Collaborative work between the team of Laurent Kodjabachian in IBDM and of Hector Escriva / Stéphanie Bertrand in the laboratory of Integrative Biology of Marine Organisms (Banyuls/mer) reveals the conserved role of Nodal signaling during neural induction. This study has been published on July 3rd 2017 in Nature Ecology and Evolution.

The first developmental step in the formation of the vertebrate Central Nervous System (CNS) is called neural induction. It is the process by which naive ectodermal cells are committed to a neural fate, in response to signals produced by the underlying dorsal organizer. Based on seminal works in the amphibian Xenopus, it was proposed that neural induction occurs by default, upon inhibition of BMP signals by soluble antagonists produced by the organizer. However, works in chick and Xenopus have also highlighted the instructive role of FGF signalling, such that no common molecular scenario of neural induction in vertebrates clearly emerged.

Resolving this issue would certainly benefit from studies in other branches of the chordate lineage. Urochordates, the sister group of vertebrates appear to lack an organizer. Thus, Le Petillon and colleagues have studied neural induction in cephalochordates (amphioxus), the third and most basal chordate phylum. By using graft and micromanipulation experiments on the tiny (80 μm) amphioxus embryo, the authors definitively demonstrated that the amphioxus dorsal blastopore lip is homologous to the vertebrate organizer, and was able to induce the formation of neural tissue from naive ectoderm.

Strikingly, although BMP inhibition was required for neural tissue formation in amphioxus, it was found that the Nodal signaling pathway also played an instructive and essential role. Moreover, gain and loss of function experiments indicated that Nodal signaling also promoted neural tissue development in Xenopus. This came as a surprise as Nodal has classically been defined as a pro-mesodermal and anti-neural factor. This study further reveals the diversity of neural inducers deployed during chordate evolution, and argues against a universally conserved molecular explanation for this process.

To know more
Nodal–Activin pathway is a conserved neural induction signal in chordates.
Yann Le Petillon, Guillaume Luxardi, Pierluigi Scerbo, Marie Cibois, Anthony Leon, Lucie Subirana, Manuel Irimia, Laurent Kodjabachian*, Hector Escriva* and Stephanie Bertrand*. Nature Ecology & Evolution (2017). doi:10.1038/s41559-017-0226-3. Published online: 03 July 2017

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Contact : laurent.kodjabachian@univ-amu.fr