Effect of gonadotropins on the ability of 5,6-dichloro-1-β-d-ribofuranosylbenzimidazole (DRB) to inhibit germinal vesicle breakdown in bovine oocytes

The role of transcription in EGF- and FSH-mediated oocyte maturation in vitro

Understanding mechanisms responsible for meiotic resumption in mammalian oocytes is critical for the identification of strategies to enhance developmental competence of in vitro-matured oocytes. Improvement of in vitro oocyte maturation systems is dependent on a better understanding of mechanisms that regulate oocyte maturation both in vivo and in vitro as well as on the identification of methods to manipulate the meiotic progression of oocytes matured in vitro in a physiological manner. The purpose of this review is two-fold: first, to examine the mechanisms that underlie the acquisition of oocyte developmental competence and regulation of oocyte maturation in vivo and in vitro; second, to present data examining the role of transcription in mediating the ability of EGF and FSH to induce oocyte maturation in vitro. Results presented support the conclusions that (1) EGF-induced oocyte maturation does not require nascent gene transcription in both mice and domestic cats; (2) FSH requires gene transcription to induce oocyte maturation in both species; (3) EGF must be present in the maturation medium to optimize the effectiveness of FSH to promote oocyte maturation; (4) the mechanism used by FSH to induce oocyte maturation in vitro appears to predominate over that used by EGF when both EGF and FSH are present in maturation medium used for either murine or feline cumulus oocyte complexes.

Easy and rapid method for the determination of gene expression in cumulus cells incubated for oocyte maturation

BACKGROUND

The objectives of this study were to develop an easy and rapid method for measuring gene expression in a small number of cells by real-time PCR without RNA extraction and purification, and to use this method to determine more precisely IGF-I gene expression in the cumulus cells surrounding oocytes.

METHODS

First, after small numbers of cumulus cells were lysed in cell lysis buffer, they were digested with various concentrations of DNase I for different periods at 37 degrees C to determine the optimal conditions for digestion of genomic DNA in the lysate. Since nonspecific amplification was liable to occur when the non-purified RT product of the cell lysate was used for real-time PCR with the given primers, the optimal conditions for Mg2+ and annealing temperature were well investigated. Further, to create the same conditions as in the actual sample reaction for measurement by real-time PCR, RT-minus product was added to the reaction mixture of the standard curve, and then the amplification efficiency was assessed. Next, IGF-I gene expression in cumulus cells collected from cumulus oocyte complexes (COCs) every 4 h during maturation was determined using the developed method.

RESULTS

The optimal conditions for measuring gene expression using the cell lysate from a small number of cells were as follows: incubation of the cell lysate with 0.16 U/microL DNase I with 10 U/microL for 30 min, an Mg concentration of 1.5 mM for amplification of target gene by real-time PCR using RT-product of the cell lysate. When the RT-minus products added to the reaction mixture for the standard curve, which was prepared for purified 18SrRNA plasmid, the PCR efficiency was similar between the sample and the standard. The IGF-I gene expression in the cumulus cells was elevated up through the first 8 h of the culture and then declined gradually by the end of maturation, with the maximal gene expression (778-fold) seen at 8 h.

CONCLUSION

It can be concluded that the method developed here, in which equivalent to cumulus cells collected from 0.03-0.075 COCs were employed per reaction, permits rapid and easy determination of target gene expression in a limited number of cells using real-time PCR without RNA extraction.

Developmental capacity of bovine cumulus oocyte complexes after transcriptional inhibition of germinal vesicle breakdown

Oocytes cultured in the presence of FSH and the transcriptional inhibitor, 5,6-dichloro-1-beta-d-ribofuranosylbenzimidazole (DRB), remain in meiotic arrest at the germinal vesicle (GV) stage. The objectives of this study were to assess the kinetics of maturation and the developmental capacity of bovine cumulus oocyte complexes (COC) following release from prolonged meiotic arrest by DRB. In Experiment I, COC were cultured for 20 h in Tissue culture medium (TCM)-199 supplemented with 10% estrus cow serum (ECS), 5 microg/ml FSH and 1 microg/ml estradiol in the presence of 120 microM DRB. COC were then released from meiotic arrest and cultured for 20 h in DRB-free medium. CONTROL COC were cultured for 20 h in DRB-free medium, with culture initiated concomitant to the release of DRB-treated COC from meiotic arrest. Nuclear maturation was assessed after 0, 5, 10, 15, and 20 h of culture in DRB-free medium. The proportion of DRB-arrested oocytes reaching metaphase II (MII) following 20 h culture in DRB-free medium was not significantly different from controls ( 96+/-4% versus 99+/-4%). In Experiment II, COC were cultured for 20 h in TCM-199 supplemented with 10% ECS, 10 microg/ml LH, 5 microg/ml FSH, and 1 microg/ml estradiol in the presence or absence of 120 microM DRB. COC in the DRB-treated group were then washed and matured coincident with a second group of control COC for 20 h in DRB-free medium. COC in both groups were fertilized and then randomly assigned to one of two culture systems: TCM-199 + 10%ECS or mSOF + 0.6% fatty acid-free BSA. Development was assessed at 72 h post insemination (hpi), 168 hpi (Day 7) and 216 hpi (Day 9). In this experiment, culture with DRB-arrested oocyte maturation at the GV stage (DRB, 85+/-3% GV; CONTROL, 2+/-3% GV; P<0.001 ). Following release from arrest, maturation and fertilization, the proportion of COC that cleaved by 72 hpi was decreased by treatment with DRB (DRB: 78+/-3% versus

CONTROL

90+/-3%; P<0.05). However, no effect of DRB was found on the proportion of cleaved zygotes that reached the blastocyst stage on either Day 7 or Day 9 of culture (Day 7: DRB 16+/-2% versus CONTROL, 21+/-2%; Day 9: DRB 23+/-3% versus CONTROL, 31+/-3%). More embryos reached the blastocyst stage in the TCM-199/serum culture system compared to the mSOF/BSA system on both Days 7 and 9 (Day 7: TCM-199, 23+/-2% versus mSOF, 13+/-2%, P<0.05; Day 9: TCM-199, 32+/-3% versus mSOF, 22+/-3%, P<0.05 ). In summary, bovine COC maintained in meiotic arrest for 20 h by culture in the presence of the transcriptional inhibitor DRB retained their capacity to develop to the blastocyst stage after fertilization in vitro.

Gene transcription and regulation of oocyte maturation

The developmental potential of an embryo is dependent on the developmental potential of the oocyte from which it originates. The process of oocyte maturation is critical for the efficient application of biotechnologies such as in vitro embryo production and mammalian cloning. However, the overall efficiency of in vitro maturation remains low because oocytes matured in vitro have a lower developmental competence than oocytes matured in vivo. Furthermore, oocytes that have been exposed to gonadotropins have greater developmental competence than oocytes matured in the absence of gonadotropins. By understanding the molecular mechanisms underlying gonadotropin-induced maturation, improvement in oocyte maturation technologies may be expected as procedures to manipulate specific factors involved in signalling for resumption of meiosis are identified. The present review will focus on transcriptional mechanisms underlying the maturation of mammalian oocytes in vitro, as well as on the acquisition of oocyte developmental competence. In addition, a working model for the transcriptional control of mammalian oocyte maturation is proposed.

Developmental capacity of Bos indicus oocytes after inhibition of meiotic resuption by 6-dimethylaminopurine

Several reports have suggested that a treatment before in vitro maturation might improve oocyte competence and increase its developmental potential. Therefore, the objectives of the present study were to establish the kinetics of IVM in Zebu oocytes, to assess the effect of 6-dimethylaminopurine (6-DMAP), a phosphorylation inhibitor, on meiotic resumption, and to verify the developmental potential of the blocked oocytes after removal of the inhibitory conditions. To establish the kinetics of in vitro maturation 1422 oocytes were obtained from Nellore cows ovaries and matured in presence and absence of gonadotropins. Samples of oocytes were taken from culture at 0, 6, 9, 12, 15, 18, 21 and 24h, and the oocytes were fixed, stained and evaluated for nuclear morphology. Germinal vesicle break down (GVBD) occurred between 6 and 12h of culture in both groups. By 21h the majority of the oocytes had reached metaphase II in presence (71%) and absence (62%) of gonadotropins. In order to examine the inhibitory effect of 6-DMAP, 585 oocytes were cultured for 12, 18 and 24h in the presence or absence of 2mM of 6-DMAP. At each time point the oocytes were evaluated for nuclear morphology. To test the reversibility of meiotic inhibition 366 oocytes were incubated for 0, 12, 18 and 24h in the presence of 6-DMAP and then were transferred to the maturation medium and cultured for further 24h. A total of 429 oocytes were used to evaluate the developmental potential after meiotic inhibition. The oocytes were cultured in the presence of 6-DMAP for 0, 12, 18 and 24h, and then were matured, fertilized and cultured in vitro. Culture of bovine oocytes in the presence of 6-DMAP up to 24h completely blocked GVBD with more than 90% of the oocytes at GV stage. The inhibitory effect of 6-DMAP was fully reversible since maturation rates were similar (P>0.05) among all treatment groups. The evaluation of embryo development after various periods of meiotic blockage showed that inhibition, regardless the time period, had no effect (P>0.05) on penetration and cleavage rates. However, the proportion of embryos at blastocyst stage was reduced after inhibition for 12 (20.2%), 18 (20.1%) and 24h (19.0%) compared with the control group (35.6%). 6-DMAP has a reversible effect on maintenance of meiotic arrest, but reduced further embryo development.