Chromatin topology during the transformation of the mouse sperm nucleus into pronucleus in vivo

Mechanisms and consequences of paternally-transmitted chromosomal abnormalities

Paternally-transmitted chromosomal damage has been associated with pregnancy loss, developmental and morphological defects, infant mortality, infertility, and genetic diseases in the offspring, including cancer. There is epidemiological evidence linking paternal exposure to occupational or environmental agents with an increased risk of abnormal reproductive outcomes. There is also a large body of literature on germ cell mutagenesis in rodents showing that treatment of male germ cells with mutagens has dramatic consequences on reproduction, producing effects such as those observed in human epidemiological studies. However, we know very little about the etiology, transmission, and early embryonic consequences of paternally-derived chromosomal abnormalities. The available evidence suggests that: 1) there are distinct patterns of germ cell-stage differences in the sensitivity of induction of transmissible genetic damage, with male postmeiotic cells being the most sensitive; 2) cytogenetic abnormalities at first metaphase after fertilization are critical intermediates between paternal exposure and abnormal reproductive outcomes; and 3) there are maternal susceptibility factors that may have profound effects on the amount of sperm DNA damage that is converted into chromosomal aberrations in the zygote and that directly affect the risk for abnormal reproductive outcomes.

Paternally Transmitted Chromosomal Aberrations in Mouse Zygotes Determine Their Embryonic Fate1

The developmental consequences of chromosomal aberrations in embryos include spontaneous abortions, morphological defects, inborn abnormalities, and genetic/chromosomal diseases. Six germ-cell mutagens with different modes of action and spermatogenic stage sensitivities were used to investigate the relationship between the types of cytogenetic damage in zygotes with their subsequent risk of postimplantation death and of birth as a translocation carrier. Independent of the mutagen used, over 98% of paternally transmitted aberrations were chromosome type, rather than chromatid type, indicating that they were formed during the period between exposure of male germ cells and initiation of the first S phase after fertilization. There were consistent one-to-one agreements between the proportions of a) zygotes with unstable aberrations and the frequencies of dead embryos after implantation (slope = 0.87, confidence interval [CI]: 0.74, 1.16) and b) zygotes with reciprocal translocations and the frequency of translocation carriers at birth (slope = 0.74, CI: 0.48, 2.11). These findings suggest that chromosomal aberrations in zygotes are highly predictive of subsequent abnormal embryonic development and that development appears to proceed to implantation regardless of the presence of chromosomal abnormalities. Our findings support the hypothesis that, for paternally transmitted chromosomal aberrations, the fate of the embryo is already set by the end of G1 of the first cell cycle of development.

Mouse Xist expression begins at zygotic genome activation and is timed by a zygotic clock

The imprinted mouse Xist (X-inactive specific transcript) gene is involved in the initiation of X-chromosome inactivation. Only the paternal Xist is expressed in preimplantation development beginning from the 4-cell stage, preceding and in correlation with paternal X-inactivation in the extraembryonic lineage of the blastocyst. To better understand the mechanisms regulating Xist expression in early development, we investigated the precise timing of its onset. We set up a single-cell RT-PCR for the simultaneous analysis on single embryos of Xist and Hprt (internal control) cDNAs and a Y-chromosome specific DNA sequence, Zfy (for embryo sexing). Applying this procedure, we demonstrate that Xist expression begins at the G2-phase of 2-cell female embryos, earlier than previously reported and at the same time of the major wave of zygotic genome activation (ZGA). We then examined, if Xist expression at the 2-cell stage is dependent on the lapse of time spent since fertilization, as previously reported for zygotic genes. One-cell embryos at the G2-phase of the first cell-cycle were cultured with cytochalasin D (inhibitor of cytokinesis but not of DNA synthesis or nuclear progression) for a time equivalent to the 4-cell stage in control, untreated embryos. We show that Xist activation occurs at a scheduled time following fertilization despite the embryos being blocked at the 1-cell stage, suggesting the existence of a zygotic clock involved in the regulation of the transcription of this imprinted gene.

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.

Comparative studies on various modes of classification of morphology of sperm heads and results in in vitro fertilization--a preliminary report

In a retrospective blind study, Papanicolaou's stained semen smears of the husbands of 105 patients who took part in the in vitro fertilization/embryo transfer (IVF/ET) program of the Department of Gynecology and Obstetrics at the Heinrich Heine University Düsseldorf in 1991, were evaluated for the rates of normal sperm heads. The patients were divided into three groups according to the results of the IVF/ET: A = no fertilized egg; B = only fertilization, no pregnancy; C = pregnancy/birth. Four different categories of the sperm heads' morphology were defined: 1) 'Ideally' normal heads (normal size, relations width to length 2/3 to 3/5, acrosomal region > or = 40 < or = 70%); 2) Normal heads with only minor deviations from the 'ideally' normal form, strictly excluding spermatozoa with inhomogeneously stained acrosomal or pointed post-acrosomal regions; 3) Sperm heads with major alterations which are nevertheless widely considered as still normal in the literature; 4) Heads which are generally accepted as pathologically formed. It could be shown that the application of the strict criteria in the categories 1 plus 2 lead to significant differences not only between the IVF/ET groups A and B or C, but also between the groups B and C.

Fluorescence energy transfer shows that various physical and chemical treatments of human sperm induce unpacking of chromatin

Fluorescence resonance energy transfer (FRET) was used to study the changes which human sperm chromatin went through after various physical and chemical treatments. This technique showed a dilatation of the spatial relationship among chromatin liner arrays, with UV and DNAse among the treatments that gave rise to the highest increase. FRET image analysis showed that the chromatin linear arrays after treatment reach a spatial disarrangement similar to that brought about by sperm decondensation. Comparison of these results with the ability of human treated sperm to form pronuclei after microinjection into hamster eggs, suggests that the highly condensed spatial organization of sperm chromatin arrays may not be a necessary prerequisite for pronucleus formation.

Propidium iodide and the thiol-specific reagent DACM as a dye pair for fluorescence resonance energy transfer analysis: an application to mouse sperm chromatin

The dyes N-(-7-dimethyl-amino-4-methyl-coumarinyl) maleimide and propidium iodide, specific for the thiol group and DNA, respectively, were considered as a donor-acceptor couple suitable for investigating "in situ" the relative spatial distribution of DNA and protamines in mouse spermatozoa chromatin. The two dyes are characterized by favourable spectral properties, so that a simplified analytical procedure, based on the measurement of both donor and acceptor emission in double-stained samples, can be applied to evaluate the relative efficiency of the energy transfer process and its topological distribution. The results obtained indicate that during the maturation process: 1) the basic arrangement of protamine-DNA complex does not undergo structure changes, and 2) the oxidation of sulfhydryl to disulfide groups, resulting in chromatin stabilization, first involves the protamine thiols spatially closer to DNA. Fluorescence energy transfer imaging suggests that chromatin stabilization starts in the midportion of the sperm head, then spreads towards the periphery.

Chromatin remodeling in mammalian zygotes

With sperm-egg fusion at the time of fertilization the gamete nuclei are remodeled from genetically quiescent structures into pronuclei capable of DNA synthesis. Features of this process that are critical to insure the genetic integrity of the zygote and the success of subsequent embryonic development include: oocyte responses that prevent polyspermy; completion of the 2nd meiotic division by the oocyte; exchange of proteins in the sperm nucleus; and, remodelling of the oocyte chromosomes and sperm nucleus into functional pronuclei. Elucidation of the biological and molecular mechanisms underlying zygote formation and chromatin remodeling should enhance our understanding of the potential vulnerability of the zygote to toxicant-induced damage.

Chromosome variability and germ cell development in the house mouse

Structural heterozygosities of the karyotype have detrimental effects on the meiotic process, resulting very often in impairment of fertility in the carriers. Both male and female germ cell development are affected by chromosomal variability although spermatogenesis seems particularly prone to be affected, probably because of the intrinsic characteristics of the male germ cell cytodifferentiation process (i.e. the histological architecture of the seminiferous epithelium). However, euploid and aneuploid sperm do not seem to differ in the molecular organization of the genome they carry, thus explaining the almost regular capacity to accomplish the first zygotic developmental stages by the aneuploid sperm (aneuploid both for gametogenic genes and for entire chromosomal arms). A survey of the molecular and morphological data available on germ cell development in conditions of chromosomal rearrangement leads to the conclusion that the current hypotheses accounting for this phenomenon can only partly explain it. A working hypothesis is proposed which considers the three-dimensional changes (produced by structural heterozygosity) in the spatial order of chromosomes within the nucleus as the primary cause potentially able to trigger distorted functioning of the germ cells.

Dynamics of paternal chromatin changes in live one-cell mouse embryo after natural fertilization

Using video-enhanced fluorescence microscopy, we describe in live mouse zygotes the paternal chromatin changes undergone after fertilization. We focus on the sperm recondensation process and the formation of the paternal pronucleus, in relationship with the progression of maternal chromatin. Chromatin is labeled with the vital fluorophore Hoechst 33342. Our conditions of dye concentration and irradiation allow a continuous following of the dynamics of changes without major perturbation. We combine these observations with ultrastructural analysis performed by electron microscopy of the same eggs fixed at chosen stages. We show that the highly recondensed state corresponds to the appearance of the nuclear envelope and therefore the beginning of the pronuclear stage.