Interprétation d’un signal graduel en une décision cellulaire binaire : une approche quantitative basée sur l’induction neurale de l’ascidie

To address how a cell interprets a graded signal to generate a threshold response, I studied the initial step of ascidian neural induction. During this process, four ectoderm cells among sixteen are selected as neural precursors. FGF9/16/20, derived from mesendoderm cells, acts as a neural inducer and activates Otx expression through the ERK pathway. Quantitative measurement of cell surface contacts (CSC) between ectoderm cells and FGF-expressing cells has revealed that each ectoderm cell is exposed to FGF, with neural precursors having the largest area of CSC with FGF-expressing cells. Using quantitative measurements of endogenous ERK activation and Otx expression, we have revealed that each ectoderm cell exhibits a level of ERK activation corresponding to its area of CSC with FGF-expressing mesendoderm cells while Otx expression is restricted to only the four neural precursors. An ephrin ligand is expressed in the ectoderm cells and act antagonistically to FGF signals. In embryos inhibited for ephrin/Eph signals, ERK levels are increased in all ectoderm cells, proportionally to their CSC with the FGF-expressing cells. Under these conditions, the spatial precision of Otx expression is lost with additional ectoderm cells exhibiting Otx expression suggesting that ephrin/Eph signals act to reduce the overall levels of ERK activation, such that the non-neural ectoderm cells remain below the threshold required for Otx activation. We tested this hypothesis by treating embryos, in which ephrin signals were blocked, with low doses of the MEK inhibitor U0126. This treatment was sufficient to re-establish the normal Otx expression profile in the four neural precursors.Afin de comprendre comment les cellules interprètent un signal graduel en une réponse binaire, j’ai étudié l’induction neurale de l’ascidie. Lors de ce processus, quatre cellules ectodermiques parmi les seize vont adopter un destin neural. FGF9/16/20, exprimé par les cellules mésendodermiques, est l’inducteur neural et active l’expression du gène Otx via ERK. Les surfaces de contact (SC) quantifiées entre les cellules ectodermiques et les cellules mésendodermiques exprimant FGF montrent que chaque cellule ectodermique est exposée à FGF et que les précurseurs neuraux ont les plus grandes SC avec les cellules mésendodermiques. En quantifiant l’activation d’ERK et de l’expression d’Otx, nous avons montré que chaque cellule ectodermique montre un niveau d’activation d’ERK proportionnel à sa SC avec les cellules exprimant FGF alors que l’expression d’Otx est restreinte aux quatre précurseurs neuraux. Un ligand ephrine est exprimé par les cellules ectodermiques et joue un rôle antagoniste à FGF. Dans les embryons où le signal ephrine a été bloqué, le niveau d’ERK augmente dans toutes les cellules ectodermiques proportionnellement aux SC avec les cellules exprimant FGF. Dans ces conditions, d’autres cellules ectodermiques montrent une expression ectopique d’Otx. Cela suggère que le signal ephrine réduit le niveau d’activation total d’ERK pour maintenir les cellules ectodermiques non neural sous le seuil requis pour l’expression d’Otx. Nous avons testé cette hypothèse en traitant des embryons, où la voie ephrine était inhibée, avec de faibles doses d’UO126 (inhibiteur de MEK). Ce traitement a suffi à rétablir l’expression normale d’Otx dans les quatre précurseurs neuraux

Model of neural induction in the ascidian embryo

Abstract How cell specification can be controlled in a reproducible manner is a fundamental question in development biology. In ascidians, a group of marine invertebrate chordates, geometry plays a key role in achieving this control. Here, we use mathematical modeling to demonstrate that geometry dictates the neural-epidermal cell fate choice in the 32-cell stage ascidian embryo by a two-step process involving first the modulation of ERK signaling and second, the expression of the neural marker gene, Otx . The model describes signal transduction by the ERK pathway that is stimulated by FGF and repressed by ephrin, and ERK-mediated control of Otx gene expression, which involves both an activator and an inhibitor of transcription. Considering the measured area of cell surface contacts with FGF- or ephrin-expressing cells as inputs, the solutions of the model reproduce the experimental observations about ERK activation and Otx expression in the different cells under normal and perturbed conditions. Sensitivity analyses and computations of Hill coefficients allow to quantify the robustness of the specification mechanism controlled by cell surface area and to identify the respective role played by each signaling input. Simulations also predict in which conditions the dual control of gene expression by an activator and an inhibitor that are both under the control of ERK can induce a robust ON/OFF control of neural fate induction. Author summary The development of a single cell zygote into a multicellular embryo occurs thanks to the combination of cell division and cell specification. The latter process corresponds to the progressive acquisition by the embryonic cells of their final physiological and functional characteristics, which rely on well-defined signaling-controlled genetic programs. The origin of the great robustness of cell specification remains poorly understood. Here, we address this question in the framework of the embryonic neural fate induction in ascidians, which are marine invertebrates. At the 32-cells stage, four cells identified by their precise location in the embryo adopt neural fate. On the basis of experimental observations, we develop a mathematical model that predicts that the choice between the neural or epidermal fate is controlled by the cell surface areas of the cells in contact with two antagonistic signals, FGF and ephrin. Our findings provide a computational confirmation of the major role played by the geometry of the embryo in controlling cell lineage acquisition during ascidian development

Cell geometry, signal dampening, and a bimodal transcriptional response underlie the spatial precision of an ERK-mediated embryonic induction

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An ERF transcription factor participates in the bimodal transcriptional response of an ERK-target gene during ascidian neural induction

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An ERF Transcription factor participates in the bimodal transcriptional response of an ERK-target gene during ascidian neural induction.

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An ERF transcription factor participates in the bimodal transcriptional response of an ERK-target gene during ascidian neural induction

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