Impact of Assisted Reproductive Technologies: A Mitochondrial Perspective of Cytoplasmic Transplantation

Mitochondria as therapeutic targets in assisted reproduction

Mitochondria are essential organelles with specialized functions, which play crucial roles in energy production, calcium homeostasis, and programmed cell death. In oocytes, mitochondrial populations are inherited maternally and are vital for developmental competence. Dysfunction in mitochondrial quality control mechanisms can lead to reproductive failure. Due to their central role in oocyte and embryo development, mitochondria have been investigated as potential diagnostic and therapeutic targets in assisted reproduction. Pharmacological agents that target mitochondrial function and show promise in improving assisted reproduction outcomes include antioxidant coenzyme Q10 and mitoquinone, mammalian target of rapamycin signaling pathway inhibitor rapamycin, and nicotinamide mononucleotide. Mitochondrial replacement therapies (MRTs) offer solutions for infertility and mitochondrial disorders. Autologous germline mitochondrial energy transfer initially showed promise but failed to demonstrate significant benefits in clinical trials. Maternal spindle transfer (MST) and pronuclear transfer hold potential for preventing mitochondrial disease transmission and improving oocyte quality. Clinical trials of MST have shown promising outcomes, but larger studies are needed to confirm safety and efficacy. However, ethical and legislative challenges complicate the widespread implementation of MRTs.

Ginsenoside Rb1 protects porcine oocytes against methylglyoxal damage thus it improves the quality of parthenogenetic activation and in vitro fertilization embryos

Panax ginseng, a functional food, has been widely used as an edible nourishment and medicinal supplement. Ginsenoside Rb1 is a major bioactive ingredient of ginseng, which shows very specific anti-apoptosis and anti-oxidant activities. Methylglyoxal (MGO) is one of intermediate products of glucose metabolism, which is absorbed easily from high sugar foods or carbonated beverages. It may involve in a variety of detrimental processes in vivo. However, it has not been fully explored the effects of ginsenoside Rb1 on MGO-induced oocytes damage. This study found that MGO-induced DNA damage and mitochondrial dysfunction result in the failure of porcine oocytes maturation and low in vitro development capacity of parthenogenetic activation (PA) and in vitro fertilization (IVF) embryos. Conversely, Rb1 supplementation recovered the rate of maturation, and improved in vitro development capacity of PA and IVF embryos. Rb1 also provided porcine oocytes a lower level of reactive oxygen species production, higher level of ATP content and mitochondrial membrane potential, and stimulated pluripotency gene expression in blastocysts. The findings of this study reveal ginsenoside Rb1 protects porcine oocyte from the cytotoxicity effects of methylglyoxal and provides novel perspectives for the protection of reproduction system by functional food of ginseng.

The protective role of ginsenoside compound K in porcine oocyte meiotic maturation failed caused by benzo(a)pyrene during in vitro maturation

Benzo(a)pyrene (BaP) is a pollutant and carcinogen derived from air pollution. It causes serious damage to reproductive system, especially ovary. Ginseng is always used in food and traditional medicine as a nutraceuticals or herbal medicine. Ginsenoside compound K (CK) is a major bioactive ingredient of ginseng, that shows very specific anti-apoptosis, anti-oxidant, and anti-inflammatory activities and thus, it protects cells from damage. The aim of this study was to investigate the effects of CK on the BaP-induced inhibition of the in vitro maturation of porcine oocytes and their subsequent embryonic development capacity. We found that supplementation with 10 μg mL^-1 CK during in vitro maturation significantly increased maturation rate (P < 0.05) and the expression level of related genes after damage induced by 40 μM BaP treatment. In addition, reactive oxygen species (ROS) levels significantly decreased and ATP content and mitochondrial membrane potential (MMP) increased after CK supplementation (P < 0.05). The competence for embryonic development was improved by the induction of pluripotency gene expression and the inhibition of apoptosis after CK supplementation of BaP-treated oocytes. Supplementation with 10 μg mL^-1 CK improved porcine oocyte maturation and subsequent embryonic development of parthenogenetic activation (33.01 vs. 20.92, P < 0.05) and in vitro fertilization (24.01 vs. 16.52, P < 0.05) by increasing antioxidant activity and improving mitochondrial function after BaP-induced damage.

Baicalin improves IVM of pig oocytes and subsequent preimplantation embryo development by inhibiting apoptosis

Baicalin, a monomer of flavonoids extracted from dried roots of Scutellaria baicalensis, is used to treat female infertility. However, the effect of baicalin on oocyte maturation is unknown. In this study we investigated the effects of baicalin on the IVM of pig oocytes and subsequent embryo development following parthenogenetic activation (PA). We found that 0.1µgmL-1 baicalin significantly (P<0.05) increased the IVM rate of oocytes compared with the non-treatment (control) group by reducing levels of reactive oxygen species (ROS). In addition, the mRNA expression of genes related to nuclear maturation and cumulus cell expansion, mitochondrial membrane potential and ATP content was significantly (P<0.05) higher in baicalin-treated than control oocytes. To determine whether baicalin treatment during IVM of pig oocytes improves subsequent development of PA embryos, we measured the cleavage and blastocyst formation rates, as well as the number of cells per blastocyst. All these parameters were significantly (P<0.05) higher in the baicalin-treated than control group. In conclusion, this study demonstrates that baicalin improves pig oocyte maturation and subsequent embryo development invitro by inhibiting production of ROS and reducing apoptosis in oocytes.

The role of Rad51 in safeguarding mitochondrial activity during the meiotic cell cycle in mammalian oocytes

Rad51 is a conserved eukaryotic protein that mediates the homologous recombination repair of DNA double-strand breaks that occur during mitosis and meiosis. In addition, Rad51 promotes mitochondrial DNA synthesis when replication stress is increased. Rad51 also regulates cell cycle progression by preserving the G2/M transition in embryonic stem cells. In this study, we report a novel function of Rad51 in regulating mitochondrial activity during in vitro maturation of mouse oocytes. Suppression of Rad51 by injection of Rad51 dsRNA into germinal vesicle-stage oocytes resulted in arrest of meiosis in metaphase I. Rad51-depleted oocytes showed chromosome misalignment and failures in spindle aggregation, affecting the completion of cytokinesis. We found that Rad51 depletion was accompanied by decreased ATP production and mitochondrial membrane potential and increased DNA degradation. We further demonstrated that the mitochondrial defect activated autophagy in Rad51-depleted oocytes. Taken together, we concluded that Rad51 functions to safeguard mitochondrial integrity during the meiotic maturation of oocytes.

Ovarian ageing: the role of mitochondria in oocytes and follicles

BACKGROUND

There is a great inter-individual variability of ovarian ageing, and almost 20% of patients consulting for infertility show signs of premature ovarian ageing. This feature, taken together with delayed childbearing in modern society, leads to the emergence of age-related ovarian dysfunction concomitantly with the desire for pregnancy. Assisted reproductive technology is frequently inefficacious in cases of ovarian ageing, thus raising the economic, medical and societal costs of the procedures.

OBJECTIVE AND RATIONAL

Ovarian ageing is characterized by quantitative and qualitative alteration of the ovarian oocyte reserve. Mitochondria play a central role in follicular atresia and could be the main target of the ooplasmic factors determining oocyte quality adversely affected by ageing. Indeed, the oocyte is the richest cell of the body in mitochondria and depends largely on these organelles to acquire competence for fertilization and early embryonic development. Moreover, the oocyte ensures the uniparental transmission and stability of the mitochondrial genome across the generations. This review focuses on the role played by mitochondria in ovarian ageing and on the possible consequences over the generations.

SEARCH METHODS

PubMed was used to search the MEDLINE database for peer-reviewed original articles and reviews concerning mitochondria and ovarian ageing, in animal and human species. Searches were performed using keywords belonging to three groups: 'mitochondria' or 'mitochondrial DNA'; 'ovarian reserve', 'oocyte', 'ovary' or 'cumulus cells'; and 'ageing' or 'ovarian ageing'. These keywords were combined with other search phrases relevant to the topic. References from these articles were used to obtain additional articles.

OUTCOMES

There is a close relationship, in mammalian models and humans, between mitochondria and the decline of oocyte quality with ageing. Qualitatively, ageing-related mitochondrial (mt) DNA instability, which leads to the accumulation of mtDNA mutations in the oocyte, plays a key role in the deterioration of oocyte quality in terms of competence and of the risk of transmitting mitochondrial abnormalities to the offspring. In contrast, some mtDNA haplogroups are protective against the decline of ovarian reserve. Quantitatively, mitochondrial biogenesis is crucial during oogenesis for constituting a mitochondrial pool sufficiently large to allow normal early embryonic development and to avoid the untimely activation of mitochondrial biogenesis. Ovarian ageing also seriously affects the dynamic nature of mitochondrial biogenesis in the surrounding granulosa cells that may provide interesting alternative biomarkers of oocyte quality.

WIDER IMPLICATIONS

A fuller understanding of the involvement of mitochondria in cases of infertility linked to ovarian ageing would contribute to a better management of the disorder in the future.

Autologous Germline Mitochondrial Energy Transfer (AUGMENT) in Human Assisted Reproduction

Ovarian aging is characterized by a decline in both the total number and overall quality of oocytes, the latter of which has been experimentally tied to mitochondrial dysfunction. Clinical studies in the late 1990s demonstrated that transfer of cytoplasm aspirated from eggs of young female donors into eggs of infertile women at the time of intracytoplasmic sperm injection improved pregnancy success rates. However, donor mitochondria were identified in offspring, and the United States Food and Drug Administration raised questions about delivery of foreign genetic material into human eggs at the time of fertilization. Accordingly, heterologous cytoplasmic transfer, while promising, was in effect shut down as a clinical protocol. The recent discovery of adult oogonial (oocyte-generating) stem cells in mice, and subsequently in women, has since re-opened the prospects of delivering a rich source of pristine and patient-matched germline mitochondria to boost egg health and embryonic developmental potential without the need for young donor eggs to obtain cytoplasm. Herein we overview the science behind this new protocol, which has been patented and termed autologous germline mitochondrial energy transfer, and its use to date in clinical studies for improving pregnancy success in women with a prior history of assisted reproduction failure.

Germline energetics, aging, and female infertility

The role of metabolism in ovarian aging is poorly described, despite the fact that ovaries fail earlier than most other organs. Growing interest in ovarian function is being driven by recent evidence that mammalian females routinely generate new oocytes during adult life through the activity of germline stem cells. In this perspective, we overview the female reproductive system as a powerful and clinically relevant model to understand links between aging and metabolism, and we discuss new concepts for how oocytes and their precursor cells might be altered metabolically to sustain or increase ovarian function and fertility in women.

Adenosine triphosphate content in human unfertilized oocytes, undivided zygotes and embryos unsuitable for transfer or cryopreservation

OBJECTIVE

To investigate the relationship between adenosine triphosphate (ATP) content and developmental potential in human oocytes, zygotes and embryos.

METHODS

ATP content was determined using a bioluminescence assay in 72 unfertilized oocytes, 26 undivided zygotes and 70 embryos unsuitable for transfer or cryopreservation, obtained from 52 women undergoing in vitro fertilization and embryo transfer (IVF-ET).

RESULTS

The mean ATP content increased with development: zygotes had a significantly higher ATP content than oocytes, and embryos had a significantly higher ATP content than both zygotes and oocytes. Within the embryo group, the ATP content was significantly higher in polypronuclear embryos than in inferior embryos with substantial fragmentation. When analysed in relation to IVF-ET outcome, the ATP content of oocytes and embryos from women who became pregnant was significantly higher than in oocytes and embryos from those who did not become pregnant.

CONCLUSIONS

Reduced ATP content may be related to fertilization failure, arrested division and abnormal embryonic development. The differences observed in oocyte and embryo ATP content between women who became pregnant and those who did not suggests that mitochondrial function is correlated with individual fertility.

The contribution of mitochondrial function to reproductive aging

PURPOSE

The number of women attempting to conceive between the ages of 36 and 44 has increased significantly in the last decade. While it is well established that women's reproductive success dramatically declines with age, the underlying physiological changes responsible for this phenomenon are not well understood. With assisted reproductive technologies, it is clear that oocyte quality is a likely cause since women over 40 undergoing in vitro fertilization (IVF) with oocytes donated by younger women have success rates comparable to young patients. Apart from oocyte donation, there is no known intervention to improve the pregnancy outcome of older patients. The aim of this paper was the review the relevant data on the potential role of mitochondria in reproductive aging.

METHOD

Review of current literature on the subject.

RESULTS

We present the current evidence that associate mitochondrial dysfunction with age related decrease in female reproductive outcome.

CONCLUSIONS

The aging process is complex, driven by a multitude of factors thought to modulate cellular and organism life span. Although the factors responsible for diminished oocyte quality remain to be elucidated, the present review focuses on the potential role of impaired mitochondrial function.

Dynamic regulation of mitochondrial function in preimplantation embryos and embryonic stem cells

Mitochondrial function is dependent upon regulation of biogenesis and dynamics. A number of studies have documented the importance of these organelles in both preimplantation embryos and embryonic stem cells (ESCs), however it remains unclear how mitochondria respond to their immediate microenvironment through modulation of morphology and movement, or whether perturbations in these processes will have a significant impact following differentiation/implantation. Here we review existing literature on two key aspects of nuclear-mitochondrial cross-talk and the dynamic processes involved in mediating mitochondrial function through regulation of mitochondrial biogenesis, morphology and movement, with particular emphasis on embryos and ESCs.

The use of mitochondrial nutrients to improve the outcome of infertility treatment in older patients

We present a hypothesis emphasizing the role of mitochondrial dysfunction in reproductive senescence and suggesting the use of mitochondrial nutrients as an adjuvant treatment in older patients with infertility.

Molecular control of mitochondrial function in developing rhesus monkey oocytes and preimplantation-stage embryos

The mitochondrion undergoes significant functional and structural changes, as well as an increase in number, during preimplantation embryonic development. The mitochondrion generates ATP and regulates a range of cellular processes, such as signal transduction and apoptosis. Therefore, mitochondria contribute to overall oocyte quality and embryo developmental competence. The present study identified, for the first time, the detailed temporal expression of mRNAs related to mitochondrial biogenesis in rhesus monkey oocytes and embryos. Persistent expression of maternally encoded mRNAs was observed, in combination with transcriptional activation and mRNA accumulation at the eight-cell stage, around the time of embryonic genome activation. The expression of these transcripts was significantly altered in oocytes and embryos with reduced developmental potential. In these embryos, most maternally encoded transcripts were precociously depleted. Embryo culture and specific culture media affected the expression of some of these transcripts, including a deficiency in the expression of key transcriptional regulators. Several genes involved in regulating mitochondrial transcription and replication are similarly affected by in vitro conditions and their downregulation may be instrumental in maintaining the mRNA profiles of mitochondrially encoded genes observed in the present study. These data support the hypothesis that the molecular control of mitochondrial biogenesis, and therefore mitochondrial function, is impaired in in vitro-cultured embryos. These results highlight the need for additional studies in human and non-human primate model species to determine how mitochondrial biogenesis can be altered by oocyte and embryo manipulation protocols and whether this affects physiological function in progeny.

Regulation of mitochondrial polarity in mouse and human oocytes: the influence of cumulus derived nitric oxide

Whether exogenous factors influenced the level of mitochondrial polarity (DeltaPsim) in the subplasmalemmal cytoplasm of the oocyte was investigated with denuded and cumulus-enclosed human and mouse oocytes between the germinal vesicle and metaphase II stage. Co-culture of denuded oocytes with cumulus masses or primary cumulus cell cultures demonstrated a 'proximity' effect with respect to the detectable level of DeltaPsim in the oocyte. The specificity and reversibility of this effect on subplasmalemmal mitochondria were shown by repeated repositioning between cellular and acellular regions, which sequentially down- or up-regulated DeltaPsim. Experimental studies with a nitric oxide (NO) donor and inhibitor of NO synthase indicate that NO produced by cumulus cells has a regulatory influence on DeltaPsim in the subplasmalemmal cytoplasm of the corresponding oocyte. Culture of denuded and cumulus-enclosed (intact) oocytes in low and high oxygen atmospheres suggests that competition between oxygen and NO at the mitochondrial level may regulate the level of DeltaPsim and maintain mitochondria homeostasis in the pre-ovulatory oocyte, with a shift to higher polarity occurring after ovulation. The role of exogenous influences on oocyte DeltaPsim is discussed with respect to the regulation of developmental processes in the oocyte and early embryo.

Oocyte senescence: a firm link to age-related female subfertility

In women optimal fertility is maintained until 30 years of age and then decreases sharply. Therefore, the trend to delay childbearing until the fourth decade of life has contributed highly to the decline in total fertility rate observed in Western countries in recent decades. Much evidence supports the idea that age-related subfertility is mostly related to oocyte senescence. Based on the finding that maternal age negatively affects the storage of oocyte transcripts involved in major cellular functions, the present paper reviews the nuclear and cytoplasmic failure of the aged oocyte which can exert a negative influence on its developmental competence. Recognizing the potential role of oocyte-based technologies for improving clinical outcome for women with age-related etiologies of infertility, the importance of basic research aimed to increase knowledge of the aged oocyte and its microenvironment is also highlighted in order to set up new therapeutic strategies.

Oocyte quality and maternal control of development

The oocyte is a unique and highly specialized cell responsible for creating, activating, and controlling the embryonic genome, as well as supporting basic processes such as cellular homeostasis, metabolism, and cell cycle progression in the early embryo. During oogenesis, the oocyte accumulates a myriad of factors to execute these processes. Oogenesis is critically dependent upon correct oocyte-follicle cell interactions. Disruptions in oogenesis through environmental factors and changes in maternal health and physiology can compromise oocyte quality, leading to arrested development, reduced fertility, and epigenetic defects that affect long-term health of the offspring. Our expanding understanding of the molecular determinants of oocyte quality and how these determinants can be disrupted has revealed exciting new insights into the role of oocyte functions in development and evolution.

Stem cell plasticity: Learning from hepatogenic differentiation strategies

Many studies on stem cell plasticity are challenging the concept that stem cells contain an intrinsically predefined, unidirectional differentiation program. This means that the developmental fate of a stem cell is dependent on the general potential of the cell (pre-determined stem cell fate) as well as on microenvironmental cues, such as stimuli from growth factors (stem cell niche). Here, we reviewed reports that examined the hepatocyte differentiation ability of stem cells from two different sources: embryonic stem cells and adult stem cells. All of those stem cells revealed the ability to give rise to hepatocyte-like cells using different induction strategies. However, it is still not clear which of those stem cells would be the best source for hepatocyte replacement or which would be the best protocol. We herein present the current knowledge regarding available protocols and factors used in order to obtain functional hepatocytes from stem cells.