Comparison of oocyte vitrification using a semi-automated or a manual closed system in human siblings: survival and transcriptomic analyses

Oocyte vitrification for fertility preservation is an evolving practice requiring a new mindset: societal, technical, clinical, and basic science-driven evolutions

Infertility is a condition with profound social implications. Indeed, it is not surprising that evolutions in both medicine and society affect the way in vitro fertilization is practiced. The keywords in modern medicine are the four principles, which implicitly involve a constant update of our knowledge and our technologies to fulfill the "prediction" and "personalization" tasks, and a continuous reshaping of our mindset in view of all relevant societal changes to fulfill the "prevention" and "participation" tasks. A worldwide aging population whose life priorities are changing requires that we invest in fertility education, spreading actionable information to allow women and men to make meaningful reproductive choices. Fertility preservation for both medical and nonmedical reasons is still very much overlooked in many countries worldwide, demanding a comprehensive update of our approach, starting from academia and in vitro fertilization laboratories, passing through medical offices, and reaching out to social media. Reproduction medicine should evolve from being a clinical practice to treat a condition to being a holistic approach to guarantee patients' reproductive health and well-being. Oocyte vitrification for fertility preservation is the perfect use case for this transition. This tool is acquiring a new identity to comply with novel indications and social needs, persisting technical challenges, brand-new clinical technologies, and novel revolutions coming from academia. This "views and reviews" piece aims at outlining the advancement of oocyte vitrification from all these tightly connected perspectives.

Transcriptomic integrity of human oocytes used in ARTs: technical and intrinsic factor effects

BACKGROUND

Millions of children have been born throughout the world thanks to ARTs, the harmlessness of which has not yet been fully demonstrated. For years, efforts to evaluate the specific effects of ART have focused on the embryo; however, it is the oocyte quality that mainly dictates first and foremost the developmental potential of the future embryo. Ovarian stimulation, cryopreservation, and IVM are sometimes necessary steps to obtain a mature oocyte, but they could alter the appropriate expression of the oocyte genome. Additionally, it is likely that female infertility, environmental factors, and lifestyle have a significant influence on oocyte transcriptomic quality, which may interfere with the outcome of an ART attempt.

OBJECTIVE AND RATIONALE

The objective of this review is to identify transcriptomic changes in the human oocyte caused by interventions specific to ART but also intrinsic factors such as age, reproductive health issues, and lifestyle. We also provide recommendations for future good practices to be conducted when attempting ART.

SEARCH METHODS

An in-depth literature search was performed on PubMed to identify studies assessing the human oocyte transcriptome following ART interventions, or in the context of maternal aging, suboptimal lifestyle, or reproductive health issues.

OUTCOMES

ART success is susceptible to external factors, maternal aging, lifestyle factors (smoking, BMI), and infertility due to endometriosis or polycystic ovary syndrome. Indeed, all of these are likely to increase oxidative stress and alter mitochondrial processes in the foreground. Concerning ART techniques themselves, there is evidence that different ovarian stimulation regimens shape the oocyte transcriptome. The perturbation of processes related to the mitochondrion, oxidative phosphorylation, and metabolism is observed with IVM. Cryopreservation might dysregulate genes belonging to transcriptional regulation, ubiquitination, cell cycle, and oocyte growth pathways. For other ART laboratory factors such as temperature, oxygen tension, air pollution, and light, the evidence remains scarce. Focusing on genes involved in chromatin-based processes such as DNA methylation, heterochromatin modulation, histone modification, and chromatin remodeling complexes, but also genomic imprinting, we observed systematic dysregulation of such genes either after ART intervention or lifestyle exposure, as well as due to internal factors such as maternal aging and reproductive diseases. Alteration in the expression of such epigenetic regulators may be a common mechanism linked to adverse oocyte environments, explaining global transcriptomic modifications.

WIDER IMPLICATIONS

Many IVF factors and additional external factors have the potential to impair oocyte transcriptomic integrity, which might not be innocuous for the developing embryo. Fortunately, it is likely that such dysregulations can be minimized by adapting ART protocols or reducing adverse exposure.

Overview of Gene Expression Dynamics during Human Oogenesis/Folliculogenesis

The oocyte transcriptome follows a tightly controlled dynamic that leads the oocyte to grow and mature. This succession of distinct transcriptional states determines embryonic development prior to embryonic genome activation. However, these oocyte maternal mRNA regulatory events have yet to be decoded in humans. We reanalyzed human single-oocyte RNA-seq datasets previously published in the literature to decrypt the transcriptomic reshuffles ensuring that the oocyte is fully competent. We applied trajectory analysis (pseudotime) and a meta-analysis and uncovered the fundamental transcriptomic requirements of the oocyte at any moment of oogenesis until reaching the metaphase II stage (MII). We identified a bunch of genes showing significant variation in expression from primordial-to-antral follicle oocyte development and characterized their temporal regulation and their biological relevance. We also revealed the selective regulation of specific transcripts during the germinal vesicle-to-MII transition. Transcripts associated with energy production and mitochondrial functions were extensively downregulated, while those associated with cytoplasmic translation, histone modification, meiotic processes, and RNA processes were conserved. From the genes identified in this study, some appeared as sensitive to environmental factors such as maternal age, polycystic ovary syndrome, cryoconservation, and in vitro maturation. In the future, the atlas of transcriptomic changes described in this study will enable more precise identification of the transcripts responsible for follicular growth and oocyte maturation failures.

Alterations in the expression pattern of some epigenetic-related genes and microRNAs subsequent to oocyte cryopreservation

MicroRNAs (miRNAs) are small non-encoding RNAs that actively regulate biological and physiological processes, and play an important role in regulating gene expression in all cells, especially in most animal cells, including oocytes and embryos. The expression of miRNAs at the right time and place is crucial for the oocyte's maturation and the embryo's subsequent development. Although assisted reproductive techniques (ART) have helped to solve many infertility problems, they cause changes in the expression of miRNA and genes in oocytes and preimplantation embryos, and the effect of these changes on the future of offspring is unknown, and has caused concerns. The relevant genomic alterations commonly imposed on embryos during cryopreservation may have potential epigenetic risks. Understanding the biological functions of miRNAs in frozen maturated oocytes may provide a better understanding of embryonic development and a comparison of fertility conservation in female mammals. With the development of new techniques for genomic evaluation of preimplantation embryos, it has been possible to better understand the effects of ART. The results of various articles have shown that freezing of oocytes and the cryopreservation method are effective for the expression of miRNAs and, in some cases, cause changes in the expression of miRNAs and epigenetic changes in the resulting embryo. This literature review study aimed to investigate the effects of oocyte cryopreservation in both pre-maturation and post-maturation stages, the cryopreservation method and the type of cryoprotectants (CPA) used on the expression of some epigenetic-related genes and miRNAs.

The role of Hippo pathway in ovarian development

The follicle is the functional unit of the ovary, whereby ovarian development is largely dependent on the development of the follicles themselves. The activation, growth, and progression of follicles are modulated by a diverse range of factors, including reproductive endocrine system and multiple signaling pathways. The Hippo pathway exhibits a high degree of evolutionary conservation between both Drosophila and mammalian systems, and is recognized for its pivotal role in regulating cellular proliferation, control of organ size, and embryonic development. During the process of follicle development, the components of the Hippo pathway show temporal and spatial variations. Recent clinical studies have shown that ovarian fragmentation can activate follicles. The mechanism is that the mechanical signal of cutting triggers actin polymerization. This process leads to the disruption of the Hippo pathway and subsequently induces the upregulation of downstream CCN and apoptosis inhibitors, thereby promoting follicle development. Thus, the Hippo pathway plays a crucial role in both the activation and development of follicles. In this article, we focused on the development and atresia of follicles and the function of Hippo pathway in these processes. Additionally, the physiological effects of Hippo pathway in follicle activation are also explored.

Transcriptomics Reveal Molecular Differences in Equine Oocytes Vitrified before and after In Vitro Maturation

In the last decade, in vitro embryo production in horses has become an established clinical practice, but blastocyst rates from vitrified equine oocytes remain low. Cryopreservation impairs the oocyte developmental potential, which may be reflected in the messenger RNA (mRNA) profile. Therefore, this study aimed to compare the transcriptome profiles of metaphase II equine oocytes vitrified before and after in vitro maturation. To do so, three groups were analyzed with RNA sequencing: (1) fresh in vitro matured oocytes as a control (FR), (2) oocytes vitrified after in vitro maturation (VMAT), and (3) oocytes vitrified immature, warmed, and in vitro matured (VIM). In comparison with fresh oocytes, VIM resulted in 46 differentially expressed (DE) genes (14 upregulated and 32 downregulated), while VMAT showed 36 DE genes (18 in each category). A comparison of VIM vs. VMAT resulted in 44 DE genes (20 upregulated and 24 downregulated). Pathway analyses highlighted cytoskeleton, spindle formation, and calcium and cation ion transport and homeostasis as the main affected pathways in vitrified oocytes. The vitrification of in vitro matured oocytes presented subtle advantages in terms of the mRNA profile over the vitrification of immature oocytes. Therefore, this study provides a new perspective for understanding the impact of vitrification on equine oocytes and can be the basis for further improvements in the efficiency of equine oocyte vitrification.