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Molecular integration of casanova in the Nodal signalling pathway controlling endoderm formation

Tazu O. Aoki^1,*, Nicolas B. David^1,*, Gabriella Minchiotti², Laure Saint-Etienne¹, Thomas Dickmeis³, Graziella M. Persico², Uwe Strähle³, Philippe Mourrain¹ and Frédéric M. Rosa^1,

¹ U 368 INSERM, Ecole Normale Supérieure, 46, rue d’Ulm, F-75230 Paris Cedex 05, France
² International Institute of Genetics and Biophysics, CNR, Via G. Marconi, 12-80125 Naples, Italy
³ Institut de Génétique et de Biologie Moléculaire et Cellulaire, CNRS/INSERM/ULP, BP 163, F-67404 Illkirch Cedex, CU de Strasbourg, France
^* These two authors contributed equally to this work

Author for correspondence (e-mail: rosa{at}wotan.ens.fr)

Accepted 24 October 2001

Endoderm originates from a large endomesodermal field requiring Nodal signalling. The mechanisms that ensure segregation of endoderm from mesoderm are not fully understood. We first show that the timing and dose of Nodal activation are crucial for endoderm formation and the endoderm versus mesoderm fate choice, because sustained Nodal signalling is required to ensure endoderm formation but transient signalling is sufficient for mesoderm formation. In zebrafish, downstream of Nodal signals, three genes encoding transcription factors (faust, bonnie and clyde and the recently identified gene casanova) are required for endoderm formation and differentiation. However their positions within the pathway are not completely established. In the present work, we show that casanova is the earliest specification marker for endodermal cells and that its expression requires bonnie and clyde. Furthermore, we have analysed the molecular activities of casanova on endoderm formation and found that it can induce endodermal markers and repress mesodermal markers during gastrulation, as well as change the fate of marginal blastomeres to endoderm. Overexpression of casanova also restores endoderm markers in the absence of Nodal signalling. In addition, casanova efficiently restores later endodermal differentiation in these mutants, but this process requires, in addition, a partial activation of Nodal signalling.

Key words: casanova, Endoderm, Nodal, Cripto, Rescue, Zebrafish

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