Influence of Maternal Aging on Mitochondrial Heterogeneity, Inheritance, and Function in Oocytes and Preimplantation Embryos

Depletion of placental brain-derived neurotrophic factor (BDNF) is attributed to premature ovarian insufficiency (POI) in mice offspring

INTRODUCTION

Premature ovarian insufficiency (POI) is one of the causes of female infertility. Unexplained POI is increasingly affecting women in their reproductive years. However, the etiology of POI is diverse and remains elusive. We and others have shown that brain-derived neurotrophic factor (BDNF) plays an important role in adult ovarian function. Here, we report on a novel role of BDNF in the Developmental Origins of POI.

METHODS

Placental BDNF knockout mice were created using CRISPR/CAS9. Homozygous knockout (cKO(HO)) mice didn't survive, while heterozygous knockout (cKO(HE)) mice did. BDNF reduction in cKO(HE) mice was confirmed via immunohistochemistry and Western blots. Ovaries were collected from cKO(HE) mice at various ages, analyzing ovarian metrics, FSH expression, and litter sizes. In one-month-old mice, oocyte numbers were assessed using super-ovulation, and oocyte gene expression was analyzed with smart RNAseq. Ovaries of P7 mice were studied with SEM, and gene expression was confirmed with RT-qPCR. Alkaline phosphatase staining at E11.5 and immunofluorescence for cyclinD1 assessed germ cell number and cell proliferation.

RESULTS

cKO(HE) mice had decreased ovarian function and litter size in adulthood. They were insensitive to ovulation induction drugs manifested by lower oocyte release after superovulation in one-month-old cKO(HE) mice. The transcriptome and SEM results indicate that mitochondria-mediated cell death or aging might occur in cKO(HE) ovaries. Decreased placental BDNF led to diminished primordial germ cell proliferation at E11.5 and ovarian reserve which may underlie POI in adulthood.

CONCLUSION

The current results showed decreased placental BDNF diminished primordial germ cell proliferation in female fetuses during pregnancy and POI in adulthood. Our findings can provide insights into understanding the underlying mechanisms of POI.

Maternal aging increases offspring adult body size via transmission of donut-shaped mitochondria

Maternal age at childbearing has continued to increase in recent decades. However, whether and how it influences offspring adult traits are largely unknown. Here, using adult body size as the primary readout, we reveal that maternal rather than paternal age has an evolutionarily conserved effect on offspring adult traits in humans, Drosophila, and Caenorhabditis elegans. Elucidating the mechanisms of such effects in humans and other long-lived animals remains challenging due to their long life course and difficulties in conducting in vivo studies. We thus employ the short-lived and genetically tractable nematode C. elegans to explore the mechanisms underlying the regulation of offspring adult trait by maternal aging. By microscopic analysis, we find that old worms transmit aged mitochondria with a donut-like shape to offspring. These mitochondria are rejuvenated in the offspring's early life, with their morphology fully restored before adulthood in an AMPK-dependent manner. Mechanistically, we demonstrate that early-life mitochondrial dysfunction activates AMPK, which in turn not only alleviates mitochondrial abnormalities but also activates TGFβ signaling to increase offspring adult size. Together, our findings provide mechanistic insight into the ancient role of maternal aging in shaping the traits of adult offspring.

Cell-cell interactions during the formation of primordial follicles in humans

Gametogenesis is a complex and sex-specific multistep process during which the gonadal somatic niche plays an essential regulatory role. One of the most crucial steps during human female gametogenesis is the formation of primordial follicles, the functional unit of the ovary that constitutes the pool of follicles available at birth during the entire reproductive life. However, the relation between human fetal germ cells (hFGCs) and gonadal somatic cells during the formation of the primordial follicles remains largely unexplored. We have discovered that hFGCs can form multinucleated syncytia, some connected via interconnecting intercellular bridges, and that not all nuclei in hFGC-syncytia were synchronous regarding meiotic stage. As hFGCs progressed in development, pre-granulosa cells formed protrusions that seemed to progressively constrict individual hFGCs, perhaps contributing to separate them from the multinucleated syncytia. Our findings highlighted the cell-cell interaction and molecular dynamics between hFGCs and (pre)granulosa cells during the formation of primordial follicles in humans. Knowledge on how the pool of primordial follicle is formed is important to understand human infertility.

Molecular phenotypes of bovine blastocyst derived from in vitro-matured oocyte supplemented with PAPP-A

Insulin-like growth factor-1 (IGF-1) regulates cellular lipid content, whereas pregnancy-associated plasma protein-A (PAPP-A) increases IGF-1 bioavailability. Using in vitro-matured cumulus-oocyte complexes, we aimed to evaluate the impact of PAPP-A on the blastocyst lipid content, embryo cryotolerance and embryonic transcriptional profile. We determined that PAPP-A did not affect the lipid content of oocytes, blastocysts, or blastocyst yield (P > 0.05). However, PAPP-A modulated the embryo transcriptional profiles by downregulating PPARGC1A and AKR1B1, which are related to lipid metabolism; CASP9, a pro-apoptotic gene; and IFN-τ, a marker of embryo quality (P < 0.05). Furthermore, the use of PAPP-A improved blastocyst re-expansion in the first 3 h of culture after vitrification (P < 0.05). Although PAPP-A did not affect the blastocyst lipid content or embryo production, we suggest that embryonic transcriptional modulation could contribute to maintain the balance in embryo lipid metabolism. Furthermore, PAPP-A's approach seems to control key intracellular pathways that improve post-cryopreservation development of blastocysts.

Mitochondrial dysfunction in the offspring of obese mothers and it's transmission through damaged oocyte mitochondria: Integration of mechanisms

In vivo and in vitro studies demonstrate that mitochondria in the oocyte, are susceptible to damage by suboptimal pre/pregnancy conditions, such as obesity. These suboptimal conditions have been shown to induce mitochondrial dysfunction (MD) in multiple tissues of the offspring, suggesting that mitochondria of oocytes that pass from mother to offspring, can carry information that can programme mitochondrial and metabolic dysfunction of the next generation. They also suggest that transmission of MD could increase the risk of obesity and other metabolic diseases in the population inter- and trans-generationally. In this review, we examined whether MD observed in offspring tissues of high energetic demand, is the result of the transmission of damaged mitochondria from obese mothers' oocytes to the offspring. The contribution of genome-independent mechanisms (namely mitophagy) in this transmission were also explored. Finally, potential interventions aimed at improving oocyte/embryo health were investigated, to see if they may provide an opportunity to halter the generational effects of MD.

The role of mtDNA in oocyte quality and embryo development

The mitochondrial genome resides in the mitochondria present in nearly all cell types. The porcine (Sus scrofa) mitochondrial genome is circa 16.7 kb in size and exists in the multimeric format in cells. Individual cell types have different numbers of mitochondrial DNA (mtDNA) copy number based on their requirements for ATP produced by oxidative phosphorylation. The oocyte has the largest number of mtDNA of any cell type. During oogenesis, the oocyte sets mtDNA copy number in order that sufficient copies are available to support subsequent developmental events. It also initiates a program of epigenetic patterning that regulates, for example, DNA methylation levels of the nuclear genome. Once fertilized, the nuclear and mitochondrial genomes establish synchrony to ensure that the embryo and fetus can complete each developmental milestone. However, altering the oocyte's mtDNA copy number by mitochondrial supplementation can affect the programming and gene expression profiles of the developing embryo and, in oocytes deficient of mtDNA, it appears to have a positive impact on the embryo development rates and gene expression profiles. Furthermore, mtDNA haplotypes, which define common maternal origins, appear to affect developmental outcomes and certain reproductive traits. Nevertheless, the manipulation of the mitochondrial content of an oocyte might have a developmental advantage.

John JC. Does supplementation of oocytes with additional mtDNA influence developmental outcome?

Introducing extra mitochondrial DNA (mtDNA) into oocytes at fertilization can rescue poor quality oocytes. However, supplementation alters DNA methylation and gene expression profiles of preimplantation embryos. To determine if these alterations impacted offspring, we introduced mtDNA from failed-to-mature sister (autologous) or third party (heterologous) oocytes into mature oocytes and transferred zygotes into surrogates. Founders exhibited significantly greater daily weight gain (heterologous) and growth rates (heterologous and autologous) to controls. In weaners, cholesterol, bilirubin (heterologous and autologous), anion gap, and lymphocyte count (autologous) were elevated. In mature pigs, potassium (heterologous) and bicarbonate (autologous) were altered. mtDNA and imprinted gene analyses did not reveal aberrant profiles. Neither group exhibited gross anatomical, morphological, or histopathological differences that would lead to clinically significant lesions. Female founders were fertile and their offspring exhibited modified weight and height gain, biochemical, and hematological profiles. mtDNA supplementation induced minor differences that did not affect health and well-being.

Quercetin protects porcine oocytes from in vitro aging by reducing oxidative stress and maintaining the mitochondrial functions

Quercetin (QUE) is a component of the flavonoid family that shows various therapeutic properties, such as antioxidant effects. However, whether QUE affects porcine oocyte in vitro aging has not yet been investigated. Therefore, in this study, we applied various doses of QUE to freshly isolated porcine oocytes and found that 10 µM QUE improved the oocyte maturation rate in vitro, as reflected by the increased degree of cumulus cell expansion and first polar body extrusion. More importantly, we found that QUE reduced in vitro aging and improved the maturity level of porcine oocytes after another 24 h of culturing, accompanied by the upregulated expression levels of bone morphogenetic protein 15, growth differentiation factor 9, Moloney sarcoma oncogene, and cyclin-dependent kinase 2. In addition, we found that QUE treatment significantly reduced the intracellular reactive oxygen species levels, apoptosis, and autophagy and upregulated the expression levels of superoxide dismutase 2 and catalase in aged porcine oocytes. In addition, QUE restored impaired mitochondrial membrane potential and spindle assembly in aged porcine oocytes. Our findings demonstrate that QUE can protect porcine oocytes from in vitro aging by reducing oxidative stress and maintaining mitochondrial function.

RETRACTED: Parallel bimodal single-cell sequencing of transcriptome and methylome provides molecular and translational insights on oocyte maturation and maternal aging

This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). This article has been retracted at the request of the Editor-in-Chief. It has been brought to our attention that the authors of the article "Parallel bimodal single-cell sequencing of transcriptome and methylome provides molecular and translational insights on oocyte maturation and maternal aging" cannot agree on who should be listed as an author of the article. Further inquiry by the journal revealed that the authorship was also changed at the revision stages of the article without notifying the handling Editor, which is contrary to the journal policy on changes to authorship. The journal considers this unacceptable practice, and the Editor-in-Chief decided to retract the article.

Role of Granulosa Cells in the Aging Ovarian Landscape: A Focus on Mitochondrial and Metabolic Function

Mitochondria are at the intersection of aging and fertility, with research efforts centered largely on the role that these specialized organelles play in the relatively rapid decline in oocyte quality that occurs as females approach reproductive senescence. In addition to various roles in oocyte maturation, fertilization, and embryogenesis, mitochondria are critical to granulosa cell function. Herein, we provide a review of the literature pertaining to the role of mitochondria in granulosa cell function, with emphasis on how mitochondrial aging in granulosa cells may impact reproduction in female mammals.

The role of sperm and oocyte in fetal programming: Is Lamarck making a comeback?

Compelling evidence has shown that parental experiences and age at conception may potentially shape the future health of the next generation(s). Certain factors may affect both the female and, strikingly, the male gametes potentially causing the transmission of acquired traits, which was strongly defended by Jean-Baptiste Lamarck. Neurodevelopmental psychiatric disorders, trinucleotide repeat-associated diseases, cardiovascular pathologies, diabetes, obesity and cancer in the offspring, among others, have now been associated with events occurring at the preconception level. The potential implications of a (trans)generational inheritance of parental disease and exposure effects should be taken into account in counselling and public policy. Further research into how exactly gametes apparently deliver more than DNA to a new generation is warranted.

The Role of Mitochondria in Oocyte Maturation

With the nucleus as an exception, mitochondria are the only animal cell organelles containing their own genetic information, called mitochondrial DNA (mtDNA). During oocyte maturation, the mtDNA copy number dramatically increases and the distribution of mitochondria changes significantly. As oocyte maturation requires a large amount of ATP for continuous transcription and translation, the availability of the right number of functional mitochondria is crucial. There is a correlation between the quality of oocytes and both the amount of mtDNA and the amount of ATP. Suboptimal conditions of in vitro maturation (IVM) might lead to changes in the mitochondrial morphology as well as alternations in the expression of genes encoding proteins associated with mitochondrial function. Dysfunctional mitochondria have a lower ability to counteract reactive oxygen species (ROS) production which leads to oxidative stress. The mitochondrial function might be improved with the application of antioxidants and significant expectations are laid on the development of new IVM systems supplemented with mitochondria-targeted reagents. Different types of antioxidants have been tested already on animal models and human rescue IVM oocytes, showing promising results. This review focuses on the recent observations on oocytes’ intracellular mitochondrial distribution and on mitochondrial genomes during their maturation, both in vivo and in vitro. Recent mitochondrial supplementation studies, aiming to improve oocyte developmental potential, are summarized.

Ultrastructural heterogeneity of the mitochondrial population in rat embryonic and induced pluripotent stem cells

Even though rats are popular model animals, the ultrastructure of their pluripotent cells, that is, embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), remains unexplored, although fine structure of pluripotent stem cells of mice and humans and its changes during differentiation have been investigated well. In the present study, we carried out ultrastructural and morphometric analyses of three lines of rat ESCs and two lines of rat iPSCs. The rat pluripotent stem cells were found to have the main typical morphological features of pluripotent cells: large nuclei of irregular or nearly round shape, scanty cytoplasm with few membrane organelles, and a poorly developed Golgi apparatus and endoplasmic reticulum. The cytoplasm of the rat pluripotent cells contains clusters of glycogen, previously described in human ESCs. To identify possible differences between rat ESCs and iPSCs, we performed a morphometric analysis of cell parameters. The mean area of cells and nuclei, the nuclear/cytoplasmic ratio, distributions of glycogen and diversity of mitochondria showed marked variations among the lines of rat pluripotent stem cells and were more pronounced than variations between rat ESCs and iPSCs as separate types of pluripotent stem cells. We noted morphological heterogeneity of the mitochondrial population in the rat pluripotent stem cells. The cells contained three types of mitochondria differing in the structure of cristae and in matrix density, and our morphometric analysis revealed differences in cristae structure.

Maternal recognition of pregnancy in the mare: does it exist and why do we care?

Maternal recognition of pregnancy (MRP) is a process by which an early conceptus signals its presence to the maternal system and prevents the lysis of the corpus luteum, thus ensuring a maternal milieu supportive of pregnancy continuation. It is a fundamental aspect of reproductive biology, yet in the horse, the mechanism underlying MRP remains unknown. This review seeks to address some of the controversies surrounding the evidence and theories of MRP in the equine species, such as the idea that the horse does not conform to the MRP paradigm established in other species or that equine MRP involves a mechanical, rather than chemical, signal. The review examines the challenges of studying this particularly clandestine phenomenon along with the new tools in scientific research that will drive this quest forward in coming years, and discusses the value of knowledge gleaned along this path in the context of clinical applications for improving breeding outcomes in the horse industry.

Imperatorin Ameliorates the Aging-Associated Porcine Oocyte Meiotic Spindle Defects by Reducing Oxidative Stress and Protecting Mitochondrial Function

Imperatorin (IMP) exhibits a variety of pharmacological properties, including antioxidant, anti-inflammatory, antibacterial, anti-cancer, and anti-hypertension activities. However, its effects on animal reproduction systems, especially oocyte development, maturation, and aging are not yet clear. In this study, the effects of IMP on oocyte development and aging as well as the underlying molecular mechanisms were explored. Oocytes were cultured for an additional 24 h for aging. Results revealed that the blastocyst formation and hatching rates of embryos, which were parthenogenetically activated aged oocytes, were significantly increased with IMP treatment (40 μM). Simultaneously, well-distributed cortical granules but no significant difference in zona pellucida hardness were observed after IMP treatment. During this stage, intracellular reactive oxygen species, apoptosis, and autophagy levels were decreased, while mitochondrial membrane potential, glutathione level, and activity of superoxide dismutase and catalase were increased. IMP-treated aged oocytes also showed significantly higher expression of MOS, CCNB1, BMP15, and GDF9 than non-IMP-treated aged oocytes although their levels were still lower than those in the fresh oocytes. These results suggest that IMP can effectively ameliorate the quality of aged porcine oocytes by reducing oxidative stress and protecting mitochondrial function.

Cytoplasmic Transfer Improves Human Egg Fertilization and Embryo Quality: an Evaluation of Sibling Oocytes in Women with Low Oocyte Quality

The aim of this study was to evaluate if cytoplasmic transfer can improve fertilization and embryo quality of women with oocytes of low quality. During ICSI, 10–15% of the cytoplasm from a fresh or frozen young donor oocyte was added to the recipient oocyte. According to the embryo quality, we defined group A as patients in which the best embryo was evident after cytoplasmic transfer and group B as patients in which the best embryo was evident after a simple ICSI. We investigated in the period of 2002–2018, 125 in vitro fertilization cycles involving 1011 fertilized oocytes. Five hundred fifty-seven sibling oocytes were fertilized using ICSI only and 454 oocytes with cytoplasmic transfer. Fertilization rates of oocytes were 67.2% in the cytoplasmic transfer and 53.5% in the ICSI groups (P < 0.001). A reduction in fertilization rate was observed with increased women age in the ICSI but not in the cytoplasmic transfer groups. The best embryo quality was found after cytoplasmic transfer in 78 cycles (62.4%) and without cytoplasmic transfer in 40 cycles (32%, P < 0.001). No significant differences were detected between the age, hormonal levels, dose of stimulation drugs, number of transferred embryos, pregnancy rate and abortion rate between A and B groups. Cytoplasmic transfer improves fertilization rates and early embryo development in humans with low oocyte quality. All 28 children resulting from cytoplasmic transfer are healthy.

Combining Bioinformatics and Experiments to Identify CREB1 as a Key Regulator in Senescent Granulosa Cells

Aging of functional ovaries occurs many years before aging of other organs in the female body. In recent years, a greater number of women continue to postpone their pregnancies to later stages in their lives, raising concerns of the effect of ovarian aging. Mitochondria play an important role in the connection between the aging granulosa cells and oocytes. However, the underlying mechanisms of mitochondrial dysfunction in these cells remain poorly understood. Therefore, we evaluated the molecular mechanism of the aging granulosa cells, including aspects such as accumulation of mitochondrial reactive oxygen species, reduction of mtDNA, imbalance of mitochondrial dynamics, and diminished cell proliferation. Here, we applied bioinformatics approaches, and integrated publicly available resources, to investigate the role of CREB1 gene expression in reproduction. Senescence hallmark enrichment and pathway analysis suggested that the downregulation of bioenergetic-related genes in CREB1. Gene expression analyses showed alterations in genes related to energy metabolism and ROS production in ovary tissue. We also demonstrate that the biogenesis of aging granulosa cells is subject to CREB1 binding to the PRKAA1 and PRKAA2 upstream promoters. In addition, cofactors that regulate biogenesis significantly increase the levels of SIRT1 and PPARGC1A mRNA in the aging granulosa cells. These findings demonstrate that CREB1 elevates an oxidative stress-induced senescence in granulosa cells by reducing the mitochondrial function.

Developmental programming of mitochondrial biology: a conceptual framework and review

Research on mechanisms underlying the phenomenon of developmental programming of health and disease has focused primarily on processes that are specific to cell types, organs and phenotypes of interest. However, the observation that exposure to suboptimal or adverse developmental conditions concomitantly influences a broad range of phenotypes suggests that these exposures may additionally exert effects through cellular mechanisms that are common, or shared, across these different cell and tissue types. It is in this context that we focus on cellular bioenergetics and propose that mitochondria, bioenergetic and signalling organelles, may represent a key cellular target underlying developmental programming. In this review, we discuss empirical findings in animals and humans that suggest that key structural and functional features of mitochondrial biology exhibit developmental plasticity, and are influenced by the same physiological pathways that are implicated in susceptibility for complex, common age-related disorders, and that these targets of mitochondrial developmental programming exhibit long-term temporal stability. We conclude by articulating current knowledge gaps and propose future research directions to bridge these gaps.

The obligate need for accuracy in reporting preclinical studies relevant to clinical trials: autologous germline mitochondrial supplementation for assisted human reproduction as a case study

A now large body of work has solidified the central role that mitochondria play in oocyte development, fertilization, and embryogenesis. From these studies, a new technology termed autologous germline mitochondrial energy transfer was developed for improving pregnancy success rates in assisted reproduction. Unlike prior clinical studies that relied on the use of donor, or nonautologous, mitochondria for microinjection into eggs of women with a history of repeated in vitro fertilization failure to enhance pregnancy success, autologous germline mitochondrial energy transfer uses autologous mitochondria collected from oogonial stem cells of the same woman undergoing the fertility treatment. Initial trials of autologous germline mitochondrial energy transfer during - in vitro fertilization at three different sites with a total of 104 patients indicated a benefit of the procedure for improving pregnancy success rates, with the birth of children conceived through the inclusion of autologous germline mitochondrial energy transfer during in vitro fertilization. However, a fourth clinical study, consisting of 57 patients, failed to show a benefit of autologous germline mitochondrial energy transfer-in vitro fertilization versus in vitro fertilization alone for improving cumulative live birth rates. Complicating this area of work further, a recent mouse study, which claimed to test the long-term safety of autologous mitochondrial supplementation during in vitro fertilization, raised concerns over the use of the procedure for reproduction. However, autologous mitochondria were not actually used for preclinical testing in this mouse study. The unwarranted fears that this new study's erroneous conclusions could cause in women who have become pregnant through the use of autologous germline mitochondrial energy transfer during-in vitro fertilization highlight the critical need for accurate reporting of preclinical work that has immediate bearing on human clinical studies.

Inherent mitochondrial activity influences specification of the germ line in pluripotent stem cells

Herein we investigated whether inherent differences in mitochondrial activity in mouse pluripotent cells could be used to identify populations with an intrinsic ability to differentiate into primordial germ cells (PGCs). Notably, we determined that stem cells sorted based on differences in mitochondrial membrane activity exhibited altered germline differentiation capacity, with low-mitochondrial membrane potential associated with an increase in PGC-like cells. This specification was not further enhanced by hypoxia. We additionally noted differences between these populations in metabolism, transcriptome, and cell-cycle. These data contribute to a growing body of work demonstrating that pluripotent cells exhibit a large range of mitochondrial activity, which impacts cellular function and differentiation potential. Furthermore, pluripotent cells possess a subpopulation of cells with an improved ability to differentiate into the germ lineage that can be identified based on differences in mitochondrial membrane potential.

Reporting on the Role of miRNAs and Affected Pathways on the Molecular Backbone of Ovarian Insufficiency: A Systematic Review and Critical Analysis Mapping of Future Research

Ovarian insufficiency is identified as a perplexing entity in the long list of pathologies impairing fertility dynamics. The three distinct classifications of ovarian insufficiency are poor ovarian response, premature ovarian insufficiency/failure, and advanced maternal age, sharing the common denominator of deteriorated ovarian reserve. Despite efforts to define clear lines among the three, the vast heterogeneity and overlap of clinical characteristics renders their diagnosis and management challenging. Lack of a consensus has prompted an empirically based management coupled by uncertainty from the clinicians' perspective. Profiling of patients in the era of precision medicine seems to be the way forward, while the necessity for a novel approach is underlined. Implicating miRNAs in the quest for patient profiling is promising in light of their fundamental role in cellular and gene expression regulation. To this end, the current study sets out to explore and compare the three pathophysiologies-from a molecular point of view-in order to enable profiling of patients in the context of in vitro fertilization treatment and enrich the data required to practice individualized medicine. Following a systematic investigation of literature, data referring to miRNAs were collected for each patient category based on five included studies. miRNA-target pairs were retrieved from the DIANA-TarBase repository and microT-CDS. Gene and miRNA annotations were derived from Ensembl and miRbase. A subsequent gene-set enrichment analysis of miRNA targets was performed for each category separately. A literature review on the most crucial of the detected pathways was performed to reveal their relevance to fertility deterioration. Results supported that all three pathophysiologies share a common ground regarding the affected pathways, naturally attributed to the common denominator of ovarian insufficiency. As evidenced, miRNAs could be employed to explore the fine lines and diverse nature of pathophysiology since they constitute invaluable biomarkers. Interestingly, it is the differentiation through miRNAs and not through the molecular affected pathways that corresponds to the three distinctive categories. Alarming discrepancies among publications were revealed, pertaining to employment of empirical and arbitrary criteria in categorizing the patients. Following bioinformatic analysis, the final step of the current study consisted of a critical analysis of the molecular data sourced, providing a clear and unique insight into the physiological mechanisms involved. It is our intention to contribute to mapping future research dedicated to ovarian insufficiency and to help researchers navigate the overwhelming information published in molecular studies.

The deteriorating soma and the indispensable germline: gamete senescence and offspring fitness

The idea that there is an impenetrable barrier that separates the germline and soma has shaped much thinking in evolutionary biology and in many other disciplines. However, recent research has revealed that the so-called 'Weismann Barrier' is leaky, and that information is transferred from soma to germline. Moreover, the germline itself is now known to age, and to be influenced by an age-related deterioration of the soma that houses and protects it. This could reduce the likelihood of successful reproduction by old individuals, but also lead to long-term deleterious consequences for any offspring that they do produce (including a shortened lifespan). Here, we review the evidence from a diverse and multidisciplinary literature for senescence in the germline and its consequences; we also examine the underlying mechanisms responsible, emphasizing changes in mutation rate, telomere loss, and impaired mitochondrial function in gametes. We consider the effect on life-history evolution, particularly reproductive scheduling and mate choice. Throughout, we draw attention to unresolved issues, new questions to consider, and areas where more research is needed. We also highlight the need for a more comparative approach that would reveal the diversity of processes that organisms have evolved to slow or halt age-related germline deterioration.

Effect of PPARGC1A on the development and metabolism of early rabbit embryos in vitro

Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PPARGC1A) is a central regulator of mitochondrial biogenesis and metabolism, and its expression is closely related to embryo development. To gain insights into the possible mechanisms of PPARGC1A during early embryogenesis, the development potential, mitochondrial biogenesis, and the culture medium metabolomics of embryos were evaluated when PPARGC1A overexpressed or suppressed in rabbit zygotes. Results showed that different PPARGC1A levels in rabbit zygotes could affect blastocyst percentage, and the expressions of mitochondrial biogenesis and metabolic-related genes, as well as the glutathione and adenosine triphosphate levels during early embryo development. In addition, compared with the controls, 12 and 10 different metabolites involved in carbohydrate, amino acid, and fatty acid metabolism were screened in the 5 day's spent culture medium of PPARGC1A overexpressed and suppressed embryos by gas chromatography-mass spectrometer, respectively. Consistent with these metabolite changes, the transcriptions of genes encoding glucose transporters and fatty acid biosynthetic proteins in the embryos from different groups were regulated by PPARGC1A during rabbit embryo development. Taken together, these data provide evidence that PPARGC1A may regulate early rabbit embryo development through mitochondrial biogenesis and metabolism.

The Effect of Melatonin on Mitochondrial Function and Autophagy in In Vitro Matured Oocytes of Aged Mice

Objective

This study examined the in vitro effect of melatonin on the protein synthesis of mitochondria, as well as autophagy in matured oocytes of aged mice.

Materials and Methods

In this experimental study, germinal vesicles (GV) oocytes were collected from aged (with the age of six-months-old) and young mice (with age range of 6-8 weeks old) and then cultured in the in vitro culture medium (IVM) for 24 hours to each metaphase II (MII) oocytes and then supplemented with melatonin at a concentration of 10 μM. The culture medium of MII oocytes was devoid of melatonin. Afterward, the expression of the SIRT-1 and LC3 was assessed by immunocytochemistry. ATP-dependent luciferin-luciferase bioluminescence assay was employed for the measurement of the ATP contents. Intracellular reactive oxygen specious (ROS) was detected by DCFH-DA, and the total antioxidant capacity (TAC) level was determined by TAC assay.

Results

The expression of SIRT-1 and LC3, as well as the measurement of the ATP content, was significantly increased in oocytes treated with melatonin compared with the oocytes receiving no treatment. Moreover, TAC was considerably higher in melatonin-treated oocytes than oocytes receiving no treatment. On the other hand, the level of ROS was significantly decreased in oocytes treated with melatonin in comparison with the untreated oocytes. The results indicated that melatonin considerably improved the development of oocytes as well.

Conclusion

According to the data, melatonin increased mitochondrial function and autophagy via an increase in the expression of SIRT1 and LC3, as well as the ATP contents while it decreased the levels of ROS and increased TAC in oocytes derived from aged mice.

Mitochondrial Network State Scales mtDNA Genetic Dynamics

Mitochondrial DNA (mtDNA) mutations cause severe congenital diseases but may also be associated with healthy aging. mtDNA is stochastically replicated and degraded, and exists within organelles which undergo dynamic fusion and fission. The role of the resulting mitochondrial networks in the time evolution of the cellular proportion of mutated mtDNA molecules (heteroplasmy), and cell-to-cell variability in heteroplasmy (heteroplasmy variance), remains incompletely understood. Heteroplasmy variance is particularly important since it modulates the number of pathological cells in a tissue. Here, we provide the first wide-reaching theoretical framework which bridges mitochondrial network and genetic states. We show that, under a range of conditions, the (genetic) rate of increase in heteroplasmy variance and de novo mutation are proportionally modulated by the (physical) fraction of unfused mitochondria, independently of the absolute fission-fusion rate. In the context of selective fusion, we show that intermediate fusion:fission ratios are optimal for the clearance of mtDNA mutants. Our findings imply that modulating network state, mitophagy rate, and copy number to slow down heteroplasmy dynamics when mean heteroplasmy is low could have therapeutic advantages for mitochondrial disease and healthy aging.

Maternal ageing impairs mitochondrial DNA kinetics during early embryogenesis in mice

STUDY QUESTION

Does ageing affect the kinetics of the mitochondrial pool during oogenesis and early embryogenesis?

SUMMARY ANSWER

While we found no age-related change during oogenesis, the kinetics of mitochondrial DNA content and the expression of the factors involved in mitochondrial biogenesis appeared to be significantly altered during embryogenesis.

WHAT IS KNOWN ALREADY

Oocyte mitochondria are necessary for embryonic development. The morphological and functional alterations of mitochondria, as well as the qualitative and quantitative mtDNA anomalies, observed during ovarian ageing may be responsible for the alteration of oocyte competence and embryonic development.

STUDY DESIGN, SIZE, DURATION

The study, conducted from November 2016 to November 2017, used 40 mice aged 5–8 weeks and 45 mice aged 9–11 months (C57Bl6/CBA F(1)). A total of 488 immature oocytes, with a diameter ranging from 20 μm to more than 80 μm, were collected from ovaries, and 1088 mature oocytes or embryos at different developmental stages (two PN, one-cell, i.e. syngamy, two-cell, four-cell, eight-cell, morula and blastocyst) were obtained after ovarian stimulation and, for embryos, mating.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Mitochondrial DNA was quantified by quantitative PCR. We used quantitative reverse transcriptase PCR (RT-PCR) (microfluidic method) to study the relative expression of three genes involved in the key steps of embryogenesis, i.e. embryonic genome activation (HSPA1) and differentiation (CDX2 and NANOG), two mtDNA genes (CYB and ND2) and five genes essential for mitochondrial biogenesis (PPARGC1A, NRF1, POLG, TFAM and PRKAA). The statistical analysis was based on mixed linear regression models applying a logistic link function (STATA v13.1 software), with values of P < 0.05 being considered significant.

MAIN RESULTS AND THE ROLE OF CHANCE

During oogenesis, there was a significant increase in oocyte mtDNA content (P < 0.0001) without any difference between the two groups of mice (P = 0.73). During the first phase of embryogenesis, i.e. up to the two-cell stage, embryonic mtDNA decreased significantly in the aged mice (P < 0.0001), whereas it was stable for young mice (young/old difference P = 0.015). The second phase of embryogenesis, i.e. between the two-cell and eight-cell stages, was characterized by a decrease in embryonic mtDNA for young mice (P = 0.013) only (young/old difference P = 0.038). During the third phase, i.e. between the eight-cell and blastocyst stage, there was a significant increase in embryonic mtDNA content in young mice (P < 0.0001) but not found in aged mice (young/old difference P = 0.002). We also noted a faster expression of CDX2 and NANOG in the aged mice than in the young mice during the second (P = 0.007 and P = 0.02, respectively) and the third phase (P = 0.01 and P = 0.008, respectively) of embryogenesis. The expression of mitochondrial genes CYB and ND2 followed similar kinetics and was equivalent for both groups of mice, with a significant increase during the third phase (P < 0.01). Of the five genes involved in mitochondrial biogenesis, i.e. PPARGC1A, NRF1, POLG, TFAM and PRKAA, the expression of three genes decreased significantly during the first phase only in young mice (NRF1, P = 0.018; POLGA, P = 0.002; PRKAA, P = 0.010), with no subsequent difference compared to old mice. In conclusion, during early embryogenesis in the old mice, we suspect that the lack of a replicatory burst before the two-cell stage, associated with the early arrival at the minimum threshold value of mtDNA, together with the absence of an increase of mtDNA during the last phase, might potentially deregulate the key stages of early embryogenesis.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

Because of the ethical impossibility of working on a human, this study was conducted only on a murine model. As superovulation was used, we cannot totally exclude that the differences observed were, at least partially, influenced by differences in ovarian response between young and old mice.

WIDER IMPLICATIONS OF THE FINDINGS

Our findings suggest a pathophysiological explanation for the link observed between mitochondria and the deterioration of oocyte quality and early embryonic development with age.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by the University of Angers, France, by the French national research centres INSERM and the CNRS and, in part, by PHASE Division, INRA. There are no competing interests.

Female Fertility Preservation through Stem Cell-based Ovarian Tissue Reconstitution In Vitro and Ovarian Regeneration In Vivo

Historically, approaches designed to offer women diagnosed with cancer the prospects of having a genetically matched child after completion of their cytotoxic treatments focused on the existing oocyte population as the sole resource available for clinical management of infertility. In this regard, elective oocyte and embryo cryopreservation, as well as autologous ovarian cortical tissue grafting posttreatment, have gained widespread support as options for young girls and reproductive-age women who are faced with cancer to consider. In addition, the use of ovarian protective therapies, including gonadotropin-releasing hormone agonists and sphingosine-1-phosphate analogs, has been put forth as an alternative way to preserve fertility by shielding existing oocytes in the ovaries in vivo from the side-effect damage caused by radiotherapy and many chemotherapeutic regimens. This viewpoint changed with the publication of now numerous reports that adult ovaries of many mammalian species, including humans, contain a rare population of oocyte-producing germ cells-referred to as female germline or oogonial stem cells (OSCs). This new line of study has fueled research into the prospects of generating new oocytes, rather than working with existing oocytes, as a novel approach to sustain or restore fertility in female cancer survivors. Here, we overview the history of work from laboratories around the world focused on improving our understanding of the biology of OSCs and how these cells may be used to reconstitute "artificial" ovarian tissue in vitro or to regenerate damaged ovarian tissue in vivo as future fertility-preservation options.

Maternal Protein Restriction in Two Successive Generations Impairs Mitochondrial Electron Coupling in the Progeny's Brainstem of Wistar Rats From Both Sexes

Maternal protein deficiency during the critical development period of the progeny disturbs mitochondrial metabolism in the brainstem, which increases the risk of developing cardiovascular diseases in the first-generation (F1) offspring, but is unknown if this effect persists in the second-generation (F2) offspring. The study tested whether mitochondrial health and oxidative balance will be restored in F2 rats. Male and female rats were divided into six groups according to the diet fed to their mothers throughout gestation and lactation periods. These groups were: (1) normoprotein (NP) and (2) low-protein (LP) rats of the first filial generation (F1-NP and F1-LP, respectively) and (3) NP and (4) LP rats of the second filial generation (F2-NP and F2-LP, respectively). After weaning, all groups received commercial chow and a portion of each group was sacrificed on the 30th day of life for determination of mitochondrial and oxidative parameters. The remaining portion of the F1 group was mated at adulthood and fed an NP or LP diet during the periods of gestation and lactation, to produce progeny belonging to (5) F2R-NP and (6) F2R-LP group, respectively. Our results demonstrated that male F1-LP rats suffered mitochondrial impairment associated with an 89% higher production of reactive species (RS) and 137% higher oxidative stress biomarkers, but that the oxidative stress was blunted in female F1-LP animals despite the antioxidant impairment. In the second generation following F0 malnutrition, brainstem antioxidant defenses were restored in the F2-LP group of both sexes. However, F2R-LP offspring, exposed to LP in the diets of the two preceding generations displayed a RS overproduction with a concomitant decrease in mitochondrial bioenergetics. Our findings demonstrate that nutritional stress during the reproductive life of the mother can negatively affect mitochondrial metabolism and oxidative balance in the brainstem of F1 progeny, but that restoration of a normal diet during the reproductive life of those individuals leads toward a mitochondrial recovery in their own (F2) progeny. Otherwise, if protein deprivation is continued from the F0 generation and into the F1 generation, the F2 progeny will exhibit no recovery, but instead will remain vulnerable to further oxidative damage.

Implications and Current Limitations of Oogenesis from Female Germline or Oogonial Stem Cells in Adult Mammalian Ovaries

A now large body of evidence supports the existence of mitotically active germ cells in postnatal ovaries of diverse mammalian species, including humans. This opens the possibility that adult stem cells naturally committed to a germline fate could be leveraged for the production of female gametes outside of the body. The functional properties of these cells, referred to as female germline or oogonial stem cells (OSCs), in ovaries of women have recently been tested in various ways, including a very recent investigation of the differentiation capacity of human OSCs at a single cell level. The exciting insights gained from these experiments, coupled with other data derived from intraovarian transplantation and genetic tracing analyses in animal models that have established the capacity of OSCs to generate healthy eggs, embryos and offspring, should drive constructive discussions in this relatively new field to further exploring the value of these cells to the study, and potential management, of human female fertility. Here, we provide a brief history of the discovery and characterization of OSCs in mammals, as well as of the in-vivo significance of postnatal oogenesis to adult ovarian function. We then highlight several key observations made recently on the biology of OSCs, and integrate this information into a broader discussion of the potential value and limitations of these adult stem cells to achieving a greater understanding of human female gametogenesis in vivo and in vitro.

Updating the markers for oocyte quality evaluation: intracellular temperature as a new index

BACKGROUND

The developmental competence of an embryo is principally dictated by the oocyte. Usually, oocyte selection is based on morphological properties; however, all morphological criteria that are currently used for the grading and screening of oocytes are not able to eliminate the subjectivity. Despite recent studies of the molecular factors related to oocyte quality, it is technically difficult to develop an index based on these factors, and new indices that reflect intracellular conditions are necessary.

METHODS

Morphological and molecular factors influencing developmental competence were comprehensively reviewed, and intracellular temperature was evaluated as a new marker of oocyte quality.

MAIN FINDINGS

The intracellular temperature of mature oocytes was high in fresh oocytes and decreased with time after polar body release. Under the same conditions, the intracellular temperature and its distribution differed among oocytes, suggesting that temperature represents the state of each oocyte.

CONCLUSION

Intracellular temperature is advantageous as an objective and quantitative indicator of oocyte quality. Further studies should evaluate the link between temperature and cellular phenomena to establish its use as an indicator of quality.

Calorie restriction does not influence oocyte quality in oocytes from POLG mitochondrial mutator mice

It has recently been demonstrated that moderate adult onset caloric restriction (e.g. calorie restriction; CR) has a positive impact on female fertility in aged mice, due in large to preventing the age-associated decline in the quality of oocytes available for fertilization. The impact of CR on oocyte quality has been attributed, at least in part, to mitochondrial functions. In mitochondrial DNA (mtDNA) mutator mice (Polg^D257A/D257A), which harbor a mutation in the proofreading mtDNA polymerase-gamma (POLG), mitochondrial mutations rapidly accumulate, resulting in a premature aging phenotype and female infertility. As CR has been shown to extend both lifespan and ‘healthspan’ as well as improve oocyte quality in aged mice, we investigated whether adult onset CR could improve oocyte quality in the POLG mouse. Female Polg^D257A/D257A mice exhibited infertility based on an inability to produce litters through natural mating. Analysis of oocytes from 8–9-month-old Polg^D257A/D257A mice on CR following hormone stimulation revealed no improvement in the number of oocytes ovulated. Furthermore, CR did not result in a greater percentage of metaphase II oocytes, with the majority of the oocytes prematurely arrested at the germinal vesicle stage. Finally, CR did not improve the abnormal mitochondrial distribution or pronounced defects in meiotic spindle assembly and chromosomal distribution observed in the ad libitum fed Polg^D257A/D257A. Taken together, these data suggest that although CR benefits oocyte quality and fertility outcomes in naturally aged female mice, it does not sufficiently improve oocyte quality in Polg^D257A/D257A.