Decondensation of the mouse sperm nucleus within the interphase nucleus

J. Nucleus reprogramming/remodeling through selective enucleation (SE) of immature oocytes and zygotes: a nucleolus point of view

It is now approximately 25 years since the sheep Dolly, the first cloned mammal where the somatic cell nucleus from an adult donor was used for transfer, was born. So far, somatic cell nucleus transfer, where G1-phase nuclei are transferred into cytoplasts obtained by enucleation of mature metaphase II (MII) oocytes followed by the activation of the reconstructed cells, is the most efficient approach to reprogram/remodel the differentiated nucleus. In general, in an enucleated oocyte (cytoplast), the nuclear envelope (NE, membrane) of an injected somatic cell nucleus breaks down and chromosomes condense. This condensation phase is followed, after subsequent activation, by chromatin decondensation and formation of a pseudo-pronucleus (i) whose morphology should resemble the natural postfertilization pronuclei (PNs). Thus, the volume of the transferred nuclei increases considerably by incorporating the content released from the germinal vesicles (GVs). In parallel, the transferred nucleus genes must be reset and function similarly as the relevant genes in normal embryo reprogramming. This, among others, covers the relevant epigenetic modifications and the appropriate organization of chromatin in pseudo-pronuclei. While reprogramming in SCNT is often discussed, the remodeling of transferred nuclei is much less studied, particularly in the context of the developmental potential of SCNT embryos. It is now evident that correct reprogramming mirrors appropriate remodeling. At the same time, it is widely accepted that the process of rebuilding the nucleus following SCNT is instrumental to the overall success of this procedure. Thus, in our contribution, we will mostly focus on the remodeling of transferred nuclei. In particular, we discuss the oocyte organelles that are essential for the development of SCNT embryos.

Mice produced by mitotic reprogramming of sperm injected into haploid parthenogenotes

Sperm are highly differentiated and the activities that reprogram them for embryonic development during fertilization have historically been considered unique to the oocyte. We here challenge this view and demonstrate that mouse embryos in the mitotic cell cycle can also directly reprogram sperm for full-term development. Developmentally incompetent haploid embryos (parthenogenotes) injected with sperm developed to produce healthy offspring at up to 24% of control rates, depending when in the embryonic cell cycle injection took place. This implies that most of the first embryonic cell cycle can be bypassed in sperm genome reprogramming for full development. Remodelling of histones and genomic 5′-methylcytosine and 5′-hydroxymethylcytosine following embryo injection were distinct from remodelling in fertilization and the resulting 2-cell embryos consistently possessed abnormal transcriptomes. These studies demonstrate plasticity in the reprogramming of terminally differentiated sperm nuclei and suggest that different epigenetic pathways or kinetics can establish totipotency.

It is unclear what regulates gamete reprogramming competence. Here, the authors inject sperm into parthenogenetic embryos, generating viable offspring and show that mouse embryos in the mitotic cell cycle can reprogram sperm for full term development.

The role of redox environment in neurogenic development

The dynamic changes of cellular redox elements during neurogenesis allow the control of specific programs for selective lineage progression. There are many redox couples that influence the cellular redox state. The shift from a reduced to an oxidized state and vice versa may act as a cellular switch mechanism of stem cell mode of action from proliferation to differentiation. The redox homeostasis ensures proper functioning of redox-sensitive signaling pathways through oxidation/reduction of critical cysteine residues on proteins involved in signal transduction. This review presents the current knowledge on the relation between changes in the cellular redox environment and stem cell programming in the course of commitment to a restricted neural lineage, focusing on in vivo neurogenesis and in vitro neuronal differentiation. The first two sections outline the main systems that control the intracellular redox environment and make it more oxidative or reductive. The last section provides the background on redox-sensitive signaling pathways that regulate neurogenesis.

The presence of heparan sulfate in the mammalian oocyte provides a clue to human sperm nuclear decondensation in vivo

BACKGROUND

Previous results from our laboratory have led us to propose heparan sulfate (HS) as a putative protamine acceptor during human sperm decondensation in vivo. The aim of this paper was to investigate the presence of glycosaminoglycans in the mammalian oocyte in an effort to better support this contention.

METHODS

Two experimental approaches are used: oocyte labeling to identify the presence of HS and analysis of sperm decondensing ability of fresh oocytes in the presence or absence of specific glycosidases.

RESULTS

Staining of mouse zona-intact oocytes with the fluorescent cationic dye, Rubipy, at pH 1.5 allowed for the detection of sulfate residues in the ooplasm by confocal microscopy. HS was detected in the ooplasm by immunocytochemistry. A sperm decondensation microassay using heparin and glutathione was successfully developed. The same level of sperm decondensation could be attained when heparin was replaced by mouse zona-free oocytes. Addition of heparinase to the oocyte/glutathione mixture significantly reduced sperm decondensation (P = 0.0159), while there was no effect following addition of either chondroitinase ABC or hyaluronidase.

CONCLUSIONS

The results presented in this paper demonstrate for the first time that HS is present in the mammalian oocyte and show that HS is necessary for fresh oocytes to express their sperm decondensing ability in vitro.

Further evidence on the role of heparan sulfate as protamine acceptor during the decondensation of human spermatozoa

BACKGROUND

Human spermatozoa decondense in vitro upon exposure to heparin and glutathione. Glutathione is also the disulfide bond reducer in vivo, and heparan sulfate, a functional analogue of heparin, has been proposed as the protamine acceptor. The aim of this study was to evaluate the decondensing ability of chemically modified heparins and different glycosaminoglycans (GAGs) on isolated sperm nuclei in vitro, and to analyse the possible role of different GAGs as protamine acceptors.

METHODS

Capacitated spermatozoa and isolated sperm nuclei from normospermic semen samples were decondensed in the presence of heparin (or its equivalent) and glutathione. After fixation with glutaraldehyde, the percentage of decondensed spermatozoa and nuclei was determined under phase-contrast. Proteins were extracted from sperm nuclei previously incubated in the presence of gluhathione and different GAGs by incubation with urea-beta-meracptoethanol-NaCl, and analysed by acid polyacrylamide gel electrophoresis.

RESULTS

The ability of desulfated heparins and other GAGs to decondense isolated nuclei mirrored exactly the decondensation of capacitated spermatozoa, the only difference being the level of maximum decondensation achieved. Heparan sulfate and heparin, but not other GAGs, were able to release protamines from sperm chromatin.

CONCLUSIONS

Heparan sulfate could be functioning as protamine acceptor in vivo during human sperm nuclear decondensation.

Remodelling the paternal chromatin at fertilization in mammals

At fertilization, the highly condensed and transcriptionally inert chromatin of the spermatozoa becomes remodelled into the decondensed and transcriptionally competent chromatin of the male pronucleus. The chromatin initially becomes dispersed and then transiently recondenses into a small mass upon entry into the ooplasm. This morphological change is coincident with and likely dependent on the replacement of the sperm-specific protamines by oocyte-supplied histones and the organization of the chromatin into nucleosomes. The chromatin then extensively decondenses within the male pronucleus and acquires many of the proteins that are associated with the maternal chromatin. Nonetheless, the paternal chromatin manifests distinct characteristics, including transient hyperacetylation of histone H4, increased transcription of endogenous and microinjected genes, and replication-independent demethylation of DNA. Sperm chromatin remodelling is controlled by an oocyte activity that appears during meiotic maturation and disappears approximately 3 h after activation (release from metaphase II arrest), and which requires factors associated with the germinal vesicle of the oocyte. The molecular components of this activity remain largely unknown. In frogs, nucleoplasmin is required to assemble histones H2A and H2B onto the paternal chromatin. Evidence is presented that related proteins may perform similar functions in mammals. Identifying the mechanisms that underlie sperm chromatin remodelling at fertilization may be relevant for understanding reprogramming of somatic cell nuclei after transfer into oocytes.

Roles of NPM2 in chromatin and nucleolar organization in oocytes and embryos

Upon fertilization, remodeling of condensed maternal and paternal gamete DNA occurs to form the diploid genome. In Xenopus laevis, nucleoplasmin 2 (NPM2) decondenses sperm DNA in vitro. To study chromatin remodeling in vivo, we isolated mammalian NPM2 orthologs. Mouse NPM2 accumulates in oocyte nuclei and persists in preimplantation embryos. Npm2 knockout females have fertility defects owing to failed preimplantation embryo development. Although sperm DNA decondensation proceeds without NPM2, abnormalities are evident in oocyte and early embryonic nuclei. These defects include an absence of coalesced nucleolar structures and loss of heterochromatin and deacetylated histone H3 that normally circumscribe nucleoli in oocytes and early embryos, respectively. Thus, Npm2 is a maternal effect gene critical for nuclear and nucleolar organization and embryonic development.

Penetration in vitro of zona-free pig oocytes by homologous and heterologous spermatozoa

We examined the penetrability of pig, rat and bull spermatozoa into zona-free pig oocytes. Frozen-thawed boar spermatozoa penetrated into both zona-intact and zona-free oocytes with similar efficacy in a modified Tris-buffered medium (mTBM) supplemented with BSA and caffeine, but not in medium without caffeine. Rat epididymal spermatozoa did not readily penetrate into zona-free pig oocytes in mTBM with BSA. However, when a modified Krebs-Ringer bicarbonate solution was used, penetration rate varied with sperm concentrations at insemination: 79% of the oocytes were penetrated at 1.0 x 10(6) cells/ml, but very few at 0.1 x 10(6) and 10.0 x 10(6) cells/ml. In all oocytes penetrated, no activation was observed and the sperm nucleus was fully decondensed but did not transform into a male pronucleus. Frozen-thawed bull spermatozoa were also found to penetrate into zona-free pig oocytes in mTBM with BSA, caffeine and heparin: higher penetration rates were obtained with 1.0 x 106 and 10.0 x 10(6) spermatozoa/ml compared with 0.1 x 10(6) spermatozoa/ml. The penetration rate with 1.0 x 10(6) spermatozoa/ml was stable in five different bulls. All oocytes penetrated were activated and male pronuclear formation was observed in 57-79% of the penetrated oocytes. These results suggest that capacitation or the acrosome reaction is required for boar, rat, and possibly, bull spermatozoa to penetrate into zona-free pig oocytes. Bull spermatozoa can easily induce activation of pig oocytes and form male pronuclei, but rat spermatozoa cannot do so, indicating species differences in the ability of spermatozoa to activate pig oocytes and to transform to male pronuclei in the ooplasm.

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.

The ability to develop an activity that transfers histones onto sperm chromatin is acquired with meiotic competence during oocyte growth

Following fertilization, the oocyte remodels the sperm chromatin into the male pronucleus. As a component of this process, during meiotic maturation, oocytes develop an activity that transfers histones onto sperm DNA. To further characterize this activity, we tested whether oocytes at different stages of growth could, upon entry into metaphase of maturation, transfer histones onto sperm DNA, as judged by chromatin morphology and immunocytochemistry. Meiotically competent growing oocytes, which spontaneously enter metaphase upon culture, transferred histones onto sperm chromatin, whereas incompetent oocytes did not, even when treated with okadaic acid to induce germinal vesicle breakdown (GVBD) and chromosome condensation. When incompetent oocytes were cultured until they acquired the ability to undergo GVBD, only a small proportion also developed histone-transfer activity during maturation. However, this proportion significantly increased when the oocytes were cultured as granulosa-oocyte complexes. The failure of histone-transfer activity to develop in incompetent oocytes treated with okadaic acid was not linked to low H1 kinase activity nor rescued by injected histones. Because competent, but not incompetent, oocytes produce natural calcium oscillations, incompetent oocytes were exposed to SrCl2. One-third of treated oocytes produced at least one Ca2+ oscillation and, following insemination, the same proportion transferred histones onto sperm DNA. Histone transfer did not occur in oocytes pretreated with the Ca2+ chelator, BAPTA-AM. These results indicate that the ability to develop histone-transfer activity is acquired by growing oocytes near the time of meiotic competence, that it is separable from this event, and that it may be regulated through a Ca2+-dependent process.

Somatic linker histone H1 is present throughout mouse embryogenesis and is not replaced by variant H1 degrees

A striking feature of early embryogenesis in a number of organisms is the use of embryonic linker histones or high mobility group proteins in place of somatic histone H1. The transition in chromatin composition towards somatic H1 appears to be correlated with a major increase in transcription at the activation of the zygotic genome. Previous studies have supported the idea that the mouse embryo essentially follows this pattern, with the significant difference that the substitute linker histone might be the differentiation variant H1 degrees, rather than an embryonic variant. We show that histone H1 degrees is not a major linker histone during early mouse development. Instead, somatic H1 was present throughout this period. Though present in mature oocytes, somatic H1 was not found on maternal metaphase II chromatin. Upon formation of pronuclear envelopes, somatic H1 was rapidly incorporated onto maternal and paternal chromatin, and the amount of somatic H1 steadily increased on embryonic chromatin through to the 8-cell stage. Microinjection of somatic H1 into oocytes, and nuclear transfer experiments, demonstrated that factors in the oocyte cytoplasm and the nuclear envelope, played central roles in regulating the loading of H1 onto chromatin. Exchange of H1 from transferred nuclei onto maternal chromatin required breakdown of the nuclear envelope and the extent of exchange was inversely correlated with the developmental advancement of the donor nucleus.

Nuclear chaperones

We discuss nuclear chaperones that bind correctly folded protein subunits and mediate molecular interactions, particularly between proteins and nucleic acids. The charge of these chaperones helps to prevent non-specific electrostatic interactions between the components. Thus, an ordered assembly of macromolecular complexes is mediated, most notably in the formation and maintenance of chromatin, though similar principles are likely to apply in ribonucleoprotein assembly. Here, we discuss roles for nuclear chaperones in mediating nucleosome assembly and remodelling during DNA replication and transcription, and upon fertilisation.

Paternal contributions to the mammalian zygote: fertilization after sperm-egg fusion

Mammalian fertilization has traditionally been regarded as a simple blending of two gametes, during which the haploid genome of the fertilizing spermatozoon constitutes the primary paternal contribution to the resulting embryo. In contrast to this view, new research provides evidence of important cytoplasmic contributions made by the fertilizing spermatozoon to the zygotic makeup, to the organization of preimplantation development, and even reproductive success of new forms of assisted fertilization. The central role of the sperm-contributed centriole in the reconstitution of zygotic centrosome has been established in most mammalian species and is put in contrast with strictly maternal centrosomal inheritance in rodents. The complementary reduction or multiplication of sperm and oocyte organelles during gametogenesis, exemplified by the differences in the biogenesis of centrosome in sperm and oocytes, represents an intriguing mechanism for avoiding their redundancy during early embryogenesis. New studies on perinuclear theca of sperm revealed its importance for both spermatogenesis and fertilization. Remodeling of the sperm chromatin into a male pronucleus is guided by oocyte-produced, reducing peptide glutathione and a number of molecules required for the reconstitution of the functional nuclear envelope and nuclear skeleton. Although some of the sperm structures are transformed into zygotic components, the elimination of others is vital to early stages of embryonic development. Sperm mitochondria, carrying potentially harmful paternal mtDNA, appear to be eliminated by a ubiquitin-dependent mechanism. Other accessory structures of the sperm axoneme, including fibrous sheath, microtubule doublets, outer dense fibers, and the striated columns of connecting piece, are discarded in an orderly fashion. The new methods of assisted fertilization, represented by intracytoplasmic sperm injection and round spermatid injection, bypass multiple steps of natural fertilization by introducing an intact spermatozoon or spermatogenic cell into oocyte cytoplasm. Consequently, the carryover of sperm accessory structures that would normally be eliminated before or during the entry of sperm into oocyte cytoplasm persist therein and may interfere with early embryonic development, thus decreasing the success rate of assisted fertilization and possibly causing severe embryonic anomalies. Similarly, foreign organelles, proteins, messenger RNAs, and mitochondrial DNAs, which may have a profound impact on the embryonic development, are propagated by the nuclear transfer of embryonic blastomeres and somatic cell nuclei. This aspect of assisted fertilization is yet to be explored by a focused effort.

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.

Intracytoplasmic injection of porcine, bovine, mouse, or human spermatozoon into porcine oocytes

We determined the incidence of activation, male pronuclear formation, and apposition of pronuclei in porcine oocytes following intracytoplasmic injection of various porcine sperm components and foreign species spermatozoa, such as that of cattle, mouse or human. The porcine oocytes were activated by injection of a spermatozoon or an isolated sperm head. In contrast, injection of either sperm tail or a trypsin- or NaOH-treated sperm head failed to induce oocyte activation. Because injection of mouse, bovine, or human spermatozoon activated porcine oocytes, the sperm-borne activation factor(s) is not strictly species-specific. Male pronuclear formation and pronuclear apposition were observed in porcine oocytes following injection of porcine, bovine, mouse or human spermatozoa. Electrical stimulation following sperm cell injection did not enhance the incidence of male pronuclear formation or pronuclear apposition compared with sperm cell injection alone (P > 0.1). Following porcine sperm injection, the microtubular aster was organized from the neck of the spermatozoon, and filled the whole cytoplasm. In contrast, following injection of bovine, mouse, or human spermatozoon, the maternal-derived microtubules were organized from the cortex to the center of the oocytes, which seems to move both pronuclei to the center of oocytes. Cleavage to the two-cell stage was observed at 19-21 hr after injection of porcine spermatozoon. However, none of the oocytes following injection of mouse, bovine, or human spermatozoa developed to the mitotic metaphase or the two-cell stage. These results suggested that the oocyte activating factor(s) is present in the perinuclear material and that it is not species-specific for the porcine oocyte. Self-organized microtubules seemed to move the pronuclei into center of oocytes when foreign species spermatozoa were injected into porcine oocytes.