Roles of brachyury and hairy2 in dorsal-ventral axis development in Xenopus laevis

AGUIRRE, CECILIA E.; CARRASCO, ANDRÉS E.; MURGAN, SABRINA; LÓPEZ, SILVIA L.

Montevideo

Congreso; VI International Meeting of the Latin American Society for Developmental Biology; 2012

Latin American Society for Developmental Biology

Hairy2 is a transcriptional repressor known to play important roles in the formation of boundaries in neural tissue territories and is an effector of the Notch pathway. Hairy2a overexpression represses the expression of dorsal midline (DML) markers, such as chordin and promotes their ectopic expression in the ventral and lateral marginal zone (VLMZ). On the other hand, hairy2a overexpression represses the general-mesoderm marker brachyury (bra) in all dorso-ventral locations, while knockdown of hairy2 by antisense morpholino oligonucleotides expands the bra domain. Bra is a transcriptional activator that activates the mesoderm-specific genes and promotes convergence and extension movements. In early gastrula, we show that bra and hairy2 are expressed in adjacent and mutually exclusive domains. To investigate the functional relationships between hairy2 and bra in the development of the dorsal-ventral axis, we performed experiments of gain or loss of function and analyzed both the embryo?s morphology and the expression of molecular markers by in situ hybridization (ISH). Our results indicate that dorsal overexpression of bra represses hairy2. In contrast, interfering with bra function by injection of braEnR mRNA (a construction that transforms Bra into an active repressor) increases hairy2 expression and expands the hairy2 domain, which occupies regions normally corresponding to the bra domain. Notably, braEnR mRNA injection, like hairy2a overexpression, promotes ectopic expression of DML markers in the VLMZ. These embryos later develop secondary axes which have neural tube (sox2+, nrp1+), posterior midbrain (en2+, krox20+) and somitic mesoderm (myoD+) but lack notochord and forebrain, as revealed by the absence of chd in the dorsal midline and otx2 expression in the anterior region, respectively. Taken together, these results suggest that hairy2 and brachyury are involved in a regulatory mechanism that implies mutual repression. Moreover, the VLMZ has the potential to develop dorsal midline characteristics, but the strong expression of bra in this area is necessary to maintain hairy2 repressed and to inhibit the formation of secondary axes. Thus, apart from the maternal mechanisms that normally repress dorsal-anterior development on the ventral side of the embryo, bra ensures the normal development by controlling the transcription of hairy2 at the zygotic level.