On October 7, a paper entitled “Precise temporal regulation of Dux is important for embryo development” waspublished on Cell Research by two Tongji research teams headed by Prof. GAO Shaorong and Prof. YANG Peng, respectively.
Zygotic genome activation (ZGA) is one of the necessary conditions for zygote to acquire totipotency. However, due to the limited number of early embryos in mammals, the key factors and specific regulatory mechanisms of ZGA process are still unclear. Recently, Nature Genetics has published three research papers simultaneously, reporting that Dux/DUX4 transcription protein may be the key factor to activate mouse ZGA and human ZGA. These three papers found that overexpression of Dux/Dux4 in muscle cells or ES cells might activate some ZGA genes, but the role of DUX in vivo embryo development is still unclear.
This paper makes a comprehensive study on the specific role of Dux in early embryos. The Dux-knockout mice obtained by designing sgRNA directly targeting all Dux repeat sequence regions are consistent with the phenotype of Dux knockout mice recently reported by ZHANG Yi’s Laboratory and Didier Trono’s research group. Because the transient expression of Dux in early 2- cell embryos and the developmental trend of Dux knockout embryos are different, the team found that compared with normal embryos, Dux knockout embryos show a decrease in the expression of MERVL and a certain number of ZGA genes in early, middle and late stages of 2- cell. However, these ZGA genes, which declined in the early and middle stages of Dux knockout 2- cell embryos, reached the expression level equivalent to that of normal embryos in the late 2- cell stage. Dux knocks out some ZGA genes in late 2- cell embryos with lower expression than normal embryos, but the expression is increased compared with early 2- cell embryos, which is consistent with the results recently published inNature Geneticsby Zhang Yi's research team. These results indicate that activation of some ZGA genes is delayed in Dux-deleted embryos, and Dux plays a role in enhancing ZGA in early embryo development rather than the role of initial ZGA inferred from previous in vitro research results. Due to the presence of other factors, ZGA still occurs normally in the late stage of Dux knockout 2- cell embryos, so the phenotype of reduced litter size due to Dux knockout embryos either developing normally, being born or dying may be determined by the expression dose threshold effect of other factors.
The team also injected mRNA in the late 2- cell stage and 4- cell stage of normal embryos to extend the expression of Dux in vivo. They found that Dux can indeed enhance the expression of ZGA gene and MERVL in embryos, which is consistent with overexpression of Dux in ES cells in vitro, but the blastomeres that extend Dux expression stagnate and eventually die after one division. They also found that exogenous injected Dux would be rapidly degraded in the embryo, which is consistent with the transient expression of endogenous Dux. If injected at the 1- cell stage of the embryo, Dux will be degraded more rapidly so as not to affect the development of the embryo, which further illustrates the importance of timely degradation of Dux in the early embryo development process. However, it is not clear how the endogenous or exogenous Dux mRNA and protein are rapidly degraded in the embryo.
To conclude, the in vivo knockout and overexpression experiments prove that Dux plays a role in enhancing rather than initiating ZGA in the early stage of 2- cell embryos. When Dux is absent, the dose effect of substitution factors may lead to different developmental trends of Dux knockout embryos, while Dux must be degraded in time after its short-term function. Prolonged expression of Dux will damage the normal development of embryos, and the fast degradation pathway and mechanism of Dux mRNA and protein are unclear.
About the authors:
GUO Mingyue, a PhD candidate of Tongji University, and ZHANG Yanping, an assistant researcher, are the first authors of this paper. Other authors of this article include doctoral students ZHOU Jianfeng and BI Yan of Professor GAO Shaorong's Laboratory, technicians KOU Xiaochen and ZHAO Yanhong, PhD candidates LI Yanhe, TU Zhifen, LIU Kuisheng, Master’s student LIN Jiaming as well as XU Junqin and XU Ce, Master’s students of Professor YANG Peng's laboratory. Professor GAO, Professor WANG Yixuan and Professor YANG are the corresponding authors of this paper.
Full-text paper“Precise temporal regulation of Dux is important for embryo development” is available at https://www.nature.com/articles/s41422-019-0238-4.