A Pou5f1/Oct4 dependent Klf2a, Klf2b, and Klf17 regulatory sub-network contributes to EVL and ectoderm development during zebrafish embryogenesis.

In mammalian ES cells, the transcription factors Klf4 and Klf2 contribute to maintenance of pluripotency and self-renewal and are regulated by Pou5f1/Oct4. In the early zebrafish embryo Pou5f1/Oct4 is necessary for expression of three Klf2/4 family members, klf2a, klf2b and klf17 (previously klf4b), similar to the regulation reported for mammalian ES cells. In this study, we analyzed blastula and gastrula stage Klf regulatory networks and their influence on zebrafish embryonic patterning. We show that Pou5f1 acts in combination with region-specific factors to activate klf2a, klf2b, and klf17 in the superficial cell layer of the embryo. In addition, Pou5f1 acts together with the BMP signaling pathway to activate and maintain expression of klf2a and klf2b in a ventral ectodermal domain. We used microarray expression profiles of klf2a, klf2b and klf17 knockdown and overexpression embryos to identify Klf target genes, which reveals that Klfs participate in specification of the extraembryonic enveloping layer (EVL). We discuss mechanistic implications of simultaneous activation of transcriptional targets by ubiquitous, like Pou5f1, and region-specific inducers, emerging as a common regulatory motif in early development.

Apparent role of Tribolium orthodenticle in anteroposterior blastoderm patterning largely reflects novel functions in dorsoventral axis formation and cell survival.

In the short-germ beetle Tribolium castaneum, the head gap gene orthodenticle (Tc-otd) has been proposed to functionally substitute for bicoid, the anterior morphogen unique to higher dipterans. In this study we reanalyzed the function of Tc-otd. We obtained a similar range of cuticle phenotypes as in previously described RNAi experiments; however, we noticed unexpected effects on blastodermal cell fates. First, we found that Tc-otd is essential for dorsoventral patterning. RNAi depletion results in lateralized embryos, a fate map change that by itself can explain the observed loss of the anterior head, which is a ventral anlage in Tribolium. We find that this effect is due to diminished expression of short gastrulation (sog), a gene essential for establishment of the Decapentaplegic (Dpp) gradient in this species. Second, we found that gnathal segment primordia in Tc-otd RNAi embryos are shifted anteriorly but otherwise appear patterned normally. This anteroposterior (AP) fate map shift might largely be due to diminished zen-1 expression and is not responsible for the severe segmentation defects observed in some Tc-otd RNAi embryos. As neither Tc-sog nor Tc-zen-1 probably requires Otd gradient-mediated positional information, we posit that the blastoderm function of Tc-Otd depends on its initial homogeneous maternal expression and that this maternal factor does not provide significant positional information for Tribolium blastoderm embryos.

Pou5f1/Oct4 promotes cell survival via direct activation of mych expression during zebrafish gastrulation.

Myc proteins control cell proliferation, cell cycle progression, and apoptosis, and play important roles in cancer as well in establishment of pluripotency. Here we investigated the control of myc gene expression by the Pou5f1/Oct4 pluripotency factor in the early zebrafish embryo. We analyzed the expression of all known zebrafish Myc family members, myca, mycb, mych, mycl1a, mycl1b, and mycn, by whole mount in situ hybridization during blastula and gastrula stages in wildtype and maternal plus zygotic pou5f1 mutant (MZspg) embryos, as well as by quantitative PCR and in time series microarray data. We found that the broad blastula and gastrula stage mych expression, as well as late gastrula stage mycl1b expression, both depend on Pou5f1 activity. We analyzed ChIP-Seq data and found that both Pou5f1 and Sox2 bind to mych and mycl1b control regions. The regulation of mych by Pou5f1 appears to be direct transcriptional activation, as overexpression of a Pou5f1 activator fusion protein in MZspg embryos induced strong mych expression even when translation of zygotically expressed mRNAs was suppressed. We further showed that MZspg embryos develop enhanced apoptosis already during early gastrula stages, when apoptosis was not be detected in wildtype embryos. However, Mych knockdown alone did not induce early apoptosis, suggesting potentially redundant action of several early expressed myc genes, or combination of several pathways affected in MZspg. Experimental mych overexpression in MZspg embryos did significantly, but not completely suppress the apoptosis phenotype. Similarly, p53 knockdown only partially suppressed apoptosis in MZspg gastrula embryos. However, combined knockdown of p53 and overexpression of Mych completely rescued the MZspg apoptosis phenotype. These results reveal that Mych has anti-apoptotic activity in the early zebrafish embryo, and that p53-dependent and Myc pathways are likely to act in parallel to control apoptosis at these stages.