Expression of zebrafish pax6b in pancreas is regulated by two enhancers containing highly conserved cis-elements bound by PDX1, PBX and PREP factors

BACKGROUND: PAX6 is a transcription factor playing a crucial role in the development of the eye and in the differentiation of the pancreatic endocrine cells as well as of enteroendocrine cells. Studies on the mouse Pax6 gene have shown that sequences upstream from the P0 promoter are required for expression in the lens and the pancreas; but there remain discrepancies regarding the precise location of the pancreatic regulatory elements. RESULTS: Due to genome duplication in the evolution of ray-finned fishes, zebrafish has two pax6 genes, pax6a and pax6b. While both zebrafish pax6 genes are expressed in the developing eye and nervous system, only pax6b is expressed in the endocrine cells of the pancreas. To investigate the cause of this differential expression, we used a combination of in silico, in vivo and in vitro approaches. We show that the pax6b P0 promoter targets expression to endocrine pancreatic cells and also to enteroendocrine cells, retinal neurons and the telencephalon of transgenic zebrafish. Deletion analyses indicate that strong pancreatic expression of the pax6b gene relies on the combined action of two conserved regulatory enhancers, called regions A and C. By means of gel shift assays, we detected binding of the homeoproteins PDX1, PBX and PREP to several cis-elements of these regions. In constrast, regions A and C of the zebrafish pax6a gene are not active in the pancreas, this difference being attributable to sequence divergences within two cis-elements binding the pancreatic homeoprotein PDX1. CONCLUSION: Our data indicate a conserved role of enhancers A and C in the pancreatic expression of pax6b and emphasize the importance of the homeoproteins PBX and PREP cooperating with PDX1, in activating pax6b expression in endocrine pancreatic cells. This study also provides a striking example of how adaptative evolution of gene regulatory sequences upon gene duplication progressively leads to subfunctionalization of the paralogous gene pair

Effet de fortes teneurs en hydrogène sur les propriétés métallurgiques et mécaniques des gaines en alliage de zirconium après incursion à haute température

Under hypothetical loss-of-coolant accident conditions, fuel cladding tubes made of zirconium alloys can be exposed to steam at high temperature (up 1 200°C) before being cooled and then quenched in water. In some conditions, after burst occurrence the cladding can rapidly absorb a significant amount of hydrogen (secondary hydriding), up to 3 000wt.ppm locally, during steam exposition at HT.The study deals with the effect, poorly studied up to date, of high contents of hydrogen on the metallurgical and mechanical properties of two zirconium alloys, Zircaloy-4 and M5®, during and after cooling from high temperatures, at which zirconium is in its β phase. A specific facility was developed to homogeneously charge in hydrogen up to ~3 000wt.mass. cladding tube samples of several centimeters in length. Phase transformations, chemical element partitioning and hydrogen precipitation during cooling from the β temperature domain of zirconium were studied by using several techniques, for the materials containing up to ~3 000wt.ppm of hydrogen in average: in-situ neutron diffraction upon cooling from 700°C, X-ray diffraction, µ-ERDA, EPMA and electron microscopy in particular. The results were compared to thermodynamic predictions. In order to study the effect of high hydrogen contents on the mechanical behavior of the (prior-)β phase of zirconium, axial tensile tests were performed à various temperatures between 20 and 700°C upon cooling from the β temperature domain, on samples with mean hydrogen contents up to ~3 000ppm-mass.The results show that metallurgical and mechanical properties of the (prior-)β phase of zirconium alloys strongly depend on temperature and hydrogen content.En conditions hypothétiques d’accident par perte de réfrigérant primaire, la gaine en alliage de zirconium des crayons combustibles des réacteurs nucléaires à eau pressurisée peut être temporairement exposée à de la vapeur d’eau à haute température (jusqu’à 1 200°C) avant d’être refroidie puis trempée à l’eau. Dans certaines conditions, après éclatement, la gaine peut absorber une quantité très importante d’hydrogène (hydruration secondaire), pouvant atteindre 3 000ppm mass. localement, lors du maintien sous vapeur d’eau à haute température.Cette étude porte sur l’effet, peu étudié jusqu’alors, de fortes concentrations en hydrogène sur les caractéristiques métallurgiques et mécaniques de deux alliages de zirconium, le Zircaloy-4 et le M5®, au cours et après refroidissement depuis le domaine des hautes températures, auxquelles le zirconium est en phase β. Un protocole a été mis au point afin de charger en hydrogène, de manière homogène jusqu’à ~3 000ppm-mass., des tronçons de tube de gainage de plusieurs centimètres de long. Les transformations de phases, la ségrégation des éléments chimiques et la précipitation des hydrures lors du refroidissement depuis le domaine d’existence de la phase β du zirconium ont été étudiées, pour les matériaux contenant jusqu’à ~3 000ppm-mass. d’hydrogène en moyenne, au moyen de différentes techniques : diffraction de neutrons in-situ en cours de refroidissement depuis 700°C, diffraction de rayons X, µ-ERDA, et microscopie électronique notamment. Les résultats ont été confrontés à des prévisions thermodynamiques. Puis, pour étudier l’effet de fortes teneurs en hydrogène sur le comportement mécanique de la phase (ex-)β du zirconium, des essais de traction axiale ont été effectués à différentes températures entre 20 et 700°C, au cours du refroidissement depuis le domaine de phase β, sur des échantillons contenant jusqu’à ~3 000ppm-mass. d’hydrogène en moyenne. Les résultats montrent que les propriétés métallurgiques et mécaniques de la phase (ex-)β des alliages de zirconium dépendent fortement de la température et de la teneur en hydrogène

Influence of high hydrogen content, on microstructure and mechanical behaviour, of zirconium alloys fuel cladding upon and after cooling from high temperature

En conditions hypothétiques d’accident par perte de réfrigérant primaire, la gaine en alliage de zirconium des crayons combustibles des réacteurs nucléaires à eau pressurisée peut être temporairement exposée à de la vapeur d’eau à haute température (jusqu’à 1 200°C) avant d’être refroidie puis trempée à l’eau. Dans certaines conditions, après éclatement, la gaine peut absorber une quantité très importante d’hydrogène (hydruration secondaire), pouvant atteindre 3 000ppm mass. localement, lors du maintien sous vapeur d’eau à haute température.Cette étude porte sur l’effet, peu étudié jusqu’alors, de fortes concentrations en hydrogène sur les caractéristiques métallurgiques et mécaniques de deux alliages de zirconium, le Zircaloy-4 et le M5®, au cours et après refroidissement depuis le domaine des hautes températures, auxquelles le zirconium est en phase β. Un protocole a été mis au point afin de charger en hydrogène, de manière homogène jusqu’à ~3 000ppm-mass., des tronçons de tube de gainage de plusieurs centimètres de long. Les transformations de phases, la ségrégation des éléments chimiques et la précipitation des hydrures lors du refroidissement depuis le domaine d’existence de la phase β du zirconium ont été étudiées, pour les matériaux contenant jusqu’à ~3 000ppm-mass. d’hydrogène en moyenne, au moyen de différentes techniques : diffraction de neutrons in-situ en cours de refroidissement depuis 700°C, diffraction de rayons X, µ-ERDA, et microscopie électronique notamment. Les résultats ont été confrontés à des prévisions thermodynamiques. Puis, pour étudier l’effet de fortes teneurs en hydrogène sur le comportement mécanique de la phase (ex-)β du zirconium, des essais de traction axiale ont été effectués à différentes températures entre 20 et 700°C, au cours du refroidissement depuis le domaine de phase β, sur des échantillons contenant jusqu’à ~3 000ppm-mass. d’hydrogène en moyenne. Les résultats montrent que les propriétés métallurgiques et mécaniques de la phase (ex-)β des alliages de zirconium dépendent fortement de la température et de la teneur en hydrogène.Under hypothetical loss-of-coolant accident conditions, fuel cladding tubes made of zirconium alloys can be exposed to steam at high temperature (up 1 200°C) before being cooled and then quenched in water. In some conditions, after burst occurrence the cladding can rapidly absorb a significant amount of hydrogen (secondary hydriding), up to 3 000wt.ppm locally, during steam exposition at HT.The study deals with the effect, poorly studied up to date, of high contents of hydrogen on the metallurgical and mechanical properties of two zirconium alloys, Zircaloy-4 and M5®, during and after cooling from high temperatures, at which zirconium is in its β phase. A specific facility was developed to homogeneously charge in hydrogen up to ~3 000wt.mass. cladding tube samples of several centimeters in length. Phase transformations, chemical element partitioning and hydrogen precipitation during cooling from the β temperature domain of zirconium were studied by using several techniques, for the materials containing up to ~3 000wt.ppm of hydrogen in average: in-situ neutron diffraction upon cooling from 700°C, X-ray diffraction, µ-ERDA, EPMA and electron microscopy in particular. The results were compared to thermodynamic predictions. In order to study the effect of high hydrogen contents on the mechanical behavior of the (prior-)β phase of zirconium, axial tensile tests were performed à various temperatures between 20 and 700°C upon cooling from the β temperature domain, on samples with mean hydrogen contents up to ~3 000ppm-mass.The results show that metallurgical and mechanical properties of the (prior-)β phase of zirconium alloys strongly depend on temperature and hydrogen content

A micromechanical analysis of swelling-induced embrittlement in neutron-irradiated austenitic stainless steels

International audienceSwelling is commonly observed in austenitic stainless steels irradiated at high doses and high temperatures, such as fuel claddings in Fast-Breeder Reactors FBR), as a consequence of the formation of irradiation-induced nanovoids. Macroscopic embrittlement has been reported above a critical swelling level. Several physical explanations have been proposed such as void-related fracture mechanisms and phase transformation associated with local changes of microchemistry.In addition, structural effects can affect the critical swelling level, tentatively explained by local corrosion, swelling gradient and bending effects. In order to ddress these phenomena, an extended experimental database is first presented, based on fuel pins irradiated in the Phénix fast reactor, showing the evolution of conventional tensile properties with swelling. SEM and TEM analysis are also summarized to highlight specific embrittlement mechanisms related to thepresence of voids. Micromechanical simulations are then performed based on porous materials constitutive equations in order to rationalize the experimental observations. Analytical and numerical results show that swelling-induced embrittlement can be understood from a mechanical perspective as a transition from void growth to void coalescence deformation mode. The effect of the spatial heterogeneity of void distribution is quantified and shown to be a key parameter. Structural effects coming from tests performed on ring and axial tensile specimens and from the presence of a swelling gradient in cladding thickness are also quantified. Numerical results are compared to experimental data, and a good agreement is observed

Met909 plays a key role in the activation of the progesterone receptor and also in the high potency of 13-ethyl progestins

ABSTRACT Many progestins have been developed for use in contraception, menopausal hormone therapy, and treatment of gynecological diseases. They are derived from either progesterone or testosterone, and they act by binding to the progesterone receptor (PR), a hormone-inducible transcription factor belonging to the nuclear receptor superfamily. Unlike mineralocorticoid, glucocorticoid, and androgen receptors, the steroid-receptor contacts that trigger the switch of the ligand-binding domain from an inactive to an active conformation have not yet been identified for the PR. With this aim, we solved the crystal structure of the ligand-binding domain of the human PR complexed with levonorgestrel, a potent testosterone-derived progestin characterized by a 13-ethyl substituent. Via mutagenesis analysis and functional studies, we identified Met909 of the helix 12 as the key residue for PR activation by both testosterone-and progesterone-derived progestins with a 13-methyl or a 13-ethyl substituent. We also showed that Asn719 contributes to PR activation by testosterone-derived progestins only, and that Met759 and Met909 are responsible for the high potency of 19-norprogestins and of 13-ethyl progestins, respectively. Our findings provide a structural guideline for the rational synthesis of potent PR agonist and antagonist ligands that could have therapeutic uses in women's health

Mechanical behavior at high temperatures of highly oxygen- or hydrogen-enriched α and (Prior-) β phases of zirconium alloys

International audienceMechanical behavior at high temperature of highly oxygen-or hydrogen-enriched α and (prior-) β phases of zirconium alloys ABSTRACT: During a hypothetical loss-of-coolant accident (LOCA), zirconium alloy fuel claddings can be loaded by internal pressure and exposed to steam at high temperature (HT, potentially up to 1200°C), then cooled and water quenched. A significant fraction of the oxygen reacting with the cladding during HT oxidation diffuses beneath the oxide through the metallic substrate. This induces a progressive transformation of the metallic βZr phase layer into an intermediate layer of αZr(O) phase containing up to 7 wt.% of oxygen. Furthermore, in some specific conditions, the cladding may rapidly absorb a significant amount of hydrogen during steam exposition at HT. Being a βZr-stabilizer, hydrogen would mainly diffuse and concentrate up to several thousands of wt.ppm into the inner βZr phase layer. Oxygen and hydrogen are known to modify the metallurgical and mechanical properties of zirconium alloys but data are scarce for high contents, especially at HT. However, such data are important basic components to improve the assessment of the oxidized cladding mechanical behavior and integrity during and after LOCA-like thermal-mechanical transients. This study intended to provide new, more comprehensive data on the HT mechanical behavior of the αZr(O) and the (prior-) βZr phases containing high contents of oxygen and hydrogen, respectively. Model samples, produced from M5® 5 and Zircaloy-4 cladding tubes, homogeneously charged in oxygen (≤6 wt.%) and in hydrogen (≤3000 wt.ppm) respectively, were prepared. Their mechanical behavior was determined under vacuum between 800 and 1100°C for the oxygen-enriched αZr phase, and in air between 700 and 20°C, after cooling from the βZr temperature domain, for the hydrogen-enriched (prior-) βZr phase. The αZr phase is substantially strengthened and embrittled by oxygen. Power-law and nearly linear creep regimes are observed and were modelled for stress levels beyond and below 15 MPa, respectively. The model αZr(O) material experiences a ductile-to-brittle transition at 1000-1100°C for oxygen contents between 3.4 and 4.3 wt.%. The viscoplastic behavior of the αZr(O) phase was used to evaluate the contribution of the αZr(O) layer to the HT creep behavior of an oxidized fuel cladding tube subjected to internal pressure. The model (prior-) βZr phase becomes macroscopically brittle at temperatures ≤135°C and ≤350-400°C for average hydrogen content

Phase transformations during cooling from the βZr phase temperature domain in several hydrogen-enriched zirconium alloys studied by in situ and ex situ neutron diffraction

International audienceIn hypothetical accidental conditions, zirconium-based nuclear fuel claddings can absorb high hydrogen contents (up to several thousand wppm) and be exposed to high temperatures (βZr phase temperature range) before being cooled. This paper thoroughly investigates the microstructural and microchemical evolutions that take place in such conditions. Two zirconium-based alloys and unalloyed zirconium were pre-charged with hydrogen at various contents up to 3300 wppm and heat-treated at 1000 or 850 °C. Neutron diffraction analyses were performed in situ upon slow cooling from 700 °C and at room temperature. In the materials containing 3300 wppm of hydrogen, βZr progressively transforms into αZr during slow cooling then extensively transforms into αZr and δZrH2-x hydrides precipitate via a eutectoid reaction. Thermodynamic predictions at equilibrium are in good agreement with the experimental results. However, depending on the cooling scenario and the average hydrogen content, the precipitation of γZrH hydrides, potentially metastable, is evidenced below 350 °C and a significant amount of hydrogen can remain in solid solution in αZr. These metallurgical evolutions and the evolution of the different phase lattice parameters are strongly influenced by the partitioning of oxygen and hydrogen (revealed by electron probe and elastic recoil detection microanalyses) that occurs during the βZr to αZr transformation and hydride precipitation