Application of auxin-inducible degron technology to mouse oocyte activation with PLCζ

Phospholipase C Zeta 1 (PLCZ1): The Function and Potential for Fertility Assessment and In Vitro Embryo Production in Cattle and Horses

Phospholipase C Zeta 1 (PLCZ1) is considered a major sperm-borne oocyte activation factor. After gamete fusion, PLCZ1 triggers calcium oscillations in the oocyte, resulting in oocyte activation. In assisted fertilization, oocyte activation failure is a major cause of low fertility. Most cases of oocyte activation failures in humans related to male infertility are associated with gene mutations and/or altered PLCZ1. Consequently, PLCZ1 evaluation could be an effective diagnostic marker and predictor of sperm fertilizing potential for in vivo and in vitro embryo production. The characterization of PLCZ1 has been principally investigated in men and mice, with less known about the PLCZ1 impact on assisted reproduction in other species, such as cattle and horses. In horses, sperm PLCZ1 varies among stallions, and sperm populations with high PLCZ1 are associated with cleavage after intracytoplasmic sperm injection (ICSI). In contrast, bull sperm is less able to initiate calcium oscillations and undergo nuclear remodeling, resulting in poor cleavage after ICSI. Advantageously, injections of PLCZ1 are able to rescue oocyte failure in mouse oocytes after ICSI, promoting full development and birth. However, further research is needed to optimize PLCZ1 diagnostic tests for consistent association with fertility and to determine whether PLCZ1 as an oocyte-activating treatment is a physiological, efficient, and safe method for improving assisted fertilization in cattle and horses.

Aberrant histone methylation in mouse early preimplantation embryos derived from round spermatid injection

Round spermatid injection (ROSI) is the last resort and recourse for men with nonobstructive azoospermia to become biological fathers of their children. However, the ROSI-derived offspring rate is lower than intracytoplasmic sperm injection (ICSI) in mice (20% vs. 60%). This low success rate has hindered the spread of ROSI in ART (Assisted Reproductive Technology). However, the cause of the ROSI-zygote-derived low offspring rate is currently unknown. In the previous studies, we reported that H3K9me3 and H3K27me3 exhibited ectopic localizations in male pronuclei (mPN) of ROSI-zygotes, suggesting that the carried over histone to zygotes conveys epigenetic information. In this study, we analyzed other histone modifications to explore unknown abnormalities. H3K36me3 showed an increased methylation state compared to ICSI-derived embryos but not for H3K4me3. Abnormal H3K36me3 was corrected until 2-cell stage embryos, suggesting a long window of reprogramming ability in ROSI-embryos. Treatment with TSA of ROSI-zygotes, which was reported to be capable of correcting ectopic DNA methylation in ROSI-zygotes, caused abnormalities of H3K36me3 in male and female PN (fPN) of the zygotes. In contrast, round spermatid TSA treatment before ROSI, which was reported to improve the preimplantation development of ROSI-zygotes, showed beneficial effects without toxicity in fPN. Therefore, the results suggest that TSA has some negative effects, but overall, it is effective in the correction of epigenetic abnormalities in ROSI-zygotes. When attempting to correct epigenetic abnormalities, attention should be paid to epigenomes not only in male but also in female pronuclei.

The auxin-inducible degron 2 technology provides sharp degradation control in yeast, mammalian cells, and mice

Protein knockdown using the auxin-inducible degron (AID) technology is useful to study protein function in living cells because it induces rapid depletion, which makes it possible to observe an immediate phenotype. However, the current AID system has two major drawbacks: leaky degradation and the requirement for a high dose of auxin. These negative features make it difficult to control precisely the expression level of a protein of interest in living cells and to apply this method to mice. Here, we overcome these problems by taking advantage of a bump-and-hole approach to establish the AID version 2 (AID2) system. AID2, which employs an OsTIR1(F74G) mutant and a ligand, 5-Ph-IAA, shows no detectable leaky degradation, requires a 670-times lower ligand concentration, and achieves even quicker degradation than the conventional AID. We demonstrate successful generation of human cell mutants for genes that were previously difficult to deal with, and show that AID2 achieves rapid target depletion not only in yeast and mammalian cells, but also in mice.

Adaptation of the AID system for stem cell and transgenic mouse research

The auxin-inducible degron (AID) system is becoming a widely used method for rapid and reversible degradation of target proteins. This system has been successfully used to study gene and protein functions in eukaryotic cells and common model organisms, such as nematode and fruit fly. To date, applications of the AID system in mammalian stem cell research are limited. Furthermore, standard mouse models harboring the AID system have not been established. Here we have explored the utility of the H11 safe-harbor locus for integration of the TIR1 transgene, an essential component of auxin-based protein degradation system. We have shown that the H11 locus can support constitutive and conditional TIR1 expression in mouse and human embryonic stem cells, as well as in mice. We demonstrate that the AID system can be successfully employed for rapid degradation of stable proteins in embryonic stem cells, which is crucial for investigation of protein functions in quickly changing environments, such as stem cell proliferation and differentiation. As embryonic stem cells possess unlimited proliferative capacity, differentiation potential, and can mimic organ development, we believe that these research tools will be an applicable resource to a broad scientific audience.

Auxin-inducible protein degradation as a novel approach for protein depletion and reverse genetic discoveries in mammalian oocytes†

The disruption of protein expression is a major approach used for investigating protein function in mammalian oocytes. This is often achieved with RNAi/morpholino-mediated knockdown or gene knockout, leading to long-term loss of proteins of interest. However, these methods have noteworthy limitations, including (a) slow protein turnover can prohibit use of these approaches; (b) essential roles in early events precludes characterization of functions in subsequent events; (c) extended protein loss can allow time for compensatory mechanisms and other unanticipated events that confound interpretation of results. The work presented here examines the use of auxin-inducible degradation, a powerful new approach that overcomes these limitations through the depletion of one's protein of interest through controllable ubiquitin-mediated degradation. This method has been employed in yeast and mammalian cell lines, and here we demonstrate the utility of auxin-inducible degradation in mouse oocytes at multiple stages of meiosis, through degradation of exogenously expressed EGFP. We also evaluate important parameters for experimental design for use of this system in oocytes. This study thus expands the toolkit of researchers in oocyte biology, establishing the use of this unique and versatile approach for depleting proteins in oocytes, and providing researchers with valuable information to make use of this system.

Production of mouse offspring from inactivated spermatozoa using horse PLCζ mRNA

Improving artificial oocyte activation is essential for assisted reproduction or animal biotechnology that can obtain healthy offspring with a high success rate. Here, we examined whether intracytoplasmic injection of equine sperm-specific phospholipase C zeta (ePLCζ) mRNA, the PLCζ with the strongest oocyte activation potential in mammals, could improve the mouse oocyte activation rate and subsequent embryonic development using inactivated spermatozoa. mRNA of mouse PLCζ (mPLCζ) or ePLCζ were injected into mouse oocytes to determine the optimal mRNA concentration to maximize the oocyte activation rate and developmental rate of parthenogenetic embryos in vitro. Full-term development was examined using NaOH-treated inactive spermatozoa using the optimal activation method. We found that the most optimal ePLCζ mRNA concentration was 0.1 ng/µl for mouse oocyte activation, which was ten times stronger than mPLCζ mRNA. The concentration did not affect parthenogenetic embryo development in vitro. Relatively normal blastocysts were obtained with the same developmental rate (52–53% or 48–51%, respectively) when inactive spermatozoa were injected into activated oocytes using ePLCζ or mPLCζ mRNA injection. However, the birth rate after embryo transfer was slightly but significantly decreased in oocytes activated by ePLCζ mRNA (24%) compared to mPLCζ mRNA (37%) or strontium treatment (40%) activation. These results suggest that the higher activation rate does not always correlate the higher birth rate, and some mechanisms might exist in the oocyte activation process that could affect the later developmental stages like full-term development.

Functions of actin in mouse oocytes at a glance

Gametes undergo a specialized and reductional cell division termed meiosis. Female gametes (oocytes) undergo two rounds of meiosis; the first meiotic division produces the fertilizable egg, while the second meiotic division occurs upon fertilization. Both meiotic divisions are highly asymmetric, producing a large egg and small polar bodies. Actin takes over various essential function during oocyte meiosis, many of which commonly rely on microtubules in mitotic cells. Specifically, the actin network has been linked to long-range vesicle transport, nuclear positioning, spindle migration and anchorage, polar body extrusion and accurate chromosome segregation in mammalian oocytes. In this Cell Science at a Glance article and the accompanying poster, we summarize the many functions of the actin cytoskeleton in oocytes, with a focus on findings from the mouse model system.