Determination of microRNAs in mouse preimplantation embryos by microarray

miR-135A regulates preimplantation embryo development through down-regulation of E3 Ubiquitin Ligase Seven In Absentia Homolog 1A (SIAH1A) expression

BACKGROUND

MicroRNAs (miRNAs) are small non-coding RNA molecules capable of regulating transcription and translation. Previously, a cluster of miRNAs that are specifically expressed in mouse zygotes but not in oocytes or other preimplantation stages embryos are identified by multiplex real-time polymerase chain reaction-based miRNA profiling. The functional role of one of these zygote-specific miRNAs, miR-135a, in preimplantation embryo development was investigated.

METHODOLOGY/PRINCIPAL FINDINGS

Microinjection of miR-135a inhibitor suppressed first cell cleavage in more than 30% of the zygotes. Bioinformatics analysis identified E3 Ubiquitin Ligase Seven In Absentia Homolog 1A (Siah1a) as a predicted target of miR-135a. Western blotting and 3'UTR luciferase functional assays demonstrated that miR-135a down-regulated the expression of Siah1 in HeLa cells and in mouse zygotes. Siah1a was expressed in preimplantation embryos and its expression pattern negatively correlated with that of miR-135a. Co-injection of Siah1a-specific antibody with miR-135a inhibitor partially nullified the effect of miR-135a inhibition. Proteasome inhibition by MG-132 revealed that miR-135a regulated proteasomal degradation and potentially controlled the expression of chemokinesin DNA binding protein (Kid).

CONCLUSIONS/SIGNIFICANCE

The present study demonstrated for the first time that zygotic specific miRNA modulates the first cell cleavage through regulating expression of Siah1a.

MicroRNA regulation via DNA methylation during the morula to blastocyst transition in mice

Epigenetic regulation is responsible for transcriptional silencing of genes and parental imprinting. This study addresses the question whether microRNAs (miRNAs) could be affected by DNA methylation during morula-blastocyst transition. Mouse embryos were treated with/without a DNA methyltransferase inhibitor (5-aza-2'-deoxycytidine, 5-aza-dC, 10 nM-5 μM). Changes of miRNAs were analyzed by quantitative real-time (Q-PCR)-based megaplex pre-amp microRNA assays. Development from morula to blastocyst in mice was inhibited by 5-aza-dC in a dose-dependent manner (10 nM-5 μM), with half of the embryos arrested at morula stage when treated with levels of 5-aza-dC as low as 50 nM. In total, 48 down-regulated microRNAs and 17 up-regulated microRNAs (≥5-fold changes) were identified after 5-aza-dC treatment, including let-7e, mir-20a, mir-21, mir-34b, mir-128b and mir-452. Their predicted targets were selected based on software analysis, published databases and further confirmed by Q-PCR. At least eight targets, including dnmt3a, jagged 1, sp1, edg2, abcg4, numa1, tmsb10 and csf1r were confirmed. In conclusion, 5-aza-dC-modified microRNA profiles and identification of the microRNA's targets during the morula to blastocyst stage in mice provide information that helps us to explore the relationship between fertility, microRNA regulation and epigenetic intervention.

Wdr74 is required for blastocyst formation in the mouse

Preimplantation is a dynamic developmental period during which a combination of maternal and zygotic factors program the early embryo resulting in lineage specification and implantation. A reverse genetic RNAi screen in mouse embryos identified the WD Repeat Domain 74 gene (Wdr74) as being required for these critical first steps of mammalian development. Knockdown of Wdr74 results in embryos that develop normally until the morula stage but fail to form blastocysts or properly specify the inner cell mass and trophectoderm. In Wdr74-deficient embryos, we find activated Trp53-dependent apoptosis as well as a global reduction of RNA polymerase I, II and III transcripts. In Wdr74-deficient embryos blocking Trp53 function rescues blastocyst formation and lineage differentiation. These results indicate that Wdr74 is required for RNA transcription, processing and/or stability during preimplantation development and is an essential gene in the mouse.

Epigenetic regulatory mechanisms during preimplantation embryo development

Within the past few years, developmental scientists have switched their attention from the study of DNA sequencing to the epigenetic state of the genome. Studying epigenetics could present the key to understanding gene expression changes without altering the basic structure of DNA. For example, the blastocyst, trophectoderm, and inner cell mass grow within the same environment, having the same genome, but differentiate differently. Each stage of embryo development is characterized by a specific epigenetic pattern. These modifications give the embryos the ability to interact with the uterus. Gene expression profiles change dramatically, and chromatin remodeling allows for core histone elements to undergo significant modifications. In the past, epigenetic mechanisms were studied using less accurate technologies such as PCR techniques and gel electrophoresis. Today microarray, DNA analyzers, and other in silico approaches give us the capability to understand the epigenetic impact on differentiation and cell fate.

Epigenetic aspects of fertilization and preimplantation development in mammals: lessons from the mouse

During gametogenesis, the parental genomes are separated and are epigenetically marked by modifications that will direct the expression profile of genes necessary for meiosis as well as for the formation of the oocyte and sperm cell. Immediately after sperm-egg fusion, the parental haploid genomes show great epigenetic asymmetry with differences in the levels of DNA methylation and histone tail modifications. The epigenetic program acquired during oogenesis and spermatogenesis must be reset for the zygote to successfully proceed through preimplantation development and this occurs as the two genomes approach each other in preparation for karyogamy. During preimplantation development, the embryo is vested with the responsibility of maintaining the primary imprints. In addition, female embryos must silence one of the X-chromosomes in order to transcribe equal levels of X-linked genes as their male counterparts. This review is intended as a survey of the epigenetic modifications and mechanisms present in zygotes and preimplantation mouse embryos, namely DNA methylation, histone modifications, dosage compensation, genomic imprinting, and regulation by non-coding RNAs.

MicroRNA expression profiles are altered by gonadotropins and vitamin C status during in vitro follicular growth

MicroRNAs (miRs) are known to repress target genes at posttranscriptional level and play important roles in the maturation of cells. However, the expression profiles of miRs during follicular maturation have not been fully elucidated. This study was designed to investigate the expression profiles of miRs in murine follicles according to human chorionic gonadotropin (hCG) treatment and vitamin C status during in vitro culture. Ovaries were removed from the 12-day-old wild-type and vitamin C-deficient (L-gulonogammalactone oxidase knockout, Gulo-/-) C57BL6 mice. Preantral follicles were isolated and cultured in 20 µL droplets of culture media supplemented with follicle-stimulating hormone and luteinizing hormone (FSH + LH). After their full maturation, follicles were divided into 2 groups: with and without hCG treatment. Real-time polymerase chain reaction (PCR) was performed using oocytes and granulosa cells (G-cells) to evaluate the miRs known to be expressed mainly in the mouse ovary. After the addition of hCG, miR profiles showed divergent changes between oocytes and G-cells. These profiles significantly differed from those of hCG(-) group. Compared to wild type, Gulo-/- mice showed altered miR profiles in matured oocytes and G-cells. Conclusively, hCG supplementation and vitamin C status alter the miR expression profiles in oocytes and G-cells during in vitro growth of murine follicles.

MicroRNA expression profiling of elongated cloned and in vitro-fertilized bovine embryos

The objective of this study was to identify microRNAs (miRNAs) expressed in bovine (Bos Taurus) cloned embryos at Day 17 of development (Day 0=day of nucleus transfer or in vitro fertilization) during elongation. Day 7 bovine expanded blastocysts produced by hand made cloning (HMC) or in vitro fertilization were bulk-transferred to synchronized recipient cattle (48 HMC embryos to 10 recipients and 28 in vitro-produced embryos to four recipients). Elongated embryos were retrieved at Day 17; miRNAs were isolated and subjected to microarray screening using custom composite slides spotted with human, mouse, and rat and in silico-predicted miRNAs. An initial profile of expressed miRNAs was determined in cloned embryos and somatic donor cells; this profile changed after somatic cell nucleus transfer, identifying differentially expressed miRNAs between cloned and in vitro-produced bovine embryos. Furthermore, microarray data were validated using a miRNA-specific quantitative reverse transcription-polymerase chain reaction (qRT-PCR) approach (miR-Q). There was an 83% correlation (P=0.01) between microarray and qPCR data. Based on qRT-PCR, correct reprogramming of some miRNAs from the donor cells was confirmed in cloned bovine embryos, whereas other somatic miRNAs were not appropriately reprogrammed. Some of the miRNAs that were equally reprogrammed clustered on the same chromosomal location in the bovine genome. In conclusion, reprogramming of miRNAs seemed to occur in cloned bovine embryos. This could have profound implications for elucidating nuclear reprogramming in somatic cloning, as well as for the role of miRNAs in preimplantation mammalian development.

microRNA expression during trophectoderm specification

Background

Segregation of the trophectoderm from the inner cell mass of the embryo represents the first cell-fate decision of mammalian development. Transcription factors essential for specifying trophectoderm have been identified, but the role of microRNAs (miRNAs) in modulating this fate-choice has been largely unexplored. We have compared miRNA expression in embryonic stem cell (ESC)-derived trophectoderm and in staged murine embryos to identify a set of candidate miRNAs likely to be involved in trophectoderm specification.

Results

We profiled embryonic stem cells (ESCs) as they were induced to differentiate into trophectodermal cells by ectopic expression of HRas/Q61L. We also profiled murine embryos at progressive stages of preimplantation development (zygote, 2-cell, 4-cell, 8-cell, morula, and blastocyst), which includes the time window in which the trophectoderm is specified in vivo. Q61L/H

Conclusions

We describe miRNA expression changes that occur during trophectoderm specification and validate that our in vitro system faithfully recapitulates trophectoderm specification in vivo. By comparing our in vitro and in vivo datasets, we have identified a minimal set of candidate miRNAs likely to play a role in trophectoderm specification. These miRNAs are predicted to regulate a host of development-associated target genes, and many of these miRNAs have previously reported roles in development and differentiation. Additionally, we highlight a number of miRNAs whose tight developmental regulation may reflect a functional role in other stages of embryogenesis. Our embryo profiling data may be useful to investigators studying trophectoderm specification and other stages of preimplantation development.

Epigenetic regulation in mammalian preimplantation embryo development

Preimplantation embryo development involves four stages: fertilization, cell cleavage, morula and blastocyst formation. During these stages, maternal and zygotic epigenetic factors play crucial roles. The gene expression profile is changed dramatically, chromatin is modified and core histone elements undergo significant changes. Each preimplantation embryo stage has its own characteristic epigenetic profile, consistent with the acquisition of the capacity to support development. Moreover, histone modifications such as methylation and acetylation as well as other epigenetic events can act as regulatory switches of gene transcription. Because the epigenetic profile is largely related to differentiation, epigenetic dysfunction can give rise to developmental abnormalities. Thus, epigenetic profiling of the embryo is of pivotal importance clinically. Given the importance of these aspects, this review will mainly focus on the epigenetic profile during preimplantation embryo development, as well as interactions between epigenetic and genetic regulation in these early developmental stages.