Rôle et variations de l'expression de la superfamille des protéines IGFBP par les cellules bêta des îlots pancréatiques, notamment en cas d'atteinte pancréatique comme celle induite par le diabète

L'insuline est une hormone sécrétée par les cellules bêta des îlots pancréatiques, qui joue un rôle majeur dans le métabolisme du glucose, notamment en régulant l'augmentation de la glycémie suite à la prise alimentaire. Sa sécrétion est principalement stimulée par l'augmentation du glucose dans le sang, qui joue un rôle direct sur les cellules bêta du pancréas. En parallèle, les gluco-incrétines, GLP-1 et GIP, sécrétées par l'intestin en réponse à la prise alimentaire, jouent également un rôle important dans la sécrétion de l'insuline. Elles agissent en potentialisant l'effet du glucose sur la sécrétion de l'insuline par les cellules bêta pancréatiques, tout en favorisant leur prolifération et leur protection contre l'apoptose. Il a d'ailleurs été estimé qu'environ 50% de la sécrétion d'insuline en réponse à un repas était lié à l'effet des incrétines (M. Virally et al., 2008). Il a alors été montré que le rôle de GLP-1 sur la prolifération et la protection contre l'apoptose des cellules bêta du pancréas était en grande partie assuré par une boucle autocrine médiée par IGF2. En effet, GLP-1 stimule l'expression de récepteurs à IGF à la surface des cellules bêta et l'activation de ces récepteurs est assurée par une sécrétion autocrine d'IGF-2, sécrété en concomitance avec l'insuline dans les granules de sécrétion (M. Cornu et al., 2009). Les IGF, plus précisément IGF-1 et IGF-2, sont des hormones principalement sécrétées par le foie qui influencent le métabolisme cellulaire via leur récepteur spécifique IGF1-R. Elles sont connues pour agir comme stimulateur de la croissance, de la prolifération et de la différenciation des cellules normales ainsi que des cellules malignes. La majeure partie des IGF circulantes dans le plasma se trouve lié à des protéines de liaison, au   2 nombre de 6, IGFBP (Insulin-like Growth Factor Binding Protein) 1 à 6, qui augmentent leur stabilité et ainsi leur demi-vie mais diminuent également leur biodisponibilité. C'est ainsi que ces protéines IGFBP assurent une régulation de l'activité des IGF en régulant leur disponibilité pour leurs récepteurs spécifiques. Pour finir, il a été montré que les protéines IGFBP pouvaient avoir une action intrinsèque, indépendante de l'IGF. Même si leur rôle est encore mal défini, certaines données ont montré l'existence d'une action intrinsèque, en particulier pour IGFBP3 et IGFBP5 (Vivian HWA et al., 1999). Le but de mon travail consiste alors à étudier si les cellules bêta pancréatiques expriment elles-mêmes des protéines IGFBP, et si c'est le cas, d'essayer d'identifier les mécanismes qui régulent leur expression en imaginant que leur sécrétion pourrait soit jouer un rôle sur la voie de signalisation des gluco-incrétines et la boucle autocrine qu'elles induisent en agissant sur la biodisponibilité d'IGF-2, soit agir de manière intrinsèque indépendamment de l'IGF-2. Pour cela, après avoir montré que les cellules bêta pancréatiques expriment des protéines IGFBP, nous étudierons l'effet des gluco-incrétines sur leur expression, puis les variations susceptibles d'être induites par les mécanismes responsables de la diminution de la masse cellulaire bêta observée dans les îlots pancréatiques des patients diabétiques

Methanol exchange between grassland and the atmosphere

International audienceConcentrations and fluxes of methanol were measured above two differently managed grassland fields (intensive and extensive) in central Switzerland during summer 2004. The measurements were performed with a proton-transfer-reaction mass-spectrometer and fluxes were determined by the eddy covariance method. The observed methanol emission showed a distinct diurnal cycle and was strongly correlated with global radiation and water vapour flux. Mean and maximum daily emissions were found to depend on grassland species composition and, for the intensive field, also on the growing state. The extensive field with a more complex species composition had higher emissions than the graminoid-dominated intensive field, both on an area and on a biomass basis. A simple parameterisation depending on the water vapour flux and the leaf area index allowed a satisfying simulation of the temporal variation of methanol emissions over the growing phase. Accumulated carbon losses due to methanol emissions accounted for 0.024 and 0.048% of net primary productivity for the intensive and extensive field, respectively. The integral methanol emissions over the growing periods were more than one order of magnitude higher than the emissions related to cut and drying events

The annual ammonia budget of fertilised cut grassland – Part 1: Micrometeorological flux measurements and emissions after slurry application

Two commercial ammonia (NH<sub>3</sub>) analysers were customised to allow continuous measurements of vertical concentration gradients. The gradients were used to derive ammonia exchange fluxes above a managed grassland site at Oensingen (Switzerland) by application of the aerodynamic gradient method. The measurements from July 2006 to October 2007 covered five complete growth-cut cycles and included six applications of liquid cattle slurry. The average accuracy of the flux measurements during unstable and near-neutral conditions was 20% and the detection limit was 10 ng NH<sub>3</sub> m<sup>−2</sup> s<sup>−1</sup>. Hence the flux measurements are considered sufficiently accurate for studying typical NH<sub>3</sub> deposition rates over growing vegetation. Quantifying the overall emissions after slurry applications required the application of elaborate interpolations because of difficulties capturing the initial emissions during broadspreading of liquid manure. The emissions were also calculated with a mass balance method yielding similar fluxes. NH<sub>3</sub> losses after slurry application expressed as percentage of emitted nitrogen versus applied total ammoniacal nitrogen (TAN) varied between 4 and 19%, which is roughly a factor of three lower than the values for broadspreading of liquid manure in emission inventories. The comparatively low emission factors appear to be a consequence of the low dry matter content of the applied slurry and soil properties favouring ammonium adsorption

The annual ammonia budget of fertilised cut grassland – Part 2: Seasonal variations and compensation point modeling

The net annual NH<sub>3</sub> exchange budget of a fertilised, cut grassland in Central Switzerland is presented. The observation-based budget was computed from semi-continuous micrometeorological fluxes over a time period of 16 months and using a process-based gap-filling procedure. The data for emission peak events following the application of cattle slurry and for background exchange were analysed separately to distinguish short-term perturbations from longer-term ecosystem functioning. A canopy compensation point model of background exchange is parameterised on the basis of measured data and applied for the purposes of gap-filling. The data show that, outside fertilisation events, grassland behaves as a net sink for atmospheric NH<sub>3</sub> with an annual dry deposition flux of −3.0 kg N ha<sup>−1</sup> yr<sup>−1</sup>, although small NH<sub>3</sub> emissions by the canopy were measured in dry daytime conditions. The median Γ<sub><i>s</i></sub> ratio in the apoplast (=[NH<sub>4</sub><sup>+</sup>]/[H<sup>+</sup>]) estimated from micrometeorological measurements was 620, equivalent to a stomatal compensation point of 1.3 μg NH<sub>3</sub> m<sup>−3</sup> at 15 °C. Non-stomatal resistance to deposition <i>R<sub>w</sub></i> was shown to increase with temperature and decrease with surface relative humidity, and <i>R<sub>w</sub></i> values were among the highest published for European grasslands, consistent with a relatively high ratio of NH<sub>3</sub> to acid gases in the boundary layer at this site. Since the gross annual NH<sub>3</sub> emission by slurry spreading was of the order of +20 kg N ha<sup>−1</sup> yr<sup>−1</sup>, the fertilised grassland was a net NH<sub>3</sub> source of +17 kg N ha<sup>−1</sup> yr<sup>−1</sup>. A comparison with the few other measurement-based budget values from the literature reveals considerable variability, demonstrating both the influence of soil, climate, management and grassland type on the NH<sub>3</sub> budget and the difficulty of scaling up to the national level

Technical note: Water vapour concentration and flux measurements with PTR-MS

International audienceThe most direct approach for measuring the exchange of biogenic volatile organic compounds between terrestrial ecosystems and the atmosphere is the eddy covariance technique. It has been applied several times in the last few years using fast response proton-transfer-reaction mass spectrometry (PTR-MS). We present an independent validation of this technique by applying it to measure the water vapour flux in comparison to a common reference system comprising an infra-red gas analyser (IRGA). Water vapour was detected in the PTR-MS at mass 37 (atomic mass units) corresponding to the cluster ion H3O+·H2O. During a five-week field campaign at a grassland site, we obtained a non-linear but stable calibration function between the mass 37 signal and the reference water vapour concentration. With a correction of the high-frequency damping loss based on empirical ogive analysis, the eddy covariance water vapour flux obtained with the PTR-MS showed a very good agreement with the flux of the reference system. The application of the empirical ogive method for high-frequency correction led to significantly better results than using a correction based on theoretical spectral transfer functions. This finding is attributed to adsorption effects on the tube walls that are presently not included in the theoretical correction approach. The proposed high-frequency correction method can also be used for other trace gases with different adsorption characteristics

Atmosphere-snow transfer function for H2O2: microphysical considerations

H2O2 analyses of polar ice cores show an increase in concentration from 200 years to the present. In order to quantitatively relate the observed trend in the ice to atmospheric levels, the atmosphere-snow transfer behavior and postdepositional changes must be known. Atmosphere-snow transfer was studied by investigating uptake and release of H2O2 in a series of laboratory column experiments in the temperature range −3°C to −45°C. Experiments consisted of passing H2O2-containing air through a column packed with 200-μm diameter ice spheres and measuring the change in gas phase H2O2 concentration with time. The uptake of H2O2 was a slow process requiring several hours to reach equilibrium. Uptake involved incorporation of H2O2 into the bulk ice as well as surface accumulation. The amount of H2O2 taken up by the ice was greater at the lower temperatures. The sticking coefficient for H2O2 on ice in the same experiments was estimated to be of the order of 0.02 to 0.5. Release of H2O2 from the ice occurred upon passing H2O2-free air through the packed columns, with the time scale for degassing similar to that for uptake. These results suggest that systematic losses of H2O2 from polar snow could occur under similar conditions, when atmospheric concentrations of H2O2 are low, that is, in the winter

Flux measurements of biogenic VOCs during ECHO 2003

International audienceWithin the framework of the AFO 2000 project ECHO, two PTR-MS instruments were operated in combination with sonic anemometers to determine biogenic VOC fluxes from a mixed deciduous forest site in North-Western Germany using the eddy covariance (EC) technique. The measurement site was characterised by a forest of inhomogeneous composition, complex canopy structure, limited extension in certain wind directions and frequent calm wind conditions during night time. As a consequence, a considerable fraction of the measurements did not qualify for flux calculations by EC and had to be discarded. The validated results show light and temperature dependent emissions of isoprene and monoterpenes from this forest, with average emissions (normalised to 30°C and 1000 µmoles m?2 s?1 PAR) of 1.5 and 0.39 µg m?2 s?1, respectively. Emissions of methanol reached on average 0.087 µg m?2 s?1 during daytime, but fluxes were too small to be detected during night time. Upward fluxes of the isoprene oxidation products methyl vinyl ketone (MVK) and methacrolein (MACR) were also found, being two orders of magnitude lower than those of isoprene. The observed fluxes are consistent with upscalings from leaf-level emission measurements of representative tree species in this forest and, in the case of MVK and MACR, can plausibly be explained by chemical production through oxidation of isoprene within the canopy. Calculations with an analytical footprint model indicate that the observed isoprene fluxes correlate with the fraction of oaks within the footprints of the flux measurement

Diel variations of H2O2 in Greenland: A discussion of the cause and effect relationship

Atmospheric hydrogen peroxide (H2O2) measurements at Summit, Greenland, in May–June, 1993 exhibited a diel variation, with afternoon highs typically 1–2 parts per billion by volume (ppbv) and nighttime lows about 0.5 ppbv lower. This variation closely followed that for temperature; specific humidity exhibited the same general trend. During a 17-day snowfall-free period, surface snow was accumulating H2O2, apparently from nighttime cocondensation of H2O and H2O2. Previous photochemical modeling (Neftel et al., 1995) suggests that daytime H2O2 should be about 1 ppbv, significantly lower than our measured values. Previous equilibrium partitioning measurements between ice and gas phase (Conklin et al., 1993) suggest that air in equilibrium with H2O2 concentrations measured in surface snow (15–18 μM) should have an H2O2 concentration 2–3 times what we measured 0.2–3.5 m above the snow surface. A simple eddy diffusion model, with vertical eddy diffusion coefficients calculated from balloon soundings, suggested that atmospheric H2O2 concentrations should be affected by any H2O2 degassed from surface snow. However, field measurements showed the absence of either high concentrations of H2O2 or a measurable concentration gradient between inlets 0.2 and 3 m above the snow. A surface resistance to degassing, that is, slow release of H2O2 from the ice matrix, is a plausible explanation for the differences between observations and modeled atmospheric profiles. Degassing of H2O2 at a rate below our detection limit would still influence measured atmospheric concentrations and help explain the difference between measurements and photochemical modeling. The cumulative evidence suggests that surface snow adjusts slowly to drops in atmospheric H2O2 concentration, over timescales of at least weeks. The H2O2 losses previously observed in pits sampled over more than 1 year are thought to have occurred later in the summer or fall, after the May–July field season

Abyssal Atlantic circulation during the Last Glacial Maximum: Constraining the ratio between transport and vertical mixing

The ocean’s role in regulating atmospheric carbon dioxide on glacial‐interglacial timescales remains an unresolved issue in paleoclimatology. Reduced mixing between deep water masses may have aided oceanic storage of atmospheric CO_2 during the Last Glacial Maximum (LGM), but data supporting this idea have remained elusive. The δ^(13)C of benthic foraminifera indicate the Atlantic Ocean was more chemically stratified during the LGM, but the nonconservative nature of δ^(13)C complicates interpretation of the LGM signal. Here we use benthic foraminiferal δ^(18)O as a conservative tracer to constrain the ratio of meridional transport to vertical diffusivity in the deep Atlantic. Our calculations suggest that the ratio was at least twice as large at the LGM. We speculate that the primary cause was reduced mixing between northern and southern component waters, associated with movement of this water mass boundary away from the zone of intense mixing near the seafloor. The shallower water mass boundary yields an order of magnitude increase in the volume of southern component water, suggesting its residence time may have increased substantially. Our analysis supports the idea that an expanded volume of Antarctic Bottom Water and limited vertical mixing enhanced the abyssal ocean’s ability to trap carbon during glacial times