Nuclear transfer of adult and genetically modified fetal cells of the rat

Reactive oxygen species signalling in the deterioration of quality of mammalian oocytes cultured in vitro: Protective effect of antioxidants

The in vitro fertilization (IVF) is the first choice of infertile couples worldwide to plan for conception. Besides having a significant advancement in IVF procedure, the success rate is still poor. Although several approaches have been tested to improve IVF protocol, minor changes in culture conditions, physical factors and/or drug treatment generate reactive oxygen species (ROS) in oocytes. Due to large size and huge number of mitochondria, oocyte is more susceptible towards ROS-mediated signalling under in vitro culture conditions. Elevation of ROS levels destabilize maturation promoting factor (MPF) that results in meiotic exit from diplotene as well as metaphase-II (M-II) arrest in vitro. Once meiotic exit occurs, these oocytes get further arrested at metaphase-I (M-I) stage or metaphase-III (M-III)-like stage under in vitro culture conditions. The M-I as well as M-III arrested oocytes are not fit for fertilization and limits IVF outcome. Further, the generation of excess levels of ROS cause oxidative stress (OS) that initiate downstream signalling to initiate various death pathways such as apoptosis, autophagy, necroptosis and deteriorates oocyte quality under in vitro culture conditions. The increase of cellular enzymatic antioxidants and/or supplementation of exogenous antioxidants in culture medium protect ROS-induced deterioration of oocyte quality in vitro. Although a growing body of evidence suggests the ROS and OS-mediated deterioration of oocyte quality in vitro, their downstream signalling and related mechanisms remain poorly understood. Hence, this review article summarizes the existing evidences concerning ROS and OS-mediated downstream signalling during deterioration of oocyte quality in vitro. The use of various antioxidants against ROS and OS-mediated impairment of oocyte quality in vitro has also been explored in order to increase the success rate of IVF during assisted reproductive health management.

Selection of Rattus norvegicus cumulus-oocyte complex for vitrification by brilliant cresyl blue

The influence of the method of evaluating developmentally competent oocytes on their viability after cryopreservation still needs to be better understood. The objective of this study was to determine the cleavage and embryo developmental rates after parthenogenetic activation of cumulus-oocyte complexes (COCs) selected by different concentrations of brilliant cresyl blue (BCB) and cryopreservation. In the first experiment, COCs were separated into groups and incubated for 1 h in medium containing BCB (13 μM, 16 μM, or 20 μM). The control group was not exposed to BCB staining. In the second experiment, COCs were divided into four groups: 13 μM BCB(+), 13 μM BCB(-), fresh control (selected by morphologic observation and immediately in vitro matured) and vitrified control (selected by morphologic evaluation, vitrified, and in vitro matured). In the first experiment, the 13 μM BCB group displayed greater development rates at the morula stage (65.45%, 36/55) when compared with the other groups. In the second experiment, cleavage (47.05%, 72/153) and morula development (33.55%, 51/153) of the control group of fresh COCs were increased compared with the other groups. However, when comparing morula rates between vitrified COC control and BCB(+) groups, the BCB(+) group had better results (19.23%, 5/26 and 64.7%, 11/17, respectively). Our best result in rat COC selection by BCB staining was obtained using a concentration of 13 μM. This selection could be a valuable tool to improve vitrification outcomes, as observed by the BCB(+) group that demonstrated better results compared with the vitrified COC control.

Oocyte Spontaneous Activation: An Overlooked Cellular Event That Impairs Female Fertility in Mammals

In mammals, including humans, mature oocytes are ovulated into the oviduct for fertilization. Normally, these oocytes are arrested at metaphase of the second meiosis (MII), and this arrest can be maintained for a certain period, which is essential for fertilization in vivo and oocyte manipulations in vitro, such as assisted reproduction in clinics and nuclear/spindle transfer in laboratories. However, in some species and under certain circumstances, exit from MII occurs spontaneously without any obvious stimulation or morphological signs, which is so-called oocyte spontaneous activation (OSA). This mini-review summarizes two types of OSA. In the first type (e.g., most rat strains), oocytes can maintain MII arrest in vivo, but once removed out, oocytes undergo OSA with sister chromatids separated and eventually scattered in the cytoplasm. Because the stimulation is minimal (oocyte collection itself), this OSA is incomplete and cannot force oocytes into interphase. Notably, once re-activated by sperm or chemicals, those scattered chromatids will form multiple pronuclei (MPN), which may recapitulate certain MPN and aneuploidy cases observed in fertility clinics. The second type of OSA occurs in ovarian oocytes (e.g., certain mouse strains and dromedary camel). Without ovulation or fertilization, these OSA-oocytes can initiate intrafollicular development, but these parthenotes cannot develop to term due to aberrant genomic imprinting. Instead, they either degrade or give rise to ovarian teratomas, which have also been reported in female patients. Last but not the least, genetic models displaying OSA phenotypes and the lessons we can learn from animal OSA for human reproduction are also discussed.

Meiotic Instability Generates a Pathological Condition in Mammalian Ovum

Maintenance of metaphase-II (M-II) arrest in ovum is required to present itself as a right gamete for successful fertilization in mammals. Surprisingly, instability of meiotic cell cycle results in spontaneous exit from M-II arrest, chromosomal scattering and incomplete extrusion of second polar body (PB-II) without forming pronuclei so called abortive spontaneous ovum activation (SOA). It remains unclear what causes meiotic instability in freshly ovulated ovum that results in abortive SOA. We propose the involvement of various signal molecules such as reactive oxygen species (ROS), cyclic 3',5' adenosine monophosphate (cAMP) and calcium (Ca²⁺) in the induction of meiotic instability and thereby abortive SOA. These signal molecules through their downstream pathways modulate phosphorylation status and activity of cyclin dependent kinase (cdk1) as well as cyclin B1 level. Changes in phosphorylation status of cdk1 and its activity, dissociation and degradation of cyclin B1 destabilize maturation promoting factor (MPF). The premature MPF destabilization and defects in other cell cycle regulators possibly cause meiotic instability in ovum soon after ovulation. The meiotic instability results in a pathological condition of abortive SOA and deteriorates ovum quality. These ova are unfit for fertilization and limit reproductive outcome in several mammalian species including human. Therefore, global attention is required to identify the underlying causes in greater details in order to address the problem of meiotic instability in ova of several mammalian species icluding human. Moreover, these activated ova may be used to create parthenogenetic embryonic stem cell lines in vitro for the use in regenerative medicine.Graphical abstract.

Optimized Protocols for In Vitro Maturation of Rat Oocytes Dramatically Improve Their Developmental Competence to a Level Similar to That of Ovulated Oocytes

The developmental capacity of in vitro-matured (IVM) oocytes is markedly lower than that of their in vivo-matured (IVO) counterparts, suggesting the need for optimization of IVM protocols in different species. There are few studies on IVM of rat oocytes, and there are even fewer attempts to improve ooplasmic maturation compared to those reported in other species. Furthermore, rat oocytes are well known to undergo spontaneous activation (SA) after leaving the oviduct; however, whether IVM rat oocytes have lower SA rates than IVO oocytes and can potentially be used for nuclear transfer is unknown. In this study, we investigated the effects of maturation protocols on cytoplasmic maturation of IVM rat oocytes and observed the possibility to reduce SA by using IVM rat oocytes. Ooplasmic maturation was assessed using multiple markers, including pre- and postimplantation development, meiotic progression, CG redistribution, redox state, and the expression of developmental potential- and apoptosis-related genes. The results showed that the best protocol consisting of modified Tissue Culture Medium-199 (TCM-199) supplemented with cysteamine/cystine and the cumulus cell monolayer dramatically improved the developmental competence of rat oocytes and supported both pre- and postimplantation development and other ooplasmic maturation makers to levels similar to that observed in ovulated oocytes. Rates of SA were significantly lower in IVM oocytes than in IVO oocytes when observed at the same intervals after nuclear maturation. In conclusion, we have optimized protocols for IVM of rat oocytes that sustain ooplasmic maturation to a level similar to ovulated oocytes. The results suggest that IVM rat oocytes might be used to reduce SA for rat cloning.

Role of Na+/Ca2+ exchanger (NCX) in modulating postovulatory aging of mouse and rat oocytes

We studied the role of the Na+/Ca2+ exchanger (NCX) in modulating oocyte postovulatory aging by observing changes in NCX contents and activities in aging mouse and rat oocytes. Whereas the NCX activity was measured by observing oocyte activation following culture with NCX inhibitor or activator, the NCX contents were determined by immunohistochemical quantification. Although NCX was active in freshly-ovulated rat oocytes recovered 13 h post hCG injection and in aged oocytes recovered 19 h post hCG in both species, it was not active in freshly-ovulated mouse oocytes. However, NCX became active when the freshly-ovulated mouse oocytes were activated with ethanol before culture. Measurement of cytoplasmic Ca2+ revealed Ca2+ increases always before NCX activation. Whereas levels of the reactive oxygen species (ROS) and the activation susceptibility increased, the density of NCX member 1 (NCX1) decreased significantly with oocyte aging in both species. While culture with H2O2 decreased the density of NCX1 significantly, culture with NaCl supplementation sustained the NCX1 density in mouse oocytes. It was concluded that (a) the NCX activity was involved in the modulation of oocyte aging and spontaneous activation; (b) ROS and Na+ regulated the NCX activity in aging oocytes by altering its density as well as functioning; and (c) cytoplasmic Ca2+ elevation was essential for NCX activation in the oocyte.

An insufficient increase of cytosolic free calcium level results postovulatory aging-induced abortive spontaneous egg activation in rat

PURPOSE

The present study was aimed to find out whether postovulatory aging-induced abortive spontaneous egg activation (SEA) is due to insufficient increase of cytosolic free Ca(2+) level.

METHODS

Immature female rats (22-24 days old) were subjected to superovulation induction protocol. Eggs were collected 14, 17 and 19 h post-hCG surge to induce in vivo egg aging. The eggs were collected 14 h post-hCG surge and cultured in vitro for 3, 5 and 7 h to induce in vitro egg aging. The morphological changes, rate of abortive SEA, chromosomal status and cytosolic free Ca(2+) levels were analyzed.

RESULTS

Postovulatory aging induced morphological features characteristics of abortive SEA in a time-dependent manner in vivo as well as in vitro. The extracellular Ca(2+) increased rate of abortive SEA during initial period of culture, while co-addition of a nifedipine (L-type Ca(2+) channel blocker) protected against postovulatory aging-induced abortive SEA. However, CI induced morphological features characteristics of egg activation (EA) in a dose-dependent manner. As compare to control, an increase of cytosolic free Ca(2+) level (1.42 times) induced abortive SEA, while further increase of cytosolic free Ca(2+) level (2.55 times) induced EA.

CONCLUSION

Our results show that an insufficient cytosolic free Ca(2+) level is associated with postovulatory aging -induced abortive SEA, while furthermore increase is required to induce EA in rat.

Roles of MAPK and spindle assembly checkpoint in spontaneous activation and MIII arrest of rat oocytes

Rat oocytes are well known to undergo spontaneous activation (SA) after leaving the oviduct, but the SA is abortive with oocytes being arrested in metaphase III (MIII) instead of forming pronuclei. This study was designed to investigate the mechanism causing SA and MIII arrest. Whereas few oocytes collected from SD rats at 13 h after hCG injection that showed 100% of mitogen-activated protein kinase (MAPK) activities activated spontaneously, all oocytes recovered 19 h post hCG with MAPK decreased to below 75% underwent SA during in vitro culture. During SA, MAPK first declined to below 45% and then increased again to 80%; the maturation-promoting factor (MPF) activity fluctuated similarly but always began to change ahead of the MAPK activity. In SA oocytes with 75% of MAPK activities, microtubules were disturbed with irregularly pulled chromosomes dispersed over the spindle and the spindle assembly checkpoint (SAC) was activated. When MAPK decreased to 45%, the spindle disintegrated and chromosomes surrounded by microtubules were scattered in the ooplasm. SA oocytes entered MIII and formed several spindle-like structures by 6 h of culture when the MAPK activity re-increased to above 80%. While SA oocytes showed one Ca²⁺ rise, Sr²⁺-activated oocytes showed several. Together, the results suggested that SA stimuli triggered SA in rat oocytes by inducing a premature MAPK inactivation, which led to disturbance of spindle microtubules. The microtubule disturbance impaired pulling of chromosomes to the spindle poles, caused spindle disintegration and activated SAC. The increased SAC activity reactivated MPF and thus MAPK, leading to MIII arrest.

Rat eggs cannot wait: Spontaneous exit from meiotic metaphase-II arrest

Mammalian eggs await fertilisation while arrested at the second metaphase stage of meiotic division. A network of signalling pathways enables the establishment and maintenance of this metaphase-II arrest. In the absence of fertilisation, mammalian eggs can spontaneously exit metaphase II when parthenogenetically stimulated, or sometimes without any obvious stimulation. Ovulated rat eggs abortively release from metaphase-II arrest once removed from egg donors. Spontaneously activated rat eggs extrude the second polar body and proceed to the so-called metaphase III-'like' stage, with clumps of condensed chromatin scattered in the egg cytoplasm. It is still unclear what makes rat eggs susceptible to spontaneous activation; however, a vague picture of the signalling pathways involved in the process of spontaneous activation is beginning to emerge. Such cell cycle instability is one of the major reasons why it is more difficult to establish nuclear transfer in the rat. This review examines the known predisposing factors and biochemical mechanisms involved in spontaneous activation. The strategies used to prevent spontaneous metaphase-II release in rat eggs will also be discussed.

Effect of culture conditions on viability of mouse and rat embryos developed in vitro

Currently in vitro culture of mouse preimplantation embryos has become a very important technique to investigate different mechanisms of early embryogenesis. However, there is a big difference in the preimplantation development between mammalian species. Despite close relatedness to mice, in vitro cultivation of rat preimplantation embryos is still delicate and needs further investigation and optimizations. In this study we have compared the in vitro developmental potential of mouse and rat embryos cultured at different culture conditions in parallel experiments. Interestingly, mouse zygotes developed in vitro until blastocyst stage even in inadequate medium without any phosphates and with low osmolarity which was formulated especially for cultivation of rat embryos. Rat parthenotes and zygotes developed in M16 medium formulated for mouse embryos only till 2-cell stage and further development is blocked completely at this stage. Moreover, developmental ability of rat embryos in vitro was significantly lower in comparison with mouse even in special rat mR1ECM medium. Mouse and rat embryos at 2-cell stage obtained in vivo developed until blastocyst stages significantly more efficiently compared to zygotes. Culture of mouse zygotes in glass capillaries resulted in a significantly higher rate of morula and blastocyst development compared with dishes. The Well-of-the-Well system resulted in a significant improvement when compared with dishes for the culture of rat zygotes only until morula stage. Reduced oxygen tension increased the developmental rate of rat but not mouse zygotes until blastocyst stage. This study demonstrates that development of early preimplantation embryos is altered by different culture conditions and show strong differences even between two related species such as mice and rats. Therefore, for understanding the fundamental mechanisms of early mammalian development it is very important to use embryos of various species.

Superovulatory response, oocyte spontaneous activation, and embryo development in WMN/Nrs inbred rats

WMN/Nrs inbred rats have been widely used in radiation biology for years. However, their reproductive profile has never been examined. We examined various reproductive characteristics of WMN/Nrs inbred rats such as superovulatory response, oocytes spontaneous activation (OSA), and embryo development in vitro and in vivo. Superovulation was induced in 3- to 9-week-old females by injection of 150 IU/kg PMSG and 150 IU/Kg hCG by 48 h apart. Only 8- and 9-week-old animals superovulated averaging 31.4 and 43.9 oocytes, respectively, and superovulation did not depend on estrous cycle. Animals 3-7 weeks of age did not superovulate. Because Wistar strains have been known to show a high incidence of OSA, factors expected to affect OSA in WMN/Nrs, including the time interval of various steps from euthanasia to oocyte recovery, incubation media, estrous cycle, and anesthetic treatments, were examined. The time from animal euthanasia to oviduct excision was the only factor shown to affect OSA. We also compared in vitro and in vivo embryo developmental competence between embryos obtained by natural ovulation and superovulation. Although percent in vitro development of 2-cell embryos to blastocysts was similar for embryos obtained by natural ovulation (63.7%) and superovulation (69.7%), fetus development after oviductal transfer of 2-cell embryos was significantly lower in embryos obtained by superovulation than in those obtained by natural ovulation (60.2% vs. 87.5%, P=0.02). Our results provide important normative data regarding future applications of rat assisted reproductive technologies (ARTs) such as in vitro fertilization and cryopreservation in WMN/Nrs strain and may be applicable to other strains of laboratory rats.

Evaluation of in vitro cultured rat oocytes, from different strains, by spindle morphology and maturation-promoting-factor activity combined with nuclear-transfer experiments

Although successful nuclear transfer (NT) has been reported in the rat 6 years ago, somatic cell nuclear transfer (SCNT) in the rat could not be repeated. Our experiments with rat SCNT reveal the difficulties related to rat cloning. We first focussed on the most appropriate rat strain that could be used as an oocyte donor. Then we describe how rat oocytes can be kept in a nonactivated state during in vitro culture, because the latter undergo spontaneous partial activation through rapid extrusion of the second polar body after isolation from the oviduct. In the SCNT experiments performed with the one-step manipulation technique it was possible to produce rat embryos, which developed in vivo up to the blastocyst stage. In addition, we identified the implantation sites of SCNT rat embryos reconstructed with Sprague-Dawley (SD) oocytes. Furthermore, different rat strains were used as oocyte donors and their oocytes were cultured under different conditions to establish a stable nonactivating oocyte culture system. The ratio of activated to nonactivated oocytes was measured by spindle-stability and maturation promoting factor (MPF) activity. These measurements indicated that a substrain of the SD rat strain, the so-called OFA-SD strain, is the one providing the most stable oocytes, when their oocytes are cultured in the presence of the proteasome inhibitor MG132. However, it was not possible to obtain any implantation sites with reconstructed oocytes derived from the OFA-SD strain transferred to foster mothers. This goal was not achieved, even when the trichostatin A (TSA) treatment was used, which is known to enhance the cloning efficiency of reconstructed mouse, porcine, bovine, and rabbit oocytes both in vitro and in vivo by enhancing the reprogramming efficiency of the recipient nucleus.

The effect of the time interval between injection and parthenogenetic activation on the spindle formation and the in vitro developmental potential of somatic cell nuclear-transferred rat oocytes

We examined the optimal conditions for somatic cell nuclear transfer (SCNT) in the rat. First, we examined the effect of preincubation time before activation on SCNT rat oocytes produced in the presence of MG132 with regard to spindle formation and the potential to develop into blastocysts. The spindles of SCNT oocytes continued to elongate with an increase in the culture duration and, in approximately half of oocytes, the chromosomes were distributed along the spindles at 120 min after incubation. Such abnormal spindle formation in SCNT oocytes is a possible reason for the low developmental potential of SCNT rat oocytes. To inhibit the formation of abnormal spindle formation, we examined secondly the developmental potential of rat SCNT oocytes that had been preincubated with nocodazole and demecolcine instead of MG132. The developmental rates in SCNT oocytes, however, were decreased. For successful rat somatic cell cloning, two steps might be required: (1) to culture the somatic cell nuclei for a sufficient time in MII oocyte cytoplasm to enhance nuclear reprogramming; and (2) to induce normal spindle formation with normal chromosomal construction.

Development and spindle formation in rat somatic cell nuclear transfer (SCNT) embryos in vitro using porcine recipient oocytes

Cloning that uses somatic cell nuclear transfer (SCNT) technology with gene targeting could be a potential alternative approach to obtain valuable rat models. In the present study, we determined the developmental competence of rat SCNT embryos constructed using murine and porcine oocytes at metaphase II (MII). Further, we assessed the effects of certain factors, such as: (i) the donor cell type (fetal fibroblasts or cumulus cells); and (ii) premature chromosome condensation (PCC) with normal spindle formation, on the developmental competence of rat interspecies SCNT (iSCNT) embryos. iSCNT embryos that had been constructed using porcine oocytes developed to the blastocyst stage, while those embryos made using murine MII oocytes did not. Rat iSCNT embryos constructed with green fluorescent protein (GFP)-expressing fetal fibroblasts injected into porcine oocytes showed considerable PCC with a normal bipolar spindle formation. The total cell number of iSCNT blastocyst derived from GFP-expressing fetal fibroblasts was higher than the number derived from cumulus cells. In addition, these embryos expressed GFP at the blastocyst stage. This paper is the first report to show that rat SCNT embryos constructed using porcine MII oocytes have the potential to develop to the blastocyst stage in vitro. Thus the iSCNT technique, when performed using porcine MII oocytes, could provide a new bioassay system for the evaluatation of the developmental competence of rat somatic cells.

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.

Efficient production of nuclear transferred rat embryos by modified methods of reconstruction

In this study we investigated spontaneous oocyte activation and developmental ability of rat embryos of the SD-OFA substrain. We also tried to improve the somatic cell nuclear transfer (SCNT) technique in the rat by optimizing methods for the production of reconstructed embryos. About 20% of oocytes extruded the second polar body after culture for 3 hr in vitro and 84% of oocytes were at the MII stage. MG132 blocked spontaneous activation but decreased efficiency of parthenogenetic activation. Pronuclear formation was more efficient in strontium-activated oocytes (66.1-80.9%) compared to roscovitine activation (24.1-54.5%). Survival rate after enucleation was significantly higher (89.4%) after slitting the zona pellucida and then pressing the oocyte with a holding pipette in medium without cytochalasin B (CB) compared to the conventional protocol using aspiration of the chromosomes after CB treatment (67.7%). Exposure of rat ova to UV light for 30 sec did not decrease their in vitro developmental capacity. Intracytoplasmic cumulus cell injection dramatically decreased survival rate of oocytes (42%). In contrast, 75.9% of oocytes could be successfully electrofused. Development to the 2-cell stage was reduced after SCNT (24.6% compared 94.6% in controls) and none from 244 reconstructed embryos developed in vitro beyond this stage. After overnight in vitro culture, 74.4% of the SCNT embryos survived and 56.1% formed pronuclei. The pregnancy rate of 33 recipients after the transfer of 695 of these cloned embryos was, however, very low (18.2%) and only six implantation sites could be detected (0.9%) without any live fetuses and offspring.

Overcoming MIII arrest from spontaneous activation in cultured rat oocytes

The rat oocyte spontaneously activates under a wide variety of conditions. This process progresses to MIII arrest that is not responsive to parthenogenetic activation and development. Insofar as activation involves extrusion of the second polar body (PBII), we set out to determine if preventing this step by inhibiting microfilaments would change the course of spontaneous activation (SA). In particular, how long does the effect of SA persist while retaining reversibility of PBII extrusion once inhibitors are removed? We wanted to determine if the eggs would be responsive to parthenogenetic activation and capable of resuming development once a permanent inhibition is achieved. We set out to determine whether SA would depend on the ovular age of oocytes. Inhibiting of PBII extrusion was achieved by affecting microtubules with demecolcine or nocodazole or actin filaments with cytochalasin B (CB) and cytochalasin D (CD). We found that all oocytes undergo SA and progression to MIII; however, the rapidity of spontaneous activation is a function of the ovular age of the oocyte. The resumption of the meiosis period changes dramatically from 20 to 180 min with decreasing ovular age. We established that suppression of PB formation can be effectively achieved in oocytes of younger ovular age, and that inhibition of PB extrusion became irreversible after 3.5 h of treatment. We established that drug-treated oocytes could undergo subsequent reactivation and in vitro development to blastocysts. The rate of in vitro development of cytochalasin-treated group was comparable to parthenogenetic controls, while nocodazole and demecolcine produced oocytes that developed at lower frequencies. Thus, the application of the microfilament inhibiting drugs helps to overcome the negative effect of SA that results in MIII arrest. Here we also show optimized parthenogenetic stimulation that resulted in development to the blastocyst stage at frequency comparable to development of fertilized embryos.

Extracellular calcium induces activation of Ca(2+)/calmodulin-dependent protein kinase II and mediates spontaneous activation in rat oocytes

Ovulated rat oocytes are activated spontaneously soon after recovery from the oviducts. To investigate the kinetics and mechanism of rat oocyte spontaneous activation (OSA), we investigated the effect of aging in oviducts, hyaluronidase treatment, and extracellular and intracellular calcium, and examined the activity of CaMKII and the effect of its inhibitor on OSA. Oocyte aging in oviducts and hyaluronidase did not affect OSA. However, OSA was significantly decreased in calcium-free medium and in calcium-containing medium containing L-type calcium channel blocker and IP(3)R inhibitor. Moreover, significantly lower OSA was shown with an inhibitor of CaMKII. There was a significant increase of CaMKII activity at 30min after oocyte recovery and constitutively active CaMKII was located near the meiotic spindle in freshly recovered oocytes. Therefore, CaMKII is one of the upstream signals to activate rat oocytes spontaneously after recovery and rat oocytes respond very sensitively to extracellular calcium.

Effect of Enucleation on Inactivation of Cytostatic Factor Activity in Matured Rat Oocytes

In mammals, matured oocytes are arrested at the MII stage until fertilization, which is regulated by cytostaticfactor (CSF) activity. Maturation-promoting factor (MPF) and the mitogen-activated protein kinase (MAPK) pathway are known as candidates for CSF. Despite of the results that nuclear and perinuclear materials were dispensable for activation of MPF and MAPK in other species, our previous study in rats demonstrated that MPF activity was rapidly decreased after enucleation. We showed here for the first time that nuclear and perinuclear materials were indispensable for CSF activity in matured rat oocytes. In both cytoplasm-removed and enucleated oocytes, high activity of p34(cdc2) kinase was observed immediately after manipulation, but the activity of enucleated oocytes was dramatically reduced within 1 h. Cyclin B level was also decreased, corresponding with inactivation of p34(cdc2) kinase. In enucleated oocytes, the Mos level was dramatically decreased, and both MEK and MAPK dephosphorylation were also induced. A combined treatment with a proteasome inhibitor, MG132, and a protein phosphatase inhibitor, okadaic acid, dramatically improved both levels of p-MAPK and cyclin B in these enucleated oocytes. These data suggest that nuclear and perinuclear materials of matured rat oocytes suppress proteasome and protein phosphatase activation, which is indispensable for stability of CSF.

Spindle formation and microtubule organization during first division in reconstructed rat embryos produced by somatic cell nuclear transfer

The present study was conducted to demonstrate the spindle formation and behavior of chromosomes and microtubules during first division in reconstructed rat embryos produced by somatic cell nuclear transfer (SCNT) with cumulus cell nuclei. To demonstrate the effect of oocyte aging after ovulation on the cleavage of SCNT embryos, micromanipulation was carried out 11, 15 and 18 h after injection of hCG. SCNT oocytes were activated by incubation in culture medium supplemented with 5 microM ionomycin for 5 min followed by treatment with 2 mM 6-dimethylaminopurine (6-DMAP) in mR1ECM for 2-3 h. For immunocytochemical observation, the SCNT embryos were incubated with monoclonal anti-alpha-tubulin antibody and then fluorescein isothiocyanate (FITC)-conjugated goat anti-mouse IgG. Cleavage rates were significantly higher for oocytes collected after 15 and 18 h rather than for those collected 11 h after injection of hCG (56 and 53%, respectively vs. 28%; P<0.05). Premature chromosome condensation occurred before activation of the SCNT oocytes, but adequate spindle formation was only rarely observed. The distribution of microtubules in SCNT embryos after activation was different from those of fertilized and parthenogenic oocytes, i.e., a dense microtubule organization shaped like a ring was observed. Eighteen to 20 h post-activation, most SCNT embryos were in the 2-cell stage, but no nucleoli were clearly visible, which was quite different from the fertilized oocytes. In addition, first division with and without small cellular bodies containing DNA was observed in the rat SCNT embryos in some cases. The present study suggests that reorganization of transferred nuclei in rat SCNT embryos may be inadequate in terms of formation of the mitotic assembly and nucleolar reorganization.

Oocyte spontaneous activation in different rat strains

Oocyte spontaneous activation (OSA) has been reported to occur during in vitro culture of ovulated rat oocytes. The objective of this study was to compare the rate of oocyte spontaneous activation and the level of maturation promoting factor (MPF) activity in oocytes from different strains. Twelve strains were selected from two commercial sources. Females were superovulated and oocytes collected 17 h after hCG injection. Denuded oocytes were cultured in M16 medium under oil at 37 degrees C and 5% CO(2) in air. The proportion of activated oocytes was determined after 6 h of in vitro culture. Data were compared by analysis of variance (ANOVA), considering each animal as an experimental unit. MPF activity was determined in oocytes from the different strains at 0, 1.5, and 3 h after oocyte collection. The log ratio of the MPF activity at 1.5 and 3 h relative to 0 hours for each animal was analyzed by ANOVA. While significant (p < 0.01) differences were observed between strains in the rate of OSA, there were no differences between strains in the level of MPF during the time points measured (p > 0.3).

Cloning in companion animal, non-domestic and endangered species: can the technology become a practical reality?

Somatic cell nuclear transfer (SCNT) can provide a unique alternative for the preservation of valuable individuals, breeds and species. However, with the exception of a handful of domestic animal species, successful production of healthy cloned offspring has been challenging. Progress in species that have little commercial or research interest, including many companion animal, non-domestic and endangered species (CANDES), has lagged behind. In this review, we discuss the current and future status of SCNT in CANDES and the problems that must be overcome to improve pre- and post-implantation embryo survival in order for this technology to be considered a viable tool for assisted reproduction in these species.

Transgenic modifications of the rat genome

The laboratory rat (R. norvegicus) is a very important experimental animal in several fields of biomedical research. This review describes the various techniques that have been used to generate transgenic rats: classical DNA microinjection and more recently described techniques such as lentiviral vector-mediated DNA transfer into early embryos, sperm-mediated transgenesis, embryo cloning by nuclear transfer and germline mutagenesis. It will also cover techniques associated to transgenesis such as sperm cryopreservation, embryo freezing and determination of zygosity. The availability of several technologies allowing genetic manipulation in the rat coupled to genomic data will allow biomedical research to fully benefit from the rat as an experimental animal.

Parthenogenetic activation of rat oocytes and their development (in vitro)

The present study was performed to determine suitable methods for parthenogenetic activation and subsequent development of rat oocytes in vitro. In the first series of experiments, the ability of electrical pulses, strontium, ethanol and ionomycin to activate Sprague-Dawley (SD) rat oocytes was examined. The synergistic effect of strontium and cycloheximide or puromycin was also examined in the second series of experiments. In the third series of experiments, the development of F1 hybrid (SD x Dark Agouti) parthenotes activated with different concentrations of strontium (10-0.08 mM) was compared with that of SD parthenotes. The effect of the timing of activation (10 min and 2, 4 and 6 h after cervical dislocation) was also assessed in a fourth series of experiments. The oocytes activated by strontium showed higher pronuclear formation and cleavage rates than those in the other groups (P < 0.05). Higher blastocyst development was obtained from parthenotes activated by strontium and strontium-cycloheximide compared with the strontium-puromycin group (P < 0.01). However, the total cell number of blastocysts from the strontium-cycloheximide activation group was higher than that of other groups (P < 0.05). With strontium (2.5-10 mM) treatment, 40.9% of blastocysts were obtained from F1 hybrid oocytes, whereas 22.9% were obtained from SD (P < 0.01). The oocytes activated 10 min or 2 h following cervical dislocation showed higher blastocyst development than those of the 4 and 6 h groups (P < 0.01). These results suggest that strontium-cycloheximide produces the highest parthenogenetic activation rate in the rat and that oocytes must be activated by 2 h after cervical dislocation.

Genetic manipulation in nutrition, metabolism, and obesity research

We summarize the current standard methods for overexpressing, inactivating, or manipulating genes, with special focus on nutritional and obesity research. These molecular biology procedures can be carried out with the maintenance of the genetic information to subsequent generations (transgenic technology) or devised to exclusively transfer the genetic material to a given target animal, which cannot be transmitted to the future progeny (gene therapy). On the other hand, the RNA interference (RNAi) approach allows for the creation of new experimental models by transient ablation of gene expression by degrading specific mRNA, which can be applied to assess different biological functions and mechanisms. The combination of these technologies contributes to the study of the function and regulation of different metabolism- and obesity-related genes as well as the identification of new pharmacologic targets for nutritional and therapeutic approaches.

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.

Review of Enucleation Methods and Procedures Used in Animal Cloning: State of the Art

Enucleation of a recipient oocyte is a crucially important process for nuclear transfer efficiency. Several procedures have been developed and used in the production of nuclear transfer embryos. Although the use of excitable fluorochromes and ultraviolet (UV) light are commonly used for complete enucleation, they also pose the risk of damaging the maternal cytoplast. Telophase and chemically assisted enucleation have also been used for cloning, but the quality and quantity of the recipient cytoplasm varies with the procedure used. This paper reviews various methods used for enucleation, and discusses their benefits and limitations with respect to cloning efficiency.

Determination of Optimal Conditions for Parthenogenetic Activation and Subsequent Development of Rat Oocytes In Vitro

The present study was undertaken to determine optimal conditions for parthenogenetic activation and subsequent development of rat oocytes. Oocytes from immature Wistar-Imamichi (WI) and Sprague Dawley (SD) rats were activated by electrical stimulation in combination with 6-dimethylaminopurine (6-DMAP) to assess whether different rat strains display different responses to activation treatment. Since the cleavage rates of activated oocytes were significantly higher in WI than SD strain rats, WI rats were used for the subsequent experiments to determine the effects of post-hCG time, culture duration, different activation protocols (electrical stimulation with 6-DMAP or ionomycin with 6-DMAP) and osmolarity of the activation medium on the activation and subsequent development of WI rat oocytes. For oocytes activated by electrical stimulation combined with 6-DMAP, the percentages of oocytes that were activated and that developed to blastocysts were higher when oocytes were collected at 18-20 h than at any other time points after hCG injection (16, 22-24 h). Culturing for 2-6 h before activation treatment markedly decreased the percentage of activated oocytes that developed to beyond the four-cell stage. There were no differences in the percentages of oocytes with pronuclear formation and subsequent development to the two-cell and blastocyst stages between oocytes that were activated by electrical stimulation or ionomycin, both followed by 6-DMAP treatment. Activation of oocytes by ionomycin and 6-DMAP, both in low osmolarity media (246 mOsM), markedly increased the cleavage rates and percentages of high quality blastocysts (71%). The optimal conditions determined in the present study with simplified activation protocols and high efficiency of activation and subsequent development of WI rat oocytes will be helpful for further research involving nuclear transfer in the rat.

Human cloning: can it be made safe?

There are continued claims of attempts to clone humans using nuclear transfer, despite the serious problems that have been encountered in cloning other mammals. It is known that epigenetic and genetic mechanisms are involved in clone failure, but we still do not know exactly how. Human reproductive cloning is unethical, but the production of cells from cloned embryos could offer many potential benefits. So, can human cloning be made safe?

Factors influencing chromosome condensation and development of cloned rat embryos

Factors influencing premature chromosome condensation (PCC) in transferred rat nuclei have been examined. Chromosome condensation of rat cumulus cell nuclei did not occur when the cell nuclei were injected into enucleated rat oocytes. By contrast, chromosome condensation did occur after transfer to enucleated mouse oocytes or intact rat oocytes. In the first serial NT experiment, rat somatic cell nuclei were injected into enucleated mouse oocytes, and the reconstructed oocytes were activated by strontium chloride. From these reconstructed embryos, karyoplasts containing pronucleus-like vesicles were transferred into pronuclear zygote-derived cytoplasts by a DC pulse. Transfer of a total of 340 serial NT zygotes into recipient females, including 206 two-cell embryos, resulted in only seven implantation sites. In the second serial NT experiment, rat somatic cell nuclei were injected into intact rat oocytes; the recipient metaphase-plate was then aspirated under UV light from the NT oocytes in which PCC of injected nuclei was observed. After activation of the NT oocytes, karyoplasts were introduced into zygote-derived cytoplasts. Transfer of a total of 115 serial NT zygotes, including 37 two-cell embryos, resulted in four implantation sites but no live offspring. These results establish a mean of inducing chromosome condensation in rat oocytes and demonstrate that reconstructed rat zygotes can be prepared by serial NT procedures. Developmental competence of these embryos remains to be clarified.

Development of parthenogenetic rat embryos

In an effort to establish cloning technology for the rat, we tested several methods (electric stimulation, treatment with ethanol or strontium) for the parthenogenetic activation of rat oocytes. We observed marked individual differences among rats of the outbred Wistar strain in their ability to yield activatable oocytes. These differences were independent of the activation protocol and may be due to a genetic predisposition that is crucial for the parthenogenetic activation of oocytes. The activation of oocytes was dependent upon the time between superovulation of the donor animal and the collection of the embryos. Aged oocytes (derived about 24 h after superovulation) were more prone to activation by each method than were younger oocytes, and some even underwent spontaneous activation without treatment and exhibited pronuclear formation and blastocyst development. All activation methods were effective in generating parthenogenetic rat embryos, and rat parthenotes developed until implantation. However, in general, short-term (15 min) and long-term (2 h) strontium treatment was superior to stimulation by ethanol or electric pulse for parthenogenetic activation. These results will be helpful in achieving successful cloning in the rat.

Factors Affecting Premature Chromosome Condensation of Cumulus Cell Nuclei Injected Into Rat Oocytes

To date, production of cloned rats by somatic cell nuclear transfer (NT) has not yet been successful. Inducing premature chromosome condensation (PCC) of injected cell nuclei in recipient cytoplasm is considered essential for successful mouse cloning by the Honolulu method. In the present study, some factors affecting PCC of rat cumulus cell nuclei injected into rat oocytes were examined. Wistar female rats (young: 4 to 5-week-old, mature: > or =10-week-old) were superovulated by injections of eCG and hCG, and oocytes recovered 14 or 17 h after hCG injection were received with cumulus cell nuclei using piezo-driven micromanipulator. When the oocytes were recovered 14 h post-hCG injection from young rats and the nuclear injection into oocytes was completed within 45 min, PCC was observed in 44-49% of NT oocytes. In the case of oocytes from mature rats, PCC occurred in 11-19% of the NT oocytes. Oocytes recovered 17 h post-hCG injection did not support PCC of the injected nuclei (0-7%) regardless of the donor age. Treatment of oocytes with a neutral cysteine protease inhibitor, N-acetylleucylleucylnorleucinal, slightly increased the incidence of PCC (48 vs 37%). Comparison of rat strains for oocyte donors indicated that proportions of NT oocytes undergoing PCC in Wistar and LEW oocytes (41-46%) were higher than those in Donryu and F344 oocytes (17-25%). Thus, ability of rat oocytes to promote PCC of the injected nuclei is dependent on the characteristics of oocytes, such as age or strain of donor rats, and timing of oocyte recovery.

Effect of Activation Methods for Bovine Oocytes after Intracytoplasmic Injection

A principal nuclear transfer procedure is to inject a donor cell into the perivitelline space in an enucleated oocyte and then electric fusion is performed (cell fusion method). The effects of activation methods in reconstructed oocytes for the serum-starved somatic cell cloning procedure were investigated in this study by means of intracytoplasmic injection (i.c.i.). Bovine oocytes were enucleated at 18-22 h for in vitro maturation, and subsequently the nucleus of cumulus cell collected from Japanese Black Bulls (JBCC) after 5-7 days of starved culture was injected into the recipient cytoplast with a piezo-micromanipulator. At 1 h after i.c.i., reconstructed oocytes were stimulated with ethanol (ET) or calcium ionophore (CaI) as the first activation treatment, followed by cycloheximide (CHX) or 6-dimethylaminopurin (DMAP) treatment as the second activation. In the experiment on the first activation method, the proportion of reconstructed oocytes developing to the blastocyst stage was significantly (p<0.01) higher in the ET activation method than that with CaI (10.5% and 4.7%, respectively). And the experiment on the second activation method after ET treatment showed similar proportions of blastocyst development in both CHX and DMAP treatments (5.9% and 2.8%, respectively). The present results indicated that combined activation treatment with ET and CHX was efficient for reconstructed bovine oocytes by i.c.i.

Somatic cell nuclear transfer: recent progress and challenges

Somatic cell nuclear transfer (NT) offers new and exciting opportunities in many areas of research and biotechnology. However, the field as a whole is still in its infancy, with continuing inefficiencies in the process proving many early expectations premature. The technical steps of NT are complex, and success is highly susceptible to minor variations. Furthermore, the biological process of reprogramming is not fully understood, making it difficult to optimize the protocols for providing ideal recipient oocytes and donor cells. In this paper, we describe recent advances and novel approaches, which resulted in progress during the last year, including the birth of cloned piglets and farm animals with precise genetic changes. Key problems hindering further progress are addressed.

Production of cloned pigs from adult somatic cells by chemically assisted removal of maternal chromosomes

The present study demonstrated that brief treatment of in vitro-matured porcine oocytes with demecolcine results in a membrane protrusion that contains a condensed chromosome mass, which can be easily removed by aspiration. This simple, chemically assisted method for removing maternal chromosomes enabled the production of a large number of nuclear-transferred porcine eggs. The development of eggs whose chromosomes were removed by this procedure following transfer of somatic cell nuclei to the blastocyst stage was not significantly different among groups activated using different procedures (6% to 11%) and was also not different among donor cells of different origins (3% to 9%), except for cumulus cells (0.4%). After transfer of 180 to 341 nuclear-transferred eggs that received somatic cells to 6 recipients, 2 of the recipients produced 8 healthy cloned piglets from the heart cells of a female pig. The chemically assisted method for removing maternal chromosomes was also effective for bovine and rabbit eggs.

Rat genetics: attaching physiology and pharmacology to the genome

During the past five years, the Rat Genome Project has been rapidly gaining momentum, especially since the announcement in August 2000 of plans to sequence the rat genome. Combined with the wealth of physiological and pharmacological data for the rat, the genome sequence should facilitate the discovery of mammalian genes that underlie the physiological pathways that are involved in disease. Most importantly, this combined physiological and genomic information should also lead to the development of better pre-clinical models of human disease, which will aid in the development of new therapeutics.