma membranes indicate its key role in the direct cellular iron uptake and in the transferrin-receptor mediates pathway. Though DCT1 expression is regulated at a post-transcriptional level via the IRP/IRE system, in order to prevent iron overload in oxidative stress condition, a direct regulation of the protein in the plasma membrane could represent an important defensive mechanism of the cell.
To study the influence of redox reagents on DCT1 function we used the Xenopus laevis oocyte expression system and analysed the transport activity with radiotracer 55Fe2+ and 65Zn2+ -uptake assays and electrophysiology experiments.
Our radiotracer experiments confirmed the capability of DCT1 to mediate Zn uptake previously suggested on the basis of electrophysiology experiments. Since the characteristics of Zn2+ induced DCT1 mediated current resembled the characteristics of the iron current, we performed our experiments with Zn2+ which does not react with the used redox reagents.
Hydrogen peroxide treatment resulted in an inhibition of about 40 % of both Zn2+ uptake and Zn2+ induced DCT1 mediated current in oocytes. DCT1 sequence contains several amino acid residues that could be targets for oxidant reagents. Therefore Hg2+ administration exerted the same inhibition than H2O2 and, in both cases, DCT1 function was fully recovered after perfusion with dithiotreitol strongly indicating the involvement of cysteine residues. Mutagenesis experiments allowed us to identify a cysteine that is involved in the mechanism of DCT1 inhibition. C248 is located in the forth-outer loop of the predicted secondary structure of the transporter and could represent a sensor that allows the cell to activate defensive mechanism. In fact reactive oxygen species can be released from some kind of cells like phagocytes to affect target cells. The finding of a direct inhibition of iron transport appears to have relevant physiological interest to understand the regulation of iron metabolism and the defensive mechanisms to preserve cell from oxidative injury
Modulation of DMT1 activity by red-ox compounds
No full textIron(II) exacerbates the effects of oxidative stress via the Fenton reaction. A number of human diseases are associated with iron accumulation including ischemia-reperfusion injury, inflammation and certain neurodegenerative diseases. The functional properties and localization in plasma membrane of cells and endosomes suggest an important role for the divalent metal transporter DMT1 (also known as DCT1 and Nramp2) in iron transport and cellular iron homeostasis. Although iron metabolism is strictly controlled and the activity of DMT1 is central in controlling iron homeostasis, no regulatory mechanisms for DMT1 have been so far identified. Our studies show that the activity of DMT1 is modulated by compounds that affect its redox status. We also show that both iron and zinc are transported by DMT1 when expressed in Xenopus laevis oocytes. Radiotracer uptake and electrophysiological measurements revealed that H(2)O(2) and Hg(2+) treatments result in substantial inhibition of DMT1. These findings may have a profound relevance from a physiological and pathophysiological standpoin
Reactive oxygen species inhibit high-affinity glutamate uptake: molecular mechanism and neuropathological implications.
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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