Maternal Eomesa Regulates Zebrafish Zygotic Nodal Gene Expression And Mesendoderm Induction

In vertebrates, maternal factors produced during oogenesis and deposited in eggscontrol various processes during early embryonic development, including fertilization,blastomere cleavage, germ layer formation and differentiation and axis formation.Nodal signals produced during blastulation play a pivotal role in induction ofmesendodermal fates. Two zebrafish nodal genes ndr1and ndr2start to express afterthe midblastula transition and are essential for mesendoderm induction. It is unknownwhether maternal transcriptional factors control the initiation of nodal gene expressionin the ventrolateral blastodermal margin. This project aimed to explore function andmolecular mechanisms of the maternal T-box transcriptor factor Eomesodermin A(Eomesa) in regulating nodal gene expression and mesendoderm induction duringzebrafish embryogenesis.In this study, by knocking down eomesa in maturing oocytes, depleted of maternalEomesa resulted in the loss of expression of several mesendodermal genes in someblastodermal margins during late blastula and early gastrula stages; similar defects werealso detected in the maternal and zygotic eomesa mutants (MZeomesa). Furthermore,loss of maternal eomesa impaired ventrolateral expression of the nodal genes ndr1andndr2at and prior to the30%epiboly stage, suggesting an essential role in timelyactivation of ndr1and ndr2transcription particularly in ventrolateral blastodermmargin. Transgenic and biochemical analyses demonstrated that Eomesa could bind toEomes binding sites in the ndr1promoter and in the first intron of ndr2and as a resultpromotes ndr1/ndr2transcription.Zygotic expression of the transcription factor Mxtx2, which can promote nodalgene expression, is initiated after the midblastula transition in zebrafish embryos.Depletion of maternal Eomesa via maternal knockdown or in MZeomesa mutantsresulted in the loss of expression of mxtx2in the ventrolateral margin, and delayedintiation of mxtx2in the dorsal margin by the end of late blastulation, which indicatesan essential role in timely activating mxtx2trasncription. Epistatic analysis resultsrevealed that maternal Eomesa acted primarily by in cooporation with its downstreamtarget Mxtx2to regulate nodal gene expression.In summary, these data uncover for the first time that the maternal transcriptionfactor Eomesa acts to activate the initial expression of nodal genes in the ventrolateral blastodermal margin in the zebrafish blastula. The finding also suggests a conservedrole of maternal T-box transcription factors in activating nodal gene expression invertebrate embryos.