Mitochondrial superoxide generation enhances P2X7R-mediated loss of cell surface CD62L on naive human CD4+ T lymphocytes

Migration of naive CD4(+) T lymphocytes into lymphoid tissue is essential for their activation and subsequent roles in adaptive immunity. The adhesion molecule L-selectin (CD62L), critical for this process, is highly expressed on naive CD4(+) T lymphocytes and is downregulated upon T lymphocyte activation. We demonstrate protein expression of P2X7R on naive CD4(+) T lymphocytes and show functional channel activity in whole-cell patch clamp recordings. CD62L downregulation occurs rapidly in response to extracellular ATP, a process that is blocked by selective antagonists of P2X7R. This loss of surface CD62L expression was not associated with externalization of phosphatidylserine. While investigating the mechanisms for this process, we revealed that pharmacological modulation of mitochondrial complex I or III, but not inhibition of NADPH oxidase, enhanced P2X7R-dependent CD62L downregulation by increasing ATP potency. Enhanced superoxide generation in the mitochondria of rotenone- and antimycin A-treated cells was observed and may contribute to the enhanced sensitivity of P2X7R to ATP. P2X7R-dependent exposure of phosphatidylserine was also revealed by preincubation with mitochondrial uncouplers prior to ATP treatment. This may present a novel mechanism whereby P2X7R-dependent phosphatidylserine exposure occurs only when cells have enhanced mitochondrial reactive oxygen species generation. The clearance of apoptotic cells may therefore be enhanced by this mechanism which requires functional P2X7R expression.

A novel role for P2X7 receptor signalling in the survival of mouse embryonic stem cells

The growth of a pluripotent embryonic stem (ES) cell population is dependent on cell survival, proliferation and self-renewal. The nucleotide ATP represents an important extracellular signalling molecule that regulates the survival of differentiated cells, however, its role is largely undefined in embryonic stem cells. Here we report a role for ATP-gated P2X7 receptors in ES cell survival. The functional expression of P2X7 receptors in undifferentiated mouse ES cells is demonstrated using a selective P2X7 antagonist and small interfering RNA knockdown of these receptors. Our data illustrate a key role for the P2X7 receptor as an essential pro-survival signal required for optimal ES cell colony growth in the presence of leukemia inhibitor factor (LIF). However, chronic exposure to exogenous ATP leads to rapid P2X7-dependent cell death via necrosis. Together, these data demonstrate a novel role for P2X7 receptors in regulation of ES cell behaviour where they can mediate either a pro-survival or pro-death signal depending on the mode of activation.

Spatial and temporal variations in Pb concentrations and isotopic composition in road dust, farmland soil and vegetation in proximity to roads since cessation of use of leaded petrol in the UK

Results are presented for a study of spatial distributions and temporal trends in concentrations of lead (Pb) from different sources in soil and vegetation of an arable farm in central Scotland in the decade since the use of leaded petrol was terminated. Isotopic analyses revealed that in all of the samples analysed, the Pb conformed to a binary mixture of petrol Pb and Pb from industrial or indigenous geological sources and that locally enhanced levels of petrol Pb were restricted to within 10 m of a motorway and 3 m of a minor road. Overall, the dominant source of Pb was historical emissions from nearby industrial areas. There was no discernible change in concentration or isotopic composition of Pb in surface soil or vegetation over the decade since the ban on the sale of leaded petrol. There was an order of magnitude decrease in Pb concentrations in road dust over the study period, but petrol Pb persisted at up to 43% of the total Pb concentration in 2010. Similar concentrations and spatial distributions of petrol Pb and non petrol Pb in vegetation in both 2001 and 2010, with enhanced concentrations near roads, suggested that redistribution of previously deposited material has operated continuously over that period, maintaining a transfer pathway of Pb into the biosphere. The results for vegetation and soil transects near minor roads provided evidence of a non petrol Pb source associated with roads/traffic, but surface soil samples from the vicinity of a motorway failed to show evidence of such a source.

Analytical and sampling constraints in ²¹⁰Pb dating

²¹⁰Pb dating provides a valuable, widely used means of establishing recent chronologies for sediments and other accumulating natural deposits. The Constant Rate of Supply (CRS) model is the most versatile and widely used method for establishing ²¹⁰Pb chronologies but, when using this model, care must be taken to account for limitations imposed by sampling and analytical factors. In particular, incompatibility of finite values for empirical data, which are constrained by detection limit and core length, with terms in the age calculation, which represent integrations to infinity, can generate erroneously old ages for deeper sections of cores. The bias in calculated ages increases with poorer limit of detection and the magnitude of the disparity increases with age. The origin and magnitude of this effect are considered below, firstly for an idealized, theoretical ²¹⁰Pb profile and secondly for a freshwater lake sediment core. A brief consideration is presented of the implications of this potential artefact for sampling and analysis.

NADPH oxidase NOX2 mediates rapid cellular oxidation following ATP stimulation of endotoxin-primed macrophages

The phagocytic NADPH oxidase (NOX2) plays a fundamental role in host defense and innate immunity. Here we demonstrate that external ATP triggers rapid cellular oxidation inhibited by diphenyleneiodonium in endotoxin-primed J774 macrophages and primary murine bone marrow-derived macrophages. To identify the source of reactive oxygen species (ROS), we compared responses between wild-type and NOX2-deficient macrophages. ATP-mediated ROS production was strongly attenuated in NOX2-deficient macrophages where responses were comparable to inhibition with diphenyleneiodonium. Notably, spatial differences in superoxide anion formation were observed where ROS formation was partially antagonized by extracellular superoxide dismutase in primary bone marrow-derived macrophages but unaffected in J774 macrophages. Loss of NOX2 was not observed to affect ATP-induced cell death. However, ATP-evoked cell death was found to be partially dependent on caspase-1 and cathepsin B activation. In conclusion, NOX2 plays a fundamental role in conferring macrophages with the ability to respond to extracellular ATP stimulation with robust changes in cellular oxidation.

A key role for redox signaling in rapid P2X7 receptor-induced IL-1 beta processing in human monocytes

P2X(7) receptors (P2X(7)Rs) are ATP-gated ion channels that trigger caspase-1 activation in the presence of TLR ligands. Inflammatory caspase-1 is responsible for the proteolytic activation of IL-1beta. However, the signaling events that couple P2X(7)Rs to caspase-1 activation remain undefined. In this study we demonstrate that ATP-induced cellular oxidation is critical for caspase-1 activation and subsequent IL-1beta processing. Purinergic receptor stimulation, including P2X(7)Rs, of endotoxin-primed human monocytes augments NADPH oxidase activity whereas concurrent purinergic receptor stimulation triggers protein denitroyslation, leading to the formation of peroxynitrite. IL-1beta cleavage is blocked under conditions where superoxide anion formation is blocked or monocytes are treated with antioxidants or a peroxynitrite scavenger. Nigericin, a K(+)/H(+) antiporter, also increases NADPH oxidase activity, leading to IL-1beta and caspase-1 processing that is blocked by a peroxynitrite scavenger or inhibition of NADPH oxidase. These data demonstrate that signaling via NADPH oxidase activity is fundamental for the processing of mature IL-1beta induced by P2X(7)R stimulation.

Constraints on the use of anthropogenic radionuclide-derived chronologies for saltmarsh sediments

Numerous studies have employed anthropogenic radionuclides deposited in accumulating sediments to derive chronologies for use in investigations of geomorphological processes and in reconstructing temporal trends in contaminant deposition. However, relatively few have interrogated their use in systems that erode as well as accrete sediment, or have addressed limitations in their applicability in systems that experience variable rates of accumulation. This paper examines the utility of Sellafield-derived radionuclides for reconstructing sedimentary processes in two contrasting saltmarsh systems in the south-west of Scotland. Sedimentation rates and patterns derived from the radionuclide chronologies are outlined and compared to results established through conventional geomorphological methods. The results confirm that the vertical distribution of anthropogenic radionuclides can be useful in determining sedimentation rates, but that these cannot always be used as indicators of contemporary processes, particularly where erosion is occurring. Their use is also limited unless profiles are obtained from spatially diverse geomorphological units. Integrating the use of chronologies with other independent methods secures more robust data for assessing both marsh sustainability and their longevity as radionuclide sinks.

Murine macrophage P2X7 receptors support rapid prothrombotic responses

Non-apoptotic externalization of phosphatidylserine (PS) can act as a reactive surface for the efficient assembly of the prothrombinase complex leading to thrombin generation and coagulation. Here we show that extracellular ATP, acting at the macrophage P2X(7) receptor, drives the rapid Ca(2+)-dependent formation and release of PS-rich microvesicles that enhance the assembly of the prothrombinase complex and subsequent formation of thrombin. Incubation with P2X(7) receptor antagonists (KN-62 and Brilliant Blue G) attenuates ATP induced prothrombotic responses. Consistent with the hypothesis that exposed PS enhances prothrombinase activity; pre-incubation with annexin V blocks the increase in thrombin formation. The rapid translocation of PS and formation of pro-thrombotic microvesicles occurs in the absence of cell lysis. These data demonstrate that the pro-inflammatory P2X(7) receptor can also support and propagate rapid increases in thrombin formation.

Human teeth as historical biomonitors of environmental and dietary lead: some lessons from isotopic studies of 19th and 20th century archival material

The lead isotopic composition of various sections (crown, crown base, root) of teeth was determined in specimens collected from 19th century skulls preserved in museum collections and, upon extraction or exfoliation, from humans of known ages residing in Scotland in the 1990s. For most 20th century samples, calculation of accurate crown-complete or root-complete dates of tooth formation ranging from the 1920s to the 1990s enabled comparison of (206)Pb/(207)Pb ratios for teeth sections (crown base root) with corresponding decadally averaged data for archival herbarium Sphagnum moss samples. This showed that the teeth sections had been significantly influenced by incorporation of non-contemporaneous (more recent) lead subsequent to the time of tooth formation, most probably via continuous uptake by dentine. This finding confirmed that separation of enamel from dentine is necessary for the potential of teeth sections as historical biomonitors of environmental (and dietary) lead exposure at the time of tooth formation to be realised. Nevertheless, the mean 19th century value of 1.172+/-0.007 for the (206)Pb/(207)Pb ratio in teeth was very similar to the corresponding mean value of 1.173+/-0.004 for 19th century archival moss, although relative contributions from environmental sources - whether direct, by inhalation/ingestion of dust contaminated by local lead smelting ((206)Pb/(207)Pb~1.17) and coal combustion ((206)Pb/(207)Pb~1.18) emissions, or indirect, through ingestion of similarly contaminated food - and drinking/cooking water contaminated by lead pipes of local origin, cannot readily be determined. In the 20th century, however, the much lower values of the (206)Pb/(207)Pb ratio (range 1.100-1.166, mean 1.126+/-0.013, median 1.124) for the teeth collected from various age groups in the 1990s reflect the significant influence of imported Australian lead of lower (206)Pb/(207)Pb ratio (~1.04) and released to the environment most notably through car-exhaust emissions arising from the use of alkyl lead additives ((206)Pb/(207)Pb~1.06-1.09) in petrol in the U.K. from ca. 1930 until the end of the 20th century.

Manmade and natural radionuclides in north east Atlantic shelf and slope sediments: Implications for rates of sedimentary processes and for contaminant dispersion

Results are presented for a study of manmade and natural radionuclides in north east Atlantic continental shelf and slope sediments to the west of Scotland. The data are interpreted in the context of sediment mixing and accumulation processes and are used to establish the westward extent of contamination of the sediment system. Offshore shelf and slope sediments were found to have post-glacial sedimentation rates of the order of 1 cm ky(-1) but nearshore sediments had much higher accumulation rates of the order of 0.1 cm y(-1). Surface mixed layer depths of up to 6 cm were observed and non-local mixing affected most of the slope sediments, resulting in advective transport of surface sediment to depths of up to 10 cm. Biodiffusion coefficients for offshore shelf and slope sediments were dominantly in the range 10(-8) to 10(-9) cm2 s(-1). The study confirmed that seawater contaminated with Sellafield waste radionuclides is dominantly entrained to the east of 7 degrees W and, consistent with this, higher levels of Sellafield derived radionuclides were confined to nearshore sediments, with lower levels to the west of 7 degrees W. 238Pu/(239,240)Pu data indicated that Sellafield contributed 75-91% of the total plutonium in coastal sediment but only about 4-8% of the total in slope sediments. By analogy, it can be concluded that a similar situation will apply to other contaminants in seawater entering the north east Atlantic via the North Channel.

Lake sediments record sensitivity of two hydrologically closed upland lakes in Mexico to human impact

We provide evidence of rapid, recent environmental change in two lakes in the highlands of central Mexico. Multiple sediment cores were obtained from Lago de Zirahuén (Michoacán) and Laguna de Juanacatlán (Jalisco). Analysis of diatom assemblages, magnetic susceptibility, and metal concentrations was carried out, with the chronology provided by 210Pb dating, 14C dating, and tephrochronology. There is evidence of catchment disturbance during the colonial period in both basins, but the most striking feature at both sites is the rapid change in diatom assemblages during the last 20 y, indicating the onset of eutrophication. Limnological data from Lago de Zirahuén support this interpretation, although none are available from Laguna de Juanacatlán. Paleolimnology is a powerful tool in tracking recent change, particularly in the absence of regular limnological monitoring programs. These lakes appear to be highly sensitive to changes in catchment exploitation, which must be considered in future drainage basin management.

14C and δ13C characteristics of organic matter and carbonate in saltmarsh sediments from south west Scotland

The distribution of contaminant radionuclides from the Sellafield nuclear fuel reprocessing plant was used to establish chronologies for three saltmarsh sediment cores from south west Scotland. delta(13)C and (14)C analyses indicated that the cores provided a useful archive record of variations in input of organic matter and carbonate. The results imply that prior to major releases of contaminant (14)C from Sellafield, the (14)C specific activity of organic matter in Irish Sea offshore sediments was about 24 Bq kg(-1) C, while that of the carbonate component was below the limit of detection. These results provide baseline data for modelling the uptake of contaminant (14)C by the Irish Sea sediment system. The study confirmed that small(13)C analyses provide a sensitive means of apportioning the origin of saltmarsh organic matter between C(3) terrigenous plants, C(4) terrigenous plants and suspended particulate marine organic matter. For the <2 mm fraction of sediment, a clear pattern of decreasing marine organic input was observed in response to increasing elevation of the marsh surface as a result of sediment accumulation. Bulk sediment, including detrital vegetation, had a dominant input from terrigenous plants. The combined use of delta(13)C and (14)C data revealed that organic matter in the marine organic component of the <2 mm fraction of contemporary surface sediments of the saltmarshes is dominated by recycled old organic material.

Stable lead isotopic characterisation of the historical record of environmental lead contamination in dated freshwater lake sediment cores from northern and central Scotland

Sediment cores from three Scottish freshwater lakes, Loch Ness in the remote north and Loch Lomond and the Lake of Menteith, much closer to the heavily populated and industrialised central belt were analysed for 210Pb, 137Cs, Pb and stable Pb isotopic composition (206Pb/207Pb). The radionuclide data were used to establish chronologies for the Loch Ness and Loch Lomond cores, but a chronology could not be developed for the Lake of Menteith core, in which the surface sediment had been subject to intense mixing. Although Pb concentrations generally started increasing during the mid-17th Century, a small peak occurred for Loch Ness in the early 16th Century, perhaps attributable to the influence of medieval mining and smelting in mainland Europe. Temporal trends in the pattern of Pb accumulation were similar for Loch Ness and both sites in Loch Lomond, with 40-50% of the anthropogenic Pb deposited prior to the 20th Century. Fluxes of anthropogenic Pb to the lake sediments peaked during the 1950s at all locations where chronologies could be established. The 5-fold increase in anthropogenic Pb inventory for the southern basin of Loch Lomond relative to Loch Ness reflected geographical proximity to the main polluting sources. The 206Pb/207Pb data for anthropogenic Pb in the sediments from Loch Ness and Loch Lomond exhibited largely similar trends related to five different time periods. Pre-1820, the 206Pb/207Pb ratio was close to that for coal (1.181). From 1820 to 1900, a fairly constant 206Pb/207Pb ratio of approximately 1.17 probably resulted from a combination of emissions from the smelting of indigenous Pb ore (1.170) and coal burning (1.181) in Scotland, and industrial activity to the south in England, where Australian Pb of characteristically low 206Pb/207Pb ratio (1.04) was already in use. From 1901 to 1930, the 206Pb/207Pb ratio declined by <0.01, due to the increasing influence of Australian Pb. From 1931 to 1975/1985, the 206Pb/207Pb ratio of anthropogenic Pb declined by a further 0.03 to 0.04, to minimum values from approximately 1975 to 1985, primarily a consequence of car-exhaust emissions of Pb arising from the introduction of alkyl Pb petrol additives (206Pb/207Pb approximately 1.06-1.09). From 1975/1985 to the mid-1990s, the 206Pb/207Pb ratio of anthropogenic Pb increased by up to 0.015, a consequence of a reduction in car-exhaust emissions of Pb, resulting from reductions in the maximum permitted concentration of Pb in petrol, and the introduction and increasing uptake of unleaded petrol. Source apportionment calculations, on the basis of 206Pb/207Pb values in surface sediment, suggested that the contribution of Pb emissions from the use of leaded petrol was 27-40% of the atmospheric burden by the mid-1990s, in line with estimates from rainwater 206Pb/207Pb data.

Environmental radioactivity: experience from the 20th century--trends and issues for the 21st century

Environmental radioactivity has been an important area of research throughout the 20th century, with recent work having been stimulated mainly by: (i) the remarkable power of radionuclides as tracers of the rates and mechanisms of environmental processes and (ii) the potential health implications of contaminant radionuclides in the environment. A review is presented of salient aspects of environmental radioactivity, including consideration of tracer applications, sources and environmental impact of anthropogenic radionuclides, radioactive waste disposal and future exploitation of nuclear energy.

Functional and molecular diversity of purinergic ion channel receptors

P2X receptors are membrane ion channels gated by extracellular adenosine 5'-triphosphate (ATP); nucleotides also activate a family of seven transmembrane G protein-coupled receptors (P2Y). P2X receptors are widely expressed on mammalian cells, where they can be broadly differentiated into three groups. The first group is almost equally well activated by ATP and its analog alpha beta methyleneATP (alpha beta meATP), whereas a second group is not activated by alpha beta meATP. A third-group type of receptor (termed P2Z) is distinguished by the fact that the channel opening is followed by cell permeabilization and lysis if the agonist application is continued for more than a few seconds. Seven cDNAs have been cloned that encode P2X receptor subunits. When expressed individually in heterologous systems, P2X1 and P2X3 subunits form channels activated by ATP or alpha beta meATP; whereas P2X2, P2X4, and P2X5 form channels activated by ATP but not alpha beta meATP. P2X6 receptors do not express readily, and P2X7 receptors correspond closely in their properties to P2Z. Further phenotypes can be produced when two subunits are coexpressed, indicating hetero-multimerization. This chapter compares the properties of the native P2X receptors with those of the cloned and expressed subunits.

Purinoceptor-evoked calcium signalling in human platelets

ADP evokes a rise in platelet cytosolic Ca2+ concentration by stimulating Ca2+ entry and releasing Ca2+ from intracellular stores. Single cell studies indicate that the response consists of a series of spikes in cytosolic Ca2+. The release of stored Ca2+ is mediated by the generation of inositol 1,4,5-trisphosphate. Store depletion in turn leads to activation of a store-regulated Ca2+ entry pathway via a mechanism which appears to involve a protein tyrosine phosphorylation step. Preceding these events, ADP activates a receptor-operated non-selective cation channel, which mediates the entry of Ca2+ and Na+ with a latency of just a few milliseconds. Recent studies indicate that this channel is activated via a P2X1 purinoceptor at which ATP and diadenosine tetraphosphate are agonists. This receptor is distinct from that leading to the release of stored Ca2+ and to store-regulated Ca2+ entry.

Activation of receptor-operated cation channels via P2X1 not P2T purinoceptors in human platelets

We have investigated the purinoceptor subtypes responsible for calcium signaling in human platelets, which previous studies have shown to involve both Ca2+ influx via receptor-operated cation channels and release of Ca2+ from intracellular stores. Fura-2 measurements of [Ca2+]i in stirred platelet suspensions showed that both ADP (40 microM) and the non-hydrolyzable ATP analogue alphabeta-meATP (alpha, beta-methyleneadenosine 5-triphosphate, 10 microM) activated a rapid Ca2+ influx whereas only ADP mobilized Ca2+ from internal stores. In "nystatin" whole-cell patch clamp recordings, ATP, ADP, and the non-hydrolyzable ATP analogues, alpha, beta-meATP and ATPgammaS (adenosine 5 -O-(3-thiotriphosphate), all activated a cation channel permeable to both monovalent and divalent cations with a single-channel conductance of 11 picosiemens in NaCl saline. The current response to ATP (40 microM) was activated within 20 ms and desensitized with a time constant of 47-107 ms in the continued presence of agonist, which are characteristics of P2X1 receptors in other tissues. We conclude that human platelets possess a P2X1 purinoceptor, which mediates a rapid phase of ADP- or ATP-evoked Ca2+ entry via a cation channel, whereas one or more separate ADP-selective P2 purinoceptors evoke release of calcium from intracellular stores.

Chloride channels in excised membrane patches from human platelets: effect of intracellular calcium

Human platelets were studied by patch clamp recordings from inside-out membranes; there were formed by briefly dipping the platelet, in cell-attached mode, into silicone grease. At 20 degrees C in symmetrical 150 mM NaCl, spontaneous channel openings were rarely observed at negative potentials, whereas depolarised potentials (+ 60 to + 100 mV) elicited sustained channel activity in 38% of patches. The single channel conductance was 53 +A- 1 pS at + 80 mV (outward current), decreasing to 20 +/- 2 pS at -80 mV (inward current). Ion substitution experiments indicated that this channel conducts Cl- and not Na+. Furthermore, 5-nitro-2-(3-phenylpropylamino)benzoate (100 microM), a recognized inhibitor of anion channels, induced a reversible 'flickery' channel block. We estimate that each platelet possesses < or = 30 such channels. Kinetic analysis suggested at least two open channel states (tau = 0.8 +/- 0.2 ms, tau = 22 +/- 14 ms, n = 4) and two closed states (tau = 0.8 +/- 0.2 ms, tau = 12 +/- 0.6 ms, n = 4). Increasing [Ca2+]i to 10 microM, following channel activation by depolarisation, had no significant effect on channel kinetics or open probability, however, elevated [Ca2+]i (300 nM-10 microM) increased the number of anion channels activated by subsequent depolarisation. This study represents the first recordings of ionic currents in excised, inside-out membrane patches from human platelets, and provides further evidence for the existence of chloride channels in these cells.