Increase of mitochondria surrounding spindle causes mouse oocytes arrested at metaphase I stage

Comprehensive atlas of mitochondrial distribution and dynamics during oocyte maturation in mouse models

BACKGROUND

Oocytes, the largest cells in mammals, harbor numerous mitochondria within their cytoplasm. These highly dynamic organelles are crucial for providing energy resources and serving as central regulators during oogenesis. Mitochondrial dynamics ensure proper energy distribution for various cellular processes involved in oocyte maturation. Previous studies have used alterations in mitochondrial distribution as a biomarker to assess the oocyte health. However, there are discrepancies between studies regarding mitochondrial distribution profiles in healthy oocytes. Consequently, a comprehensive mitochondrial distribution profile in oocytes during maturation has not been fully characterized. Additionally, there is a lack of objective, quantitative methods to evaluate alterations in mitochondrial distribution profiles in oocytes.

METHODS

This study aims to provide an in-depth overview of mitochondrial distribution profiles in mouse oocytes at different maturation stages: germinal vesicle (GV) stage, metaphase I (MI), and mature metaphase II (MII). Freshly collected mouse GV, MI and MII oocytes were stained with MitoTracker Red. Confocal microscopy was used to obtain images of mitochondrial distribution profiles in these oocytes. Using the Imaris software, we reconstructed three-dimensional (3D) surface renderings of each oocyte and quantitatively illustrated the mitochondrial distribution profiles.

RESULTS

At the GV stage, mitochondria in oocytes were evenly distributed throughout the ooplasm. As oocytes progressed to MI and MII stages, mitochondria aggregated and formed clusters, the mean size of mitochondrial clusters and the proportions of clustered mitochondria increased along with the maturation of oocytes.

CONCLUSIONS

Our findings reveal that mitochondria in mouse oocytes are highly dynamic, undergoing significant reorganizations during oocyte maturation. We for the first time provided comprehensive mitochondrial distribution profiles in mouse oocytes at the GV, MI and MII stages. These mitochondrial distribution profiles were further quantitatively evaluated. Our methods provide an objective and standardized approach for evaluating alterations in mitochondrial dynamics, which can be used as biomarkers to monitor oocyte conditions during maturation.

Mitochondria as determinants of reproductive senescence and competence: implications for diagnosis of embryo competence in assisted reproduction

Mitochondria are commonly recognized as the powerhouses of the cell, primarily responsible for energy production through oxidative phosphorylation. Alongside this vital function, they also play crucial roles in regulating calcium signaling, maintaining membrane potential, and modulating apoptosis. Their involvement in various cellular pathways becomes particularly evident during oogenesis and embryogenesis, where mitochondrial quantity, morphology, and distribution are tightly controlled. The efficiency of the mitochondrial network is maintained through multiple quality control mechanisms that are essential for reproductive success. These include mitochondrial unfolded protein response, mitochondrial dynamics, and mitophagy. Not surprisingly, mitochondrial dysfunction has been implicated in infertility and ovarian aging, prompting investigation into mitochondria as diagnostic and therapeutic targets in assisted reproduction. To date, mitochondrial DNA copy number in oocytes, cumulus cells, and trophectoderm biopsies, and fluorescent lifetime imaging microscopy-based assessment of NADH and flavin adenine dinucleotide content have been explored as potential predictors of embryo competence, yielding limited success. Despite challenges in the clinical application of mitochondrial diagnostic strategies, these enigmatic organelles have a significant impact on reproduction, and their potential role as diagnostic targets in assisted reproduction is likely to remain an active area of investigation in the foreseeable future.

An aggregated mitochondrial distribution in preimplantation embryos disrupts nuclear morphology, function, and developmental potential

A dispersed cytoplasmic distribution of mitochondria is a hallmark of normal cellular organization. Here, we have utilized the expression of exogenous Trak2 in mouse oocytes and embryos to disrupt the dispersed distribution of mitochondria by driving them into a large cytoplasmic aggregate. Our findings reveal that aggregated mitochondria have minimal impact on asymmetric meiotic cell divisions of the oocyte. In contrast, aggregated mitochondria during the first mitotic division result in daughter cells with unequal sizes and increased micronuclei. Further, in two-cell embryos, microtubule-mediated centering properties of the mitochondrial aggregate prevent nuclear centration, distort nuclear shape, and inhibit DNA synthesis and the onset of embryonic transcription. These findings demonstrate the motor protein-mediated distribution of mitochondria throughout the cytoplasm is highly regulated and is an essential feature of cytoplasmic organization to ensure optimal cell function.

The role of mitochondrial dynamics in oocyte and early embryo development

Mitochondrial dysfunction is widely implicated in various human diseases, through mechanisms that go beyond mitochondria's well-established role in energy generation. These dynamic organelles exert vital control over numerous cellular processes, including calcium regulation, phospholipid synthesis, innate immunity, and apoptosis. While mitochondria's importance is acknowledged in all cell types, research has revealed the exceptionally dynamic nature of the mitochondrial network in oocytes and embryos, finely tuned to meet unique needs during gamete and pre-implantation embryo development. Within oocytes, both the quantity and morphology of mitochondria can significantly change during maturation and post-fertilization. These changes are orchestrated by fusion and fission processes (collectively known as mitochondrial dynamics), crucial for energy production, content exchange, and quality control as mitochondria adjust to the shifting energy demands of oocytes and embryos. The roles of proteins that regulate mitochondrial dynamics in reproductive processes have been primarily elucidated through targeted deletion studies in animal models. Notably, impaired mitochondrial dynamics have been linked to female reproductive health, affecting oocyte quality, fertilization, and embryo development. Dysfunctional mitochondria can lead to fertility problems and can have an impact on the success of pregnancy, particularly in older reproductive age women.

Loss of the mitochondrial protein SPD-3 elevates PLK-1 levels and dysregulates mitotic events

In metazoans, Polo-like kinase (PLK1) controls several mitotic events including nuclear envelope breakdown, centrosome maturation, spindle assembly and progression through mitosis. Here we show that a mutation in the mitochondria-localized protein SPD-3 affects mitotic events by inducing elevated levels of PLK-1 in early Caenorhabditis elegans embryos. SPD-3 mutant embryos contain abnormally positioned mitotic chromosomes, show a delay in anaphase onset and asymmetrically disassemble the nuclear lamina. We found that more PLK-1 accumulated on centrosomes, nuclear envelope, nucleoplasm, and chromatin before NEBD, suggesting that PLK-1 overexpression is responsible for some of the observed mitotic phenotypes. In agreement with this, the chromosome positioning defects of the spd-3(oj35) mutant could be rescued by reducing PLK-1 levels. Our data suggests that the mitochondrial SPD-3 protein affects chromosome positioning and nuclear envelope integrity by up-regulating the endogenous levels of PLK-1 during early embryogenesis in C. elegans This finding suggests a novel link between mitochondria and nuclear envelope dynamics and chromosome positioning by increasing the amount of a key mitotic regulator, PLK-1, providing a novel link between mitochondria and mitosis.

Mitotic events depend on regulation of PLK-1 levels by the mitochondrial protein SPD-3

In metazoans, Polo Kinase (Plk1) controls several mitotic events including nuclear envelope breakdown, centrosome maturation and kinetochore assembly. Here we show that mitotic events regulated by Polo Like Kinase (PLK-1) in early C. elegans embryos depend on the mitochondrial-localized protein SPD-3. spd-3 mutant one-cell embryos contain abnormally positioned mitotic chromosomes and prematurely and asymmetrically disassemble the nuclear lamina. Nuclear envelope breakdown (NEBD) in C. elegans requires direct dephosphorylation of lamin by PLK-1. In spd-3 mutants PLK-1 levels are ~6X higher in comparison to control embryos and PLK-1::GFP was highly accumulated at centrosomes, the nuclear envelope, nucleoplasm, and chromosomes prior to NEBD. Partial depletion of plk-1 in spd-3 mutant embryos rescued mitotic chromosome and spindle positioning defects indicating that these phenotypes result from higher PLK-1 levels and thus activity. Our data suggests that the mitochondrial SPD-3 protein controls NEBD and chromosome positioning by regulating the endogenous levels of PLK-1 during early embryogenesis in C. elegans . This finding suggests a novel link between mitochondria and mitotic events by controlling the amount of a key mitotic regulator, PLK-1 and thus may have further implications in the context of cancers or age-related diseases and infertility as it provides a novel link between mitochondria and mitosis.

Azoxystrobin exposure impairs meiotic maturation by disturbing spindle formation in mouse oocytes

Fungicides are a type of pesticide used to protect plants and crops from pathogenic fungi. Azoxystrobin (AZO), a natural methoxyacrylate derived from strobilurin, is one of the most widely used fungicides in agriculture. AZO exerts its fungicidal activity by inhibiting mitochondrial respiration, but its cytotoxicity to mammalian oocytes has not been studied. In this study, we investigated the effect of AZO exposure on mouse oocyte maturation to elucidate the underlying mechanisms of its possible reproductive toxicity. We found that AZO exposure disturbed meiotic maturation by impairing spindle formation and chromosome alignment, which was associated with decreased microtubule organizing center (MTOC) integrity. Moreover, AZO exposure induced abnormal mitochondrial distribution and increased oxidative stress. The AZO-induced toxicity to oocytes was relieved by melatonin supplementation during meiotic maturation. Therefore, our results suggest that AZO exposure impairs oocyte maturation not only by increasing oxidative stress and mitochondrial dysfunction, but also by decreasing MTOC integrity and subsequent spindle formation and chromosome alignment.

Miro1 depletion disrupts spatial distribution of mitochondria and leads to oocyte maturation defects

Mitochondria are dynamic organelles that undergo regulated microtubule- and actin-mediated trafficking to meet local energy and metabolic needs. Mitochondrial trafficking may be particularly critical in large cells such as eggs and early embryos where spindle formation and polar body extrusion occur in specific regions of the cytoplasm. To investigate the role of mitochondrial distribution in oocytes we have targeted the mitochondrial membrane protein, MIRO1, which couples mitochondria to the motor protein-TRAK complex. Oocyte-specific deletion of MIRO1 leads to the formation of large aggregates of mitochondria in perinuclear and cortical compartments. Mitochondria remain capable of long-range trafficking during maturation, indicating redundancy in the mechanisms coupling mitochondria to motor proteins. Polar body extrusion in the absence of MIRO1 was reduced by approximately 20%. In MIRO1-deleted zygotes, mitochondria showed increased accumulation around the pronuclei but this did not affect mitochondrial distribution to daughter blastomeres. In vitro development of parthenogenetic embryos was also reduced, although no differences were found in the fertility of oocyte-specific Miro1 KO mice. These findings demonstrate MIRO1 acts as a mitochondrial adaptor, setting mitochondrial distribution in oocytes and early embryos, and disrupting this process compromises in vitro oocyte maturation and embryo development.

Effects of putrescine on the quality and epigenetic modification of mouse oocytes during in vitro maturation

CONTEXT

Low ovarian putrescine levels and decreased peak values following luteinising hormone peaks are related to poor oocyte quantity and quality in ageing women.

AIMS

To investigate the effects of putrescine supplementation in in vitro maturation (IVM) medium on oocyte quality and epigenetic modification.

METHODS

Germinal vesicle oocytes retrieved from the ovaries of 8-week-old and 9-month-old mice were divided into four groups (the young, young+difluoromethylornithine (DFMO), ageing and ageing+putrescine groups) and cultured in IVM medium with or without 1mM putrescine or DFMO for 16h. The first polar body extrusion (PBE), cleavage and embryonic development were evaluated. Spindles, chromosomes, mitochondria and reactive oxygen species (ROS) were measured. The expression levels of SIRT1, H3K9ac, H3K9me2, H3K9me3, and 5mC levels were evaluated. Sirt1 and imprinted genes were detected.

RESULTS

The PBE was higher in the ageing+putrescine group than in the ageing group. Putrescine increased the total and inner cell mass cell numbers of blastocysts in ageing oocytes. Putrescine decreased aberrant spindles and chromosome aneuploidy, increased the mitochondrial membrane potential and decreased ROS levels. Putrescine increased SIRT1 expression and attenuated the upregulation of H3K9ac levels in ageing oocytes. Putrescine did not affect 5mC, H3K9me2 or H3K9me3 levels or imprinted gene expression.

CONCLUSIONS

Putrescine supplementation during IVM improved the maturation and quality of ageing oocytes and promoted embryonic development by decreasing ROS generation, maintaining mitochondrial and spindle function and correcting aberrant epigenetic modification.

IMPLICATIONS

Putrescine shows application potential for human-assisted reproduction, especially for IVM of oocytes from ageing women.

Oocyte maturation abnormalities - A systematic review of the evidence and mechanisms in a rare but difficult to manage fertility pheneomina

A small proportion of infertile women experience repeated oocyte maturation abnormalities (OMAS). OMAS include degenerated and dysmorphic oocytes, empty follicle syndrome, oocyte maturation arrest (OMA), resistant ovary syndrome and maturation defects due to primary ovarian insufficiency. Genetic factors play an important role in OMAS but still need specifications. This review documents the spectrum of OMAS and to evaluate the multiple subtypes classified as OMAS. In this review, readers will be able to understand the oocyte maturation mechanism, gene expression and their regulation that lead to different subtypes of OMAs, and it will discuss the animal and human studies related to OMAS and lastly the treatment options for OMAs. Literature searches using PubMed, MEDLINE, Embase, National Institute for Health and Care Excellence were performed to identify articles written in English focusing on Oocyte Maturation Abnormalities by looking for the following relevant keywords. A search was made with the specified keywords and included books and documents, clinical trials, animal studies, human studies, meta-analysis, randomized controlled trials, reviews, systematic reviews and options written in english. The search detected 3,953 sources published from 1961 to 2021. After title and abstract screening for study type, duplicates and relevancy, 2,914 studies were excluded. The remaining 1,039 records were assessed for eligibility by full-text reading and 886 records were then excluded. Two hundred and twenty seven full-text articles and 0 book chapters from the database were selected for inclusion. Overall, 227 articles, one unpublished and one abstract paper were included in this final review. In this review study, OMAS were classified and extensively evaluatedand possible treatment options under the light of current information, present literature and ongoing studies. Either genetic studies or in vitro maturation studies that will be handled in the future will lead more informations to be reached and may make it possible to obtain pregnancies.

Mitofusins: from mitochondria to fertility

Germ cell formation and embryonic development require ATP synthesized by mitochondria. The dynamic system of the mitochondria, and in particular, the fusion of mitochondria, are essential for the generation of energy. Mitofusin1 and mitofusin2, the homologues of Fuzzy onions in yeast and Drosophila, are critical regulators of mitochondrial fusion in mammalian cells. Since their discovery mitofusins (Mfns) have been the source of significant interest as key influencers of mitochondrial dynamics, including membrane fusion, mitochondrial distribution, and the interaction with other organelles. Emerging evidence has revealed significant insight into the role of Mfns in germ cell formation and embryonic development, as well as the high incidence of reproductive diseases such as asthenospermia, polycystic ovary syndrome, and gestational diabetes mellitus. Here, we describe the key mechanisms of Mfns in mitochondrial dynamics, focusing particularly on the role of Mfns in the regulation of mammalian fertility, including spermatogenesis, oocyte maturation, and embryonic development. We also highlight the role of Mfns in certain diseases associated with the reproductive system and their potential as therapeutic targets.

A coarse-grained NADH redox model enables inference of subcellular metabolic fluxes from fluorescence lifetime imaging

Mitochondrial metabolism is of central importance to diverse aspects of cell and developmental biology. Defects in mitochondria are associated with many diseases, including cancer, neuropathology, and infertility. Our understanding of mitochondrial metabolism in situ and dysfunction in diseases are limited by the lack of techniques to measure mitochondrial metabolic fluxes with sufficient spatiotemporal resolution. Herein, we developed a new method to infer mitochondrial metabolic fluxes in living cells with subcellular resolution from fluorescence lifetime imaging of NADH. This result is based on the use of a generic coarse-grained NADH redox model. We tested the model in mouse oocytes and human tissue culture cells subject to a wide variety of perturbations by comparing predicted fluxes through the electron transport chain (ETC) to direct measurements of oxygen consumption rate. Interpreting the fluorescence lifetime imaging microscopy measurements of NADH using this model, we discovered a homeostasis of ETC flux in mouse oocytes: perturbations of nutrient supply and energy demand of the cell do not change ETC flux despite significantly impacting NADH metabolic state. Furthermore, we observed a subcellular spatial gradient of ETC flux in mouse oocytes and found that this gradient is primarily a result of a spatially heterogeneous mitochondrial proton leak. We concluded from these observations that ETC flux in mouse oocytes is not controlled by energy demand or supply, but by the intrinsic rates of mitochondrial respiration.

Physical bioenergetics: Energy fluxes, budgets, and constraints in cells

Cells are the basic units of all living matter which harness the flow of energy to drive the processes of life. While the biochemical networks involved in energy transduction are well-characterized, the energetic costs and constraints for specific cellular processes remain largely unknown. In particular, what are the energy budgets of cells? What are the constraints and limits energy flows impose on cellular processes? Do cells operate near these limits, and if so how do energetic constraints impact cellular functions? Physics has provided many tools to study nonequilibrium systems and to define physical limits, but applying these tools to cell biology remains a challenge. Physical bioenergetics, which resides at the interface of nonequilibrium physics, energy metabolism, and cell biology, seeks to understand how much energy cells are using, how they partition this energy between different cellular processes, and the associated energetic constraints. Here we review recent advances and discuss open questions and challenges in physical bioenergetics.