Vincent Fleury, SLT, 10-01-2007 à 17:27 : Bon évidemment, mon point de vue a légèrement évolué depuis le premier papier: pour faire un peu plaisir aux biologistes, j’ai laissé le pôle attracteur. Je crois qu’il n’est même pas nécessaire. Le mouvement est générique.
Une fois que c’est parti, le pôle est réduit en bouillie par le mouvement lui-même. De toute façon on ne peut pas sans sortir sans de l’advection diffusion, et quand vous voyez les moulinets que fait la blastula, ça laisse songeur sur le rôle des molécules.
Tsukushi cooperates with VG1 to induce primitive streak and Hensen’s node formation in the chick embryo
Kunimasa Ohta, Sei Kuriyama, Tatsuya Okafuji, Ryu Gejima, Shin-ichi Ohnuma and Hideaki Tanaka
Development 133, 3777-3786 (2006) doi:10.1242/dev.02579
Three classes of signaling molecule, VG1, WNT and BMP, play crucial roles in axis formation in the chick embryo. Although VG1 and WNT signals have a pivotal function in inducing the primitive streak and Hensen’s node in the embryo midline, their action is complemented by that of BMP antagonists that protect the prospective axial tissue from the inhibitory influence of BMPs secreted from the periphery. We have previously reported that a secreted factor, chick Tsukushi (TSK), is expressed in the primitive streak and Hensen’s node, where it works as a BMP antagonist. Here, we describe a new crucial function for TSK in promoting formation of the primitive streak and Hensen’s node by positively regulating VG1 activity. We provide evidence that TSK directly binds VG1 in vitro, and that TSK and VG1 functionally interact in axis formation, as shown by biological assays performed in chick and Xenopus embryos. Furthermore, we show that alternative splicing of TSKRNA leads to the formation of two isoforms (TSKA, originally designated as TSK, and TSKB) that differ in their C-terminal region. Biochemical and biological assays indicate that TSKB is a much weaker BMP antagonist than TSKA, although both isoforms efficiently interact with VG1. Remarkably, although both TSKA and TSKB are expressed throughout the early extending primitive streak, their expression patterns diverge during gastrulation. TSKA expression concentrates in Hensen’s node, a well-known source of anti-BMP signals, whereas TSKBaccumulates in the middle primitive streak (MPS), a region known to work as a node-inducing center where VG1 expression is also specifically localized. Loss-of-function experiments demonstrate that TSKB, but not TSKA, function is required in the MPS for induction of Hensen’s node. Taken together, these results indicate that TSK isoforms play a crucial role in chick axis formation by locally modulating VG1 and BMP activities during gastrulation.