Nuclear accumulation of pyruvate dehydrogenase alpha 1 promotes histone acetylation and is essential for zygotic genome activation in porcine embryos

Porcine zygotic genome activation (ZGA) occurs along with global epigenetic remodeling at the 4-cell stage. These processes are regulated by histone acetylation, which requires acetyl-coenzyme A (CoA). Pyruvate dehydrogenase complex (PDC) is a crucial enzyme in glucose metabolism that converts pyruvate into acetyl-CoA. In mammalian cells, acetyl-CoA is produced by pyruvate dehydrogenase alpha 1 (PDHA1) translocated into the nucleus in special conditions. To determine whether zygotic PDHA1 plays a critical role in promoting histone acetylation during ZGA, a CRISPR/Cas9 genome editing system using multiple guide RNAs was employed to generate a PDHA1-targeted parthenogenetic embryo model. Results of immunofluorescent staining showed that the nuclear accumulation of PDHA1 during ZGA was significantly inhibited by PDHA1 targeting. Meanwhile, the 4-cell arrest rate significantly increased at 72 h after activation, indicating impeded embryonic development. In addition, nuclear histone acetylation significantly decreased when PDHA1 was targeted, and quantitative PCR showed that expression of several zygotic genes was significantly decreased in the PDHA1-targeting group compared to the control group. Overexpression of PDHA1 recovered the nuclear PDHA1, H3K9Ac and H3K27Ac and EIF1A expression levels. Moreover, the 5-to-8-cell-stage embryo development rate was only partially rescued. In conclusion, expression of zygotic origin PDHA1 contributes to porcine ZGA by maintaining histone acetylation in porcine embryos.     

早期胚胎发育合子基因组激活调控

动物早期胚胎发育始于分化成熟的雌雄配子经受精后重编程为全能性合子。在胚胎发育的初期,合子基因组的转录水平处于静默状态,母源物质调控占据主导地位。随着胚胎发育的进行,母源物质会经历分阶段的降解,合子基因组开始逐渐激活转录,标志着早期胚胎发育从母源性调控向合子基因组调控的转变,也称为母源-合子转换（maternal-zygotic transition,MZT）。其中一个关键的转折性事件就是合子基因组激活（zygotic genome activation,ZGA）,ZGA的正确发生对于早期胚胎发育和细胞命运决定至关重要。然而,目前对于ZGA的调控因子和具体的分子机制仍知之甚少。研究表明,ZGA在不同物种中存在较大差异,可能受到DNA甲基化、组蛋白修饰、非编码RNA、染色质重塑以及ZGA相关因子等多种调控因素的影响。本文探讨了上述几种调控因素影响合子基因组激活的研究进展,对进一步研究早期胚胎ZGA的相关机制具有借鉴意义。     

Transcriptome Analysis of Honeybee (Apis Mellifera) Haploid and Diploid Embryos Reveals Early Zygotic Transcription during Cleavage

In honeybees, the haplodiploid sex determination system promotes a unique embryogenesis process wherein females develop from fertilized eggs and males develop from unfertilized eggs. However, the developmental strategies of honeybees during early embryogenesis are virtually unknown. Similar to most animals, the honeybee oocytes are supplied with proteins and regulatory elements that support early embryogenesis. As the embryo develops, the zygotic genome is activated and zygotic products gradually replace the preloaded maternal material. The analysis of small RNA and mRNA libraries of mature oocytes and embryos originated from fertilized and unfertilized eggs has allowed us to explore the gene expression dynamics in the first steps of development and during the maternal-to-zygotic transition (MZT). We localized a short sequence motif identified as TAGteam motif and hypothesized to play a similar role in honeybees as in fruit flies, which includes the timing of early zygotic expression (MZT), a function sustained by the presence of the zelda ortholog, which is the main regulator of genome activation. Predicted microRNA (miRNA)-target interactions indicated that there were specific regulators of haploid and diploid embryonic development and an overlap of maternal and zygotic gene expression during the early steps of embryogenesis. Although a number of functions are highly conserved during the early steps of honeybee embryogenesis, the results showed that zygotic genome activation occurs earlier in honeybees than in Drosophila based on the presence of three primary miRNAs (pri-miRNAs) (ame-mir-375, ame-mir-34 and ame-mir-263b) during the cleavage stage in haploid and diploid embryonic development.     

Atypical GATA protein TRPS1 plays indispensable roles in mouse two-cell embryo

Zygotic genome activation (ZGA) is one of the most critical events at the beginning of mammalian preimplantation embryo development (PED). The mechanisms underlying mouse ZGA remain unclear although it has been widely studied. In the present study, we identified that tricho-rhino-phalangeal syndrome 1 (TRPS1), an atypical GATA family member, is an important factor for ZGA in mouse PED. We found that the Trps1 mRNA level peaked at the one-cell stage while TRPS1 protein did so at the two/four-cell stage. Knockdown of Trps1 by the microinjection of Trps1 siRNA reduced the developmental rate of mouse preimplantation embryos by approximately 30%, and increased the expression of ZGA marker genes MuERV-L and Zscan4d via suppressing the expression of major histone markers H3K4me3 and H3K27me3. Furthermore, Trps1 knockdown decreased the expression of Sox2 but increased Oct4 expression. We conclude that TRPS1 may be indispensable for zygotic genome activation during mouse PED.     

Maternal effects and temperature-sensitive period of mutations affecting embryogenesis in Caenorhabditis elegans

We have used standard tests to investigate the nature of gene expression of a new set of temperature-sensitive mutants defining 30 emb genes (essential for embryogenesis) in the nematode Caenorhabditis elegans. The mode of gene expression as determined by progeny tests for parental effects divides the genes into four classes. For 18 genes maternal gene expression is necessary and sufficient for normal embryogenesis; for 2 genes zygotic expression is necessary and sufficient; for 7 genes either maternal or zygotic expression is sufficient; for 3 genes both maternal and zygotic expression are necessary. One mutant displayed partial paternal sufficiency. The results of temperature-shift experiments define two “execution stages,” corresponding to the limits of the temperature-sensitive period (TSP), and indicate the nature and the time of action or synthesis of the gene products. Most of the maternally expressed genes have very early execution stages indicating translation before fertilization, but some are temperature sensitive late in embryogenesis. Early execution stages for 2 zygotically necessary genes demonstrate that the zygotic genome can be active in the earliest stages of embryogenesis. All taken together, the mode of gene expression, TSP, and arrest stage (terminal phenotype) allow us to classify functionally and begin to order the genes essential for embryogenesis. The results indicate a preeminent role for maternal genes and gene products in embryogenesis, in agreement with the results of others.     

Maternal-Zygotic Lethal Interactions in DROSOPHILA MELANOGASTER: The Effects of Deficiencies in the Zeste-White Region of the X Chromosome

The possibility that essential loci in the zeste-white region of the Drosophila melanogaster X chromosome are expressed both maternally and zygotically has been tested. Maternal gene activity was varied by altering gene dose, and zygotic gene activity was manipulated by use of position-effect variegation of a duplication. Viability is affected when both maternal and zygotic gene activity are reduced, but not when either maternal or zygotic gene activity is normal. Tests of a set of overlapping deficiencies demonstrate that at least three sections of the zeste-white region yield maternal zygotic lethal interactions. Single-cistron mutations at two loci in one of these segments have been tested, and maternal heterozygosity for mutations at both loci give lethal responses of mutant-duplication zygotes. Thus, at least four of the 13 essential functions coded in the zeste-white region are active both maternally and zygotically, suggesting that a substantial fraction of the genome may function at both stages. The normal survival of zygotes when either maternal gene expression or zygotic gene expression is normal, and their inviability when both are depressed, suggests that a developmental stage exists when maternally determined functions and zygotically coded functions are both in use.