Incomplete activation of Alyref and Gabpb1 leads to preimplantation arrest in cloned mouse embryos.

Differentiated cell nuclei can be reprogrammed after nuclear transfer (NT) to oocytes and the produced NT embryos can give rise to cloned animals. However, development of NT embryos is often hampered by recurrent reprogramming failures, including the incomplete activation of developmental genes, yet specific genes responsible for the arrest of NT embryos are not well understood. Here, we searched for developmentally important genes among the reprogramming-resistant H3K9me3-repressed genes and identified Alyref and Gabpb1 by siRNA screening. Gene knockout of Alyref and Gabpb1 by the CRISPR/Cas9 system resulted in early developmental arrest in mice. Alyref was needed for the proper formation of inner cell mass by regulating Nanog, whereas Gabpb1 deficiency led to apoptosis. The supplement of Alyref and Gabpb1 mRNA supported efficient preimplantation development of cloned embryos. Alyref and Gabpb1 were silenced in NT embryos partially because of the repressed expression of Klf16 by H3K9me3. Thus, our study shows that the H3K9me3-repressed genes contain developmentally required genes, and the incomplete activation of such genes results in preimplantation arrest of cloned embryos.

Cell division- and DNA replication-free reprogramming of somatic nuclei for embryonic transcription.

Nuclear transfer systems represent the efficient means to reprogram a cell and in theory provide a basis for investigating the development of endangered species. However, conventional nuclear transfer using oocytes of laboratory animals does not allow reprogramming of cross-species nuclei owing to defects in cell divisions and activation of embryonic genes. Here, we show that somatic nuclei transferred into mouse four-cell embryos arrested at the G2/M phase undergo reprogramming toward the embryonic state. Remarkably, genome-wide transcriptional reprogramming is induced within a day, and ZFP281 is important for this replication-free reprogramming. This system further enables transcriptional reprogramming of cells from Oryx dammah, now extinct in the wild. Thus, our findings indicate that arrested mouse embryos are competent to induce intra- and cross-species reprogramming. The direct induction of embryonic transcripts from diverse genomes paves a unique approach for identifying mechanisms of transcriptional reprogramming and genome activation from a diverse range of species.

Developmental competence of interspecies cloned embryos produced using cells from large Japanese field mice (Apodemus speciosus) and oocytes from laboratory mice (Mus musculus domesticus).

The large Japanese field mouse (Apodemus speciosus) is endemic to Japan and may be used as an animal model for studies related to environmental pollution, medical science, and basic biology. However, the large Japanese field mouse has low reproductive ability due to the small number of oocytes ovulated per female. To produce experimental models, we investigated the in vitro developmental potential of interspecies somatic cell nuclear transfer (iSCNT) embryos produced by fusing tail tip cells from the large Japanese field mouse with enucleated oocytes from laboratory mice (Mus musculus domesticus). Only a small number of iSCNT embryos developed to the 4-cell (0-4%) and blastocysts (0-1%) stages under sequential treatment using trichostatin A (TSA) and vitamin C (VC) supplemented with deionized bovine serum albumin (d-BSA). This sequential treatment led to the reduction in H3K9 trimethylation and did not affect H3K4 trimethylation in at least the 2-cell stage of the iSCNT embryos. Moreover, iSCNT embryos that received tail tip cells with exposure treatment to ooplasm from cell fusion to oocyte activation or VC treatment prior to cell fusion did not exhibit significant in vitro development improvement compared to that of each control group. This suggests that large Japanese field mice/laboratory mice iSCNT embryos that received sequential treatment using TSA and VC with d-BSA may have slightly better developmental potential beyond the 4-cell stage. Our results provide insights into the reprogramming barriers impeding the wider implementation of iSCNT technology.

Signs of biological activities of 28,000-year-old mammoth nuclei in mouse oocytes visualized by live-cell imaging.

The 28,000-year-old remains of a woolly mammoth, named 'Yuka', were found in Siberian permafrost. Here we recovered the less-damaged nucleus-like structures from the remains and visualised their dynamics in living mouse oocytes after nuclear transfer. Proteomic analyses demonstrated the presence of nuclear components in the remains. Nucleus-like structures found in the tissue homogenate were histone- and lamin-positive by immunostaining. In the reconstructed oocytes, the mammoth nuclei showed the spindle assembly, histone incorporation and partial nuclear formation; however, the full activation of nuclei for cleavage was not confirmed. DNA damage levels, which varied among the nuclei, were comparable to those of frozen-thawed mouse sperm and were reduced in some reconstructed oocytes. Our work provides a platform to evaluate the biological activities of nuclei in extinct animal species.

Development of feline embryos produced by Piezo-actuated intracytoplasmic sperm injection of elongated spermatids.

Piezo-actuated intracytoplasmic sperm injection (Piezo-ICSI) is used as an efficient in vitro fertilization method with various animals. With this method, elongated spermatids are collected from testicular tissues and are easier to obtain from animals that unexpectedly die than ejaculate sperm. Additionally, elongated spermatid injection often results in the development of embryos and offspring. To develop assisted reproductive techniques (ARTs) for domestic cats, we examined the effects of oocyte activation on cleavage and embryo development after Piezo-ICSI with motile sperm (experiment 1) and after Piezo-ICSI with either testicular sperm or elongated spermatids (experiment 2). In experiment 1, the proportions of cleaved embryos, morulas, and blastocysts following Piezo-ICSI with ethanol activation were significantly higher (P < 0.05) than in the non-activated groups. However, the proportion of blastocysts and the blastocyst quality did not differ significantly (P > 0.05) between the ethanol-activated and non-activated groups. In experiment 2, the cleavage frequencies of oocytes after Piezo-ICSI of testicular sperm or elongated spermatids and ethanol activation were higher (P < 0.05) than that of oocytes in the non-activated group, but the occurrence of blastocyst formation and quality of blastocysts did not differ between the activated and non-activated groups. In summary, cat embryos can be produced by Piezo-actuated microinjection of elongated spermatids. Ethanol activation increased the frequency of cleavage, but it affected neither the occurrence of blastocyst development nor the quality of blastocysts. These results represent an expansion in the repertoire of ARTs that are potentially applicable to both domestic and endangered species of cats.

Combinational Treatment of Trichostatin A and Vitamin C Improves the Efficiency of Cloning Mice by Somatic Cell Nuclear Transfer.

Somatic cell nuclear transfer (SCNT) provides a unique opportunity to directly produce a cloned animal from a donor cell, and it requires the use of skillful techniques. Additionally, the efficiencies of cloning have remained low since the successful production of cloned animals, especially mice. There have been many attempts to improve the cloning efficiency, and trichostatin A (TSA), a histone deacetylase inhibitor, has been widely used to enhance the efficiency of cloning. Here, we report a dramatically improved cloning method in mice. This somatic cell nuclear transfer method involves usage of Hemagglutinating virus of Japan Envelope (HVJ-E), which enables easy manipulation. Moreover, the treatment using two small molecules, TSA and vitamin C (VC), with deionized bovine serum albumin (dBSA), is highly effective for embryonic development. This approach requires neither additional injection nor genetic manipulation, and thus presents a simple, suitable method for practical use. This method could become a technically feasible approach for researchers to produce genetically modified animals from cultured cells. Furthermore, it might be a useful way for the rescue of endangered animals via cloning.

Peroxiredoxin as a functional endogenous antioxidant enzyme in pronuclei of mouse zygotes.

Antioxidant mechanisms to adequately moderate levels of endogenous reactive oxygen species (ROS) are important for oocytes and embryos to obtain and maintain developmental competence, respectively. Immediately after fertilization, ROS levels in zygotes are elevated but the antioxidant mechanisms during the maternal-to-zygotic transition (MZT) are not well understood. First, we identified peroxiredoxin 1 (PRDX1) and PRDX2 by proteomics analysis as two of the most abundant endogenous antioxidant enzymes eliminating hydrogen peroxide (H2O2). We here report the cellular localization of hyperoxidized PRDX and its involvement in the antioxidant mechanisms of freshly fertilized oocytes. Treatment of zygotes at the pronuclear stage with H2O2 enhanced pronuclear localization of hyperoxidized PRDX in zygotes and concurrently impaired the generation of 5-hydroxymethylcytosine (5hmC) on the male genome, which is an epigenetic reprogramming event that occurs at the pronuclear stage. Thus, our results suggest that endogenous PRDX is involved in antioxidant mechanisms and epigenetic reprogramming during MZT.

Ubiquitin-proteasome system modulates zygotic genome activation in early mouse embryos and influences full-term development.

Maternal RNA/protein degradation and zygotic genome activation (ZGA), occurring during maternal-to-zygotic transition (MZT), are the first essential events for the development of pre-implantation embryos. Previously, we have shown the importance of the ubiquitin-proteasome system (UPS) for initiation of minor ZGA at the 1-cell stage of mouse embryos. However, little is known about the mechanism of involvement of the UPS-degraded maternal proteins in ZGA. In this study, we investigated the effect of inhibiting maternal protein degradation by the reversible proteasome inhibitor, MG132, on post-implantation development and ZGA regulation during early cleavage stages. Our study revealed that zygotic transcription by RNA polymerase II (Pol II) at the 1-cell stage was delayed and the full-term development was affected by transient proteasome inhibition during 1 to 9 h post-insemination (hpi). Furthermore, we found that the transient inhibition of proteasome activity at the 2-cell stage delayed the onset of transcription of some major ZGA genes. These results support the model hypothesizing the requirement of sequential degradation of maternal proteins by UPS for the proper onset of ZGA and normal progression of MZT in early mouse embryos.

Reprogramming towards totipotency is greatly facilitated by synergistic effects of small molecules.

Animal cloning has been achieved in many species by transplanting differentiated cell nuclei to unfertilized oocytes. However, the low efficiencies of cloning have remained an unresolved issue. Here we find that the combination of two small molecules, trichostatin A (TSA) and vitamin C (VC), under culture condition with bovine serum albumin deionized by ion-exchange resins, dramatically improves the cloning efficiency in mice and 15% of cloned embryos develop to term by means of somatic cell nuclear transfer (SCNT). The improvement was not observed by adding the non-treated, rather than deionized, bovine serum. RNA-seq analyses of SCNT embryos at the two-cell stage revealed that the treatment with TSA and VC resulted in the upregulated expression of previously identified reprogramming-resistant genes. Moreover, the expression of early-embryo-specific retroelements was upregulated by the TSA and VC treatment. The enhanced gene expression was relevant to the VC-mediated reduction of histone H3 lysine 9 methylation in SCNT embryos. Our study thus shows a simply applicable method to greatly improve mouse cloning efficiency, and furthers our understanding of how somatic nuclei acquire totipotency.

The Clock mutation reduces reproductive performance of mice by affecting the implantation capacity: Maternal Clock mutation is not the only factor affecting implantation.

Here, we showed that the Clock gene was important for reproductive performance in mice. We compared outcomes from the four possible mating combinations between wild-type mice (WT) and mice homozygous for the Clock delta-19 mutation (CL). We found that the only significant differences were between the WT♂ × WT♀ and CL♂ × CL♀ mating groups; these groups differed with regard to elongation of the pregnancy period (19.3 vs. 20.5 days, respectively, P < 0.05) and the number of newborn pups (13.4 ± 0.8 vs. 8.6 ± 1.5, respectively, P < 0.05). Because CL dams impregnated by male CLs exhibited normal continuous increases in body weight during the entire gestation period and did not show any signs of spontaneous abortion from mid to late gestation, we reasoned that some embryos were lost before or at the time of implantation. Immediately before implantation (88 hours after fertilization), neither the number of embryos collected from uteri nor the percentage of the embryos that reached the blastocyst stage differed significantly among mating groups. In contrast, immediately after implantation (160 hours after fertilization), the average number of implantation sites was significantly lower for the CL♂ × CL♀ mating group than that for the WT♂ × WT♀ mating group (7.0 vs. 13.0, P < 0.05); this decrease was accompanied by a significant lowering of the positions of implantation sites in uteri, and this lowering of the implantation sites was more severe when mothers and embryos bore more CL alleles (WT♂ × WT♀ >CL♂ × WT♀ > WT♂ × CL♀ >CL♂ × CL♀), suggesting that the Clock mutation reduced the reproduction performance of the parents by affecting the implantation capacity via such as embryos' ability to implant.

Cryopreservation of a small number of human sperm using enzymatically fabricated, hollow hyaluronan microcapsules handled by conventional ICSI procedures.

PURPOSE: We investigated whether enzymatically fabricated hyaluronan (HA) microcapsules were feasible for use in the cryopreservation of a small number of sperm. METHODS: HA microcapsules were fabricated using a system of water-immiscible fluid under laminar flow. Three sperm were injected into a hollow HA microcapsule using a micromanipulator. Capsules containing injected sperm were incubated in a freezing medium composed of sucrose as the cryoprotectant and then placed in a Cryotop® device and plunged into liquid nitrogen. After thawing, the capsule was degraded by hyaluronidase, and the recovery rate of sperm and their motility were investigated. RESULTS: The HA microcapsule measuring 200 µm in diameter and with a 30-µm thick membrane was handled using a conventional intracytoplasmic sperm injection (ICSI) system, and the procedure involved the injection of sperm into the capsule. The HA microcapsules containing sperm were cryopreserved in a Cryotop® device and decomposed by the addition of hyaluronidase. The recovery rate of sperm after cryopreservation and degradation of HA microcapsules was sufficient for use in clinical practice (90 %). CONCLUSIONS: Hollow HA microcapsules can be used for the cryopreservation of a small number of sperm without producing adverse effects on sperm quality.

Possible role of ZPAC, zygote-specific proteasome assembly chaperone, during spermatogenesis in the mouse.

In the mammalian testis, the ubiquitin-proteasome system plays important roles in the process that promotes the formation of mature sperm. We recently identified zygote-specific proteasome assembly chaperone (ZPAC), which is specifically expressed in the mouse gonads and zygote. ZPAC mediates a unique proteasome assembly pathway in the zygote, but the expression profile and function of ZPAC in the testis is not fully understood. In this study, we investigated the possible role of ZPAC during mouse spermatogenesis. First, we analyzed the expression of ZPAC and 20S proteasome subunit α4/PSMA7 in the adult mouse testis. ZPAC and α4 were expressed in spermatogonia, spermatocytes, and round spermatids. In elongating spermatids, ZPAC was expressed until step 10, whereas expression of α4 persisted until step 12. We then examined the expression profile of ZPAC and α4 in a mouse model of experimental unilateral cryptorchidism. Consistent with appearance of morphologically impaired germ cells following cryptorchidism, the ZPAC protein level was significantly decreased at 4 days post induction of experimental cryptorchidism (D4) compared with the intact testis, although the amount of α4 protein persisted at least until D10. Moreover, intense ZPAC staining was co-localized with staining of annexin V, an early indicator of apoptosis in mammalian cells, in germ cells of cryptorchid testis, but ZPAC was also expressed in germ cells showing no detectable expression of annexin V. These results suggest that ZPAC plays a role during spermatogenesis and raises the possibility that 20S proteasome mediated by ZPAC may be involved in the regulation of germ cell survival during spermatogenesis.

GSE is a maternal factor involved in active DNA demethylation in zygotes.

After fertilization, the sperm and oocyte genomes undergo extensive epigenetic reprogramming to form a totipotent zygote. The dynamic epigenetic changes during early embryo development primarily involve DNA methylation and demethylation. We have previously identified Gse (gonad-specific expression gene) to be expressed specifically in germ cells and early embryos. Its encoded protein GSE is predominantly localized in the nuclei of cells from the zygote to blastocyst stages, suggesting possible roles in the epigenetic changes occurring during early embryo development. Here, we report the involvement of GSE in epigenetic reprogramming of the paternal genome during mouse zygote development. Preferential binding of GSE to the paternal chromatin was observed from pronuclear stage 2 (PN2) onward. A knockdown of GSE by antisense RNA in oocytes produced no apparent effect on the first and second cell cycles in preimplantation embryos, but caused a significant reduction in the loss of 5-methylcytosine (5mC) and the accumulation of 5-hydroxymethylcytosine (5hmC) in the paternal pronucleus. Furthermore, DNA methylation levels in CpG sites of LINE1 transposable elements, Lemd1, Nanog and the upstream regulatory region of the Oct4 (also known as Pou5f1) gene were clearly increased in GSE-knockdown zygotes at mid-pronuclear stages (PN3-4), but the imprinted H19-differential methylated region was not affected. Importantly, DNA immunoprecipitation of 5mC and 5hmC also indicates that knockdown of GSE in zygotes resulted in a significant reduction of the conversion of 5mC to 5hmC on LINE1. Therefore, our results suggest an important role of maternal GSE for mediating active DNA demethylation in the zygote.

Functional analysis of nocturnin, a circadian deadenylase, at maternal-to-zygotic transition in mice.

Degradation of maternally stored mRNAs after fertilization is an essential process for mammalian embryogenesis. Maternal mRNA degradation depending on deadenylases in mammalian early embryos has been mostly speculated, rather than directly demonstrated. Previously, we found that gene expression of nocturnin, which functions as a circadian clock-controlled deadenylase in mammalian cells, was clearly changed during the maternal-to-zygotic transition (MZT). Here, we investigated the possible role of nocturnin during mouse MZT. First, we examined the expression profile and localization of nocturnin in mouse oocytes and early embryos. The abundance of Nocturnin mRNA level was significantly decreased from the MII to 4-cell stages and slightly increased from the 8-cell to blastocyst stages, whereas the Nocturnin protein level was almost stable in all examined cells including GV and MII oocytes and early embryos. Nocturnin was localized in both the cytoplasm and the nucleus of all examined cells. We then examined the effect of loss or gain of Nocturnin function on early embryonic development. Knockdown of Nocturnin by injection of Nocturnin antisense expression vector into 1-cell embryos resulted in the delay of early embryonic development to the early blastocyst stage. Moreover, Nocturnin-overexpressed embryos by injection of Nocturnin expression vector impaired their development from the 1-cell to 2-cell or 4-cell stages. These results suggest that precise expression of nocturnin is critical to proper development of early mouse embryos. Functional analysis of nocturnin may contribute to the understanding of the possible role of the deadenylase at mouse MZT.

Mouse zygote-specific proteasome assembly chaperone important for maternal-to-zygotic transition.

During the maternal-to-zygotic transition (MZT), maternal proteins in oocytes are degraded by the ubiquitin-proteasome system (UPS), and new proteins are synthesized from the zygotic genome. However, the specific mechanisms underlying the UPS at the MZT are not well understood. We identified a molecule named zygote-specific proteasome assembly chaperone (ZPAC) that is specifically expressed in mouse gonads, and expression of ZPAC was transiently increased at the mouse MZT. ZPAC formed a complex with Ump1 and associated with precursor forms of 20S proteasomes. Transcription of ZPAC genes was also under the control of an autoregulatory feedback mechanism for the compensation of reduced proteasome activity similar to Ump1 and 20S proteasome subunit gene expression. Knockdown of ZPAC in early embryos caused a significant reduction of proteasome activity and decrease in Ump1 and mature proteasomes, leading to accumulation of proteins that need to be degraded at the MZT and early developmental arrest. Therefore, a unique proteasome assembly pathway mediated by ZPAC is important for progression of the mouse MZT.

Development of interspecies cloned embryos reconstructed with rabbit (Oryctolagus cuniculus) oocytes and cynomolgus monkey (Macaca fascicularis) fibroblast cell nuclei.

Interspecies somatic cell nuclear transfer (ISCNT) has been proposed as a technique to produce cloned offspring of endangered species as well as to investigate nucleus-cytoplasm interactions in mammalian embryo. However, it is still not known which embryo culture medium is optimal for ISCNT embryos for the nuclear donor or the oocyte recipient. We assessed the effects of the culture medium on the developmental competence of the ISCNT embryos by introducing cynomolgus monkey (Macaca fascicularis) fibroblast nuclei into enucleated rabbit (Oryctolagus cuniculus) oocytes (monkey-rabbit embryo). The monkey-rabbit ISCNT embryos that were cultured in mCMRL-1066 developed to the blastocyst stage, although all monkey-rabbit ISCNT embryos cultured in M199 were arrested by the 4-cell stage. When monkey-rabbit ISCNT and rabbit-rabbit somatic cell nuclear transfer (SCNT) embryos were cultured in mCMRL-1066, the blastocyst cell numbers of the monkey-rabbit ISCNT embryos corresponded to the cell numbers of the control rabbit-rabbit SCNT embryos, which were produced from a rabbit fibroblast nucleus and an enucleated rabbit oocyte. In addition, the presence of mitochondria, which were introduced with monkey fibroblasts into rabbit recipient cytoplasm, was confirmed up to the blastocyst stage by polymerase chain reaction (PCR). This study demonstrated that: (1) rabbit oocytes can reprogramme cynomolgus monkey somatic cell nuclei, and support preimplantation development; (2) monkey-rabbit ISCNT embryos developed well in monkey culture medium at early embryonic developmental stages; (3) the cell number of monkey-rabbit ISCNT embryos is similar to that of rabbit-rabbit SCNT embryos; and (4) the mitochondrial fate of monkey-rabbit ISCNT embryos is heteroplasmic from the time just after injection to the blastocyst stage that has roots in both rabbit oocytes and monkey fibroblasts.

In vivo application of an RNAi strategy for the selective suppression of a mutant allele.

Gene therapy for dominantly inherited diseases with small interfering RNA (siRNA) requires mutant allele-specific suppression when genes in which mutation causes disease normally have an important role. We previously proposed a strategy for selective suppression of mutant alleles; both mutant and wild-type alleles are inhibited by most effective siRNA, and wild-type protein is restored using mRNA mutated to be resistant to the siRNA. Here, to prove the principle of this strategy in vivo, we applied it to our previously reported anti-copper/zinc superoxide dismutase (SOD1) short hairpin RNA (shRNA) transgenic (Tg) mice, in which the expression of the endogenous wild-type SOD1 gene was inhibited by more than 80%. These shRNA Tg mice showed hepatic lipid accumulation with mild liver dysfunction due to downregulation of endogenous wild-type SOD1. To rescue this side effect, we generated siRNA-resistant SOD1 Tg mice and crossed them with anti-SOD1 shRNA Tg mice, resulting in the disappearance of lipid accumulation in the liver. Furthermore, we also succeeded in mutant SOD1-specific gene suppression in the liver of SOD1(G93A) Tg mice, a model for amyotrophic lateral sclerosis, using intravenously administered viral vectors. Our method may prove useful for siRNA-based gene therapy for dominantly inherited diseases.

Short-term storage and transport at cold temperatures of 2-cell mouse embryos produced by cryopreserved sperm.

At refrigerated temperatures, mouse embryos can maintain developmental ability for short periods. Previously, we succeeded in transporting vitrified and warmed 2-cell mouse embryos while maintaining developmental ability at refrigerated temperatures for 50 h. Transport of nonfrozen embryos is an easier and more useful means of exchanging genetically engineered mice between laboratories than is transport of cryopreserved embryos. Here we examined the developmental ability of transported 2-cell embryos that were produced through in vitro fertilization using cryopreserved sperm. Results show that 2-cell embryos produced by cryopreserved sperm can develop into blastocysts after cold storage for 24, 48, and 72 h. Transported 2-cell embryos produced by cryopreserved sperm yielded a favorable number of pups in all of the receiving laboratories after transport lasting 48 to 52 h. In summary, cold storage and transport of 2-cell embryos derived from cryopreserved sperm at refrigerated temperatures provides a novel means of transporting genetically engineered mice as an alternative to the transport of cryopreserved embryos and sperm.

Inhibition of the ubiquitin-proteasome system leads to delay of the onset of ZGA gene expression.

In mammalian oocytes, the ubiquitin-proteasome system (UPS) is suggested to play important roles in oocyte meiosis resumption, spindle assembly, polar body emission and pronuclear formation by regulating cyclin B1 degradation. However, little is known about the direct relationship between zygotic gene activation (ZGA) and degradation of maternal proteins. Here, we investigated the role of the UPS in the onset of ZGA in early mouse embryos. First, we found degradation of cyclin B1 protein in fertilized oocytes at 1 hpi by western blot analysis and used these oocytes throughout this study. Subsequently, we determined optimal experimental conditions for transient inhibition of proteasomal activity by specific and reversible proteasomal inhibitor MG132 in the G1 phase of the first cell cycle. Under the selected optimal conditions, we subjected transient MG132-treated embryos to reverse transcription (RT)-PCR analysis of expression of four ZGA genes, i.e., the hsp70.1, MuERV-L, eif-1a and zscan4d genes. As a result, we found that onset of expression of the four examined ZGA genes was delayed in both normally developed 2-cell embryos and arrested 1-cell embryos. Our results indicate that proteasomal degradation of proteins by the UPS plays a pivotal role in the molecular mechanisms of ZGA in early mouse embryos.

Deposition of acetylated histones by RNAP II promoter clearance may occur at onset of zygotic gene activation in preimplantation mouse embryos.

We investigated the contribution of phosphorylated RNA polymerase II (RNAP II) and dynamic epigenetic changes to the onset of minor zygotic gene activation (ZGA). Using immunofluorescence staining, we observed that the nuclear localization of RNAP II was initiated by 6 hours post insemination (hpi), whereas RNAP II phosphorylated at serine residue 5 of the carboxyl-terminal domain (CTD) was localized by 9 hpi, and then RNAP II phosphorylated at serine residue 2 of the CTD was localized in the nucleus of embryos by 12 hpi. In a transient gene expression assay using a plasmid reporter gene (pß-actin/luciferase+/SV40) injected during 6-9 hpi into the male pronucleus, the luciferase+ gene was actively transcribed and translated by 13 and 15 hpi, respectively, indicating that a transcriptionally silent state persisted for at least 4 hours after injection. We found that the methylation status in the chicken ß-actin promoter region of the plasmid reporter gene may not be associated with the transcriptionally silent state before minor ZGA. Exposure to trichostatin A did not induce premature expression of the silent reporter gene injected into 1-cell embryos containing histone deacetylase activity and did not affect the amount of luciferase produced per embryo. Acetylated histone H3 lysine 9/14 and acetylated histone H4 lysine 12 and 16 were enriched preferentially in the injected reporter gene at least until 13 hpi, which coincided with the transcriptionally active state. Taken together, these results suggest that deposition of selectively acetylated histones onto the chromatin of 1-cell embryos functions together with transcriptional elongation by RNAP II and that this sequential chromatin remodeling is involved in the molecular mechanism associated with the onset of minor ZGA in the preimplantation mouse embryo.