Oocyte-selective expression of MT transposon-like element, clone MTi7 and its role in oocyte maturation and embryo development

N6-methyladenosine regulates maternal RNA maintenance in oocytes and timely RNA decay during mouse maternal-to-zygotic transition

N⁶-methyladenosine (m⁶A) and its regulatory components play critical roles in various developmental processes in mammals. However, the landscape and function of m⁶A in early embryos remain unclear owing to limited materials. Here we developed a method of ultralow-input m⁶A RNA immunoprecipitation followed by sequencing to reveal the transcriptome-wide m⁶A landscape in mouse oocytes and early embryos and found unique enrichment and dynamics of m⁶A RNA modifications on maternal and zygotic RNAs, including the transcripts of transposable elements MTA and MERVL. Notably, we found that the maternal protein KIAA1429, a component of the m⁶A methyltransferase complex, was essential for m⁶A deposition on maternal mRNAs that undergo decay after zygotic genome activation and MTA transcripts to maintain their stability in oocytes. Interestingly, m⁶A methyltransferases, especially METTL3, deposited m⁶A on mRNAs transcribed during zygotic genome activation and ensured their decay after the two-cell stage, including Zscan4 and MERVL. Together, our findings uncover the essential functions of m⁶A in specific contexts during the maternal-to-zygotic transition, namely ensuring the stability of mRNAs in oocytes and the decay of two-cell-specific transcripts after fertilization.

Exploratory analysis of transposable elements expression in the C. elegans early embryo

BACKGROUND

Transposable Elements (TE) are mobile sequences that make up large portions of eukaryote genomes. The functions they play within the complex cellular architecture are still not clearly understood, but it is becoming evident that TE have a role in several physiological and pathological processes. In particular, it has been shown that TE transcription is necessary for the correct development of mice embryos and that their expression is able to finely modulate transcription of coding and non-coding genes. Moreover, their activity in the central nervous system (CNS) and other tissues has been correlated with the creation of somatic mosaicisms and with pathologies such as neurodevelopmental and neurodegenerative diseases as well as cancers.

RESULTS

We analyzed TE expression among different cell types of the Caenorhabditis elegans (C. elegans) early embryo asking if, where and when TE are expressed and whether their expression is correlated with genes playing a role in early embryo development. To answer these questions, we took advantage of a public C. elegans embryonic single-cell RNA-seq (sc-RNAseq) dataset and developed a bioinformatics pipeline able to quantify reads mapping specifically against TE, avoiding counting reads mapping on TE fragments embedded in coding/non-coding transcripts. Our results suggest that i) canonical TE expression analysis tools, which do not discard reads mapping on TE fragments embedded in annotated transcripts, may over-estimate TE expression levels, ii) Long Terminal Repeats (LTR) elements are mostly expressed in undifferentiated cells and might play a role in pluripotency maintenance and activation of the innate immune response, iii) non-LTR are expressed in differentiated cells, in particular in neurons and nervous system-associated tissues, and iv) DNA TE are homogenously expressed throughout the C. elegans early embryo development.

CONCLUSIONS

TE expression appears finely modulated in the C. elegans early embryo and different TE classes are expressed in different cell types and stages, suggesting that TE might play diverse functions during early embryo development.

PIWIs, piRNAs and Retrotransposons: Complex battles during reprogramming in gametes and early embryos

Gamete and embryo development are indispensable processes for successful reproduction. Cells involved in these processes acquire pluripotency, the ability to differentiate into multiple different cell types, through a series of events known as reprogramming that lead to profound changes in histone and DNA methylation. While essential for pluripotency, this epigenetic remodelling removes constraints that normally limit the expression of genomic sequences known as transposable elements (TEs). Unconstrained TE expression can lead to many deleterious consequences including infertility, so organisms have evolved complex and potent mechanistic arsenals to target and suppress TE expression during reprogramming. This review will focus on the control of transposable elements in gametes and embryos, and one important TE suppressing system known as the PIWI pathway. This broadly conserved, small RNA-targeted silencing mechanism appears critical for fertility in many species and may participate in multiple aspects of gene regulation in reproduction and other contexts.

Profile of the mosaic element BTMR1 in the genome of the bumble bee Bombus terrestris (Hymenoptera: Apidae)

Co-evolution involving a mariner transposon, Botmar1 and the other repeats contained in the Bombus terrestris genome was investigated. We found that the 5'-region of Botmar1 forms one of the components of a mosaic element, known as B. terrestris mosaic repeat 1 (BTMR1), which is also composed of inner segments originating from two different retrotransposons and a pseudogene corresponding to an RNA methyltransferase cDNA. The fact that BTMR1 is interspersed within chromosomes and the differences in its abundance in different species indicate that it is very probably a mobile element. Nevertheless, the absences of direct or inverted repeats at its ends and of target site duplication indicate that its mobility is not ensured by a cardinal transposable element, but putatively by a Crypton-like element.

Obox4 critically regulates cAMP-dependent meiotic arrest and MI-MII transition in oocytes

Extra follicular oocytes spontaneously resume meiosis in vitro, but the intact germinal vesicle (GV) is retained if the oocytes are cultured in medium containing phosphodiesterase (PDE) inhibitors or cAMP analogues. On the basis of our finding that Obox4 is prominently expressed in oocytes, the present study was conducted to determine the functional role of the homeodomain-containing factor Obox4 during in vitro oocyte maturation. After microinjection of Obox4 dsRNA into the cytoplasm of GV oocytes cultured in M16 medium, oocytes were arrested at metaphase I (MI, 77.7%) and metaphase II (MII, 22.3%). Surprisingly, however, 89% of Obox4 RNAi-treated oocytes resumed meiosis and developed to MI and MII when cultured in medium containing 0.2 mM 3-isobutyl-1-methyl-xanthine (IBMX), in which untreated oocytes maintain intact GVs. Spindles were aberrant, and chromosomes were severely aggregated with decreased MPF and MAP kinase activities in arrested MI oocytes after exposure to Obox4 RNAi. Oocytes overexpressing Obox4 retained intact GVs when cultured in M16 medium. Taken together, for the first time to our knowledge, these findings indicate that Obox4 plays a key role in the cAMP-dependent signaling cascades that maintain GV arrest. Oocytes not expressing Obox4 failed to maintain intact GVs in IBMX-supplemented medium, while GVs remained intact when oocytes were kept in plain medium and overexpressing Obox4, suggesting that Obox4 plays a critical role in cAMP-dependent cascade for maintaining intact GVs.

Role of Bcl2-like 10 (Bcl2l10) in Regulating Mouse Oocyte Maturation

Previously, we have shown that Bcl2l10 is highly expressed in metaphase II (MII)-stage oocytes. The objective of this study was to characterize Bcl2l10 expression in ovaries and to examine the function of Bcl2l10 in oocyte maturation using RNA interference. Bcl2l10 transcript expression was ovary and oocyte specific. Bcl2l10 was highly expressed in oocytes and pronuclear-stage embryos; however, its expression decreased at the two-cell stage and dramatically disappeared thereafter. Microinjection of Bcl2l10 double-stranded RNA into the cytoplasm of germinal vesicle oocytes resulted in a marked decrease in Bcl2l10 mRNA and protein and metaphase I (MI) arrest (78.9%). Most MI-arrested oocytes exhibited abnormalities in their spindles and chromosome configurations. Bcl2l10 RNA interference had an obvious effect on the activity of maturation-promoting factor but not on that of mitogen-activated protein kinase. We concluded that the role of Bcl2l10 is strongly associated with oocyte maturation, especially at the MI-MII transition.

SEBOX is essential for early embryogenesis at the two-cell stage in the mouse

Previously, we found high levels of skin-embryo-brain-oocyte homeobox (Sebox) gene expression in germinal vesicle (GV)-stage oocytes. The objective of the present study was to determine the role played by SEBOX in oocyte maturation and early embryogenesis using RNA interference (RNAi). Microinjection of Sebox double-stranded RNA into GV oocytes resulted in a marked decrease in Sebox mRNA and protein expression. However, Sebox RNAi affects neither oocyte maturation rate nor morphological characteristics, including spindle and chromosomal organization of metaphase II oocytes. In addition, Sebox RNAi had no discernible effect on the activities of M-phase promoting factor or mitogen-activated protein kinase. In contrast, microinjection of Sebox double-stranded RNA into pronuclear-stage embryos resulted in holding embryo development at the two-cell (84.9%) and the four- and eight-cell (15.1%) stages. We concluded that Sebox is a new addition to maternal effect genes that produced and stored in oocytes and function in preimplantation embryo development.

RNA silencing in mammalian oocytes and early embryos

RNA silencing is a common term for homology-dependent silencing phenomena found in the majority of eukaryotic species. RNA silencing pathways share several conserved components. The common denominator of these pathways is the presence of specific, short (21-25 nt) RNA molecules generated from different double-stranded RNA substrates by a specific RNase III activity. Short RNA molecules serve as a template for sequence-specific effects including transcriptional silencing, mRNA degradation, and inhibition of translation. This review will discuss possible roles of RNA silencing pathways in mouse oocytes and early embryos as well as the use of RNA silencing for experimental inhibition of gene expression in this model system.

Establishment of ovarian reconstruction system in culture for functional genomic analysis

In a previous study, we reported a list of genes expressed differentially in primordial and primary follicles [Park et al., Fertil. Steril., 83, 410-418 (2005)]. An innovative experimental system is required to evaluate the functions of these genes in folliculogenesis, particularly primordial-primary follicle transition. In this study, ovarian tissues were dissociated, and isolated cells were transfected using small interfering RNAs (siRNAs) for disrupting a specific gene, followed by ovarian reconstruction via calcium alginate encapsulation. The effects of RNA interference (RNAi) on follicular development were evaluated by a histological observation of the reconstructed ovarian tissue. Interestingly, follicular formation and development showed differences between control and experimental groups. Thus, even though this system includes some problems that need to be solved, ovarian reconstruction following the modification of gene expression of individual ovarian component cells could lead the way to methods of studying molecular mechanisms of primordial-primary follicle transition.

Role of cytosolic malate dehydrogenase in oocyte maturation and embryo development

OBJECTIVE

To elucidate the function of cytosolic malate dehydrogenase (Mor2) in oocyte maturation and embryo development using RNA interference (RNAi).

DESIGN

Experimental animal study.

SETTING

Research unit of university.

ANIMAL(S)

Female 4-week-old (C57/BL6) mice.

INTERVENTION(S)

Isolation of immature germinal vesicle (GV) oocytes or fertilized pronucleus (PN) embryos, microinjection of Mor2 double-stranded RNA (dsRNA), and reverse transcription and polymerase chain reaction (RT-PCR) analysis to investigate Mor2-specific messenger RNA (mRNA) knockdown.

MAIN OUTCOME MEASURE(S)

Relative changes in mRNA levels after microinjection of Mor2 dsRNA and in rates of oocyte maturation and preimplantation embryo development.

RESULT(S)

Mor2 mRNA mostly was knocked down in germinal vesicle- and metaphase I (MI)-arrested oocytes, compared with metaphase II (MII)-developed oocytes, after microinjection of Mor2 dsRNA and in vitro culture for 16 hours. In vitro oocyte maturation was significantly decreased (34%), compared with noninjected (73.4%) and buffer-injected (67.5%) control groups. The rate of blastocyst development (48.1%) was lower in the Mor2 dsRNA-injected group than in buffer-injected control (88.2%).

CONCLUSION(S)

In the present study, the function of Mor2 was analyzed with the aid of RNAi. On the basis of the data obtained, we propose that Mor2 is an essential factor for oocyte maturation and embryo development in mouse.

Abundant transcripts from retrotransposons are unstable in fully grown mouse oocytes

One physiological function proposed for RNA interference (RNAi) is to constrain expression of repetitive elements and thereby reduce the incidence of retrotransposition. Consistent with this model is that inhibiting the RNAi pathway results in an increase in expression of repetitive elements in preimplantation mouse embryos. Mouse oocytes are essentially transcriptionally quiescent providing a unique opportunity to assess the stability of repetitive element-derived transcripts in these cells. We compared the transcriptome of freshly isolated fully grown germinal vesicle (GV)-intact oocytes to that of oocytes in which meiotic maturation in vitro was inhibited for 48 h by milrinone. Consistent with the aforementioned function for RNAi is that the abundance of only a relatively small number of transcripts decreased in the cultured oocytes, when compared to changes that occur during maturation or following fertilization, and of those, several belonged to mobile elements.

Identification and characterization of a novel Mt-retrotransposon highly represented in the female mouse germline

The control of primordial follicle recruitment into the growing follicle population is a major limiting process in female reproduction. In order to gain insight into the molecular processes occurring at the time of primordial follicle activation, a subtractive hybridization analysis was performed between cDNAs prepared from temporally distinct mouse neonatal ovarian tissues that differed according to the state of primordial follicle activation. One highly represented clone associated with activation was an Mt retrotransposon-like sequence designated Mtfull, which was subsequently cloned and determined to be novel and restricted in expression to the ovary. The polyadenylated 1684-bp sequence has long terminal repeats, is predicted to be noncoding, and is the predominant Mti-related sequence present in the mouse ovary. In situ hybridization further localized Mtfull expression to the oocyte and confirmed that expression is concomitant with follicle activation. Together with in silico data, we predict Mtfull plays an essential role in folliculogenesis through regulation of gene expression.

Transcripts from a human primordial follicle cDNA library

BACKGROUND

Human primordial follicles (PFs) or the oocyte-pre-granulosa complex, constitute the earliest and most immature stage of human oogenesis. The factors, signalling networks and the precise role of the oocyte and the pre-granulosa cells in initiating growth and recruitment from this finite resting pool remain largely unknown at present.

METHODS

To obtain a gene resource of this oogenesis stage and thereby determine a molecular blueprint of the human PF, a cDNA library was constructed from 50 isolated human PFs using the phagemid vector pTriplEx2.

RESULTS

Sequence analysis showed that 46.67% of these clones corresponded to known genes while 29.48% were uncharacterized genes that included hypothetical proteins, human cDNA clones and novel genes. Bioinformatics analysis revealed a preponderance of mitochondrial genes and repeat elements followed by ribosomal proteins, transcription and translation genes. Transcripts for heat shock proteins, cell cycle, embryogenesis genes and apoptosis genes were identified. Members of the ubiquitin-proteasome pathway, MAPK, p38/JNK, GPCR, Wnt, NF-kappaB and notch signalling pathways were identified. A mitochondrial pathway and a transcription factor pathway in the human PF were generated. The gene networks in the transcription factor pathway provided a first glimpse of the balance between proliferation and cell death/apoptosis in this earliest stage of oogenesis.

CONCLUSIONS

The abundance and diversity of retroviral elements and transcriptional repressor genes in the human PF suggest these could contribute to the maintainance of this oogenesis stage. The role of these genes in initial recruitment and in subsequent oogenesis stages will be greatly facilitated and elucidated by printing a human PF cDNA array of the sequenced clones and using it for gene profiling.