Germinal vesicle material is essential for nucleus remodeling after nuclear transfer

Transfer of germinal vesicle to ooplasm of young mice could not rescue ageing-associated chromosome misalignment in meiosis of oocytes from aged mice

BACKGROUND

[corrected] Transferring a germinal vesicle (GV) from an aged woman's oocyte into ooplasm from a younger woman has been proposed as a possible way to overcome the problem of age-related decline in female fertility. Here we assessed this possibility by determining whether ooplasts derived from young mice could rescue ageing-associated chromosome misalignment in meiosis of oocytes from aged mice.

METHODS

Three groups of reconstructed oocytes, young GV-young cytoplast (group YY), aged GV-young cytoplast (group AY), and young GV-aged cytoplast (group YA), were created by micromanipulation and electrofusion.

RESULTS

Nuclear transplantation was successful in 89.8-94.4% of GV-ooplast complexes, and maturation rate of the reconstructed oocytes was 93.5-97.9%. Confocal microscopy analysis showed a significantly higher rate (49.2%) of chromosome misalignment in ageing mice than in young mice (16.9%), and 57.1% of oocytes in group AY exhibited chromosome misalignment, while the abnormality rate in groups YY and YA was 16.3 and 16.7% respectively. Calcium imaging showed that the three groups of reconstructed oocytes exhibited a similar pattern of calcium oscillations upon stimulation with bovine sperm extracts. Fertilization rate and developmental capacity to 2-cell embryos were also similar among the three groups of oocytes.

CONCLUSIONS

Our findings suggest that: (i) the ooplasm from young mice could not rescue ageing-associated chromosome misalignment in meiosis of GV from aged mice; and (ii) behaviour of chromosome alignment over metaphase spindle is predominantly determined by GV material.

Developmental Competence of HMC Derived Bovine Cloned Embryos Obtained from Somatic Cell Nuclear Transfer of Adult Fibroblasts and Granulosa Cells

To enable us to handle a large number of oocytes at a given time and to have an increased throughput of cloned embryos, we attempted the Handmade cloning (HMC) technique, a zona-free method of bovine somatic cell nuclear transfer. Our objective was to study the developmental competence of the HMC derived embryos obtained using different types of somatic cells. A total of 6,874 cumulus-oocyte-complexes were used with either 7th or 11th passage fibroblasts (1st and 2nd groups, respectively), which were prepared from male animals, or granulosa cells (3rd group) as nuclei donors. The average cleavage rate was 65%, accompanied by a blastocyst rate of just 2% for the cleaved products and 5% for the >8-cell embryos, and there was no significant difference between the three groups. Out of 27 blastocysts recovered, 22 blastocysts were transferred to 22 recipients, resulting in two pregnancies. One pregnancy was lost after the fourth week while the other progressed to full term with the birth of a male calf. This first successful cloning of a male calf with the HMC technique in Europe indicates the successful adoption and establishment of this technique in our laboratory, and that this technique can be successful in producing viable embryos.

The first cell cycle after transfer of somatic cell nuclei in a non-human primate

Production of genetically identical non-human primates through somatic cell nuclear transfer (SCNT) can provide diseased genotypes for research and clarify embryonic stem cell potentials. Understanding the cellular and molecular changes in SCNT is crucial to its success. Thus the changes in the first cell cycle of reconstructed zygotes after nuclear transfer (NT) of somatic cells in the Long-tailed Macaque (Macaca fascicularis) were studied. Embryos were reconstructed by injecting cumulus and fibroblasts from M. fascicularis and M. silenus, into enucleated M. fascicularis oocytes. A spindle of unduplicated premature condensed chromosome (PCC spindle) from the donor somatic cell was formed at 2 hours after NT. Following activation, the chromosomes segregated and moved towards the two PCC spindle poles, then formed two nuclei. Twenty-four hours after activation, the first cell division occurred. A schematic of the first cell cycle changes following injection of a somatic cell into an enucleated oocyte is proposed. Ninety-three reconstructed embryos were transferred into 31 recipients, resulting in 7 pregnancies that were confirmed by ultrasound;unfortunately none progressed beyond 60 days.

Nuclear transfer in rodents

Cloning is the asexual reproduction of an individual, such that the offspring have an essentially identical nuclear genome. Nuclear transfer and cloning have been achieved in a number of species, namely sheep, cows, goats, rabbits, cats and mice, but have been largely unsuccessful, so far, in dogs, primates and rats. Clearly, contributory factors which affect the outcome of successful cloning experiments are not universally applicable to all species. One theme common to all cloning experiments, however, is the overall inefficiency of the process, typically 0-4%. A number of factors contribute to nuclear transfer inefficiency, and we will review mouse cloning experiments, which address these problems, highlighting the importance of donor nucleus choice (somatic or ES cell, fetal or adult, quiescent or actively dividing). Finally, we will summarize the emerging principles which appear to govern nuclear reprogramming and production of clones, and will consider the application of nuclear transfer to the rat.

Rapid H1 linker histone transitions following fertilization or somatic cell nuclear transfer: evidence for a uniform developmental program in mice

H1 linker histones (H1s) are key regulators of chromatin structure and function. The functions of different H1s during early embryogenesis, and mechanisms regulating their associations with chromatin are largely unknown. The developmental transitions of H1s during oocyte growth and maturation, fertilization and early embryogenesis, and in cloned embryos were examined. Oocyte-specific H1FOO, but not somatic H1s, associated with chromatin in oocytes (growing, GV-stage, and MII-arrested), pronuclei, and polar bodies. H1FOO associated with sperm or somatic cell chromatin within 5 min of intracytoplasmic sperm injection (ICSI) or somatic cell nuclear transfer (SCNT), and completely replaced somatic H1s by 60 min. The switching from somatic H1s to H1FOO following SCNT was developmentally regulated. H1FOO was replaced by somatic H1s during the late two- and four-cell stages. H1FOO association with chromatin can occur in the presence of a nuclear envelope and independently of pronucleus formation, is regulated by factors associated with the spindle, and is likely an active process. All SCNT constructs recapitulated the normal sequence of H1 transitions, indicating that this alone does not signify a high developmental potential. A paucity of all known H1s in two-cell embryos may contribute to precocious gene transcription in fertilized embryos, and the elaboration of somatic cell characteristics in cloned embryos.

Improved in vitro development rates of sheep somatic nuclear transfer embryos by using a reverse-order zona-free cloning method

This paper describes a modified zona-free cloning protocol for examining the effects of short-term exposure of donor nuclei to maternal chromosomal components and associated factors. In vitro matured zona-free sheep oocytes were enucleated by micromanipulator-assisted aspiration either before or after fusion with adult fibroblast or granulosa donor cells. Subsequent kinetics of donor nuclei and maternal chromatin as well as in vitro embryo development rates were recorded. The effect of an additional activation stimulus in connection with the reverse-order cloning (fusion before enucleation) and the feasibility of manual enucleation by metal blade were also studied. As a result of the simultaneous fusion and activation, most donor nuclei remained in interphase but swelled in size. Maternal chromosomes reached anaphase II-telophase II stages within 1-2 h of activation, effectively facilitating telophase enucleation with both the micromanipulator-assisted aspiration and manual bisection. A significantly higher development rate to the blastocyst stage was achieved with the reverse-order protocol, suggesting further investigation into the possible role of oocyte nucleus-associated factors in reprogramming is warranted. Overall, the reverse-order zona-free cloning method was efficient in the production of transferable cloned sheep blastocysts and may offer yet another choice of methodology in the practical application of nuclear transfer technology.

Chromatin as a regulative architecture of the early developmental functions of mammalian embryos after fertilization or nuclear transfer

Nuclear transfer of a somatic nucleus into an enucleated oocyte has demonstrated in several mammalian species that the chromatin of a differentiated nucleus can be reprogrammed so as to be able to direct the full development of the reconstructed embryo. This review focus on the timing of the early events that allow the return of somatic chromatin to a totipotent state. Our understanding of the modifications associated with chromatin remodeling is limited by the low amount of biological material available in mammals at early developmental stages and the fact that very few genetic studies have been conducted with nuclear transfer embryos. However, the importance of several factors such as the covalent modifications of DNA through the methylation of CpG dinucleotides, the exchange of histones through a reorganized nuclear membrane, and the interaction between cytoplasmic oocyte components and nuclear complexes in the context of nuclear transfer is becoming clear. A better characterization of the changes in somatic chromatin after nuclear transfer and the identification of oocyte factors or structures that govern the formation of a functional nucleus will help us to understand the relationship between chromatin structure and cellular totipotency.