KHDC1B is a novel CPEB binding partner specifically expressed in mouse oocytes and early embryos

Heterozygous Eif4nif1 Stop-Gain Mice Replicate the Primary Ovarian Insufficiency Phenotype in Women

We created the c.1286C>G stop-gain mutation found in a family with primary ovarian insufficiency (POI) at age 30 years. The Eif4enif1 C57/Bl6 transgenic mouse model contained a floxed exon 10-19 cassette with a conditional knock-in cassette containing the c.1286C>G stop-gain mutation in exon 10. The hybrid offspring of CMV-Cre mice with Eif4enif1WT/flx mice were designated Eif4enif1WT/Δ for simplicity. A subset of female heterozygotes (Eif4enif1WT/Δ) had no litters. In those with litters, the final litter was earlier (5.4 ± 2.6 vs 10.5 ± 0.7 months; P = .02). Heterozygous breeding pair (Eif4enif1WT/Δ × Eif4enif1WT/Δ) litter size was 60% of WT litter size (3.9 ± 2.0 vs 6.5 ± 3.0 pups/litter; P < .001). The genotypes were 35% Eif4enif1WT/flx and 65% Eif4enif1WT/Δ, with no homozygotes. Homozygote embryos did not develop beyond the 4- to 8-cell stage. The number of follicles in ovaries from Eif4enif1WT/Δ mice was lower starting at the primordial (499 ± 290 vs 1445 ± 381) and primary follicle stage (1069 ± 346 vs 1450 ± 193) on day 10 (P < .05). The preantral follicle number was lower starting on day 21 (213 ± 86 vs 522 ± 227; P < .01). Examination of ribosome protected mRNAs demonstrated altered mRNA expression. The Eif4enif1 stop-gain mice replicate the POI phenotype in women based on an earlier end to reproduction due to oocyte loss. The unique mouse model provides a platform to study regulation of protein translation across oocyte and embryo development in mammals.

Heterozygous Eif4nif1 Stop Gain Mice Replicate the Primary Ovarian Insufficiency Phenotype in Women

We created the c.1286C>G stop-gain mutation found in a family with primary ovarian insufficiency (POI) at age 30 years. The Eif4enif1 C57/Bl6 transgenic mouse model contained a floxed exon 10-19 cassette with a conditional knock-in cassette containing the c.1286C>G stop-gain mutation in exon 10. The hybrid offspring of CMV- Cre mice with Eif4enif1 WT/flx mice were designated Eif4enif1 WT/ Δ for simplicity. A subset of female heterozygotes ( Eif4enif1 WT/ Δ ) had no litters. In those with litters, the final litter was earlier (5.4±2.6 vs. 10.5±0.7 months; p=0.02). Heterozygous breeding pair ( Eif4enif1 WT/ Δ x Eif4enif1 WT/ Δ ) litter size was 60% of WT litter size (3.9±2.0 vs. 6.5±3.0 pups/litter; p <0.001). The genotypes were 35% Eif4enif1 WT/flx and 65% Eif4enif1 WT/ Δ , with no homozygotes. Homozygote embryos did not develop beyond the 4-8 cell stage. The number of follicles in ovaries from Eif4enif1 WT/ Δ mice was lower starting at the primordial (499±290 vs. 1445±381) and primary follicle stage (1069±346 vs. 1450±193) on day 10 (p<0.05). The preantral follicle number was lower starting on day 21 (213±86 vs. 522±227; p<0.01). Examination of ribosome protected mRNAs (RPR) demonstrated altered mRNA expression. The Eif4enif1 stop-gain mice replicate the POI phenotype in women. The unique mouse model provides a platform to study regulation of protein translation across oocyte and embryo development in mammals.

Regulation of Oocyte mRNA Metabolism: A Key Determinant of Oocyte Developmental Competence

The regulation of mRNA transcription and translation is uncoupled during oogenesis. The reason for this uncoupling is two-fold. Chromatin is only accessible to the transcriptional machinery during the growth phase as it condenses prior to resumption of meiosis to ensure faithful segregation of chromosomes during meiotic maturation. Thus, transcription rates are high during this time period in order to produce all of the transcripts needed for meiosis, fertilization, and embryo cleavage until the newly formed embryonic genome becomes transcriptionally active. To ensure appropriate timing of key developmental milestones including chromatin condensation, resumption of meiosis, segregation of chromosomes, and polar body extrusion, the translation of protein from transcripts synthesized during oocyte growth must be temporally regulated. This is achieved by the regulation of mRNA interaction with RNA binding proteins and shortening and lengthening of the poly(A) tail. This chapter details the essential factors that regulate the dynamic changes in mRNA synthesis, storage, translation, and degradation during oocyte growth and maturation.

Preimplantation embryo gene expression: 56 years of discovery, and counting

The biology of preimplantation embryo gene expression began 56 years ago with studies of the effects of protein synthesis inhibition and discovery of changes in embryo metabolism and related enzyme activities. The field accelerated rapidly with the emergence of embryo culture systems and progressively evolving methodologies that have allowed early questions to be re-addressed in new ways and in greater detail, leading to deeper understanding and progressively more targeted studies to discover ever more fine details. The advent of technologies for assisted reproduction, preimplantation genetic testing, stem cell manipulations, artificial gametes, and genetic manipulation, particularly in experimental animal models and livestock species, has further elevated the desire to understand preimplantation development in greater detail. The questions that drove enquiry from the earliest years of the field remain drivers of enquiry today. Our understanding of the crucial roles of oocyte-expressed RNA and proteins in early embryos, temporal patterns of embryonic gene expression, and mechanisms controlling embryonic gene expression has increased exponentially over the past five and a half decades as new analytical methods emerged. This review combines early and recent discoveries on gene regulation and expression in mature oocytes and preimplantation stage embryos to provide a comprehensive understanding of preimplantation embryo biology and to anticipate exciting future advances that will build upon and extend what has been discovered so far.

The genomic architecture of NLRP7 is Alu rich and predisposes to disease-associated large deletions

NLRP7 is a major gene responsible for recurrent hydatidiform moles. Here, we report 11 novel NLRP7 protein truncating variants, of which five deletions of more than 1-kb. We analyzed the transcriptional consequences of four variants. We demonstrate that one large homozygous deletion removes NLRP7 transcription start site and results in the complete absence of its transcripts in a patient in good health besides her reproductive problem. This observation strengthens existing data on the requirement of NLRP7 only for female reproduction. We show that two other variants affecting the splice acceptor of exon 6 lead to its in-frame skipping while another variant affecting the splice donor site of exon 9 leads to an in-frame insertion of 54 amino acids. Our characterization of the deletion breakpoints demonstrated that most of the breakpoints occurred within Alu repeats and the deletions were most likely mediated by microhomology events. Our data define a hotspot of Alu instability and deletions in intron 5 with six different breakpoints and rearrangements. Analysis of NLRP7 genomic sequences for repetitive elements demonstrated that Alu repeats represent 48% of its intronic sequences and these repeats seem to have been inserted into the common NLRP2/7 primate ancestor before its duplication into two genes.

Cytoplasmic polyadenylation in mammalian oocyte maturation

Oocyte developmental competence is the ability of the mature oocyte to be fertilized and subsequently drive early embryo development. Developmental competence is acquired by completion of oocyte maturation, a process that includes nuclear (meiotic) and cytoplasmic (molecular) changes. Given that maturing oocytes are transcriptionally quiescent (as are early embryos), they depend on post-transcriptional regulation of stored transcripts for protein synthesis, which is largely mediated by translational repression and deadenylation of transcripts within the cytoplasm, followed by recruitment of specific transcripts in a spatiotemporal manner for translation during oocyte maturation and early development. Motifs within the 3' untranslated region (UTR) of messenger RNA (mRNA) are thought to mediate repression and downstream activation by their association with binding partners that form dynamic protein complexes that elicit differing effects on translation depending on cell stage and interacting proteins. The cytoplasmic polyadenylation (CP) element, Pumilio binding element, and hexanucleotide polyadenylation signal are among the best understood motifs involved in CP, and translational regulation of stored transcripts as their binding partners have been relatively well-characterized. Knowledge of CP in mammalian oocytes is discussed as well as novel approaches that can be used to enhance our understanding of the functional and contributing features to transcript CP and translational regulation during mammalian oocyte maturation. WIREs RNA 2016, 7:71-89. doi: 10.1002/wrna.1316 For further resources related to this article, please visit the WIREs website.

RNA-Seq profiling of single bovine oocyte transcript abundance and its modulation by cytoplasmic polyadenylation

Molecular changes occurring during mammalian oocyte maturation are partly regulated by cytoplasmic polyadenylation (CP) and affect oocyte quality, yet the extent of CP activity during oocyte maturation remains unknown. Single bovine oocyte RNA sequencing (RNA-Seq) was performed to examine changes in transcript abundance during in vitro oocyte maturation in cattle. Polyadenylated RNA from individual germinal-vesicle and metaphase-II oocytes was amplified and processed for Illumina sequencing, producing approximately 30 million reads per replicate for each sample type. A total of 10,494 genes were found to be expressed, of which 2,455 were differentially expressed (adjusted P < 0.05 and fold change >2) between stages, with 503 and 1,952 genes respectively increasing and decreasing in abundance. Differentially expressed genes with complete 3'-untranslated-region sequence (279 increasing and 918 decreasing in polyadenylated transcript abundance) were examined for the presence, position, and distribution of motifs mediating CP, revealing enrichment (85%) and lack thereof (18%) in up- and down-regulated genes, respectively. Examination of total and polyadenylated RNA abundance by quantitative PCR validated these RNA-Seq findings. The observed increases in polyadenylated transcript abundance within the RNA-Seq data are likely due to CP, providing novel insight into targeted transcripts and resultant differential gene expression profiles that contribute to oocyte maturation.

KHDC1A, a novel translational repressor, induces endoplasmic reticulum-dependent apoptosis

RNA binding proteins are characterized as a new family of apoptosis inducers; however, the mechanism by which they induce apoptosis is poorly understood. KHDC1 family members were recently identified as K-homology (KH)-domain containing RNA binding proteins that are unique to eutherian mammals and highly expressed in oocytes. In this study, we report that the expression of KHDC1A induces caspase-3 dependent apoptosis and inhibits mRNA translation, and the translational repression is independent of apoptosis. We demonstrate that both the N-terminus and C-terminus of KHDC1A are required for its pro-apoptotic and translational repression activities. Furthermore, in the C-terminus of KHDC1A, a putative trans-membrane motif (TMM) is critical for these activities. In addition, the ectopically expressed KHDC1A is localized to the endoplasmic reticulum (ER) and changes the morphology of the ER. The inhibition of ER-specific caspase-12 successfully rescues KHDC1A-induced apoptosis, but not Fas-induced apoptosis. Taken together, we conclude that KHDC1A functions as a global translational repressor and induces apoptosis through an ER-dependent signaling pathway.

Loss of maternal CTCF is associated with peri-implantation lethality of Ctcf null embryos

CTCF is a highly conserved, multifunctional zinc finger protein involved in critical aspects of gene regulation including transcription regulation, chromatin insulation, genomic imprinting, X-chromosome inactivation, and higher order chromatin organization. Such multifunctional properties of CTCF suggest an essential role in development. Indeed, a previous report on maternal depletion of CTCF suggested that CTCF is essential for pre-implantation development. To distinguish between the effects of maternal and zygotic expression of CTCF, we studied pre-implantation development in mice harboring a complete loss of function Ctcf knockout allele. Although we demonstrated that homozygous deletion of Ctcf is early embryonically lethal, in contrast to previous observations, we showed that the Ctcf nullizygous embryos developed up to the blastocyst stage (E3.5) followed by peri-implantation lethality (E4.5–E5.5). Moreover, one-cell stage Ctcf nullizygous embryos cultured ex vivo developed to the 16–32 cell stage with no obvious abnormalities. Using a single embryo assay that allowed both genotype and mRNA expression analyses of the same embryo, we demonstrated that pre-implantation development of the Ctcf nullizygous embryos was associated with the retention of the maternal wild type Ctcf mRNA. Loss of this stable maternal transcript was temporally associated with loss of CTCF protein expression, apoptosis of the developing embryo, and failure to further develop an inner cell mass and trophoectoderm ex vivo. This indicates that CTCF expression is critical to early embryogenesis and loss of its expression rapidly leads to apoptosis at a very early developmental stage. This is the first study documenting the presence of the stable maternal Ctcf transcript in the blastocyst stage embryos. Furthermore, in the presence of maternal CTCF, zygotic CTCF expression does not seem to be required for pre-implantation development.

The N-terminus of FILIA forms an atypical KH domain with a unique extension involved in interaction with RNA

FILIA is a member of the recently identified oocyte/embryo expressed gene family in eutherian mammals, which is characterized by containing an N-terminal atypical KH domain. Here we report the structure of the N-terminal fragment of FILIA (FILIA-N), which represents the first reported three-dimensional structure of a KH domain in the oocyte/embryo expressed gene family of proteins. The structure of FILIA-N revealed a unique N-terminal extension beyond the canonical KH region, which plays important roles in interaction with RNA. By co-incubation with the lysates of mice ovaries, FILIA and FILIA-N could sequester specific RNA components, supporting the critical roles of FILIA in regulation of RNA transcripts during mouse oogenesis and early embryogenesis.

Post-transcriptional control of gene expression during mouse oogenesis

Post-transcriptional mechanisms play a central role in regulating gene expression during oogenesis and early embryogenesis. Growing oocytes accumulate an enormous quantity of messenger RNAs (mRNAs), but transcription decreases dramatically near the end of growth and is undetectable during meiotic maturation. Following fertilization, the embryo is initially transcriptionally inactive and then becomes active at a species-specific stage of early cleavage. Meanwhile, beginning during maturation and continuing after fertilization, the oocyte mRNAs are eliminated, allowing the embryonic genome to assume control of development. How the mammalian oocyte manages the storage, translation, and degradation of the huge quantity and diversity of mRNAs that it harbours has been the focus of enormous research effort and is the subject of this review. We discuss the roles of sequences within the 3'-untranslated region of certain mRNAs and the proteins that bind to them, sequence-non-specific RNA-binding proteins, and recent studies implicating ribonucleoprotein processing (P-) bodies and cytoplasmic lattices. We also discuss mechanisms that may control the temporally regulated translational activation of different mRNAs during meiotic maturation, as well as the signals that trigger silencing and degradation of the oocyte mRNAs. We close by highlighting areas for future research including the potential key role of small RNAs in regulating gene expression in oocytes.