Extracellular vesicles of follicular fluid from heat-stressed cows modify the gene expression of in vitro-matured oocytes

Seasonal influence on miRNA expression dynamics of extracellular vesicles in equine follicular fluid

BACKGROUND

Ovarian follicular fluid (FF) is a dynamic environment that changes with the seasons, affecting follicle development, ovulation, and oocyte quality. Cells in the follicles release tiny particles called extracellular vesicles (EVs) containing vital regulatory molecules, such as microRNAs (miRNAs). These miRNAs are pivotal in facilitating communication within the follicles through diverse signaling and information transfer forms. EV-coupled miRNA signaling is implicated to be associated with ovarian function, follicle and oocyte growth and response to various environmental insults. Herein, we investigated how seasonal variations directly influence the ovulatory and anovulatory states of ovarian follicles and how are they associated with follicular fluid EV-coupled miRNA dynamics in horses.

RESULTS

Ultrasonographic monitoring and follicular fluid aspiration of preovulatory follicles in horses during the anovulatory (spring: non-breeding) and ovulatory (spring, summer, and fall: breeding) seasons and subsequent EV isolation and miRNA profiling identified significant variation in EV-miRNA cargo content. We identified 97 miRNAs with differential expression among the groups and specific clusters of miRNAs involved in the spring transition (miR-149, -200b, -206, -221, -328, and -615) and peak breeding period (including miR-143, -192, -451, -302b, -100, and let-7c). Bioinformatic analyses showed enrichments in various biological functions, e.g., transcription factor activity, transcription and transcription regulation, nucleic acid binding, sequence-specific DNA binding, p53 signaling, and post-translational modifications. Cluster analyses revealed distinct sets of significantly up- and down-regulated miRNAs associated with spring anovulatory (Cluster 1) and summer ovulation-the peak breeding season (Clusters 4 and 6).

CONCLUSIONS

The findings from the current study shed light on the dynamics of FF-EV-coupled miRNAs in relation to equine ovulatory and anovulatory seasons, and their roles in understanding the mechanisms involved in seasonal shifts and ovulation during the breeding season warrant further investigation.

Extracellular vesicles as mediators of stress response in embryo-maternal communication

Introduction: The pivotal role of extracellular vesicles (EVs) in facilitating effective communication between the embryo and maternal cells during the preimplantation stage of pregnancy has been extensively explored. Nonetheless, inquiries persist regarding the alterations in EV cargo from endometrial cells under stress conditions and its potential to elicit specific stress responses in trophoblast cells. Thus, the aim of this study was to elucidate the involvement of EV miRNA miRNAs in transmitting stress signals from maternal cells to trophoblasts. Methods: The receptive endometrial epithelium analogue RL95-2 cells were subjected to stress induction with 200 µM CoCl2 for 24 h before EV isolation. JAr trophoblast spheroids, which serve as embryos, were subjected to treatment with stressed or unstressed EVs derived from RL95-2 cells for 24 h. Transcriptomic alterations in the treated JAr spheroids as well as in the untreated group, as a negative control, were investigated by mRNA sequencing. Furthermore, the changes in EV miRNAs were assessed by sequencing EV samples. Results: A comprehensive analysis comparing the miRNA profiles between stressed and unstressed EVs revealed significant changes in 25 miRNAs. Furthermore, transcriptomic analysis of JAr spheroids treated with stressed RL95-2EVs versus unstressed EVs or the untreated group demonstrated 6 and 27 differentially expressed genes, respectively. Pathway enrichment analysis showed that stressed EVs induce alterations in gene expression in trophoblast cells, which is partially mediated by EV microRNAs. Discussion: Our results suggest that EVs can transfer stress signals from endometrial cells to the embryo. These discoveries shed new light on the mechanism underlying implantation failures under stress conditions. Unraveling the role of EVs in transmitting stress signals, can extend our knowledge to pave the way for targeted interventions to manage stress-related implantation failures.

Programming effects of intrauterine hyperthermia on adrenal gland development

Maternal heat stress during late pregnancy can lead to intrauterine hyperthermia and affect fetal hypothalamic-pituitary-adrenal axis development and function. Herein, we investigated the effects of chronic environmental heat stress exposure of Holstein cows in the last 2 mo of gestation on their offspring's adrenal gland histomorphology and transcriptome. Cows in their last 54 ± 5 d of gestation were either heat stressed (housed under the shade of a freestall barn) or provided heat stress abatement via active cooling (via water soakers and fans) during a subtropical summer (temperature-humidity index >68). Respiration rate (RR) and skin temperature (ST) were elevated in heat-stressed dams relative to the cows with access to heat abatement (23 breaths/min and 2°C higher for RR and ST, respectively). Heifers born to heat-stressed cows experienced heat stress in utero (HS), whereas heifers born to actively cooled cows did not (CL). The adrenal gland was harvested from 6 heifers per group that were euthanized at birth (d 0; n = 12) or 1 wk after weaning (d 63; n = 12). Circulating cortisol was measured from blood samples collected weekly throughout the preweaning period. At d 63, heifers that experienced HS while developing in utero had heavier adrenal glands, with a greater total tissue surface area and thickness of the zona glomerulosa (ZG), fasciculata (ZF), and reticularis (ZR), compared with CL heifers. In addition, the adrenal gland of HS heifers had fewer cells in the ZG, more and larger cells in the ZF, and larger cells in the ZR, relative to CL heifers. Although no changes in circulating cortisol were observed through the preweaning period, the transcriptomic profile of the adrenal tissue was altered by fetal exposure to hyperthermia. Both at birth and on d 63, approximately 30 pathways were differentially expressed in the adrenal glands of HS heifers relative to CL. These pathways were associated with immune function, inflammation, prolactin signaling, cell function, and calcium transport. Upstream regulators significantly activated or inhibited in the adrenal glands of heifers exposed to intrauterine hyperthermia were identified. Maternal exposure to heat stress during late gestation caused an enlargement of their offspring's adrenal glands by inducing ZG and ZF cell hypertrophy, and caused gene expression changes. These phenotypic, histological, and molecular changes in the adrenal gland might lead to alterations in stress, immune, and metabolic responses later in life.

Heat-Stress Impacts on Developing Bovine Oocytes: Unraveling Epigenetic Changes, Oxidative Stress, and Developmental Resilience

Extreme temperature during summer may lead to heat stress in cattle and compromise their productivity. It also poses detrimental impacts on the developmental capacity of bovine budding oocytes, which halt their fertility. To mitigate the adverse effects of heat stress, it is necessary to investigate the mechanisms through which it affects the developmental capacity of oocytes. The primary goal of this study was to investigate the impact of heat stress on the epigenetic modifications in bovine oocytes and embryos, as well as on oocyte developmental capacity, reactive oxygen species, mitochondrial membrane potential, apoptosis, transzonal projections, and gene expression levels. Our results showed that heat stress significantly reduced the expression levels of the epigenetic modifications from histone H1, histone H2A, histone H2B, histone H4, DNA methylation, and DNA hydroxymethylation at all stages of the oocyte and embryo. Similarly, heat stress significantly reduced cleavage rate, blastocyst rate, oocyte mitochondrial-membrane potential level, adenosine-triphosphate (ATP) level, mitochondrial DNA copy number, and transzonal projection level. It was also found that heat stress affected mitochondrial distribution in oocytes and significantly increased reactive oxygen species, apoptosis levels and mitochondrial autophagy levels. Our findings suggest that heat stress significantly impacts the expression levels of genes related to oocyte developmental ability, the cytoskeleton, mitochondrial function, and epigenetic modification, lowering their competence during the summer season.

Review: Environment of the ovulatory follicle: modifications and use of biotechnologies to enhance oocyte competence and increase fertility in cattle

The oocyte is the basis of life, supporting development from a fertilized cell to an independent multicellular organism. The oocyte's competence to drive the first cell cycles postfertilization are critical to embryonic survival and subsequent successful pregnancy. Coupled with the complex processes of follicle assembly, activation, differentiation, growth, and terminal maturation, oocyte developmental competence is gradually acquired during oocyte growth and meiotic maturation. Most reproduction management technologies and interventions are centered around these highly coordinated processes, targeting the ovarian follicle and the oocyte within. Thus, our objective was to highlight key aspects of oocyte and follicle development in cattle, and to discuss recent advances in oocyte and follicle-centered reproductive biotechnologies.

Extracellular vesicles in mammalian reproduction: a review

Over the last decades, extracellular vesicles (EVs) have been found to be implicated in a complex universal mechanism of communication between different cell types. EVs are nanostructures of lipid nature that have an exosomal or ectosomal biogenesis, responsible for the intercellular transport of proteins, lipids, carbohydrates, nucleic acids, ions, among other molecules. The content of EVs can vary due to various factors such as hormonal stimuli, non-physiological conditions, metabolic state, etc. Once EVs reach their target cell, they can modulate processes such as gene expression, metabolism, response to external factors, and can even be associated with the delivery of molecules involved in epigenetic inheritance processes in germ cells. In mammalian reproduction, EVs have been shown to play an important role, either in vivo or in vitro, modulating a variety of processes in sperm, oocytes and embryos, and in their respective environments. Moreover, EVs represent a biodegradable, harmless and specific vehicle, which makes them attractive allies to consider when improving assisted reproductive technologies (ARTs). Therefore, the present review aims to describe the content of the main EVs involved in mammalian reproduction and how they can vary due to different factors, as well as to detail how EVs modulate, directly or indirectly, different molecular processes in gametes and embryos. In addition, we will highlight the mechanisms that remain to be elucidated. We will also propose new perspectives according to the characteristics of each particular EV to improve the different ARTs.

Extracellular vesicles and female reproduction

Extracellular vesicles (EVs) are nano-sized membrane bound complexes that have been identified as a mean for intercellular communication between cells and tissues both in physiological and pathological conditions. These vesicles contain numerous molecules involved in signal transduction including microRNAs, mRNAs, DNA, proteins, lipids, and cytokines and can affect the behavior of recipient cells. Female reproduction is dependent on extremely fine-tuned endocrine regulation, and EVs may represent an added layer that contributes to this regulation. This narrative review article provides an update on the research of the role of EVs in female reproduction including folliculogenesis, fertilization, embryo quality, and implantation. We also highlight potential pitfalls in typical EV studies and discuss gaps in the current literature.

Exosomal Cargo May Hold the Key to Improving Reproductive Outcomes in Dairy Cows

The reproductive status of dairy cows remains a challenge for dairy farmers worldwide, with impaired fertility linked to a significant reduction in herd profitability, due in part to impaired immunity, increased metabolic pressure, and longer postpartum anestrous interval (PPAI). Exosomes are nanovesicles released from a variety of cell types and end up in circulation, and carry proteins, bioactive peptides, lipids, and nucleic acids specific to the place of origin. As such, their role in health and disease has been investigated in humans and animals. This review discusses research into exosomes in the context of reproduction in dairy herds and introduces recent advances in mass-spectrometry (MS) based proteomics that have a potential to advance quantitative profiling of exosomal protein cargo in a search for early biomarkers of cattle fertility.

Exploiting Microfluidics for Extracellular Vesicle Isolation and Characterization: Potential Use for Standardized Embryo Quality Assessment

Recent decades have seen a growing interest in the study of extracellular vesicles (EVs), driven by their role in cellular communication, and potential as biomarkers of health and disease. Although it is known that embryos secrete EVs, studies on the importance of embryonic EVs are still very limited. This limitation is due mainly to small sample volumes, with low EV concentrations available for analysis, and to laborious, costly and time-consuming procedures for isolating and evaluating EVs. In this respect, microfluidics technologies represent a promising avenue for optimizing the isolation and characterization of embryonic EVs. Despite significant improvements in microfluidics for EV isolation and characterization, the use of EVs as markers of embryo quality has been held back by two key challenges: (1) the lack of specific biomarkers of embryo quality, and (2) the limited number of studies evaluating the content of embryonic EVs across embryos with varying developmental competence. Our core aim in this review is to identify the critical challenges of EV isolation and to provide seeds for future studies to implement the profiling of embryonic EVs as a diagnostic test for embryo selection. We first summarize the conventional methods for isolating EVs and contrast these with the most promising microfluidics methods. We then discuss current knowledge of embryonic EVs and their potential role as biomarkers of embryo quality. Finally, we identify key ways in which microfluidics technologies could allow researchers to overcome the challenges of embryonic EV isolation and be used as a fast, user-friendly tool for non-invasive embryo selection.

Heat stress reduces maturation and developmental capacity in bovine oocytes

The ovarian pool of follicles, and their enclosed oocytes, is highly sensitive to hyperthermia. Heat-induced changes in small antral follicles can later manifest as impaired follicle development and compromised competence of the enclosed oocytes to undergo maturation, fertilisation and further development into an embryo. This review describes the main changes documented so far that underlie the oocyte damage. The review discusses some cellular and molecular mechanisms by which heat stress compromises oocyte developmental competence, such as impairment of nuclear and cytoplasmic maturation and mitochondrial function, changes in the expression of both nuclear and mitochondrial transcripts and the induction of apoptosis. The review emphasises that although the oocyte is exposed to heat stress, changes are also evident in the developed embryo. Moreover, the effect of heat stress is not limited to the summer; it carries over to the cold autumn, as manifest by impaired steroid production, low oocyte competence and reduced fertility. The spontaneous recovery of oocytes from the end of the summer through the autumn until the beginning of winter suggests that only subpopulations of follicles, rather than the entire ovarian reserve, are damaged upon heat exposure.

Transcriptomic responses of porcine cumulus cells to heat exposure during oocytes in vitro maturation

The oocyte is vulnerable to various environmental stressors, including heat exposure. Cumulus-oocyte complexes (COCs) comprise functional units for oocytes in vitro maturation, and the cumulus cells provide essential supports and protect the oocyte from environmental insults. Heat exposure results in varied consequences in oocyte, presumably due to different responses of cumulus cells to heat exposure. In this study, we examined whether heat exposure of different duration affects porcine oocytes quality differently, and how such effects, if any, relate to transcriptomic profiles of cumulus cells. COCs were heat-exposed for 4 h (20-24 h, COC4) and 24 h (0-24 h, COC24), respectively, and the quality of oocytes in COC24 group showed significantly impaired with disrupted cumulus expansion and extracellular matrix (ECM) structure. The transcriptomic analysis identified 749 and 1238 differential expression genes (DEGs) in COC4 and COC24, respectively. Moreover, 852 DEGs were found when COC24 was compared with COC4, and the downregulated DEGs were mainly associated with Gene Ontology terms linked with ECM and cell proliferation. In the protein-protein interaction network, HSPE1, TNFAIP6, COL12A1, and COL18A1 were identified as hub genes playing important roles in heat-induced transcriptomic responses. These results indicate that impaired cumulus proliferation and ECM structure are responsible for heat-induced damage in oocytes quality.

Extracellular Vesicles as Mediators of Environmental and Metabolic Stress Coping Mechanisms During Mammalian Follicular Development

Extracellular vesicles are evolutionarily conserved nano-sized phospholipid membraned structures and released from virtually all types of cells into the extracellular space. Their ability to carry various molecular cargos (mRNA, miRNA, proteins, and lipids) from one cell to the other to exert functional impact on the target cells enables them to play a significant role in cell to cell communication during follicular development. As the molecular signals carried by extracellular vesicles reflect the physiological status of the cells of origin, they are expected to mediate any effect of environmental or metabolic stress on the follicualr cells and the growing oocyte. Recent studies have evidenced that reproductive cells exposed to various environmental stressors (heat and oxidative stress) released extracellular vesicles enriched with mRNA and miRNA associated with stress response mechanisms. Moreover, the metabolic status of post-calving cows could be well-reflected in the follicular extracellular vesicle's miRNA profile, which signified the potential role of extracellular cellular vesicle molecular signals in mediating the effect of metabolic stress on follicular and oocyte development. In the present review, the potential role of extracellular vesicles in mediating the effect of environmental and metabolic stress in various reproductive cells and oocytes are thoroughly discussed Moreover, considering the importance of extracellular vesicles in shuttling protective or rescuing molecular signals during stress, their potential usage as means of targeted delivery of molecules to mitigate the effect of stress on oocytes are addressed as the focus of future research.

Exosomes as a Potential Tool for Supporting Canine Oocyte Development

The canine oviduct is a unique reproductive organ where the ovulated immature oocytes complete their maturation, while the other mammals ovulate matured gametes. Due to their peculiar reproductive characteristics, the in vitro maturation of dog oocytes is still not wellestablished compared with other mammals. Investigations of the microenvironment conditions in the oviductal canal are required to establish a reliable in vitro maturation system in the dog. Previous studies have suggested that the oviduct and its derivatives play a key role in improving fertilization as well as embryo development. In particular, the biological function of oviduct-derived exosomes on sperm and early embryo development has been investigated in porcine, bovine, and murine species. However, the information about their functions on canine cumulus-oocyte complexes is still elusive. Recent canine reproductive studies demonstrated how oviduct-derived extracellular vesicles such as microvesicles and exosomes interact with oocyte-cumulus complexes and how they can play roles in regulating canine cumulus/oocyte communications. In this review, we summarize the physiological characteristics of canine oviduct-derived exosomes and their potential effects on cumulus cells development as well as oocyte in vitro maturation via molecular signaling pathways.

Role of extracellular vesicles during oocyte maturation and early embryo development

The follicle is a dynamic microenvironment in the ovary where the oocyte develops. Intercellular communication between somatic cells and the oocyte inside the follicle is essential to generate a competent gamete. Extracellular vesicles are nanoparticles secreted by cells that mediate cell-to-cell communication in the follicle microenvironment and can be obtained from the follicular fluid. These extracellular vesicles have been studied as biomarkers and supplementation tools to mimic physiological conditions during assisted reproductive techniques because they are vehicles of bioactive molecules. Therefore, this paper reviews the importance of changes in the ovarian follicle and the effects of extracellular vesicles from follicular fluid during oocyte maturation and early embryo development. Finally, we propose that is important to consider the source of the extracellular vesicles to improve diagnostic methods and to increase invitro embryo production.

Thermoprotective molecules to improve oocyte competence under elevated temperature

Heat stress is an environmental factor that challenges livestock by disturbing animal homeostasis. Despite the broad detrimental effects of heat stress on reproductive function, the germline and the early preimplantation embryo are particularly prone. There is extensive evidence that elevated temperature reduces oocyte developmental competence through a series of cellular and molecular damages. Further research revealed that the oocyte respond to stress by activating cellular mechanisms such as heat shock response, unfolded protein response and autophagy to improve survival under heat shock. Such knowledge paved the way for the identification of thermoprotective molecules that alleviate heat-induced oocyte oxidative stress, organelle damage, and apoptosis. Therefore, this review depicts the deleterious effects of heat shock on oocyte developmental competence, heat-induced cellular and molecular changes, outlines pro-survival cellular mechanisms and explores thermoprotective molecules to improve oocyte competence.

Follicular extracellular vesicles enhance meiotic resumption of domestic cat vitrified oocytes

Extracellular vesicles (EVs) contain multiple factors that regulate cell and tissue function. However, understanding of their influence on gametes, including communication with the oocyte, remains limited. In the present study, we characterized the proteome of domestic cat (Felis catus) follicular fluid EVs (ffEV). To determine the influence of follicular fluid EVs on gamete cryosurvival and the ability to undergo in vitro maturation, cat oocytes were vitrified using the Cryotop method in the presence or absence of ffEV. Vitrified oocytes were thawed with or without ffEVs, assessed for survival, in vitro cultured for 26 hours and then evaluated for viability and meiotic status. Cat ffEVs had an average size of 129.3 ± 61.7 nm (mean ± SD) and characteristic doughnut shaped circular vesicles in transmission electron microscopy. Proteomic analyses of the ffEVs identified a total of 674 protein groups out of 1,974 proteins, which were classified as being involved in regulation of oxidative phosphorylation, extracellular matrix formation, oocyte meiosis, cholesterol metabolism, glycolysis/gluconeogenesis, and MAPK, PI3K-AKT, HIPPO and calcium signaling pathways. Furthermore, several chaperone proteins associated with the responses to osmotic and thermal stresses were also identified. There were no differences in the oocyte survival among fresh and vitrified oocyte; however, the addition of ffEVs to vitrification and/or thawing media enhanced the ability of frozen-thawed oocytes to resume meiosis. In summary, this study is the first to characterize protein content of cat ffEVs and their potential roles in sustaining meiotic competence of cryopreserved oocytes.

Extracellular vesicle mediated molecular signaling in ovarian follicle: Implication for oocyte developmental competence

The bidirectional communication between the oocyte and the companion somatic cells in the follicular environment is known to be mediated by either a direct communication via gap junction or transzonal projections or indirectly through endocrine, paracrine and autocrine signaling factors. Extracellular vesicles (EVs), which are found in various biological fluids, including follicular fluid (FF) are known to play important roles in mediating the communication between the oocyte and the surrounding somatic cells through shuttling bioactive molecules to facilitate follicular growth and oocyte maturation. As vesicles in the extracellular space are known to reflect the physiological status of the donor or the releasing cells, molecules carried by the EVs in the follicular environment could be markers of the internal and external stressors. EVs exhibit greater degree of heterogeneity in their size, biogenesis and the bioactive molecule they carry. The process of biogenesis of EVs is known to be regulated by several proteins associated with the endosomal sorting complex required for transport (ESCRT) proteins. The type of EVs and surface proteins markers vary according to the type of protein involved in their biogenesis. EVs are recently reported to play indispensable role in promoting cell-to-cell communication during follicular growth. Recent advancements in EV research opened the possibilities to load EVs with specific molecules like miRNA, siRNA, CRISPR-cas9 complex and protein, which showed a new horizon for their application in therapeutics. The present review explores the biogenesis, the role and the future prospects of EVs with a special emphasis given to follicular growth and oocyte maturation.

The Role of MicroRNAs in Mammalian Fertility: From Gametogenesis to Embryo Implantation

The genetic codes inscribed during two key developmental processes, namely gametogenesis and embryogenesis, are believed to determine subsequent development and survival of adult life. Once the embryo is formed, its further development mainly depends on its intrinsic characteristics, maternal environment (the endometrial receptivity), and the embryo–maternal interactions established during each phase of development. These developmental processes are under strict genetic regulation that could be manifested temporally and spatially depending on the physiological and developmental status of the cell. MicroRNAs (miRNAs), one of the small non-coding classes of RNAs, approximately 19–22 nucleotides in length, are one of the candidates for post-transcriptional developmental regulators. These tiny non-coding RNAs are expressed in ovarian tissue, granulosa cells, testis, oocytes, follicular fluid, and embryos and are implicated in diverse biological processes such as cell-to-cell communication. Moreover, accumulated evidences have also highlighted that miRNAs can be released into the extracellular environment through different mechanisms facilitating intercellular communication. Therefore, understanding miRNAs mediated regulatory mechanisms during gametogenesis and embryogenesis provides further insights about the molecular mechanisms underlying oocyte/sperm formation, early embryo development, and implantation. Thus, this review highlights the role of miRNAs in mammalian gametogenesis and embryogenesis and summarizes recent findings about miRNA-mediated post-transcriptional regulatory mechanisms occurring during early mammalian development.