The homeodomain transcription factor rx determines gene expression territories during early hypothalamic development

DE SOUZA FS; ORQUERA, DP; NASIF S; DOMENE S; LOW MJ; RUBINSTEIN M

Misiones

Encuentro; Gene Expression and RNA Processing Meeting; 2011

During embryogenesis, brain development is controlled by a complex interplay of signalling molecules and transcription factors that pattern the neural tube and induce a myriad of distinct cell types. The homeobox gene Rx/Rax is essencial for the development of retina and ventral forebrain, including the hypothalamus, and homozygous Rx mutant mice are eyeless and display severe cranofacial abnormalities. To better understand Rx function, we performed temporal inactivation of the gene during embryogenesis using "floxed" Rx mice and a Cre recombinase that can be induced by tamoxifen injections at specific time points. Rx inactivation at gestational day 7.5 (E7.5) caused lack of eyes and hypothalamic disorganisation, as well as severe forebrain deformities, probably related to its early expression in the opened anterior neural ridge. In contrast, later Rx deletion (E8.0, 8.5) caused a milder phenotype, without extensive forebrain deformity. Eyes were absent and, at the hypothalamic level, the infundibulum (future neural lobe of the pituitary) did not evaginate. Immunohistochemistry showed that in these embryos the infundibulum region was anteriorised, as evidenced by misexpression of POMC and Isl1, two markers normally restricted to the region anterior to the infundibulum. Rx inactivation at E9.5, in contrast, did not cause extensive alterations in hypothalamic patterning or POMC expression. Antisense morpholinos against the zebrafish homologue Rx3 showed total absence of Pomca expression in the developing ventromedial zebrafish hypothalamus. Altogether our work shows that Rx regulates the early patterning of the hypothalamus and highlights the importance of studying transcription factor function within narrow developmental windows.