Single Molecule RNA Localization and Translation in the Mammalian Oocyte and Embryo

Absence of CDK12 in oocyte leads to female infertility

Transcriptional activity and gene expression are critical for the development of mature, meiotically competent oocytes. Our study demonstrates that the absence of cyclin-dependent kinase 12 (CDK12) in oocytes leads to complete female sterility, as fully developed oocytes capable of completing meiosis I are absent from the ovaries. Mechanistically, CDK12 regulates RNA polymerase II activity in growing oocytes and ensures the maintenance of the physiological maternal transcriptome, which is essential for protein synthesis that drives further oocyte growth. Notably, CDK12-deficient growing oocytes exhibit a 71% reduction in transcriptional activity. Furthermore, impaired oocyte development disrupts folliculogenesis, leading to premature ovarian failure without terminal follicle maturation or ovulation. In conclusion, our findings identify CDK12 as a key master regulator of the oocyte transcriptional program and gene expression, indispensable for oocyte growth and female fertility. A schematic illustrating the effects of loss of CDK12 in mammalian oocytes on the regulation of transcription by polymerase II and the concomitant effects on translation. This disruption leads to an aberrant transcriptome and translatome, resulting in the absence of fully mature oocytes and ultimately female sterility.

High-sensitivity whole-mount in situ Hybridization of Mouse Oocytes and Embryos Visualizes the Super-resolution Structures and Distributions of mRNA Molecules

Mammalian oocytes accumulate more than ten thousand mRNAs, of which three to four thousand mRNAs are translationally repressed. The timings and sites of translational activation of these dormant mRNAs are crucial for promoting oocyte maturation and embryonic development. How these mRNAs are accumulated and distributed in oocytes is therefore a fundamental issue to be explored. A method that enables visualization of mRNA molecules with high resolution in a simple manner would be valuable for understanding how oocytes accumulate and regulate the dormant mRNAs. We have developed a highly sensitive whole-mount in situ hybridization method using in vitro-synthesized RNA probes and the tyramide signal amplification (TSA) system optimized for mouse oocytes and embryos. By using this method, Pou5f1/Oct4, Emi2, and cyclin B1 mRNAs were detected in immature oocytes and 2-cell stage embryos. Confocal microscopy showed that these mRNAs formed granular structures in the oocyte cytoplasm. The structures of Pou5f1/Oct4 and cyclin B1 mRNAs persisted in 2-cell stage embryos. Pou5f1/Oct4 RNA granules exhibited a solid-like property in immature oocytes and became liquid-like droplets in 2-cell stage embryos. Double-staining of cyclin B1 mRNA with Emi2 or Pou5f1/Oct4 mRNA revealed that these mRNAs were distributed as different RNA granules without overlapping each other and that the size of cyclin B1 RNA granules tended to be larger than that of Emi2 RNA granules. The structures and distribution patterns of these mRNAs were further analyzed by N-SIM super-resolution microscopy. This analysis revealed that the large-sized RNA granules consist of many small-sized granules, suggesting the accumulation and regulation of dormant mRNAs as basal-sized RNA granules. The method established in this study can easily visualize the structure and distribution of mRNAs accumulated in mammalian oocytes and embryos with high sensitivity and super-resolution. This method is useful for investigating the cellular and molecular mechanisms of translational control of mRNAs by which maturation and early developmental processes are promoted.

Visualizing the translational activation of a particular mRNA in zebrafish embryos using in situ hybridization and proximity ligation assay

Fertilized eggs initiate translation of stored mRNAs in spatially and temporally controlled manners. Here, we present a protocol for visualizing spatial and temporal translation in zebrafish embryos by fluorescence in situ hybridization and proximity ligation assay. We describe steps for labeling newly synthesized proteins and mRNA, visualizing mRNA translation and mRNA, sample mounting, and observation. Coupling detection of mRNA molecules with their translation sites is useful for understanding the molecular and cellular mechanisms that drive embryo development. For complete details on the use and execution of this protocol, please refer to Sato et al.1 and Takada et al.2.

Spatiotemporal regulation of maternal mRNAs during vertebrate oocyte meiotic maturation

Vertebrate oocytes face a particular challenge concerning the regulation of gene expression during meiotic maturation. Global transcription becomes quiescent in fully grown oocytes, remains halted throughout maturation and fertilization, and only resumes upon embryonic genome activation. Hence, the oocyte meiotic maturation process is largely regulated by protein synthesis from pre-existing maternal messenger RNAs (mRNAs) that are transcribed and stored during oocyte growth. Rapidly developing genome-wide techniques have greatly expanded our insights into the global translation changes and possible regulatory mechanisms during oocyte maturation. The storage, translation, and processing of maternal mRNAs are thought to be regulated by factors interacting with elements in the mRNA molecules. Additionally, posttranscriptional modifications of mRNAs, such as methylation and uridylation, have recently been demonstrated to play crucial roles in maternal mRNA destabilization. However, a comprehensive understanding of the machineries that regulate maternal mRNA fate during oocyte maturation is still lacking. In particular, how the transcripts of important cell cycle components are stabilized, recruited at the appropriate time for translation, and eliminated to modulate oocyte meiotic progression remains unclear. A better understanding of these mechanisms will provide invaluable insights for the preconditions of developmental competence acquisition, with important implications for the treatment of infertility. This review discusses how the storage, localization, translation, and processing of oocyte mRNAs are regulated, and how these contribute to oocyte maturation progression.

Applications of advances in mRNA-based platforms as therapeutics and diagnostics in reproductive technologies

The recent COVID-19 pandemic led to many drastic changes in not only society, law, economics, but also in science and medicine, marking for the first time when drug regulatory authorities cleared for use mRNA-based vaccines in the fight against this outbreak. However, while indeed representing a novel application of such technology in the context of vaccination medicine, introducing RNA into cells to produce resultant molecules (proteins, antibodies, etc.) is not a novel principle. It has been common practice to introduce/inject mRNA into oocytes and embryos to inhibit, induce, and identify several factors in a research context, while such aspects have also been proposed as potential therapeutic and diagnostic applications to combat infertility in humans. Herein, we describe key areas where mRNA-based platforms have thus far represented potential areas of clinical applications, describing the advantages and limitations of such applications. Finally, we also discuss how recent advances in mRNA-based platforms, driven by the recent pandemic, may stand to benefit the treatment of infertility in humans. We also present brief future directions as to how we could utilise recent and current advancements to enhance RNA therapeutics within reproductive biology, specifically with relation to oocyte and embryo delivery.

High-resolution ribosome profiling reveals translational selectivity for transcripts in bovine preimplantation embryo development

High-resolution ribosome fractionation and low-input ribosome profiling of bovine oocytes and preimplantation embryos has enabled us to define the translational landscapes of early embryo development at an unprecedented level. We analyzed the transcriptome and the polysome- and non-polysome-bound RNA profiles of bovine oocytes (germinal vesicle and metaphase II stages) and early embryos at the two-cell, eight-cell, morula and blastocyst stages, and revealed four modes of translational selectivity: (1) selective translation of non-abundant mRNAs; (2) active, but modest translation of a selection of highly expressed mRNAs; (3) translationally suppressed abundant to moderately abundant mRNAs; and (4) mRNAs associated specifically with monosomes. A strong translational selection of low-abundance transcripts involved in metabolic pathways and lysosomes was found throughout bovine embryonic development. Notably, genes involved in mitochondrial function were prioritized for translation. We found that translation largely reflected transcription in oocytes and two-cell embryos, but observed a marked shift in the translational control in eight-cell embryos that was associated with the main phase of embryonic genome activation. Subsequently, transcription and translation become more synchronized in morulae and blastocysts. Taken together, these data reveal a unique spatiotemporal translational regulation that accompanies bovine preimplantation development.

Identification of embryonic RNA granules that act as sites of mRNA translation after changing their physical properties

Fertilized eggs begin to translate mRNAs at appropriate times and placements to control development, but how the translation is regulated remains unclear. Here, we found that pou5f3 mRNA encoding a transcriptional factor essential for development formed granules in a dormant state in zebrafish oocytes. Although the number of pou5f3 granules remained constant, Pou5f3 protein accumulated after fertilization. Intriguingly, signals of newly synthesized peptides and a ribosomal protein became colocalized with pou5f3 granules after fertilization and, moreover, nascent Pou5f3 was shown to be synthesized in the granules. This functional change was accompanied by changes in the state and internal structure of granules. Dissolution of the granules reduced the rate of protein synthesis. Similarly, nanog and sox19b mRNAs in zebrafish and Pou5f1/Oct4 mRNA in mouse assembled into granules. Our results reveal that subcellular compartments, termed embryonic RNA granules, function as activation sites of translation after changing physical properties for directing vertebrate development.

Exploring the Diverse Functional and Regulatory Consequences of Alternative Splicing in Development and Disease

Alternative splicing is a fundamental mechanism of eukaryotic RNA regulation that increases the transcriptomic and proteomic complexity within an organism. Moreover, alternative splicing provides a framework for generating unique yet complex tissue- and cell type-specific gene expression profiles, despite using a limited number of genes. Recent efforts to understand the negative consequences of aberrant splicing have increased our understanding of developmental and neurodegenerative diseases such as spinal muscular atrophy, frontotemporal dementia and Parkinsonism linked to chromosome 17, myotonic dystrophy, and amyotrophic lateral sclerosis. Moreover, these studies have led to the development of innovative therapeutic treatments for diseases caused by aberrant splicing, also known as spliceopathies. Despite this, a paucity of information exists on the physiological roles and specific functions of distinct transcript spliceforms for a given gene. Here, we will highlight work that has specifically explored the distinct functions of protein-coding spliceforms during development. Moreover, we will discuss the use of alternative splicing of noncoding exons to regulate the stability and localization of RNA transcripts.