Functional relationship between brachyury and hairy2a in the development of the Xenopus dorsal-ventral axis

AGUIRRE, CECILIA E.; PAGANELLI, ALEJANDRA R.; CARRASCO, ANDRÉS E.; LÓPEZ, SILVIA LILIANA

Buenos Aires

Congreso; 4th International Meeting of the Latin American Society of Developmental Biology; 2008

Latin American Society for Developmental Biology

We have previously proposed that the early Xenopus organiser contains cells equally potent to give rise to notochord and floor plate. Delta 1 activates Notch, which in turn triggers a binary decision, favouring the development of floor plate at the expense of the notochord. This is executed through the Notch- mediated activation of hairy2a in cells within the organiser, impeding their involution through the blastopore and promoting their incorporation into the hairy2a notoplate precursors in the dorsal non-involuting marginal zone (López et al. Development 132:  1035-1046).We have also shown that overexpression of hairy2a represses dorsal chordin expression and results in ectopic chordin+ and foxA4a+ cells on ventral or lateral mesodemal locations. However, injection of hairy2a mRNA represses brachyury expression in all locations; while knock-down of hairy2a increases brachyury+ cells in both the organiser and the notochord.Now we found that dorsal overexpression of brachyury represses hairy2a, while injection of Xbra-EnR mRNA (a construction that transforms Brachyury into an active repressor) notably increases hairy2a expression and furthermore, promotes ectopic expression of the dorsal midline markers chordin, foxA4a and sonic hedgehog on the ventral and lateral marginal zones. Interestingly, even hairy2a is induced in these ectopic locations.In early gastrula, in situ hibridization of brachyury and hairy2a reveals that their territories are adjacent and mutually exclusive. Together, these results suggest that hairy2a and brachyury are involved in a regulatory mechanism that implies mutual repression. Moreover, the ventral and lateral marginal zones have the potential to develop dorsal midline characteristics, but the strong expression of brachyury in these areas may be necessary to maintain hairy2a repressed, allowing ventral and lateral development to proceed.