RNA silencing in mammalian oocytes and early embryos

The oocyte-to-embryo transition in mouse: past, present, and future

The oocyte-to-embryo transition (OET) arguably initiates with formation of a primordial follicle and culminates with reprogramming of gene expression during the course of zygotic genome activation. This transition results in converting a highly differentiated cell, i.e. oocyte, to undifferentiated cells, i.e. initial blastomeres of a preimplantation embryo. A plethora of changes occur during the OET and include, but are not limited to, changes in transcription, chromatin structure, and protein synthesis; accumulation of macromolecules and organelles that will comprise the oocyte's maternal contribution to the early embryo; sequential acquisition of meiotic and developmental competence to name but a few. This review will focus on transcriptional and post-transcriptional changes that occur during OET in mouse because such changes are likely the major driving force for OET. We often take a historical and personal perspective, and highlight how advances in experimental methods often catalyzed conceptual advances in understanding the molecular bases for OET. We also point out questions that remain open and therefore represent topics of interest for future investigation.

Targeted Gene Knockdown in Early Embryos Using siRNA

RNA interference is a convenient and highly effective technique to investigate the biological function of genes. Adequately designed RNA molecules introduced into an oocyte are able to bind specific endogenous mRNAs and trigger their degradation. Subsequent fertilization of these oocytes will result in the generation of embryos in which the expression of the gene of interest is downregulated, and following the degradation of maternal proteins the role of the gene product can be studied. Here, we describe the approach how post-transcriptional gene silencing can be achieved in oocytes and early embryos using siRNA.

An oocyte-specific ELAVL2 isoform is a translational repressor ablated from meiotically competent antral oocytes

At the end of the growth phase, mouse antral follicle oocytes acquire full developmental competence. In the mouse, this event is marked by the transition from the so-called non-surrounded nucleolus (NSN) chromatin configuration into the transcriptionally quiescent surrounded nucleolus (SN) configuration, which is named after a prominent perinucleolar condensed chromatin ring. However, the SN chromatin configuration alone is not sufficient for determining the developmental competence of the SN oocyte. There are additional nuclear and cytoplamic factors involved, while a little is known about the changes occurring in the cytoplasm during the NSN/SN transition. Here, we report functional analysis of maternal ELAVL2 an AU-rich element binding protein. Elavl2 gene encodes an oocyte-specific protein isoform (denoted ELAVL2°), which acts as a translational repressor. ELAVL2° is abundant in fully grown NSN oocytes, is ablated during the NSN/SN transition and remains low during the oocyte-to-embryo transition (OET). ELAVL2° overexpression during meiotic maturation causes errors in chromosome segregation, indicating the significance of naturally reduced ELAVL2° levels in SN oocytes. On the other hand, during oocyte growth, prematurely reduced Elavl2 expression results in lower yields of fully grown and meiotically matured oocytes, suggesting that Elavl2 is necessary for proper oocyte maturation. Moreover, Elavl2 knockdown showed stimulating effects on translation in fully grown oocytes. We propose that ELAVL2 has an ambivalent role in oocytes: it functions as a pleiotropic translational repressor in efficient production of fully grown oocytes, while its disposal during the NSN/SN transition contributes to the acquisition of full developmental competence.

Age-associated metabolic and morphologic changes in mitochondria of individual mouse and hamster oocytes

Background

In human oocytes, as in other mammalian ova, there is a significant variation in the pregnancy potential, with approximately 20% of oocyte-sperm meetings resulting in pregnancies. This frequency of successful fertilization decreases as the oocytes age. This low proportion of fruitful couplings appears to be influenced by changes in mitochondrial structure and function. In this study, we have examined mitochondrial biogenesis in both hamster (Mesocricetus auratus) and mouse (Mus musculus) ova as models for understanding the effects of aging on mitochondrial structure and energy production within the mammalian oocyte.

Methodology/Principal Findings

Individual metaphase II oocytes from a total of 25 young and old mice and hamsters were collected from ovarian follicles after hormone stimulation and prepared for biochemical or structural analysis. Adenosine triphosphate levels and mitochondrial DNA number were determined within individual oocytes from young and old animals. In aged hamsters, oocyte adenosine triphosphate levels and mitochondrial DNA molecules were reduced 35.4% and 51.8%, respectively. Reductions of 38.4% and 44% in adenosine triphosphate and mitochondrial genomes, respectively, were also seen in aged mouse oocytes. Transmission electron microscopic (TEM) analysis showed that aged rodent oocytes had significant alterations in mitochondrial and cytoplasmic lamellae structure.

Conclusions/Significance

In both mice and hamsters, decreased adenosine triphosphate in aged oocytes is correlated with a similar decrease in mtDNA molecules and number of mitochondria. Mitochondria in mice and hamsters undergo significant morphological change with aging including mitochondrial vacuolization, cristae alterations, and changes in cytoplasmic lamellae.

Shortcomings of short hairpin RNA-based transgenic RNA interference in mouse oocytes

BACKGROUND

RNA interference (RNAi) is a powerful approach to study a gene function. Transgenic RNAi is an adaptation of this approach where suppression of a specific gene is achieved by expression of an RNA hairpin from a transgene. In somatic cells, where a long double-stranded RNA (dsRNA) longer than 30 base-pairs can induce a sequence-independent interferon response, short hairpin RNA (shRNA) expression is used to induce RNAi. In contrast, transgenic RNAi in the oocyte routinely employs a long RNA hairpin. Transgenic RNAi based on long hairpin RNA, although robust and successful, is restricted to a few cell types, where long double-stranded RNA does not induce sequence-independent responses. Transgenic RNAi in mouse oocytes based on a shRNA offers several potential advantages, including simple cloning of the transgenic vector and an ability to use the same targeting construct in any cell type.

RESULTS

Here we report our experience with shRNA-based transgenic RNAi in mouse oocytes. Despite optimal starting conditions for this experiment, we experienced several setbacks, which outweigh potential benefits of the shRNA system. First, obtaining an efficient shRNA is potentially a time-consuming and expensive task. Second, we observed that our transgene, which was based on a common commercial vector, was readily silenced in transgenic animals.

CONCLUSIONS

We conclude that, the long RNA hairpin-based RNAi is more reliable and cost-effective and we recommend it as a method-of-choice when a gene is studied selectively in the oocyte.

Dicer is a key player in oocyte maturation

OBJECTIVE

Apply Dicer siRNA to study functions of Dicer and miRNA during oogenesis.

MATERIALS AND METHODS

Mouse oocytes were injected with Dicer siRNA and negative control siRNA and then matured in vitro. After IVM, oocytes were examined for maturation rates, spindle and chromosomal organization, and various gene expressions.

RESULTS

Dicer siRNA significantly reduced maturation rates, increased abnormal spindle and chromosomal organization, and reduced the transcripts of Dicer miRNAs, spindle formation proteins (plk1 and AURKA) and spindle check points (Bub1, Bublb). Depletion of bulb16 markedly prohibited the first polar body extrusion and increased the incidence of misaligned chromosomes and abnormal meiotic spindle assembly.

CONCLUSION

Dicer siRNA triggered a cascade reduction for gene expressions starting from Dicer to miRNAs than to spindle assembly proteins and checkpoints which led to abnormal spindle and chromosomal organization. Thus, Dicer and miRNA appeared to play an important role during oogenesis and were essential for meiotic completion.

RNAi experiments in mouse oocytes and early embryos

The discovery of RNA interference (RNAi) in 1998 ushered in a new era in biology. RNAi currently serves as a favorite approach for inhibition of gene function in many areas of research. This article provides a brief review of RNAi and discussion of the benefits and drawbacks of using long double-stranded RNA (dsRNA) in mammalian oocytes and early embryos. We also provide an introduction to protocols for RNAi experiments in mouse, including preparation and microinjection of dsRNA into mouse oocytes and early embryos, and preparation and testing of constructs for transgenic RNAi based on long hairpin RNA expression.

P-body loss is concomitant with formation of a messenger RNA storage domain in mouse oocytes

In mammalian somatic cells, several pathways that converge on deadenylation, decapping, and 5'-3' degradation are found in cytoplasmic foci known as P-bodies. Because controlled mRNA stability is essential for oocyte-to-zygote transition, we examined the dynamics of P-body components in mouse oocytes. We report that oocyte growth is accompanied by loss of P-bodies and a subcortical accumulation of several RNA-binding proteins, including DDX6, CPEB, YBX2 (MSY2), and the exon junction complex. These proteins form transient RNA-containing aggregates in fully grown oocytes with a surrounded nucleolus chromatin configuration. These aggregates disperse during oocyte maturation, consistent with recruitment of maternal mRNAs that occurs during this time. In contrast, levels of DCP1A are low during oocyte growth, and DCP1A does not colocalize with DDX6 in the subcortical aggregates. The amount of DCP1A markedly increases during meiosis, which correlates with the first wave of destabilization of maternal mRNAs. We propose that the cortex of growing oocytes serves as an mRNA storage compartment, which contains a novel type of RNA granule related to P-bodies.

Role of Dicer in female fertility

Dicer is an RNAse III endonuclease that is essential for the biogenesis of microRNAs and small interfering RNAs. These small RNAs post-transcriptionally regulate mRNA gene expression through several mechanisms to affect key cellular events including proliferation, differentiation and apoptosis. Recently, the role of Dicer function in female reproductive tissues has begun to be elucidated through the use of knockout mouse models. Loss of Dicer within ovarian granulosa cells, luteal tissue, oocyte, oviduct and, potentially, the uterus renders females infertile. This review discusses these early studies and other data describing the current understanding of microRNAs and small interfering RNAs in female reproduction.