RESUMO
In vertebrates, the embryonic dorsal midline is a crucial signalling centre that patterns the surrounding tissues during development. Members of the FoxA subfamily of transcription factors are expressed in the structures that compose this centre. Foxa2 is essential for dorsal midline development in mammals, since knock-out mouse embryos lack a definitive node, notochord and floor plate. The related gene foxA4 is only present in amphibians. Expression begins in the blastula -chordin and -noggin expressing centre (BCNE) and is later restricted to the dorsal midline derivatives of the Spemann's organiser. It was suggested that the early functions of mammalian foxa2 are carried out by foxA4 in frogs, but functional experiments were needed to test this hypothesis. Here, we show that some important dorsal midline functions of mammalian foxa2 are exerted by foxA4 in Xenopus. We provide new evidence that the latter prevents the respecification of dorsal midline precursors towards contiguous fates, inhibiting prechordal and paraxial mesoderm development in favour of the notochord. In addition, we show that foxA4 is required for the correct regionalisation and maintenance of the central nervous system. FoxA4 participates in constraining the prospective rostral forebrain territory during neural specification and is necessary for the correct segregation of the most anterior ectodermal derivatives, such as the cement gland and the pituitary anlagen. Moreover, the early expression of foxA4 in the BCNE (which contains precursors of the whole forebrain and most of the midbrain and hindbrain) is directly required to restrict anterior neural development.
Assuntos
Sistema Nervoso Central/embriologia , Sistema Nervoso Central/metabolismo , Embrião não Mamífero/metabolismo , Fatores de Transcrição Forkhead/metabolismo , Mesoderma/embriologia , Notocorda/embriologia , Proteínas de Xenopus/metabolismo , Xenopus/embriologia , Animais , Apoptose/efeitos dos fármacos , Biomarcadores/metabolismo , Blástula/efeitos dos fármacos , Blástula/metabolismo , Padronização Corporal/efeitos dos fármacos , Embrião não Mamífero/efeitos dos fármacos , Desenvolvimento Embrionário/efeitos dos fármacos , Técnicas de Silenciamento de Genes , Glicoproteínas/metabolismo , Cabeça/anormalidades , Cabeça/embriologia , Peptídeos e Proteínas de Sinalização Intercelular/metabolismo , Mesoderma/efeitos dos fármacos , Mesoderma/metabolismo , Modelos Biológicos , Morfogênese/efeitos dos fármacos , Morfolinos/farmacologia , Placa Neural/embriologia , Placa Neural/metabolismo , Neurogênese/efeitos dos fármacos , Notocorda/efeitos dos fármacos , Notocorda/metabolismo , Fenótipo , Xenopus/metabolismoRESUMO
We have previously shown that the member of the HES family hairy2 induces the ectopic expression of dorsal markers when it is overexpressed in the ventral side of Xenopus embryos. Intriguingly, hairy2 represses the mesoderm transcription factor brachyury (bra) throughout its domain in the marginal zone. Here we show that in early gastrula, bra and hairy2 are expressed in complementary domains. Overexpression of bra repressed hairy2. Interference of bra function with a dominant-negative construct expanded the hairy2 domain and, like hairy2 overexpression, promoted ectopic expression of dorsal axial markers in the ventral side and induced secondary axes without head and notochord. Hairy2 depletion rescued the ectopic dorsal development induced by interference of bra function. We concluded that an intact bra function is necessary to exclude hairy2 expression from the non-organiser field, to impede the ectopic specification of dorsal axial fates and the appearance of incomplete secondary axes. This evidence supports a previously unrecognised role for bra in maintaining the dorsal fates inhibited in the ventral marginal zone, preventing the appearance of trunk duplications.