Demecolcine-assisted enucleation of goat oocytes: protocol optimization, mechanism investigation, and application to improve the developmental potential of cloned embryos

Application of the modified handmade cloning technique to pigs

Although somatic cell nuclear transfer (SCNT) is frequently employed to produce cloned animals in laboratories, this technique is expensive and inefficient. Therefore, the handmade cloning (HMC) technique has been suggested to simplify and advance the cloning process, however, HMC wastes many oocytes and leads to mitochondrial heteroplasmy. To solve these problems, we propose a modified handmade cloning (mHMC) technique that uses simple laboratory equipment, i.e., a Pasteur pipette and an alcohol lamp, applying it to porcine embryo cloning. To validate the application of mHMC to pig cloning, embryos produced through SCNT and mHMC are compared using multiple methods, such as enucleation efficiency, oxidative stress, embryo developmental competence, and gene expression. The results show no significant differences between techniques except in the enucleation efficiency. The 8-cell and 16-cell embryo developmental competence and Oct4 expression levels exhibit significant differences. However, the blastocyst rate is not significantly different between mHMC and SCNT. This study verifies that cloned embryos derived from the two techniques exhibit similar generation and developmental competence. Thus, we suggest that mHMC could replace SCNT for simpler and cheaper porcine cloning.

Improved functional oocyte enucleation by actinomycin D for bovine somatic cell nuclear transfer

Somatic cell nuclear transfer (SCNT) allows animal cloning but remains technically challenging. This study investigated limitations to functional oocyte enucleation by actinomycin D (AD) as a means of making SCNT easier to perform. Denuding oocytes or inhibiting transcription before AD treatment revealed that the toxicity of this compound during bovine oocyte maturation is mediated by cumulus cells. Exposure of denuded oocytes to higher concentrations of AD (5-20μgmL-1 ) and stepwise reductions of the incubation period (from 14.0 to 0.25h) led to complete inhibition of parthenogenetic development. Bovine SCNT using this improved AD enucleation protocol (NT(AD)) restored cleavage rates compared with rates in the parthenogenetic and SCNT controls (P(CTL) and NT(CTL) respectively). However, NT(AD) was associated with increased caspase-3 activity in cleavage stage embryos and did not recover blastocyst rates. The removal of AD-treated oocyte spindle before reconstruction (NT(AD+SR)) improved embryo development and reduced caspase-3 activity to levels similar to those in the P(CTL) and NT(CTL) groups. Furthermore, mid-term pregnancies were achieved using NT(AD+SR) blastocysts. In conclusion, improvements in AD functional enucleation for bovine SCNT circumvents most cellular roadblocks to early embryonic development and future investigations must focus on restoring blastocyst formation.

Bovine somatic cell nuclear transfer using mitomycin C-mediated chemical oocyte enucleation

SummaryChemical oocyte enucleation holds the potential to ease somatic cell nuclear transfer (SCNT), although high enucleation rates remain limited to micromanipulation-based approaches. Therefore, this study aimed to test mitomycin C (MMC) for use in bovine functional chemical oocyte enucleation. Incubation of denuded eggs in 10 µg ml-1 MMC for different periods did not affect most maturation rates (0.5 h: 85.78%A, 1.0 h: 72.77%B, 1.5 h: 83.87%A, and 2.0 h: 82.05%A) in comparison with non-treated controls (CTL; 85.77%A). Parthenogenetic development arrest by MMC was efficient at cleavage (CTL: 72.93%A, 0.5 h: 64.92%A,B, 1.0 h: 60.39%B,C, 1.5 h: 66.35%A,B, and 2.0 h: 53.84%C) and blastocyst stages (CTL: 33.94%A, 0.5 h: 7.58%B, 1.0 h: 2.47%C, 1.5 h: 0.46%C, and 2.0 h: 0.51%C). Blastocysts were obtained after nuclear transfer (NT) using MMC enucleation [NT(MMC): 4.54%B] but at lower rates than for the SCNT control [NT(CTL): 26.31%A]. The removal of the meiotic spindle after MMC incubation fully restored SCNT blastocyst development [NT(MMC+SR): 24.74%A]. Early pregnancies were obtained by the transfer of NT(MMC) and NT(MMC+SR) blastocysts to synchronized recipients. In conclusion, MMC leads to functional chemical oocyte enucleation during SCNT and further suggests its potential for application towards technical improvements.

Rex Rabbit Somatic Cell Nuclear Transfer with In Vitro-Matured Oocytes

Somatic cell nuclear transfer (SCNT) requires large numbers of matured oocytes. In vitro-matured (IVM) oocytes have been used in SCNT in many animals. We investigated the use of IVM oocytes in Rex rabbit SCNT using Rex rabbit ovaries obtained from a local abattoir. The meiotic ability of oocytes isolated from follicles of different diameters was studied. Rex rabbit SCNT was optimized for denucleation, activation, and donor cell synchronization. Rex rabbit oocytes grew to the largest diameter (110 μm) when the follicle diameter was 1.0 mm. Oocytes isolated from <0.5-mm follicles lacked the ability to resume meiosis. More than 90% of these oocytes remained in the germinal vesicle (GV) stage after in vitro culture (IVC) for 18 h. Oocytes isolated from >0.7-mm follicles acquired maturation ability. More than 90% of these oocytes matured after IVC for 18 h. The developmental potential of oocytes isolated from >1-mm follicles was greater than that of oocytes isolated from 0.7- to 1.0-mm follicles. The highest activation rates for IVM Rex rabbit oocytes were seen after treatment with 2.5 μM ionomycin for 5 min followed by 2 mM 6-dimethylaminopurine (6-DMAP) and 5 μg/mL cycloheximide (CHX) for 1 h. Ionomycin induced the chromatin of IVM oocytes to protrude from the oocyte surface, promoting denucleation. Fetal fibroblast cells (FFCs) and cumulus cells (CCs) were more suitable for Rex rabbit SCNT than skin fibroblast cells (SFCs) (blastocyst rate was 35.6 ± 2.2% and 38.0 ± 6.0% vs. 19.7 ± 3.1%). The best fusion condition was a 2DC interval for 1 sec, 1.6 kV/cm voltages, and 40 μsec duration in 0.28 M mannitol. In conclusion, the in vitro maturation of Rex rabbit oocytes and SCNT procedures were studied systematically and optimized in this study.

Chemically assisted somatic cell nuclear transfer without micromanipulator in the goat: effects of demecolcine, cytochalasin-B, and MG-132 on the efficiency of a manual method of oocyte enucleation using a pulled Pasteur pipette

The present study aimed to facilitate widespread application of a previously described manual method of somatic cell nuclear transfer (SCNT) by investigating the effects of demecolcine (a microtubule-depolymerizing chemical), cytochalasin-B (a microfilament-depolymerizing chemical: 2.5μg/ml for 15min) and MG-132 (a proteasome inhibitor chemical) on the (i) incidence of cytoplasmic protrusion of MII chromosomes, (ii) improvement of manual oocyte enucleation, and (iii) in vitro and in vivo developmental competence of SCNT embryos in the goat. Following in vitro maturation, around 65% of goat oocytes contained a characteristic cytoplasmic protrusion of MII-chromosomes. Treatment with demecolcine (0.4μg/ml for 30min) significantly increased this rate to 92.2±4.5%. Treatment with MG-132 (2μM for 30min) could not improve this rate when used alone (61.4±11.5%), but when combined with demecolcine (86.4±8.1%). Treatment with cytochalasin-B completely suppressed this rate whenever used, either alone (7.7±5.1%) or in combination with demecolcine (3.9±1.3%). In a direct comparison, there was no significant difference in quantity and quality of embryos propagated by the manual vs. micromanipulation-based methods of SCNT (cleavage: 85.3±4.5 vs. 89.5±8.9%, blastocyst: 19.5±4.3 vs. 24.3±4.4%, grade 1 and 2 blastocyst: 33.8±7.1 vs. 29.5±6.3%, total cell count: 125±11.1 vs. 122±10.5, respectively). Furthermore, development to live kids at term was not significant between the two SCNT methods. From both technical and economical points of view, the overall in vitro and in vivo efficiency of this manual method of SCNT proved it a simple, fast and efficient alternative for large scale production of cloned goats.

Simple, fast, and efficient method of manual oocyte enucleation using a pulled Pasteur pipette

Cloning mammals by somatic cell nuclear transfer entails the replacement of oocyte chromosomes with the nucleus of a somatic cell. A major step in this technique is to efficiently produce large batches of enucleated oocytes, a process that requires considerable micromanipulation skills and expensive equipments. Here, a simple, fast, and efficient method of manual oocyte enucleation was introduced that can be adopted in every laboratory with the minimum equipments. Common laboratory glass pipettes were pulled on the flame of a burner and then used for manual bisection or enucleation of sheep and goat zona-free oocytes by passing them through the discontinuous cutting border of culture medium and mineral oil. The described techniques showed a certain efficiency to conveniently bisect or enucleate large batches of sheep, and goat oocytes being pre-treated with demecolcine. The method may be straightforward for simple manipulation of oocytes of other species and for development of automated cloning methods as well.

Chemically assisted enucleation results in higher G6PD expression in early bovine female embryos obtained by somatic cell nuclear transfer

Despite extensive efforts, low efficiency is still an issue in bovine somatic cell nuclear transfer (SCNT). The hypothesis of our study was that the use of cytoplasts produced by chemically assisted enucleation (EN) would improve nuclear reprogramming in nuclear transfer (NT)-derived embryos because it results in lower damage and higher cytoplasm content than conventional EN. For that purpose, we investigated the expression of two X-linked genes: X inactive-specific transcript (XIST) and glucose 6-phosphate dehydrogenase (G6PD). In the first experiment, gene expression was assessed in day-7 female blastocysts from embryonic cell NT (ECNT) groups [conventional, ECNT conv; chemically assisted, ECNT deme (demecolcine)]. Whereas in the ECNT conv group, only one embryo (25%; n=4) expressed XIST transcripts, most embryos showed XIST expression (75%; n=4) in the ECNT deme group. However, no significant differences in transcript abundance of XIST and G6PD were found when comparing the embryos from all groups. In a second experiment using somatic cells as nuclear donors, we evaluated gene expression profiles in female SCNT-derived embryos. No significant differences in relative abundance (RA) of XIST transcripts were observed among the groups. Nonetheless, higher (p<0.05) levels of G6PD were observed in SCNT deme and in vitro-derived groups in comparison to SCNT conv. To know whether higher G6PD expression in embryos derived from SCNT chemically assisted EN indicates higher metabolism in embryos considered of superior quality or if the presence of higher reactive oxygen species (ROS) levels generated by the increased oxygen consumption triggers G6PD activation, the expression of genes related to stress response should be investigated in embryos produced by that technique.

Enucleation of demecolcine-treated bovine oocytes in cytochalasin-free medium: mechanism investigation and practical improvement

Demecolcine-assisted/induced enucleation has been used in nuclear transfer cloning procedures for many species, yet its mechanism of action remains unclear. Primarily because oocytoplasm protrusion induced by demecolcine is inhibited by the presence of cytochalasin, its use has had limited application. In this experiment, we investigated the microtubule and microfilament alterations in bovine oocytes after demecolcine and/or cytochalasin B (CB) treatments by immunocytochemical staining. We also examined mechanical enucleation of demecolcine-treated oocytes in cytochalasin-free medium. The results showed that demecolcine-treatment disrupts the balance between microtubule/microfilament interactions primarily by deleting microtubules and with little effect on the microfilaments that we believe accounts for the membrane protrusion. The CB treatment reduced the amount of microfilaments but had little effect on the microtubules. Most demecolcine-induced membrane protrusions disappeared when exposed to CB. Western blotting showed that CB treatment increases G-actin, which indicates a decrease in the microfilaments. High oocyte enucleation, survival, and developmental rates occurred when demecolcine-assisted enucleation was carried out in a CB-free solution. Higher blastocyst development rates and blastocyst cell numbers were achieved compared to control, indicating that CB is not necessary in the enucleation procedure of bovine oocytes. This study provides a clearer understanding of the mechanism for the demecolcine-induced oocyte membrane protrusion, and substantiates the practical use of demecolcine-assisted enucleation in a CB-free environment.

Effect of the enucleation procedure on the reprogramming potential and developmental capacity of mouse cloned embryos treated with valproic acid

Mouse recipient cytoplasts for somatic cell nuclear transfer (SCNT) are routinely prepared by mechanical enucleation (ME), an invasive procedure that requires expensive equipment and considerable micromanipulation skills. Alternatively, oocytes can be enucleated using chemically assisted (AE) or chemically induced (IE) enucleation methods that are technically simple. In this study, we compared the reprogramming potential and developmental capacity of cloned embryos generated by ME, AE, and IE procedures and treated with the histone deacetylase inhibitor valproic acid. A rapid and almost complete deacetylation of histone H3 lysine 14 in the somatic nucleus followed by an equally rapid and complete re-acetylation after activation was observed after the injection of a cumulus cell nucleus into ME and AE cytoplasts. In contrast, histone deacetylation occurred at a much lower level in IE cytoplasts. Despite these differences, the cloned embryos generated from the three types of cytoplasts developed into blastocysts of equivalent total and inner cell mass mean cell numbers, and the rates of blastocyst formation and embryonic stem cell derivation were similar among the three groups. The cloned embryos produced from ME and AE cytoplasts showed an equivalent rate of full-term development, but no offspring could be obtained from the IE group, suggesting a lower reprogramming capacity of IE cytoplasts. Our results demonstrate the usefulness of AE in mouse SCNT procedures, as an alternative to ME. AE can facilitate oocyte enucleation and avoid the need for expensive microscope optics, or for potentially damaging Hoechst staining and u.v. irradiation, normally required in ME procedures.

Mouse cloning and somatic cell reprogramming using electrofused blastomeres

Mouse cloning from fertilized eggs can assist development of approaches for the production of "genetically tailored" human embryonic stem (ES) cell lines that are not constrained by the limitations of oocyte availability. However, to date only zygotes have been successfully used as recipients of nuclei from terminally differentiated somatic cell donors leading to ES cell lines. In fertility clinics, embryos of advanced embryonic stages are usually stored for future use, but their ability to support the derivation of ES cell lines via somatic nuclear transfer has not yet been proved. Here, we report that two-cell stage electrofused mouse embryos, arrested in mitosis, can support developmental reprogramming of nuclei from donor cells ranging from blastomeres to somatic cells. Live, full-term cloned pups from embryonic donors, as well as pluripotent ES cell lines from embryonic or somatic donors, were successfully generated from these reconstructed embryos. Advanced stage pre-implantation embryos were unable to develop normally to term after electrofusion and transfer of a somatic cell nucleus, indicating that discarded pre-implantation human embryos could be an important resource for research that minimizes the ethical concerns for human therapeutic cloning. Our approach provides an attractive and practical alternative to therapeutic cloning using donated oocytes for the generation of patient-specific human ES cell lines.

MPF governs the assembly and contraction of actomyosin rings by activating RhoA and MAPK during chemical-induced cytokinesis of goat oocytes

The interplay between maturation-promoting factor (MPF), mitogen-activated protein kinase (MAPK) and Rho GTPase during actin-myosin interactions has yet to be determined. The mechanism by which microtubule disrupters induce the formation of ooplasmic protrusion during chemical-assisted enucleation of mammalian oocytes is unknown. Moreover, a suitable model is urgently needed for the study of cytokinesis. We have established a model of chemical-induced cytokinesis and have studied the signaling events leading to cytokinesis using this model. The results suggested that microtubule inhibitors activated MPF, which induced actomyosin assembly (formation of ooplasmic protrusion) by activating RhoA and thus MAPK. While MAPK controlled actin recruitment on its own, MPF promoted myosin enrichment by activating RhoA and MAPK. A further chemical treatment of oocytes with protrusions induced constriction of the actomyosin ring by inactivating MPF while activating RhoA. In conclusion, the present data suggested that the assembly and contraction of the actomyosin ring were two separable steps: while an increase in MPF activity promoted the assembly through RhoA-mediated activation of MAPK, a decrease in MPF activity triggered contraction of the ring by activating RhoA.

Colcemid-treatment of heifer oocytes enhances nuclear transfer embryonic development, establishment of pregnancy and development to term

Four experiments were designed to examine the effects of colcemid, a microtubule assembly inhibitor, on the development of bovine nuclear transfer (NT) embryos in vitro and in vivo. Recipient oocytes matured at different times were exposed to colcemid. Approximately 80-93% of the exposed oocytes, with or without the first polar body (PB1), developed obvious membrane projections. In Experiment 1, oocytes matured for either 14-15 or 16-17 hr, treated with colcemid and used as recipient cytoplasm for NT resulted in over 40% blastocyst development. In Experiment 2, oocytes matured for 16-17 hr were treated with either 0.2 or 0.4 microg/ml colcemid for 2-3 or 5-6 hr, respectively. The percentages of blastocyst development (39-42%) were not statistically different among the different colcemid treatment groups, but were both higher (P < 0.05) than the control group (30%). Colcemid concentrations and length of colcemid treatment of oocytes did not affect their ability to support NT embryo development to the blastocyst and hatched blastocyst stages. Results from Experiment 3 indicate that semi-defined medium increases morula and blastocyst development of NT embryos derived from colcemid-treated oocytes under 5% CO2 in air atmosphere. In addition, cell numbers of blastocysts in colcemid-treated groups were numerically higher than the control groups. After embryo transfer, higher (P < 0.05) pregnant rates were obtained from the colcemid-treated group than the nontreated group. Five of 40 recipients (12.5%) which received embryos from colcemid-treated oocytes delivered healthy calves, significantly higher than those recipients (3.3%) that received embryos derived from nontreated oocytes.

Antimitotic treatments for chemically assisted oocyte enucleation in nuclear transfer procedures

Chemically assisted enucleation has been successfully applied to porcine and bovine oocytes to prepare recipient cytoplasts for nuclear transfer procedures. In this study, the antimitotic drugs demecolcine, nocodazole, and vinblastine were first assessed for their ability to induce the formation of cortical membrane protrusions in mouse, goat, and human oocytes. While only 2% of the treated human oocytes were able to form a protrusion, high rates of protrusion formation were obtained both in mouse (84%) and goat oocytes (92%), once the treatment was optimized for each species. None of the antimitotics applied was superior to the others in terms of protrusion formation, but mouse oocytes treated with vinblastine were unable to restore normal spindle morphology after drug removal and their in vitro development after parthenogenetic activation was severely compromised, rendering this antimitotic useless for chemically assisted enucleation approaches. Aspiration of the protrusions in mouse oocytes treated with demecolcine or nocodazole yielded 90% of successfully enucleated oocytes and allowed the extraction of a smaller amount of cytoplasm than with mechanical enucleation, but both enucleation methods resulted in the depletion of spindle-associated gamma-tubulin from the prepared cytoplasts. Treatment of mouse oocytes with demecolcine or nocodazole had no effect on their in vitro development after parthenogenetic activation, or on their ability to repolymerize a new spindle after the removal of the drug or the reconstruction of the treated cytoplasts with a somatic nucleus. Therefore, demecolcine- and nocodazole-assisted enucleation appears as an efficient alternative to mechanical enucleation, which can simplify nuclear transfer procedures.