Mammalian oocytes and early embryos derived from in vitro production are highly susceptible to a variety of cellular stresses. During oocyte maturation and preimplantation embryo development, functional proteins must be folded properly in the endoplasmic reticulum (ER) to maintain oocyte and embryo development. However, some adverse factors negatively impact ER functions and protein synthesis, resulting in the activation of ER stress and unfolded protein response (UPR) signaling pathways. ER stress and UPR signaling have been identified in mammalian oocytes and embryos produced in vitro, suggesting that modulation of ER stress and UPR signaling play very important roles in oocyte maturation and the development of preimplantation embryos. In this review, we briefly describe the current state of knowledge regarding ER stress, UPR signaling pathways, and their roles and mechanisms in mammalian (excluding human) oocyte maturation and preimplantation embryo development.

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Lysophosphatidic acid (LPA), a member of the phospholipid autacoid family, is present in human follicular fluid. The aim of the present study was to compare the developmental competence of porcine embryos created via in vitro fertilization (IVF) and parthenogenetic activation (PA) in culture medium supplemented with LPA, in comparison with a control group. The effects of LPA on porcine oocyte maturation and pre-implantation embryonic development were also examined. Addition of 10 μM LPA to the oocyte maturation medium significantly increased the proportion of oocytes reaching metaphase I (MI) or metaphase II (MII), and enhanced embryonic developmental potential. When present during oocyte maturation, LPA significantly increased the abundance of phosphorylated ERK1/2 in MI and MII oocytes, showing that LPA enhanced nuclear maturation via activation of the mitogen-activated protein kinase (MAPK) pathway. In addition, Cyclin B1 levels were elevated in MI- and MII-stage oocytes, suggesting that LPA plays a role in both nuclear and cytoplasmic maturation of oocytes. After fertilization, the frequency of polyspermy in embryos obtained using LPA-treated oocytes was less than that in the control group. Further, blastocyst formation and blastocyst cell number were enhanced and apoptosis was reduced upon LPA treatment of embryos created either by IVF and PA. LPA treatment of blastocysts derived by IVF or PA resulted in increased expression of the anti-apoptotic BCL2L1 gene while reducing expression of the pro-apoptotic genes BAX and CASP3. Together, our data indicate that LPA supplementation improves porcine oocyte maturation and subsequent in vitro development of pre-implantation embryos.

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Cell-to-cell contact mediated by cell adhesion is fundamental to the compaction process that ensures blastocyst quality during embryonic development. In this study, we first showed that Rho-associated coiled-coil protein kinases (ROCK1 and ROCK2) were expressed both in porcine oocytes and IVF preimplantation embryos, playing different roles in oocytes maturation and embryo development. The amount of mRNA encoding ROCK1 and the protein concentration clearly increased between the eight-cell and morula stages, but decreased significantly when blastocysts were formed. Conversely, ROCK2 was more abundant in the blastocyst compared with other embryonic stages. Moreover, immunostaining showed that ROCK1 protein distribution changed as the embryo progressed through cleavage and compaction to the morula stage. Initially, the protein was predominantly associated with the plasma membrane but later became cytoplasmic. By contrast, ROCK2 protein was localized in both the cytoplasm and the spindle rotation region during oocyte meiosis, but in the cytoplasm and nucleus as the embryo developed. In addition, ROCK2 was present in the trophectoderm cells of the blastocyst. Treatment with 15 mM Y27632, a specific inhibitor of ROCKs, completely blocked further development of early four-cell stage embryos. Moreover, we did not detect the expression of ROCK1 but did detect ROCK2 expression in blastocysts. Moreover, lysophosphatidic acid an activator of ROCKs significantly improved the rates of blastocyst formation. These data demonstrate that ROCKs are required for embryo development to the blastocyst stage. Together, our results indicate that ROCK1 and ROCK2 may exert different biological functions during the regulation of compaction and in ensuring development of porcine preimplantation embryos to the blastocyst stage.

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The mRNA processing and export factor, Iws1, interacts with the histone H3/H4 chaperone, Spt6 (Supt6 in mouse gene ontology) and recruits the lysine methyltransferase, Setd2, to chromatin to regulate H3K36me3. This recruitment is known to be crucial for pre-mRNA splicing and Iws1 has been shown to interact with REF1/Aly to mediate mRNA export. However, the role of this complex has not yet been examined in embryonic development. Here, we show that knockdown of either Iws1 or Supt6 blocked embryo development, primarily at the 8/16-cell stage, indicating that Iws1 and Supt6 are crucial for mouse preimplantation development. In the knockdown embryos, we observed decreases in pre-mRNA splicing, mRNA export and the expression of the lineage-specific transcription factor, Nanog. We found that either Iws1 or Supt6 are required for H3K36 trimethylation and that concurrent knockdown of both Iws1 and Supt6 blocks embryonic development at the 2-cell stage. We show that H3K36me3 is modulated by the Pi3k/Akt pathway, as inhibition of this pathway reduced the global level of H3K36me3 while activation of the pathway increased the level of this modification in 2-cell embryos. We observed that Iws1 interacts with nuclear Akt in early embryos, and herein propose that Akt modulates H3K36me3 through interaction with Iws1. Together, our results indicate that the Iws1 and Supt6 play crucial roles in embryonic genome activation, lineage specification, and histone modification during mouse early development.

Poor-quality oocytes (those with 1-2 layers of cumulus cells) typically possess low meiotic competence and development. Prolonging the duration of in vitro maturation (IVM; 52 hr) can enhance the maturation rate of poor-quality oocytes, but it does not improve subsequent embryonic development. This likely reflects the increased reactive oxygen species (ROS) production and apoptosis seen in these oocytes compared with the non-prolonged IVM (44 hr) group. Melatonin is a free radical scavenger, anti-oxidant and anti-apoptotic agent that reported to enhance the quality of embryos by inhibiting ROS generation and apoptosis. Therefore, we herein investigated whether melatonin combined with prolonged IVM (52 hr) could improve the quality and development of poor-quality oocytes. We supplemented IVM and/or in vitro culture (IVC) media with various concentrations (0, 10 , 10 , 10 M) of melatonin, and estimated parameters related to oocyte quality and development. The addition of melatonin (10 M) to a prolonged IVM system improved the oocyte quality and development compared with those of the melatonin-free poor-quality oocytes group, and that this was due to decreases in ROS generation, apoptosis, and DNA damage. When melatonin was added during both IVM (10 M) and IVC (10 M), we observed a cumulative positive influence on the embryonic development and quality; this treatment enhanced the expression level of Oct4 and decreased the levels of ROS, DNA damage, and apoptosis. Together, these findings suggest that the combination of melatonin plus prolonged IVM can improve the quality and development of poor-quality porcine oocytes via anti-oxidative and anti-apoptotic effects.