Remodeling the sperm nucleus into a male pronucleus at fertilization

Time-lapse observation and transcriptome analysis of a case with repeated multiple pronuclei after IVF/ICSI

PURPOSE

The purpose of this study was to investigate the cause of repeated multipronucleus (MPN) formation in zygotes in a patient after both in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI).

METHOD

This is a case study. A patient had unexplained primary infertility with recurring total MPN zygotes after IVF and ICSI cycles. Time-lapse monitoring of pronucleus formation was carried out. Embryos developed from MPN zygotes were analyzed by fluorescence in situ hybridization (FISH). Single-cell RNA-seq analysis was used to identify gene expression profiles of the patient's oocyte and zygote, and these were compared to the data from oocytes and zygotes from donors with normal fertilization (patient, n = 1; donors, n = 4). Oocyte-specific genes with differential expression were selected by the Amazonia!

DATABASE

RESULTS

From time-lapse analysis, we observed the formation of multiple micronuclei near the site of the second polar body extrusion. These micronuclei migrated, expanded, and juxtaposed with the male pronucleus leading to a multipronucleus. None of these MPN zygotes could develop to the blastocyst stage, and FISH analysis revealed a chaotic chromosomal complement in the arrested embryos. RNA-seq analysis showed 113 differentially expressed genes (DEGs) between the patient and the donor oocytes and zygotes. Moreover, 25 of the 113 DEGs were unique or highly expressed in oocytes and early embryos. From 25 DEGs, three genes, DYNC2LI1, NEK2, and CCNH, which are involved in meiosis and the chromosome separation process, were further validated by real-time PCR.

CONCLUSION

We identified several candidate genes affecting pronucleus formation as a new cause of infertility.

Vesicular PtdIns(3,4,5)P3 and Rab7 are key effectors of sea urchin zygote nuclear membrane fusion

Regulation of nuclear envelope dynamics is an important example of the universal phenomena of membrane fusion. The signalling molecules involved in nuclear membrane fusion might also be conserved during the formation of both pronuclear and zygote nuclear envelopes in the fertilised egg. Here, we determine that class-I phosphoinositide 3-kinases (PI3Ks) are needed for in vitro nuclear envelope formation. We show that, in vivo, PtdIns(3,4,5)P₃ is transiently located in vesicles around the male pronucleus at the time of nuclear envelope formation, and around male and female pronuclei before membrane fusion. We illustrate that class-I PI3K activity is also necessary for fusion of the female and male pronuclear membranes. We demonstrate, using coincidence amplified Förster resonance energy transfer (FRET) monitored using fluorescence lifetime imaging microscopy (FLIM), a protein-lipid interaction of Rab7 GTPase and PtdIns(3,4,5)P₃ that occurs during pronuclear membrane fusion to create the zygote nuclear envelope. We present a working model, which includes several molecular steps in the pathways controlling fusion of nuclear envelope membranes.

Functional studies of MP62 during male chromatin decondensation in sea urchins

In amphibians, sperm histone transition post-fertilization during male pronucleus formation is commanded by histone chaperone Nucleoplasmin (NPM). Here, we report the first studies to analyze the participation of a Nucleoplasmin-like protein on male chromatin remodeling in sea urchins. In this report, we present the molecular characterization of a nucleoplasmin-like protein that is present in non fertilized eggs and early zygotes in sea urchin specie Tetrapygus niger. This protein, named MP62 can interact with sperm histones in vitro. By male chromatin decondensation assays and immunodepletion experiments in vitro, we have demonstrated that this protein is responsible for sperm nucleosome disorganization. Furthermore, as amphibian nucleoplasmin MP62 is phosphorylated in vivo immediately post-fertilization and this phosphorylation is dependent on CDK-cyclin activities found after fertilization. As we shown, olomoucine and roscovitine inhibits male nucleosome decondensation, sperm histone replacement in vitro and MP62 phosphorylation in vivo. This is the first report of a nucleoplasmin-like activity in sea urchins participating during male pronucleus formation post-fecundation.

Kinetics of human male pronuclear development in a heterologous ICSI model

PURPOSE

To evaluate human sperm nuclear chromatin decondensation in a heterologous ICSI system using hamster ova injected with human sperm.

MATERIALS AND METHODS

Frozen hamster oocytes were injected with Triton X-100 treated sperm and fixed at different time points post ICSI. Oocytes injected with non-treated sperm served as controls. Male pronuclear decondensation was evaluated after staining with DAPI.

RESULTS

Sperm cells with partially destroyed membranes and depletion of the acrosome decondense more rapidly and to a greater extent than membrane/acrosome intact cells. Marked variability in pronuclear size was observed for any time point post ICSI, which most probably reflects the heterogeneity in the mature human sperm population.

CONCLUSION

Remodeling of male gamete nuclei in this heterologous ICSI mimics events that occur during natural fertilization in humans and therefore this approach may be used for studies of human sperm chromosomes transformations.

Metaphase II human oocyte morphology: contributing factors and effects on fertilization potential and embryo developmental ability in ICSI cycles

Morphologic abnormalities in the oocyte are relevant for determining its developmental fate and could be related to controlled ovarian stimulation protocols and ovarian response. The contributing factors of oocyte dysmorphism incidence and its effects on fertilization potential and embryo development are the object of discussion in this study.

Endonuclease-sensitive regions of human spermatozoal chromatin are highly enriched in promoter and CTCF binding sequences

During the haploid phase of mammalian spermatogenesis, nucleosomal chromatin is ultimately repackaged by small, highly basic protamines to generate an extremely compact, toroidal chromatin architecture that is critical to normal spermatozoal function. In common with several species, however, the human spermatozoon retains a small proportion of its chromatin packaged in nucleosomes. As nucleosomal chromatin in spermatozoa is structurally more open than protamine-packaged chromatin, we considered it likely to be more accessible to exogenously applied endonucleases. Accordingly, we have used this premise to identify a population of endonuclease-sensitive DNA sequences in human and murine spermatozoa. Our results show unequivocally that, in contrast to the endonuclease-resistant sperm chromatin packaged by protamines, regions of increased endonuclease sensitivity are closely associated with gene regulatory regions, including many promoter sequences and sequences recognized by CCCTC-binding factor (CTCF). Similar differential packaging of promoters is observed in the spermatozoal chromatin of both mouse and man. These observations imply the existence of epigenetic marks that distinguish gene regulatory regions in male germ cells and prevent their repackaging by protamines during spermiogenesis. The ontology of genes under the control of endonuclease-sensitive regulatory regions implies a role for this phenomenon in subsequent embryonic development.

Oocyte dysmorphism is not associated with aneuploidy in the developing embryo

OBJECTIVE

Controlled ovarian hyperstimulation yields numerous oocytes with different morphologic features. The purpose of this study was to assess the effect of oocyte dysmorphism on the development and ploidy status of the derived embryo.

DESIGN

Retrospective study.

SETTING

Private assisted reproduction unit.

PATIENT(S)

A retrospective analysis was performed on 616 oocytes and 266 embryos of 65 infertile couples who had undergone preimplantation genetic diagnosis and aneuploidy screening for repeated implantation failures.

INTERVENTION(S)

Oocytes with normal and abnormal morphology and the derived embryos were compared in terms of fertilization, cleavage, and blastocyst formation rates as well as the ploidy status.

RESULT(S)

Oocyte dysmorphism did not have an adverse effect on fertilization and cleavage rates and quality of day 3 embryos. Single cytoplasmic and multiple abnormalities of the oocyte significantly decreased the blastocyst formation rate of the cleaving embryo. Oocyte dysmorphism did not have a significant association with embryonic aneuploidy.

CONCLUSIONS(S)

Oocyte dysmorphism was not associated with a higher risk of aneuploidy in the developing embryo. Cytoplasmic dysmorphism or multiple morphologic abnormalities of the oocyte adversely affects the blastocyst formation potential of the derived embryo.

Can 'abnormally' fertilized zygotes give rise to viable embryos?

Conventional practice in in vitro fertilization or intracytoplasmic sperm injection is to select the best quality embryos based on their morphology and cleavage status from a cohort of fertilized oocytes in which two pronuclei were observed at the time they were checked for fertilization. However, in a small proportion of cycles, the selection is limited to embryos that appeared to be either unfertilized (displaying zero pronuclei) or abnormally fertilized (displaying one or three pronuclei) at the time they were checked for fertilization. There is a lack of consensus on whether such embryos should be transferred to the uterus. Cytogenetic analysis of embryos from oocytes with one pronucleus has shown a proportion is diploid. Transfer of such embryos has resulted in healthy births. Limited cytogenetic analysis of oocytes that divide despite the absence of pronuclei at fertilization check indicates that a proportion also have a normal cytogenetic constitution. Cytogenetic analysis of embryos from oocytes with three pronuclei has shown high rates of triploidy and chaotic cell divisions. Subsequent foetuses have extremely unfavourable outcomes. Here, we review the published literature on the cytogenetic analysis of 'unfertilized' and 'abnormally fertilized' embryos and discuss possible pathways which lead to their formation. The limited evidence indicates that oocytes with one pronucleus and oocytes that show normal onward division despite the absence of pronuclei may be considered for replacement in certain circumstances.

Detection of damage in mammalian sperm cells

Ejaculated semen is washed for in vitro fertilization or diluted and processed to allow optimal and long-term low temperature liquid- and cryo-preservation. However, sperm are vulnerable to the washing, dilution, temperature and osmotic changes involved in sperm storage. In this review, a number of techniques are considered for detecting damaged spermatozoa. Staining protocols have been developed to detect the membrane and organelle integrity of mammalian sperm cells. Plasma membrane integrity is usually assessed after staining cells with membrane-impermeable dyes or alternatively with acetylated membrane (AM) permeable probes that are selectively de-esterified and become membrane impermeable and thus entrapped into viable cells only (AM ester loading). Organelle-specific dyes are commonly used to detect functionality of mitochondria or the acrosome. A distortion in the lateral and bilayer organization of lipids as well as the peroxidation of fatty acid moieties can be quantified and localized in living sperm. The relation of a disordering in the sperm membrane's lipid architecture and sperm deterioration versus capacitation is discussed. Finally, the integrity of sperm DNA can be measured at three different levels by assessing the degree of DNA-protamine condensation, the incidence of breaks and nicks in the DNA and the frequency of fragmentation of the nuclei into sub-haploid apoptotic bodies. The relevance of detecting DNA aberrations and especially the putative link to the incidence of apoptosis is critically considered.

Failure of male pronucleus formation is the major cause of lack of fertilization and embryo development in pig oocytes subjected to intracytoplasmic sperm injection

The objectives of this study were 1) to compare the efficiency of intracytoplasmic sperm injection (ICSI) with and without additional artificial stimulation using frozen-thawed sperm and in vitro-matured porcine oocytes and 2) to determine the nuclear anomalies of ICSI oocytes that failed to fertilize or develop. In experiments 1 and 2, we evaluated the effects of additional activation treatments, e.g., electrical stimulus, Ca ionophore (A23187), and/or cycloheximide, on fertilization and development of ICSI porcine oocytes. Significantly higher fertilization, cleavage, and blastocyst rates were obtained for oocytes treated with a combination of ICSI and electrical activation (EA) (P < 0.05) than for those treated with ICSI alone. However, different combinations of electrical and chemical activation treatments did not further improve the rates of fertilization, cleavage, and blastocyst development for ICSI embryos. To elucidate the association between sperm head decondensation and oocyte activation and to investigate the cause of embryonic development failure, in experiment 3 we evaluated the nuclear morphology of oocytes 16-20 h after ICSI. Nearly 100% of oocytes showed female pronucleus formation after ICSI regardless of activation treatment. However, failure of male pronucleus formation with intact or swelling sperm heads was observed in some ICSI embryos, suggesting that these embryos underwent cell division with the female pronucleus only. Artificial activation (EA and A23187) had a beneficial effect on embryonic development, sperm decondensation was independent of the resumption of meiosis, and the failure of formation of a male pronucleus was the major cause for fertilization failure in porcine ICSI embryos.

The Drosophila nuclear lamina protein YA binds to DNA and histone H2B with four domains

Dramatic changes occur in nuclear organization and function during the critical developmental transition from meiosis to mitosis. The Drosophila nuclear lamina protein YA binds to chromatin and is uniquely required for this transition. In this study, we dissected YA's binding to chromatin. We found that YA can bind to chromatin directly and specifically. It binds to DNA but not RNA, with a preference for double-stranded DNA (linear or supercoiled) over single-stranded DNA. It also binds to histone H2B. YA's binding to DNA and histone H2B is mediated by four domains distributed along the length of the YA molecule. A model for YA function at the end of Drosophila female meiosis is proposed.

Does blastocyst culture eliminate paternal chromosomal defects and select good embryos?: inheritance of an abnormal paternal genome following ICSI

Following intracytoplasmic sperm injection (ICSI), approximately 60-70% of oocytes are fertilized and of these embryos, approximately 45% withstand in-vitro culture conditions to produce healthy blastocysts. The efficiency of implantation of 2-4-cell embryos selected at the pronuclear stage and that of blastocysts are comparable. However, prolonged selection of embryos in vitro (4-5 days), has been proposed to eliminate chromosomal abnormalities, more specifically those inherited by defective spermatozoa. This hypothesis is based upon the assumption that the paternal genetic contribution is indispensable for blastocyst development. Here we examine this hypothesis and suggest that phenotypic manifestation of paternal genomic abnormalities might not occur prior to implantation. In addition to the parent-of-origin effect during embryogenesis, blastocyst transfer may not prevent the inheritance of genetic defects involving 'male factor' loci.

Sperm nuclear activation during fertilization

The delivery of the paternal genome to the egg is a primary goal of fertilization. In preparation for this step, the nucleus of the developing spermatozoon undergoes extensive morphological and biochemical transformations during spermatogenesis to yield a tightly compacted sperm nucleus. These modifications are essentially reversed during fertilization. As a result, the incorporated sperm nucleus undergoes many steps in the egg cytoplasm as it develops into a male pronucleus. The sperm nucleus (1) loses its nuclear envelope, (2) undergoes nucleoprotein remodeling, (3) decondenses and increases in size, (4) becomes more spherical, (5) acquires a new nuclear envelope, and (6) becomes functionally competent to synthesize DNA and RNA. These changes are coordinate with meiotic processing of the maternal chromatin, and often result in behaviors asynchronous with the maternal chromatin. For example, in eggs fertilized during meiosis, the sperm nucleus decondenses while the maternal chromatin remains condensed. A model is presented that suggests some reasons why this puzzling behavior exists. Defects in any of the processes attending male pronuclear development often result in infertility. New assisted reproductive technologies have been developed that ensure delivery of the sperm nucleus to the egg cytoplasm so that a healthy embryo is produced. An emerging challenge is to further characterize the molecular mechanisms that control sperm nuclear transformations and link these to causes of human infertility. Further understanding of this basic process promises to revolutionize our understanding of the mystery of the beginning of new life.