Dual roles for ubiquitination in the processing of sperm organelles after fertilization

Semen proteome and transcriptome of the endangered black-footed ferret (Mustela nigripes) show association with the environment and fertility outcome

The ex situ population of the endangered black-footed ferret (Mustela nigripes) has been experiencing declines in reproductive success over the past 30 years of human-managed care. A potential cause may be environmental-dependent inbreeding depression with diet being one of the contributing factors since ferrets are not fed their natural diet of prairie dogs. Here, we generated and analyzed semen proteome and transcriptome data from both wild and ex situ ferrets maintained on various diets. We identified 1757 proteins across all samples, with 149 proteins unique to the semen of wild ferrets and forming a ribosomal predicted protein-protein interaction cluster. Wild ferrets also differed from ex situ ferrets in their transcriptomic profile, showing enrichment in ribosomal RNA processing and potassium ion transport. Successful fertility outcomes documented for ex situ ferrets showed the strongest association with the semen transcriptome, with enrichment in genes involved in translation initiation and focal adhesion. Fertility also synergized with the effect of diet on differentially expressed transcriptomes, mainly affecting genes enriched in mitochondrial function. Our data and functional networks are important for understanding the causes and mechanisms of declining fertility in the ex situ ferret population and can be used as a resource for future conservation efforts.

An antibody free approach to probe the presence of poly-ubiquitin chains on C. elegans sperm derived organelles after fertilization

Upon C. elegans 's oocyte fertilization, the sperm brings mitochondria and membranous organelles (MOs) which are rapidly eliminated by autophagy. Their poly-ubiquitylation is suspected to be a signal for their recognition and degradation but mitochondria poly-ubiquitylation remains debated. Using fluorescent Tandem-repeated Ubiquitin-Binding Entities (TUBEs) we confirmed the presence of K48- and K63-ubiquitin chains on MOs contrasting with the absence of signal on sperm mitochondria. This new and sensitive approach confirmed the poly-ubiquitylation of the MOs while providing additional arguments for the absence of substantial poly-ubiquitylation of sperm-derived mitochondria, suggesting that K63- and K48-poly-ubiquitylation are unlikely acting as a common targeting signal for their degradation.

Melatonin enhances the developmental potential of immature oocytes from older reproductive-aged women by improving mitochondrial function

Aims

To evaluate whether melatonin (MT) supplementation during in vitro maturation (IVM) of human oocytes can reverse the age-related decline in oocyte quality.

Main methods

We enrolled 172 patients aged ≥35 years (older reproductive-aged women) and 83 patients aged <35 years (young women) who underwent in vitro fertilization between 2019 and 2022. We conducted IVM with and without 10 μM MT in immature oocytes of different ages. Oocyte fertilization and embryo development were observed using a stereomicroscope. We assessed the immunofluorescence intensity of mitochondrial function, measured the copy number of mitochondrial DNA (mtDNA), and examined the spindle and chromosome composition in in vitro mature stage II (IVM-MII) oocytes using immunofluorescence and second-generation sequencing.

Key findings

MT supplementation significantly improved the redox level in the IVM medium and IVM-MII oocytes in older reproductive-aged women. It also significantly increased the proportion of circular mtDNA and the adenosine triphosphate content in IVM-MII oocytes. In addition, the IVM-MII oocytes obtained with MT supplementation showed a significant improvement in the normal composition of the spindle and chromosomes. Thus, the aged immature oocytes also showed significantly improved maturation and blastocyst formation rates owing to the role of MT.

Significance

Supplementation with 10 μM MT in the IVM medium reverses the age-related decline in oocyte quality. Our findings provide a viable solution for enhancing fertility in older reproductive-aged women.

STRIPAK regulation of katanin microtubule severing in the Caenorhabditis elegans embryo

Microtubule severing plays important role in cell structure and cell division. The microtubule severing protein katanin, composed of the MEI-1/MEI-2 subunits in Caenorhabditis elegans, is required for oocyte meiotic spindle formation; however, it must be inactivated for mitosis to proceed as continued katanin expression is lethal. Katanin activity is regulated by 2 ubiquitin-based protein degradation pathways. Another ubiquitin ligase, HECD-1, the homolog of human HECTD1/HECT domain E3 ubiquitin protein ligase 1, regulates katanin activity without affecting katanin levels. In other organisms, HECD-1 is a component of the striatin-interacting kinase phosphatase complex, which affects cell proliferation and a variety of signaling pathways. Here we conducted a systematic screen of how mutations in striatin-interacting kinase phosphatase components affect katanin function in C. elegans. Striatin-interacting kinase phosphatase core components (FARL-11, CASH-1, LET-92, and GCK-1) were katanin inhibitors in mitosis and activators in meiosis, much like HECD-1. By contrast, variable components (SLMP-1, OTUB-2) functioned as activators of katanin activity in mitosis, indicating they may function to alter striatin-interacting kinase phosphatase core function. The core component CCM-3 acted as an inhibitor at both divisions, while other components (MOB-4, C49H3.6) showed weak interactions with katanin mutants. Additional experiments indicate that katanin may be involved with the centralspindlin complex and a tubulin chaperone. HECD-1 shows ubiquitous expression in the cytoplasm throughout meiosis and early development. The differing functions of the different subunits could contribute to the diverse functions of the striatin-interacting kinase phosphatase complex in C. elegans and other organisms.

Autophagy: a multifaceted player in the fate of sperm

BACKGROUND

Autophagy is an intracellular catabolic process of degrading and recycling proteins and organelles to modulate various physiological and pathological events, including cell differentiation and development. Emerging data indicate that autophagy is closely associated with male reproduction, especially the biosynthetic and catabolic processes of sperm. Throughout the fate of sperm, a series of highly specialized cellular events occur, involving pre-testicular, testicular and post-testicular events. Nonetheless, the most fundamental question of whether autophagy plays a protective or harmful role in male reproduction, especially in sperm, remains unclear.

OBJECTIVE AND RATIONALE

We summarize the functional roles of autophagy in the pre-testicular (hypothalamic–pituitary–testis (HPG) axis), testicular (spermatocytogenesis, spermatidogenesis, spermiogenesis, spermiation) and post-testicular (sperm maturation and fertilization) processes according to the timeline of sperm fate. Additionally, critical mechanisms of the action and clinical impacts of autophagy on sperm are identified, laying the foundation for the treatment of male infertility.

SEARCH METHODS

In this narrative review, the PubMed database was used to search peer-reviewed publications for summarizing the functional roles of autophagy in the fate of sperm using the following terms: ‘autophagy’, ‘sperm’, ‘hypothalamic–pituitary–testis axis’, ‘spermatogenesis’, ‘spermatocytogenesis’, ‘spermatidogenesis’, ‘spermiogenesis’, ‘spermiation’, ‘sperm maturation’, ‘fertilization’, ‘capacitation’ and ‘acrosome’ in combination with autophagy-related proteins. We also performed a bibliographic search for the clinical impact of the autophagy process using the keywords of autophagy inhibitors such as ‘bafilomycin A1’, ‘chloroquine’, ‘hydroxychloroquine’, ‘3-Methyl Adenine (3-MA)’, ‘lucanthone’, ‘wortmannin’ and autophagy activators such as ‘rapamycin’, ‘perifosine’, ‘metformin’ in combination with ‘disease’, ‘treatment’, ‘therapy’, ‘male infertility’ and equivalent terms. In addition, reference lists of primary and review articles were reviewed for additional relevant publications. All relevant publications until August 2021 were critically evaluated and discussed on the basis of relevance, quality and timelines.

OUTCOMES

(i) In pre-testicular processes, autophagy-related genes are involved in the regulation of the HPG axis; and (ii) in testicular processes, mTORC1, the main gate to autophagy, is crucial for spermatogonia stem cell (SCCs) proliferation, differentiation, meiotic progression, inactivation of sex chromosomes and spermiogenesis. During spermatidogenesis, autophagy maintains haploid round spermatid chromatoid body homeostasis for differentiation. During spermiogenesis, autophagy participates in acrosome biogenesis, flagella assembly, head shaping and the removal of cytoplasm from elongating spermatid. After spermatogenesis, through PDLIM1, autophagy orchestrates apical ectoplasmic specialization and basal ectoplasmic specialization to handle cytoskeleton assembly, governing spermatid movement and release during spermiation. In post-testicular processes, there is no direct evidence that autophagy participates in the process of capacitation. However, autophagy modulates the acrosome reaction, paternal mitochondria elimination and clearance of membranous organelles during fertilization.

WIDER IMPLICATIONS

Deciphering the roles of autophagy in the entire fate of sperm will provide valuable insights into therapies for diseases, especially male infertility.

Semen evaluation: methodological advancements in sperm quality-specific fertility assessment - A review

Assessment of male fertility is based on the evaluation of sperm. Semen evaluation measures various sperm quality parameters as fertility indicators. However, semen evaluation has limitations, and it requires the advancement and application of strict quality control methods to interpret the results. This article reviews the recent advances in evaluating various sperm-specific quality characteristics and methodologies, with the help of different assays to assess sperm-fertility status. Sperm evaluation methods that include conventional microscopic methods, computer-assisted sperm analyzers (CASA), and flow cytometric analysis, provide precise information related to sperm morphology and function. Moreover, profiling fertility-related biomarkers in sperm or seminal plasma can be helpful in predicting fertility. Identification of different sperm proteins and diagnosis of DNA damage has positively contributed to the existing pool of knowledge about sperm physiology and molecular anomalies associated with different infertility issues in males. Advances in methods and sperm-specific evaluation has subsequently resulted in a better understanding of sperm biology that has improved the diagnosis and clinical management of male factor infertility. Accurate sperm evaluation is of paramount importance in the application of artificial insemination and assisted reproductive technology. However, no single test can precisely determine fertility; the selection of an appropriate test or a set of tests and parameters is required to accurately determine the fertility of specific animal species. Therefore, a need to further calibrate the CASA and advance the gene expression tests is recommended for faster and field-level applications.

The Immp2l Mutation Causes Ovarian Aging Through ROS-Wnt/β-Catenin-Estrogen Pathway: Preventive Effect of Melatonin

Mitochondria play important roles in ovarian follicle development. Mitochondrial dysfunction, including mitochondrial gene deficiency, impairs ovarian development. Here, we explored the role and mechanism of mitochondrial inner membrane gene Immp2l in ovarian follicle growth and development. Our results revealed that female Immp2l-/- mice were infertile, whereas Immp2l+/- mice were normal. Body and ovarian weights were reduced in the female Immp2l-/- mice, ovarian follicle growth and development were stunted in the secondary follicle stage. Although a few ovarian follicles were ovulated, the oocytes were not fertilized because of mitochondrial dysfunction. Increased oxidative stress, decreased estrogen levels, and altered genes expression of Wnt/β-catenin and steroid hormone synthesis pathways were observed in 28-day-old Immp2l-/- mice. The Immp2l mutation accelerated ovarian aging process, as no ovarian follicles were detected by age 5 months in Immp2l-/- mice. All the aforementioned changes in the Immp2l-/- mice were reversed by administration of antioxidant melatonin to the Immp2l-/- mice. Furthermore, our in vitro study using Immp2l knockdown granulosa cells confirmed that the Immp2l downregulation induced granulosa cell aging by enhancing reactive oxygen species (ROS) levels, suppressing Wnt16, increasing β-catenin, and decreasing steroid hormone synthesis gene cyp19a1 and estrogen levels, accompanied by an increase in the aging phenotype of granulosa cells. Melatonin treatment delayed granulosa cell aging progression. Taken together, Immp2l causes ovarian aging through the ROS-Wnt/β-catenin-estrogen (cyp19a1) pathway, which can be reversed by melatonin treatment.

The neglected part of early embryonic development: maternal protein degradation

The degradation of maternally provided molecules is a very important process during early embryogenesis. However, the vast majority of studies deals with mRNA degradation and protein degradation is only a very little explored process yet. The aim of this article was to summarize current knowledge about the protein degradation during embryogenesis of mammals. In addition to resuming of known data concerning mammalian embryogenesis, we tried to fill the gaps in knowledge by comparison with facts known about protein degradation in early embryos of non-mammalian species. Maternal protein degradation seems to be driven by very strict rules in terms of specificity and timing. The degradation of some maternal proteins is certainly necessary for the normal course of embryonic genome activation (EGA) and several concrete proteins that need to be degraded before major EGA have been already found. Nevertheless, the most important period seems to take place even before preimplantation development-during oocyte maturation. The defects arisen during this period seems to be later irreparable.

Sperm Mitochondrial DNA Copy Number Is Not a Predictor of Intracytoplasmic Sperm Injection (ICSI) Cycle Outcomes

This study is to determine if sperm mitochondrial DNA copy number (mtDNA CN) is associated with fertilization, blastulation, blastocyst euploidy, and live birth rates in in vitro fertilization (IVF) with ICSI cycles. This is a cohort study conducted on stored sperm samples which were collected prospectively and used to create blastocysts transferred in a couple's first ICSI transfer cycle between 2007 and 2013 at a single large infertility center. Samples from ICSI cycles utilizing surgical or cryopreserved sperm or day 3 embryo biopsy were excluded. The primary outcome was live birth rate. Secondary outcomes included fertilization, usable blastocyst development, and blastocyst euploidy rates. Unique sperm samples used to create transferred embryos were identified. Mitochondrial DNA CN was evaluated using TaqMan® quantitative real-time polymerase chain reaction (qPCR) assays normalized to a nuclear control for relative quantitation. Linear regression and mixed effects logistic regression used were appropriate. A total of 2062 unique sperm samples used to create transferred embryos were included. Lower relative sperm mtDNA content was associated with increased pre-wash sperm motility (p < 0.001). No significant association was identified between sperm mtDNA CN and fertilization (p = 0.40), usable blastocyst development (p = 0.36), blastocyst euploid (p = 0.10), and live birth rates (p = 0.42) while adjusting for sperm pre-wash motility and maternal age. Sperm mtDNA CN is not prognostic of fertilization, usable blastocyst development, euploidy and live birth rates in an infertile population undergoing IVF with ICSI.

New Frontiers in IVF: mtDNA and autologous germline mitochondrial energy transfer

Many infertility specialists support the existence of a relationship between the levels of mitochondrial DNA and the quality of the blastocysts. Despite the extensive use of pre-implantation genetic testing for aneuploidy, a significant percentage of euploid embryos do not implant even though the endometrium is normal. Mitochondrial DNA may be used as a new test in evaluating embryonic vitality.Ovarian aging leads to a decrease in the quantity and quality of oocytes and aged oocytes have a reduced number of mitochondria. Mitochondria are the energy factories of the cells and their lacked could leads to lower fertilization rates and poor embryonic development. Various strategies have been tested to increase the mitochondria quantity and thus improve the quality of oocytes used in in vitro fertilization. Results of ovarian rejuvenation techniques such as autologous mitochondrial transplantation have been controversial. In this review, we describe the state of the art concerning the use of mitochondrial DNA and autologous mitochondrial transplantation as new possibilities to increase success in vitro fertilization.

Fndc-1 contributes to paternal mitochondria elimination in C. elegans

Paternal mitochondria are eliminated following fertilization by selective autophagy, but the mechanisms that restrict this process to sperm-derived organelles are not well understood. FUNDC1 (FUN14 domain containing 1) is a mammalian mitophagy receptor expressed on the mitochondrial outer membrane that contributes to mitochondrial quality control following hypoxic stress. Like FUNDC1, the C. elegans ortholog FNDC-1 is widely expressed in somatic tissues and mediates hypoxic mitophagy. Here, we report that FNDC-1 is strongly expressed in sperm but not oocytes and contributes to paternal mitochondria elimination. Paternal mitochondrial DNA is normally undetectable in wildtype larva, but can be detected in the cross-progeny of fndc-1 mutant males. Moreover, loss of fndc-1 retards the rate of paternal mitochondria degradation, but not that of membranous organelles, a nematode specific membrane compartment whose fusion is required for sperm motility. This is the first example of a ubiquitin-independent mitophagy receptor playing a role in the selective degradation of sperm mitochondria.

Ubiquitination is required for the initial removal of paternal organelles in C. elegans

Elimination of paternal mitochondria after fertilization occurs in many species using the process of selective autophagy. The mechanism for targeting paternal mitochondria, but not maternal mitochondria, for elimination in the early embryo is not well understood. The results in this paper suggest that there are at least two different mechanisms for targeting paternal mitochondria for elimination: the first involving ubiquitination and a second involving a mitochondrial associated autophagy receptor, fndc-1. Elimination of paternal mitochondria can be visualized in embryos of the nematode, C. elegans. Paternal mitochondria enter the zygote at fertilization. Initially, they are closely associated with another sperm organelle, the membraneous organelle (MO). The MOs become ubiquitinated within minutes after fertilization. Simultaneous RNAi knockdown of two ubiquitin conjugating enzymes, ubc-18 and ubc-16, reduces MO ubiquitination. Loss of function of ubc-18 alone leads to loss of K48-linked polyubiquitin chains and halts the recruitment of proteasome to MOs. Interestingly, knockdown of ubc-18 or ubc-16 or the combination does not reduce the localization of K63-linked ubiquitin chains to MOs suggesting that some ubiquitin structure other than K63 chains is responsible for recruiting the autophagy machinery to MOs. Double knockdown (ubc-18/ubc-16) inhibits the recruitment of the autophagy protein, LGG-1 (homolog of LC3/GABARAP), to paternal organelles and causes the persistence of paternal mitochondria into the two cell stage. If paternal mitochondria are not eliminated via this early process, they are eventually removed from the embryo in a process that depends on the mitophagy adaptor protein, fndc-1. Thus, there are two redundant, but temporally distinct mechanisms that target paternal mitochondria for elimination in C. elegans. In addition to the involvement of ubiquitination in the elimination of paternal mitochondria, two subunits of the proteasome, rpn-10 and rad-23, are required for elimination of paternal mitochondria. These subunits are known to function as ubiquitin receptors and knockdown of either inhibits the recruitment of proteasome to ubiquitinated MOs. Their knockdown does not affect the localization of LGG-1 to paternal structures indicating that the proteasome is not required for autophagy membrane recruitment but might be involved in autophagosome maturation or its fusion with the lysosome.

Mitochondrial DNA as a biomarker for in-vitro fertilization outcome

PURPOSE OF REVIEW

This article discusses the use of mitochondrial DNA (mtDNA) copy number as a potential biomarker for embryo viability in assisted reproduction.

RECENT FINDINGS

Mitochondria have a well-established role in regulating embryo energy metabolism, and their efficiency has significant implications for reproductive success. Two recent studies suggested that elevated mtDNA copy number is associated with decreased implantation potential in human embryos generated by IVF. In the first study, Fragouli et al. reported that blastocysts that are aneuploid and those obtained from older reproductive age women have a higher mtDNA copy number. In addition, euploid blastocysts that failed to implant had a higher mtDNA copy number; and pregnancy did not occur when mtDNA copy number was above a threshold. In a subsequent study, Diez-Juan et al. found that mtDNA copy number inversely correlates with implantation potential of euploid embryos, not only for blastocysts but also for cleavage stage embryos. Instead of a threshold model, they proposed a score for embryos based on mtDNA copy number, which would be indicative of implantation potential. Unlike the previous study, Diez-Juan et al. did not find an age-associated decrease in mtDNA copy number in day 3 or day 5 embryos.

SUMMARY

Recent reports suggest that mtDNA copy number may be used as a biomarker for embryo viability. Further studies are necessary to determine whether mtDNA copy number constitutes a parameter independent of morphology and preimplantation genetic screening and whether its use may result in higher IVF pregnancy rates.

C. elegans SUP-46, an HNRNPM family RNA-binding protein that prevents paternally-mediated epigenetic sterility

BACKGROUND

In addition to DNA, gametes contribute epigenetic information in the form of histones and non-coding RNA. Epigenetic programs often respond to stressful environmental conditions and provide a heritable history of ancestral stress that allows for adaptation and propagation of the species. In the nematode C. elegans, defective epigenetic transmission often manifests as progressive germline mortality. We previously isolated sup-46 in a screen for suppressors of the hexosamine pathway gene mutant, gna-2(qa705). In this study, we examine the role of SUP-46 in stress resistance and progressive germline mortality.

RESULTS

We identified SUP-46 as an HNRNPM family RNA-binding protein, and uncovered a highly novel role for SUP-46 in preventing paternally-mediated progressive germline mortality following mating. Proximity biotinylation profiling of human homologs (HNRNPM, MYEF2) identified proteins of ribonucleoprotein complexes previously shown to contain non-coding RNA. Like HNRNPM and MYEF2, SUP-46 was associated with multiple RNA granules, including stress granules, and also formed granules on active chromatin. SUP-46 depletion disrupted germ RNA granules and caused ectopic sperm, increased sperm transcripts, and chronic heat stress sensitivity. SUP-46 was also required for resistance to acute heat stress, and a conserved "MYEF2" motif was identified that was needed for stress resistance.

CONCLUSIONS

In mammals, non-coding RNA from the sperm of stressed males has been shown to recapitulate paternal stress phenotypes in the offspring. Our results suggest that HNRNPM family proteins enable stress resistance and paternally-mediated epigenetic transmission that may be conserved across species.

F-actin prevents interaction between sperm DNA and the oocyte meiotic spindle in C. elegans

After fertilization, interactions between sperm and egg DNA must be prevented before the completion of female meiosis. Panzica et al. show that cortical tethering by F-actin prevents contact between the paternal DNA and the meiotic spindle.

Fertilization occurs during female meiosis in most animals, which raises the question of what prevents the sperm DNA from interacting with the meiotic spindle. In this study, we find that Caenorhabditis elegans sperm DNA stays in a fixed position at the opposite end of the embryo from the meiotic spindle while yolk granules are transported throughout the embryo by kinesin-1. In the absence of F-actin, the sperm DNA, centrioles, and organelles were transported as a unit with the yolk granules, resulting in sperm DNA within 2 µm of the meiotic spindle. F-actin imaging revealed a cytoplasmic meshwork that might restrict transport in a size-dependent manner. However, increasing yolk granule size did not slow their velocity, and the F-actin moved with the yolk granules. Instead, sperm contents connect to the cortical F-actin to prevent interaction with the meiotic spindle.

Stress induced nuclear granules form in response to accumulation of misfolded proteins in Caenorhabditis elegans

Background

Environmental stress can affect the viability or fecundity of an organism. Environmental stressors may affect the genome or the proteome and can cause cellular distress by contributing to protein damage or misfolding. This study examines the cellular response to environmental stress in the germline of the nematode, C. elegans.

Results

Salt stress, oxidative stress, and starvation, but not heat shock, induce the relocalization of ubiquitin, proteasome, and the TIAR-2 protein into distinct subnuclear regions referred to as stress induced nuclear granules (SINGs). The SINGs form within 1 h of stress initiation and do not require intertissue signaling. K48-linked polyubiquitin chains but not K63 chains are enriched in SINGs. Worms with a mutation in the conjugating enzyme, ubc-18, do not form SINGs. Additionally, knockdown of ubc-20 and ubc-22 reduces the level of SING formation as does knockdown of the ubiquitin ligase chn-1, a CHIP homolog. The nuclear import machinery is required for SING formation. Stressed embryos containing SINGs fail to hatch and cell division in these embryos is halted. The formation of SINGs can be prevented by pre-exposure to a brief period of heat shock before stress exposure. Heat shock inhibition of SINGs is dependent upon the HSF-1 transcription factor.

Conclusions

The heat shock results suggest that chaperone expression can prevent SING formation and that the accumulation of damaged or misfolded proteins is a necessary precursor to SING formation. Thus, SINGs may be part of a novel protein quality control system. The data suggest an interesting model where SINGs represent sites of localized protein degradation for nuclear or cytosolic proteins. Thus, the physiological impacts of environmental stress may begin at the cellular level with the formation of stress induced nuclear granules.

Electronic supplementary material

The online version of this article (doi:10.1186/s12860-017-0136-x) contains supplementary material, which is available to authorized users.

Inheritance: Male mtDNA Just Can't Catch a Break

Mitochondrial DNA (mtDNA) is actively eliminated from the developing sperm in Drosophila. New work shows that the mitochondrial DNA polymerase, which normally replicates mtDNA, plays a surprising role in mtDNA elimination.

Age-associated events in bovine oocytes and possible countermeasures

Maternal aging profoundly affects oocyte quality. This has become common knowledge in industrialized countries and extensive studies addressing the causes and possible countermeasures against age-associated deterioration of oocytes suggest that mitochondrial dysfunction is a causal factor in infertility. However, almost all studies addressing age-associated events in oocytes have used mice as an animal model, and the reproductive life of mice is very short, making it difficult to study the gradual decline in fertility observed in humans. In the present review, age-associated changes in the quality and quantity of bovine oocytes and possible countermeasures related to mitochondrial quality control are introduced.

Oocyte mitochondrial function and reproduction

PURPOSE OF REVIEW

Mitochondria are cellular organelles that are required for energy production. Emerging evidence demonstrates their role in oocyte development and reproduction. In this review, we examine recent animal and clinical studies on the role of mitochondria in fertility. We also analyse the impact of assisted reproductive techniques (ARTs) on mitochondrial function and discuss the future clinical implications of mitochondrial nutrients and mitochondrial replacement.

RECENT FINDINGS

Mitochondria affect all aspects of mammalian reproduction. They are essential for optimal oocyte maturation, fertilization and embryonic development. Mitochondrial dysfunction causes a decrease in oocyte quality and interferes with embryonic development. ART procedures affect mitochondrial function, while mitochondrial nutrients may increase mitochondrial performance in oocytes. New mitochondrial replacement procedures using mitochondria obtained from polar bodies or from the patient's own oogonial stem cells are promising and may address concerns related to the induction of high-levels of heteroplasmy, which could potentially result in negative long-term health effects.

SUMMARY

Optimal energy production is required for oocyte and embryo development, and mitochondrial abnormalities have devastating reproductive consequences. Improvement of oocyte mitochondrial function via intake of compounds that boost mitochondrial activity may have clinical benefits, and mitochondrial replacement could potentially be used for the prevention of mitochondrial diseases.

A protein binding site in the M mitochondrial genome of Mytilus galloprovincialis may be responsible for its paternal transmission

Sea mussels (genus Mytilus) have two mitochondrial genomes in obligatory co-existence, one that is transmitted through the egg and the other through the sperm. The phenomenon, known as Doubly Uniparental Inheritance (DUI) of mitochondrial DNA (mtDNA), is presently known to occur in more than 40 molluscan bivalve species. Females and the somatic tissues of males contain mainly the maternal (F) genome. In contrast, the sperm contains only the paternal (M) genome. Through electrophoretic mobility shift assay (EMSA) experiments we have identified a sequence element in the control region (CR) of the M genome that acts as a binding site for the formation of a complex with a protein factor that occurs in the male gonad. An adenine tract upstream to the element is also essential for the formation of the complex. The reaction is highly specific. It does not occur with protein extracts from the female gonad or from a male or female somatic tissue. Further experiments showed that the interaction takes place in mitochondria surrounding the nucleus of the cells of male gonads, suggesting a distinct role of perinuclear mitochondria. We propose that at a certain point during spermatogenesis mitochondria are subject to degradation and that perinuclear mitochondria with the M mtDNA-protein complex are protected from this degradation with the result that mature spermatozoa contain only the paternal mitochondrial genome.

Tools and methods to analyze autophagy in C. elegans

For a long time, autophagy has been mainly studied in yeast or mammalian cell lines, and assays for analyzing autophagy in these models have been well described. More recently, the involvement of autophagy in various physiological functions has been investigated in multicellular organisms. Modification of autophagy flux is involved in developmental processes, resistance to stress conditions, aging, cell death and multiple pathologies. So, the use of animal models is essential to understand these processes in the context of different cell types and during the whole life. For ten years, the nematode Caenorhabditis elegans has emerged as a powerful model to analyze autophagy in physiological or pathological contexts. In this article, we present some of the established approaches and the emerging tools available to monitor and manipulate autophagy in C. elegans, and discuss their advantages and limitations.

Versatile roles of k63-linked ubiquitin chains in trafficking

Modification by Lys63-linked ubiquitin (UbK63) chains is the second most abundant form of ubiquitylation. In addition to their role in DNA repair or kinase activation, UbK63 chains interfere with multiple steps of intracellular trafficking. UbK63 chains decorate many plasma membrane proteins, providing a signal that is often, but not always, required for their internalization. In yeast, plants, worms and mammals, this same modification appears to be critical for efficient sorting to multivesicular bodies and subsequent lysosomal degradation. UbK63 chains are also one of the modifications involved in various forms of autophagy (mitophagy, xenophagy, or aggrephagy). Here, in the context of trafficking, we report recent structural studies investigating UbK63 chains assembly by various E2/E3 pairs, disassembly by deubiquitylases, and specifically recognition as sorting signals by receptors carrying Ub-binding domains, often acting in tandem. In addition, we address emerging and unanticipated roles of UbK63 chains in various recycling pathways that function by activating nucleators required for actin polymerization, as well as in the transient recruitment of signaling molecules at the plasma or ER membrane. In this review, we describe recent advances that converge to elucidate the mechanisms underlying the wealth of trafficking functions of UbK63 chains.

The specification and global reprogramming of histone epigenetic marks during gamete formation and early embryo development in C. elegans

In addition to the DNA contributed by sperm and oocytes, embryos receive parent-specific epigenetic information that can include histone variants, histone post-translational modifications (PTMs), and DNA methylation. However, a global view of how such marks are erased or retained during gamete formation and reprogrammed after fertilization is lacking. To focus on features conveyed by histones, we conducted a large-scale proteomic identification of histone variants and PTMs in sperm and mixed-stage embryo chromatin from C. elegans, a species that lacks conserved DNA methylation pathways. The fate of these histone marks was then tracked using immunostaining. Proteomic analysis found that sperm harbor ∼2.4 fold lower levels of histone PTMs than embryos and revealed differences in classes of PTMs between sperm and embryos. Sperm chromatin repackaging involves the incorporation of the sperm-specific histone H2A variant HTAS-1, a widespread erasure of histone acetylation, and the retention of histone methylation at sites that mark the transcriptional history of chromatin domains during spermatogenesis. After fertilization, we show HTAS-1 and 6 histone PTM marks distinguish sperm and oocyte chromatin in the new embryo and characterize distinct paternal and maternal histone remodeling events during the oocyte-to-embryo transition. These include the exchange of histone H2A that is marked by ubiquitination, retention of HTAS-1, removal of the H2A variant HTZ-1, and differential reprogramming of histone PTMs. This work identifies novel and conserved features of paternal chromatin that are specified during spermatogenesis and processed in the embryo. Furthermore, our results show that different species, even those with diverged DNA packaging and imprinting strategies, use conserved histone modification and removal mechanisms to reprogram epigenetic information.