Absence of Nodal signaling promotes precocious neural differentiation in the mouse embryo

AbstractAfter implantation, mouse embryos deficient for the activity of the transforming growth factor-β member Nodal fail to form both the mesoderm and the definitive endoderm. They also fail to specify the anterior visceral endoderm, a specialized signaling center which has been shown to be required for the establishment of anterior identity in the epiblast. Our study reveals that Nodal−/− epiblast cells nevertheless express prematurely and ectopically molecular markers specific of anterior fate. Our analysis shows that neural specification occurs and regional identities characteristic of the forebrain are established precociously in the Nodal−/− mutant with a sequential progression equivalent to that of wild-type embryo. When explanted and cultured in vitro, Nodal−/− epiblast cells readily differentiate into neurons. Genes normally transcribed in organizer-derived tissues, such as Gsc and Foxa2, are also expressed in Nodal−/− epiblast. The analysis of Nodal−/−;Gsc−/− compound mutant embryos shows that Gsc activity plays no critical role in the acquisition of forebrain characters by Nodal-deficient cells. This study suggests that the initial steps of neural specification and forebrain development may take place well before gastrulation in the mouse and highlights a possible role for Nodal, at pregastrula stages, in the inhibition of anterior and neural fate determination

Regionalization of the mouse visceral endoderm as the blastocyst transforms into the egg cylinder.

BACKGROUND: Reciprocal interactions between two extra-embryonic tissues, the extra-embryonic ectoderm and the visceral endoderm, and the pluripotent epiblast, are required for the establishment of anterior-posterior polarity in the mouse. After implantation, two visceral endoderm cell types can be distinguished, in the embryonic and extra-embryonic regions of the egg cylinder. In the embryonic region, the specification of the anterior visceral endoderm (AVE) is central to the process of anterior-posterior patterning. Despite recent advances in our understanding of the molecular interactions underlying the differentiation of the visceral endoderm, little is known about how cells colonise the three regions of the tissue. RESULTS: As a first step, we performed morphological observations to understand how the extra-embryonic region of the egg cylinder forms from the blastocyst. Our analysis suggests a new model for the formation of this region involving cell rearrangements such as folding of the extra-embryonic ectoderm at the early egg cylinder stage. To trace visceral endoderm cells, we microinjected mRNAs encoding fluorescent proteins into single surface cells of the inner cell mass of the blastocyst and analysed the distribution of labelled cells at E5.0, E5.5 and E6.5. We found that at E5.0 the embryonic and extra-embryonic regions of the visceral endoderm do not correspond to distinct cellular compartments. Clusters of labelled cells may span the junction between the two regions even after the appearance of histological and molecular differences at E5.5. We show that in the embryonic region cell dispersion increases after the migration of the AVE. At this time, visceral endoderm cell clusters tend to become oriented parallel to the junction between the embryonic and extra-embryonic regions. Finally we investigated the origin of the AVE and demonstrated that this anterior signalling centre arises from more than a single precursor between E3.5 and E5.5. CONCLUSION: We propose a new model for the formation of the extra-embryonic region of the egg cylinder involving a folding of the extra-embryonic ectoderm. Our analyses of the pattern of labelled visceral endoderm cells indicate that distinct cell behaviour in the embryonic and extra-embryonic regions is most apparent upon AVE migration. We also demonstrate the polyclonal origin of the AVE. Taken together, these studies lead to further insights into the formation of the extra-embryonic tissues as they first develop after implantation.RIGHTS : This article is licensed under the BioMed Central licence at http://www.biomedcentral.com/about/license which is similar to the 'Creative Commons Attribution Licence'. In brief you may : copy, distribute, and display the work; make derivative works; or make commercial use of the work - under the following conditions: the original author must be given credit; for any reuse or distribution, it must be made clear to others what the license terms of this work are

Investigation of photocurrents resulting from a living unicellular algae suspension with quinones over time

International audiencePlants, algae, and some bacteria convert solar energy into chemical energy by using photosynthesis. In light of the current energy environment, many research strategies try to benefit from photosynthesis in order to generate usable photobioelectricity. Among all the strategies developed for transferring electrons from the photosynthetic chain to an outer collecting electrode, we recently implemented a method on a preparative scale (high surface electrode) based on a Chlamydomonas reinhardtii green algae suspension in the presence of exogenous quinones as redox mediators. While giving rise to an interesting performance (10-60 mA cm À2) in the course of one hour, this device appears to cause a slow decrease of the recorded photocurrent. In this paper, we wish to analyze and understand this gradual fall in performance in order to limit this issue in future applications. We thus first show that this kind of degradation could be related to over-irradiation conditions or side-effects of quinones depending on experimental conditions. We therefore built an empirical model involving a kinetic quenching induced by incubation with quinones, which is globally consistent with the experimental data provided by fluorescence measurements achieved after dark incubation of algae in the presence of quinones

miRNA in embryonic development: the taming of Nodal signaling.

International audienceThe TGF-beta family member Nodal is one of the most potent molecules known to an embryo. Studies on Nodal regulation thus far have focused on its transcription, maturation, and interaction with antagonists. Two recent studies on the role of microRNAs (miRNAs) in Nodal signaling now reveal that something important was missing from the picture

Faut-il faire évoluer la prise en charge de l'infection par helicobacter pylori?

L'infection à Helicobacter pylori (Hp) est associée à de nombreuses maladies gastro-duodénales : dyspepsie, maladie ulcéreuse gastro-duodénale liée ou non à la prise d'aspirine ou d'anti-inflammatoires, reflux gastro-oesophagien, lymphome du MALT et cancer gastrique. Selon la conférence de consensus de 1999 la recherche d'Hp et son traitement sont recommandés uniquement chez les patients ayant des lésions ulcérées et/ou un lymphome gastrique du MALT, pour qui la prise en charge recommandée est toujours d'actualité. Selon certaines études le bénéfice de l'éradication d'Hp est faible dans la dyspepsie et non justifié dans le reflux gastro-oesophagien où l'infection par Hp pourrait avoir un rôle protecteur. D'autres études suggèrent l'intérêt de l'éradication préventive avant la prise d'anti-inflammatoires ou par aspirine à faible dose. Les progrès les plus importants réalisés ces dernières années concernent Hp et le cancer gastrique en faveur d'une éradication de la bactérie chez les parents du premier degré des sujets ayant un cancer gastrique. En conclusion, il n'apparaît pas justifié pour le moment de modifier les recommandations de la conférence de consensus de 1999, mais il semble opportun de modifier la prise en charge au cas par cas.CHATENAY M.-PARIS 11-BU Pharma. (920192101) / SudocSudocFranceF

Etude fonctionnelle d'un élément auto-régulateur du gène Nodal au cours du développement précoce de la souris

Le travail décrit dans ce manuscrit est une étude du fonctionnement de la voie de signalisation du facteur de type TGF-beta Nodal pendant l embryogenèse précoce de la souris. Ce travail est centré sur l analyse de l activité de l ASE, une séquence régulatrice du gène Nodal. Cette séquence est une cible de la voie de signalisation Smad2,3, qui transduit le signal Nodal, et permet donc au gène Nodal d amplifier sa propre expression. Des lignées transgéniques où l expression de protéines fluorescentes est placée sous le contrôle de l ASE avaient été générées au laboratoire. La caractérisation de ces lignées et leur comparaison avec d autres lignées transgéniques a permis de les valider en tant que lignées rapportrices de l activité de =l élément régulateur ASE. Une lignée ASE-YFP a été retenue pour la suite de l étude. L analyse du profil d expression du transgène a révélé qu il est actif dès les stades pré-implantatoires. Ceci a suggéré implication de l ASE dans l initiation de l expression de Nodal. Entre E3,5 et E4,5, nous avons observé que l expansion de l activité du transgène passe par des périodes alternées de relative stabilité et d accroissement rapide. Deux courtes périodes de propagation rapide de l activité de l ASE semblent correspondre à la différenciation de l endoderme primitif et à l implantation, deux événements majeurs du développement embryonnaire à ces stades. L observation que le transgène ASE-YFP à E3,5 est exprimé façon poivre et sel dans les cellules de l ICM a soulevé une interrogation quand aux destins relatifs des deux populations de cellules ainsi identifiées au sein de ce tissu pluripotent. Des études de co-marquage ont permis de montrer que l activité du transgène ASE-YFP marque indifféremment aussi bien des précurseurs de l épiblaste, qui expriment le facteur de pluripotence Nanog, que des précurseurs de l endoderme primitif, qui expriment Gata4. Cependant, les co-marquages réalisés après la formation de l endoderme primitif indiquent qu au sein de l épiblaste l expansion du nombre de cellules exprimant le transgène ASE-YFP est concomitante d une diminution du nombre de cellules exprimant Nanog. De façon intéressante, le pool des cellules co-exprimant Nanog et le transgène ASE-YFP décroît plus lentement que la population des cellules exprimant Nanog seul. Ceci apporte un éclairage nouveau aux données impliquant la signalisation Nodal dans le maintien de la pluripotence, et suggère que l activité de la voie de signalisation de Nodal et le facteur Nanog, pourraient être liés via l ASE au sein d une même boucle de régulation.Enfin, la régulation de l ASE aux stades pré et péri-implantatoires a été étudiée par une approche combinant génétique et inhibition pharmacologique. L utilisation d un inhibiteur spécifique des récepteurs ALK4, 5, 7 qui normalement participent à l activation de Smad2,3, a permis de démontrer que l activation du transgène est bien dépendante de cette voie de signalisation à ces stades. Cependant, le maintien de l activité du transgène ASE-YFP en contexte mutant pour Nodal indique qu un autre ligand que Nodal contribue alors à l activité de la voie. De même, le maintien de l activité du transgène en contexte mutant pour Foxh1, l effecteur classique de Smad2,3 dans l épiblaste, confirme que d autres acteurs que ceux déjà identifiés sont impliqués dans l activité de la voie Smad2,3 à ces stades. Il est à noter que l extinction du transgène par l inhibiteur pharmacologique ne s accompagne pour les embryons traités d aucun effet visible sur leur croissance ou leur physiologie. L ensemble de ces travaux précise la façon dont Nodal est exprimé et régulé aux stades pré et péri-implantatoires mais pose in fine la question du rôle de cette molécule et de sa voie de signalisation à ces stades.PARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Activin/Nodal signalling before implantation: setting the stage for embryo patterning.

International audienceActivins and Nodal are members of the transforming growth factor beta (TGF-β) family of growth factors. Their Smad2/3-dependent signalling pathway is well known for its implication in the patterning of the embryo after implantation. Although this pathway is active early on at preimplantation stages, embryonic phenotypes for loss-of-function mutations of prominent components of the pathway are not detected before implantation. It is only fairly recently that an understanding of the role of the Activin/Nodal signalling pathway at these stages has started to emerge, notably from studies detailing how it controls the expression of target genes in embryonic stem cells. We review here what is currently known of the TGF-β-related ligands that determine the activity of Activin/Nodal signalling at preimplantation stages, and recent advances in the elucidation of the Smad2/3-dependent mechanisms underlying developmental progression

[Epigenetic regulation of pluripotency in the early mouse embryo]

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A novel Nodal enhancer dependent on pluripotency factors and Smad2/3 signaling conditions a regulatory switch during epiblast maturation

During early development, modulations in the expression of Nodal, a TGFβ family member, determine the specification of embryonic and extra-embryonic cell identities. Nodal has been extensively studied in the mouse, but aspects of its early expression remain unaccounted for. We identified a conserved hotspot for the binding of pluripotency factors at the Nodal locus and called this sequence “highly bound element” (HBE). Luciferase-based assays, the analysis of fluorescent HBE reporter transgenes, and a conditional mutation of HBE allowed us to establish that HBE behaves as an enhancer, is activated ahead of other Nodal enhancers in the epiblast, and is essential to Nodal expression in embryonic stem cells (ESCs) and in the mouse embryo. We also showed that HBE enhancer activity is critically dependent on its interaction with the pluripotency factor Oct4 and on Activin/Nodal signaling. Use of an in vitro model of epiblast maturation, relying on the differentiation of ESCs into epiblast stem cells (EpiSCs), revealed that this process entails a shift in the regulation of Nodal expression from an HBE-driven phase to an ASE-driven phase, ASE being another autoregulatory Nodal enhancer. Deletion of HBE in ESCs or in EpiSCs allowed us to show that HBE, although not necessary for Nodal expression in EpiSCs, is required in differentiating ESCs to activate the differentiation-promoting ASE and therefore controls this regulatory shift. Our findings clarify how early Nodal expression is regulated and suggest how this regulation can promote the specification of extra-embryonic precusors without inducing premature differentiation of epiblast cells. More generally, they open new perspectives on how pluripotency factors achieve their function