Introduction
Melatonin (N-acetyl-5-hydroxytyrptamine) is an indole found in vertebrates (Stehle et al., Reference Stehle, Saade, Rawashdeh, Ackermann, Jilg, Sebestény and Maronde2011), which modulates circadian and circannual rhythms such as the sleep/wake cycle and seasonal reproduction (Lincoln et al., Reference Lincoln, Clarke, Hut and Hazlerigg2006; Reiter et al., Reference Reiter, Tan and Fuentes-Broto2010). Additionally, melatonin and its metabolites are potent free radical scavengers and antioxidants (Chen et al., Reference Chen, Chen, Lee, Chen, Hsu, Kuo, Chang, Wu and Lee2006; Manda et al., Reference Manda, Ueno and Anzai2007). In recent years, researchers have begun to pay attention to the in vitro and in vivo effects of melatonin on the maturation of oocytes and development of mammalian embryos (Berlinguer et al., Reference Berlinguer, Leoni, Succu, Spezzigu, Madeddu, Satta, Bebbere, Contreras-Solis, Gonzalez-Bulnes and Naitana2009; Shi et al., Reference Shi, Tian, Zhou, Wang, Gao, Zhu, Zeng, Tian and Liu2009).
High levels of melatonin have been detected in human (Brzezinski et al., Reference Brzezinski, Seibel, Lynch, Deng and Wurtman1987; Rönnberg et al., Reference Rönnberg, Kauppila, Leppäluoto, Martikainen and Vakkuri1990) and porcine follicular fluid (Shi et al., Reference Shi, Tian, Zhou, Wang, Gao, Zhu, Zeng, Tian and Liu2009), suggesting that melatonin may influence mammalian ovarian and reproductive function. The fertilization rate of retrieved oocytes, compared with the previous in vitro fertilization and embryos transfer (IVF-ET) cycle, was increased significantly when patients were administered 3 mg/day melatonin (Tamura et al., Reference Tamura, Takasaki, Miwa, Taniguchi, Maekawa, Asada, Taketani, Matsuoka, Yamagata, Shimamura, Morioka, Ishikawa, Reiter and Sugino2008), and melatonin implants could significantly increase the developmental abilities of goat (Berlinguer et al., Reference Berlinguer, Leoni, Succu, Spezzigu, Madeddu, Satta, Bebbere, Contreras-Solis, Gonzalez-Bulnes and Naitana2009) and ovine (Vázquez et al., Reference Vázquez, Abecia, Forcada and Casao2010) oocytes after IVF.
As a free radical scavenger, antioxidant and anti-apoptotic agent (Chen et al., Reference Chen, Chen, Lee, Chen, Hsu, Kuo, Chang, Wu and Lee2006), melatonin has been added to culture medium to improve the development of mouse (Ishizuka et al., Reference Ishizuka, Kuribayashi, Murai, Amemiya and Itoh2000), porcine (Rodriguez-Osorio et al., Reference Rodriguez-Osorio, Kim, Wang, Kaya and Memili2007) and bovine (Papis et al., Reference Papis, Poleszczuk, Wenta-Muchalska and Modlinski2007) embryos. Meanwhile, Manjunatha et al. (Reference Manjunatha, Devaraj, Gupta, Ravindra and Nandi2009) reported that supplementation of in vitro maturation (IVM) medium for cumulus–oocyte complexes (COCs) with melatonin improved in vitro embryo production efficiency in buffalo, with a high transferable embryo yield obtained by enriching culture medium with 10 μM melatonin. Optimal cleavage (79%) and blastocyst rates (35%) were obtained when 10−9 M melatonin was added to both the IVM medium of porcine COCs and the culture medium for parthenogenetic embryos (Shi et al., Reference Shi, Tian, Zhou, Wang, Gao, Zhu, Zeng, Tian and Liu2009). Furthermore, melatonin supplementation during IVM of COCs resulted in a greater proportion of oocytes extruding the polar body and ROS levels of melatonin-treated oocytes were significantly lower than untreated oocytes in porcine (Kang et al., Reference Kang, Koo, Kwon, Park, Jang, Kang and Lee2009) and bovine (El-Raey et al., Reference El-Raey, Geshi, Somfai, Kaneda, Hirako, Abdel-Ghaffar, Sosa, El-Roos and Nagai2011). However, little information is available about the approaches via which melatonin promotes the maturation of oocytes.
Therefore, to investigate the direct effect of melatonin on oocyte maturation and eliminate the ability of cumulus cells to affect oocyte maturation, denuded oocytes (DOs) were used as a research model in our study. The effect of melatonin on the metaphase-II (MII) rate of DOs was assessed, compared with DOs and COCs in control media. ROS levels, the apoptotic rates and expression levels of ATP6, ATP8, BMP-15 and GDF-9 were determined in MII oocytes. Additionally, the parthenogenetic blastocyst rate and expression levels of IFN-τ, Na+/K+-ATPase, CTNNBL1 and AQP3 were quantified in parthenogenetic blastocysts to assess the effect of melatonin on the formation rate and quality of parthenogenetic blastocysts.
Materials and methods
Unless otherwise indicated, chemicals and media were purchased from Sigma-Aldrich (St. Louis, MO, USA). The authors assert that all procedures contributing to this work comply with the ethical standards of the relevant national and institutional guides on the care and use of laboratory animals
Oocytes collection and IVM
Bovine ovaries were collected from the local abattoir and transported to the laboratory within 2 h. COCs were aspirated from follicles 2–8 mm in diameter and those with at least three layers of compact cumulus cells were used for IVM. To obtain DOs, COCs were denuded of cumulus cells by vortexing in 0.1% (w/v) hyaluronidase for 2–3 min.
Groups of 50 COCs or DOs were cultured in four-well dishes in an incubator at 38.5°C with 5% CO2. IVM was performed for 22–24 h in 500 μl IVM medium that contained medium 199 (Gibco BRL, Carlsbad, CA, USA) supplemented with 10 μg/ml follicle stimulating hormone (FSH), 10 μg/ml luteinising hormone (LH), 10% (v/v) fetal bovine serum (FBS, HyClone; Gibco BRL), 10 μg/ml estradiol and 10 μg/ml heparin.
In our preliminary study, the IVM medium of DOs was supplemented with 10−7 M to 10−11 M melatonin and the highest MII rate of DOs was achieved with 10−9 M melatonin (Table 1), so 10−9 M melatonin was supplemented in the DOs + MT group in this experiment.
Effect of concentrations of melatonin on the nuclear maturation of oocytes denuded of the cumulus oophorus (DOs)
a,b,cValues with different superscripts indicate significant difference within the same column (P < 0.05).
MT, melatonin.
Assessment of oocyte maturation
The MII oocyte phase was determined by evaluating the presence of the first polar body, according to the method described by Kang et al. (Reference Kang, Koo, Kwon, Park, Jang, Kang and Lee2009) with some modifications. After 22–24 h IVM, DOs were fixed in methanol for 10 min, mounted on a slide, stained with 10 μg/ml Hoechst 33342 and the presence or absence of polar bodies was determined by a fluorescence microscopy (Olympus IX70; Olympus, Tokyo, Japan), as shown in Fig. 1.
Nuclear staining of bovine oocytes after IVM. PB: the first polar body. Bars = 20 μm.
Analysis of ROS Levels in MII oocytes
The ROS levels in MII oocytes were measured according to the method described by Rahimi et al. (Reference Rahimi, Isachenko, Sauer, Isachenko, Wartenberg, Hescheler, Mallmann and Nawroth2003) with some modifications. The intracellular redox state was measured using the fluorescent dye, 2′,7′-dichlorodihydrofluorescein diacetate (H2DCF-DA; Genmed Scientifics, Inc., Arlington, MA, USA).
Briefly, 10 to 20 oocytes were washed three times in washing solution, transferred to 50 μl working solution containing dye and dilution buffer (in a ratio of 1:200) and stained for 20 min in an incubator at 38.5°C with 5% CO2. The oocytes were washed three times in storage solution, transferred into 96-well dishes that contained 100 μl pre-warmed storage solution per well and fluorescence was immediately measured using a luminometer (Infinite M200; Tecan Group Ltd., Untersbergstrasse, Austria) at fluorescence excitation and emission wavelengths of at 488 nm and 530 nm, respectively. The ROS level per oocyte was calculated by dividing the total ROS level by the number of oocytes in each sample and expressed as photon counts per second (c.p.s).
Quantification of the apoptotic rate of MII oocytes using the annexin-V assay
MII oocytes were stained with annexin-V–fluorescein isothiocyanate (FITC) reagent (Biovision, Mountain View, CA, USA) in accordance with the manufacturer's instructions. Briefly, MII oocytes were washed three times in phosphate-buffered saline (PBS), incubated in 500 μl binding buffer containing 1 μl annexin-V–FITC and 1 μl propidium iodide (PI) for 5–10 min at room temperature in the dark, mounted and examined with a fluorescence microscope (Olympus) equipped with a CoolSNAP HQ CCD (Photometrics/Roper Scientific, Inc., Tucson, AZ, USA). According to the method described by Anguita et al. (Reference Anguita, Vandaele, Mateusen, Maes and Soom2007), bovine DOs were classified into three groups after annexin-V staining, representing: (1) early apoptotic oocytes with a homogeneous annexin positive signal in the membrane (Fig. 2A); (2) viable oocytes no annexin staining (Fig. 2B); and (3) necrotic oocytes which showed PI-positive red nuclei (Fig. 2C).
Classification of annexin-V stained bovine oocytes after IVM. (A) Annexin-V positive: a clear green signal was observed in the cytoplasmic membrane. (B) Annexin-V negative: no signal was observed in the cytoplasmic membrane. (C) Necrotic oocyte: Propidium iodide (PI)-positive nucleus was observed. Bars = 20 μm.
Quantification of the relative expression of ATP6, ATP8, BMP-15 and GDF-9 mRNA in MII oocytes
Total RNA was extracted from 100 MII oocytes from the COCs, DOs and DOs + MT groups using Trizol reagent (Invitrogen, Carlsbad, CA, USA) in accordance with the manufacturer's instructions. The RNA pellet was dissolved in sterile water, RNA concentration was measured using a Beckman DU® 640 spectrophotometer (Beckman, Fullerton, CA, USA) and cDNA was synthesized using M-MLV reverse transcriptase (Promega, Madison, WI, USA). Briefly, 2 μg total RNA was mixed with 5 mM random hexamers and 8 μl diethylpyrocarbonate (DEPC) water, incubated at 70°C for 5 min to denature secondary structure and cooled rapidly to 0°C. Then 10 μl 5× RT buffer, 250 mM dNTPs, 40 U RNase inhibitor (Promega) and 400 U M-MLV reverse transcriptase were added to a total volume of 20 μl. The mixture was incubated at 50°C for 60 min, then at 95°C for 5 min to inhibit RNase activity and treated with RNase-free DNase (Promega).
The PCR primers for each gene were listed in Table 2. PCR reactions were carried out in a total volume of 25 μl, that contained 1.2 μl cDNA, 0.5 μl (10 μM) each primer and 0.5 μl 20× Master SYBR Green mix (Invitrogen) at 95°C for 2 min, followed by 45 cycles of 95°C for 10 s and 60°C for 30s. Quantitative real-time PCR was performed on the ABI7500 SDS (Applied Biosystems, Foster, CA, USA) using the comparative Ct (2−ΔΔCt) method (Schmittgen & Livak, Reference Schmittgen and Livak2008) with β-actin as a reference gene.
Primers sequences, expected fragment sizes of detected genes of MII oocytes
F, Forward primer; R, reverse primer.
Parthenogenetic activation
MII oocytes were treated with 5 μM of the Ca-ionophore A23187 for 5 min, then incubated in 2 μM 6-dimethylaminopurine (6-DMAP) for 4 h. After activation, oocytes were cultured in 100 μl CR1aa (Rosenkrans & First, Reference Rosenkrans and First1994) supplemented with 0.1% (w/v) bovine serum albumin (BSA) under mineral oil for 48 h in 35 × 10 mm culture dishes (FALCON, NJ, USA) in an incubator at 38.5ºC with 5% CO2, then cultured for 5 days in CR1aa supplemented with 10% (v/v) FBS, changing the medium every 2 days.
Total nuclear counts
Blastocysts were stained with Hoechst 33342 as mentioned above. Then they were viewed under ultraviolet light, and the total nuclear number of every blastocyst was counted under fluorescence microscopy.
Quantification of relative expression of IFN-τ, Na+/K+-ATPase, CTNNBL1 and AQP3 mRNA in blastocysts
RNA was isolated from pools of 60 blastocysts from the COCs, DOs and DOs + MT groups. RNA isolation, reverse transcription and quantitative real-time PCR were performed as described previously, using the primers listed in Table 3.
Primer sequences, expected fragment sizes of detected genes of blastocysts
F, Forward primer; R, reverse primer.
Statistical analysis
All results were presented as mean ± standard error and each experiment was repeated at least three times. All percentage data were subjected to arcsine transformation before statistical analysis using Statistical Analysis System (SAS) software (SAS Institute; Cary, NC, USA). One-way analysis of variance (ANOVA) was used to determine significant differences in data levels, and Duncan's test was followed to determine statistical differences between groups. P-values < 0.05 were considered statistically significant.
Results
The effect of melatonin on the nuclear maturation of bovine DOs
As shown in Table 4, the MII rate of the DOs + MT group (82.29 ± 3.92%) was significantly higher than the DOs group (65.67 ± 3.59%; P < 0.05) and similar to the COCs group (85.33 ± 8.84%; P > 0.05).
Effect of melatonin on the MII rate of bovine oocytes denuded of the cumulus oophorus
a,bValues with different superscripts indicate significant difference within the same column (P < 0.05).
COCs, cumulus–oocyte complexes; DOs, oocytes denuded of the cumulus oophorus; MT, melatonin.
The effect of melatonin on the ROS level of MII oocytes
The ROS level of DOs + MT group (3.78 ± 0.29 c.p.s) was significantly lower than that of the DOs group (4.83 ± 0.42 c.p.s; P < 0.05) and higher than the COCs group (2.95 ± 0.13 c.p.s; P < 0.05; Table 5).
Effect of melatonin on the reactive oxygen species (ROS) levels of MII oocytes
a,b,cValues with different superscripts indicate significant difference within the same column (P < 0.05).
COCs, cumulus–oocyte complexes; DOs, oocytes denuded of the cumulus oophorus; MT, melatonin.
The effect of melatonin on the apoptotic rate of MII oocytes
Representative positive and negative annexin-V staining images were presented in Fig. 2. The apoptotic rate of DOs + MT group (21.88 ± 2.08%) was significantly lower than the DOs group (36.99 ± 3.62%; P < 0.05) and higher than the COCs group (12.94 ± 0.83%; P < 0.05; Fig. 3).
Apoptotic rates of MII oocytes of COCs, DOs + MT and DOs groups. a,b,cValues with different superscripts indicate significant difference (P < 0.05). COCs, cumulus–oocyte complexes; DOs, oocytes denuded of the cumulus oophorus; MT, melatonin.
The effect of melatonin on expression of ATP6, ATP8, BMP-15 and GDF-9 mRNA in MII oocytes
The relative expression levels of ATP6 and ATP8 mRNA in MII oocytes from DOs + MT group were significantly higher than the DOs group (P < 0.05) and lower than the COCs group (P < 0.05). Significant differences of BMP-15 and GDF-9 mRNA expression levels were also observed in the DOs + MT, DOs and COCs groups (P < 0.05; Fig. 4).
Relative expression levels of ATP6, ATP8, BMP-15 and GDF-9 in MII oocytes. a,b,cValues of different superscripts indicate significant difference within the expression level of each gene (P < 0.05). COCs, cumulus–oocyte complexes; DOs, oocytes denuded of the cumulus oophorus; MT, melatonin.
The effect of melatonin on the developmental potential in MII oocytes
As shown in Table 6, there was no significant difference in the cleavage rates of the COCs (91.30 ± 6.08%), DOs + MT (92.42 ± 8.21%) and DOs groups (92.66 ± 5.53%; P > 0.05). However, the blastocyst rate of the DOs MT group (35.26 ± 4.87%) was significantly higher than the DOs group (24.17 ± 3.54%; P < 0.05) and lower than the COCs group (46.32 ± 4.29%; P < 0.05). The cell number of the DOs + MT group (74.92 ± 5.82) was significantly higher than that of the DOs group (52.74 ± 6.13; P < 0.05), and similar to that of COCs group (78.25 ± 8.37; P > 0.05).
Effect of melatonin on the developmental potential of MII oocytes
a,b,cValues with different superscripts indicate significant difference within the same column (P < 0.05).
COCs, cumulus–oocyte complexes; DOs, oocytes denuded of the cumulus oophorus; MT, melatonin.
The effect of melatonin on the expression of IFN-τ, Na+/K+-ATPase, CTNNBL1 and AQP3 in parthenogenetic blastocysts
The relative expression level of IFN-τ, Na+/K+-ATPase, CTNNBL1 and AQP3 in blastocysts from the DOs + MT group were significantly higher than those from the DOs group (P < 0.05; Fig. 5). The relative expression levels of IFN-τ and Na+/K+-ATPase in blastocysts from the DOs + MT group were significantly lower than those from COCs group (P < 0.05). The relative expression levels of CTNNBL1 in blastocysts from DOs + MT group and COCs groups were similar (P > 0.05), and AQP3 expression level in blastocysts from the DOs + MT group was significantly higher than those from the COCs group (P < 0.05).
Relative expression levels of IFN-τ, Na+/K+-ATPase, CTNNBL1 and AQP3 in parthenogenetic blastocysts. a,b,cValues of different superscripts indicate significant difference within the expression level of each gene (P < 0.05). COCs, cumulus–oocyte complexes; DOs, oocytes denuded of the cumulus oophorus; MT, melatonin.
Discussion
Melatonin increased the nuclear maturation of bovine DOs
The removal of cumulus cells significantly decreased the MII rate from 85.33 ± 8.84% to 65.67 ± 3.59% (P < 0.05; Table 4), similar to the effects observed in bovine (Zhang et al., Reference Zhang, Jiang, Wozniak, Yang and Godke1995; Geshi et al., Reference Geshi, Takenouchi, Yamauchi and Nagai2000), porcine (Maedomari et al., Reference Maedomari, Kikuchi, Ozawa, Noguchi, Kaneko, Ohnuma, Nakai, Shino, Nagai and Kashiwazaki2007) and mouse (Ge et al., Reference Ge, Han, Lan, Zhou, Liu, Zhang, Sui and Tan2008a,Reference Ge, Han, Lan, Zhou, Liu, Zhang, Sui and Tanb) oocytes. However, 10−9 M melatonin significantly increased the MII rate of DOs from 65.67 ± 3.59% to 82.29 ± 3.92% in the present study (P < 0.05).
It has been reported that melatonin acts via three distinct mechanisms of action: receptor-mediated, protein-mediated and non-receptor-mediated effects and the receptor-mediated action of melatonin involves both membrane and nuclear melatonin binding sites (Acuña-Castroviejo et al., Reference Acuña-Castroviejo, Reiter, Menéndez-Peláez, Pablos and Burgos1994, Reference Acuña-Castroviejo, Martín, Macías, Escames, León, Khaldy and Reiter2001). Two distinct subtypes of the melatonin receptor (melatonin receptor 1A (MTNR1A) and melatonin receptor 1B (MTNR1B)) have been cloned and mapped in several animal species (Messer et al., Reference Messer, Wang, Tuggle, Yerle, Chardon, Pomp, Womack, Barendse, Crawford, Notter and Rothschild1997; von Gall et al., Reference von Gall, Stehle and Weaver2002). MTNR1A is more strongly associated with reproductive activity than MTNR1B (Weaver et al., Reference Weaver, Liu and Reppert1996). MTNR1A is detected in bovine oocytes and cumulus cells, whereas MTNR1B is expressed in bovine oocytes but not bovine cumulus cells (El-Raey et al., Reference El-Raey, Geshi, Somfai, Kaneda, Hirako, Abdel-Ghaffar, Sosa, El-Roos and Nagai2011). As we removed the cumulus cells from oocytes in our experiments and both MTNR1A and MTNR1B can be detected in bovine oocytes (El-Raey et al., Reference El-Raey, Geshi, Somfai, Kaneda, Hirako, Abdel-Ghaffar, Sosa, El-Roos and Nagai2011), it was possible that melatonin directly promoted the MII rate of bovine DOs via its receptors on the oocyte membrane. Additionally, melatonin also accelerated the formation of maturation-promoting factor, which could improve the nuclear maturation of oocytes (Shi et al., Reference Shi, Tian, Zhou, Wang, Gao, Zhu, Zeng, Tian and Liu2009).
Melatonin reduced ROS levels of MII oocytes
Cumulus cells were linked to oocytes by gap junction communications, via which cumulus cells regulated the synthesis of glutathione (GSH) (de Matos et al., Reference de Matos, Furnus and Moses1997) to maintain the redox state in cells and protect them against harmful effects caused by oxidative injuries (Meister, Reference Meister1983; Gasparrini et al., Reference Gasparrini, Boccia, Marchandise, Di Palo, George, Donnay and Zicarelli2006). The GSH content is significantly lower in DOs than in cumulus-enclosed oocytes in cattle (de Matos et al., Reference de Matos, Furnus and Moses1997) and pigs (Cui et al., Reference Cui, Fan, Wu, Hao, Liu, Chen and Zeng2009). The ability of COCs to protect against the induction of oxidative stress in oocytes may explain the significantly higher ROS levels observed in DOs group (4.83 ± 0.42 c.p.s) than COCs group (2.95 ± 0.13 c.p.s; Table 5) in this study.
Melatonin was reported to influence the activity and cellular mRNA expression levels of antioxidant enzyme enzymes in neuronal cell lines (Mayo et al., Reference Mayo, Sainz, Antoli, Herrera, Martin and Rodriguez2002) and human oocytes (Tamura et al., Reference Tamura, Takasaki, Miwa, Taniguchi, Maekawa, Asada, Taketani, Matsuoka, Yamagata, Shimamura, Morioka, Ishikawa, Reiter and Sugino2008). Superoxide dismutase activities were increased in the liver (Ozturk et al., Reference Ozturk, Coşkun, Erbaş and Hasanoglu2000), kidney and brain (Liu & Ng, Reference Liu and Ng2000) of melatonin-treated rats. Metabolites of melatonin are likewise excellent scavengers of ROS (Manda et al., Reference Manda, Ueno and Anzai2007). All this evidence helped to explain the decreased ROS level in DOs + MT group (3.78 ± 0.29 c.p.s) compared with the DOs group (4.83 ± 0.42 c.p.s), similar to the observations in porcine (Kang et al., Reference Kang, Koo, Kwon, Park, Jang, Kang and Lee2009) and bovine COCs (El-Raey et al., Reference El-Raey, Geshi, Somfai, Kaneda, Hirako, Abdel-Ghaffar, Sosa, El-Roos and Nagai2011).
The presence of antioxidant enzyme transcripts at the germinal vesicle/MII stage in mouse oocytes and MII stage in human oocytes (El Mouatassim et al., Reference El Mouatassim, Guérin and Ménézo1999) suggested that antioxidant defence mechanism were important for further oocyte maturation (Tamura et al., Reference Tamura, Takasaki, Miwa, Taniguchi, Maekawa, Asada, Taketani, Matsuoka, Yamagata, Shimamura, Morioka, Ishikawa, Reiter and Sugino2008). Moreover, Tamura et al. (Reference Tamura, Takasaki, Miwa, Taniguchi, Maekawa, Asada, Taketani, Matsuoka, Yamagata, Shimamura, Morioka, Ishikawa, Reiter and Sugino2008) reported that incubation of mouse oocytes with 300 μM H2O2 for 12 h significantly reduced the percentage of MII oocytes. In this study, a lower MII rate was observed in DOs group with a higher ROS level compared with DOs + MT group, indicating that high ROS levels could negatively influence oocyte nuclear maturation as discussed above. It was inferred from these results that melatonin may influence the MII rate of bovine DOs via a non-receptor-mediated mechanisms linked altered ROS accumulation besides the receptor-mediated approach mentioned above.
Melatonin decreased the apoptotic rates of MII oocytes
Externalization of phosphatidylserine from the cytoplasmic to the extracellular side of the membrane occurs when apoptosis is activated (Lahorte et al., Reference Lahorte, Vanderheyden, Steinmetz, Van de Wiele, Dierckx and Slegers2004) and can be detected using the annexin-V assay (Chan et al., Reference Chan, Reiter, Wiese, Fertig and Gold1998). The annexin-V assay could be used to detect the initiation of apoptosis at the MII stage in bovine oocytes and the rate of first cleavage and subsequent embryonic development of oocytes during IVF decreased with annexin-V binding increasing (Kalo & Roth, Reference Kalo and Roth2011).
Chen et al. (Reference Chen, Chen, Lee, Chen, Hsu, Kuo, Chang, Wu and Lee2006) reported that 5 mg/kg melatonin-treated mice had reduced cellular shrinkage and chromatin condensation in boundary zones of ischemic infarct, suggesting melatonin had an anti-apoptotic effect and could counteract post-ischemic radical-mediated apoptosis in the neurovascular unit. Similarly, our experiments demonstrated that melatonin significantly decreased the apoptotic rate of oocytes from 36.99 ± 3.62% to 21.88 ± 2.08% during IVM (Fig. 3). The lower apoptotic rate of DOs + MT group compared with the DOs group may be due to the ability of melatonin to act as a potent free radical scavenger and antioxidant (Chen et al., Reference Chen, Chen, Lee, Chen, Hsu, Kuo, Chang, Wu and Lee2006) and/or the inhibition of apoptosis by melatonin via MTNR1A or MTNR1B receptor-mediated mechanisms (Lanoix et al., Reference Lanoix, Lacasse, Reiter and Vaillancourt2012).
The lower rate of apoptosis observed in the COCs group (12.94 ± 0.83%) compared with the DOs (36.99 ± 3.62%) and DOs + MT (21.88 ± 2.08%) groups may be due partially to the decreased ROS levels, as the higher levels of ROS observed in DOs and DOs + MT groups may activate a cascade of molecular event, leading to apoptosis (Simon et al., Reference Simon, Haj-Yehia and Levi-Schaffer2000; Shimizu et al., Reference Shimizu, Numata and Okada2004).
Melatonin increased the expression of ATP6, ATP8, BMP-15 and GDF-9 in MII oocytes
The expression of mitochondrial genes is known to affect the quality, fertilization and embryo development of oocytes (Hsieh et al., Reference Hsieh, Au, Yeh, Chang, Cheng and Tzeng2004). Also, the expression of GDF-9 and BMP-15 is essential for the development and function of mouse (Su et al., Reference Su, Wu, O’Brien, Pendola, Denegre, Matzuk and Eppig2004) and human (Wei et al., Reference Wei, Liang, Fang and Zhang2011) oocytes, and supplementation of exogenous BMP-15 or GDF-9 during the IVM significantly increased the development potential of bovine (Hussein et al., Reference Hussein, Thompson and Gilchrist2006) and mouse (Yeo et al., Reference Yeo, Gilchrist, Thompson and Lane2008) oocytes. Based on the above-mentioned finding, the expression levels of ATP6, ATP8, BMP-15 and GDF-9 mRNA could be used to assess the oocytes quality.
This study showed that during the IVM of DOs, 10−9 M melatonin significantly moderated the reduction of relative mRNA levels of ATP6, ATP8, BMP-15 and GDF-9 caused by oocyte denudation (Fig. 4), indicating that melatonin significantly improved the quality of oocytes. This moderated reduction may be due to decreased ROS levels in melatonin-treated oocyte, as ROS may degrade the mRNA in oocytes. To our knowledge, this was the first report concerning the effect of melatonin on the gene expression on bovine oocytes.
Melatonin increased the developmental potential of MII oocytes
Many in vivo or in vitro studies have shown that melatonin could increase the development potential of COCs. When goats (Berlinguer et al., Reference Berlinguer, Leoni, Succu, Spezzigu, Madeddu, Satta, Bebbere, Contreras-Solis, Gonzalez-Bulnes and Naitana2009) or sheep (Vázquez et al., Reference Vázquez, Abecia, Forcada and Casao2010) received the melatonin implants, the cleavage and blastocyst rates of COCs after IVF were increased significantly. Manjunatha et al. (Reference Manjunatha, Devaraj, Gupta, Ravindra and Nandi2009) reported that enrichment of buffalo COCs maturation medium with 10 μM melatonin improved their cleavage and blastocyst rates after IVF.
As removal of the cumulus before IVM caused a precocious exocytosis of cortical granules, leading to zona hardening and reduced penetrability of oocytes by sperm (Ge et al., Reference Ge, Sui, Lan, Liu, Wang and Tan2008b), parthenogenetic activation was used to assess the development potential of oocytes in this study. In our experiments, supplementation of DOs IVM medium with 10−9 M melatonin increased the blastocyst rate after parthenogenetic activation (24.17 ± 3.54% vs. 35.26 ± 4.87%; P < 0.05) and the cell number of parthenogenetic blastocysts (52.74 ± 6.13 vs. 74.92 ± 5.82; P < 0.05), similar to results obtained in porcine COCs (Shi et al., Reference Shi, Tian, Zhou, Wang, Gao, Zhu, Zeng, Tian and Liu2009). In contrast with the results of Shi et al. (Reference Shi, Tian, Zhou, Wang, Gao, Zhu, Zeng, Tian and Liu2009), 10−9 M melatonin did not significantly increase the cleavage rates of oocytes after parthenogenetic activation in this study, similar to the report of Kang et al. (Reference Kang, Koo, Kwon, Park, Jang, Kang and Lee2009). Several factors, such as the species (porcine vs. bovine) and cell types tested (COCs vs. DOs), may explain these differing results.
Excessive ROS can exacerbate mitochondrial dysfunction (Lin & Beal, Reference Lin and Beal2006), decrease intracellular ATP concentrations, reduce the GSH/GSH disulphide (GSSG) ratios and increase the cytosolic calcium ion concentration, leading to the detrimental effects on oocyte and embryo development (Tarín, Reference Tarín1996). Many studies have indicated that the endogenous antioxidant systems (Gupta et al., Reference Gupta, Uhm and Lee2010) and mitochondrial function (Zhao et al., Reference Zhao, Du, Wang, Hao, Liu, Qin and Zhu2011a,Reference Zhao, Du, Wang, Hao, Liu, Qin and Zhub) could directly influence the developmental potential of oocytes or embryos; therefore, the increased developmental potential of DOs treated with melatonin may be due to the lower level of ROS observed the DOs + MT group, as discussed previously.
Although the development potential of DOs was increased significantly by melatonin, it remained significantly lower than COCs, indicating that the oocyte maturation and quality were also influenced by other cumulus cell-derived factors. The increased developmental potential of the COCs group was probably due to a combination of cumulus cell-derived factors, and were associated with a lower apoptotic rate (Fig. 3) and increased expression of the oocyte quality-markers ATP6, ATP8, BMP-15 and GDF-9 (Fig. 4), compared with the DOs and DOs + MT groups.
Melatonin increased the expression of IFN-τ, Na+/K+-ATPase, CTNNBL1 and AQP3 in parthenogenetic blastocysts
To investigate the effect of melatonin on gene expression in parthenogenetic blastocysts, we quantified the expression levels of IFN-τ, Na+/K+-ATPase, CTNNBL1 and AQP3. As an important factor in regulating bovine blastocysts implantation, embryonic IFN-τ expression and secretion level can be utilized as useful tools to evaluate the developmental competence and quality of embryos (Yao et al., Reference Yao, Wan, Hao, Gao, Yang, Cui, Wu, Liu, Liu, Chen and Zeng2009). Expressions of Na+/K+-ATPase and aquaporin play an important role in the formation of tight junctions between trophoblast cells (Kidder & Watson, Reference Kidder and Watson2005), and CTNNBL1 (an important gene in intracellular signalling) has been described as an important marker of compaction and trophectoderm differentiation (Mamo et al., Reference Mamo, Mehta, McGettigan, Fair, Spencer, Bazer and Lonergan2011).
The increased expression levels of IFN-τ, Na+/K+-ATPase, CTNNBL1 and AQP3 in blastocysts from the DOs+ MT group (Fig. 5) indicated that melatonin may improve the quality of the parthenogenetic blastocysts, which may be a result of the improved oocyte quality of melatonin-treated DOs (Fig. 3 and Table 4). In 2009, Berlinguer et al. observed that subcutaneous melatonin implants (18 mg) in goats did not significantly affect the expression levels of β-actin, heat shock protein 90β, cyclin b1, Na+/K+-ATPase, Type I cadherin or AQP3 in blastocysts produced by oocyte IVF. The differences between our results and the report of Berlinguer et al. (Reference Berlinguer, Leoni, Succu, Spezzigu, Madeddu, Satta, Bebbere, Contreras-Solis, Gonzalez-Bulnes and Naitana2009) suggested that the effect of melatonin may depend on the treatment method (supplement vs. implant), species (bovine vs. goat) and the method of producing blastocysts (parthenogenetic activation vs. IVF).
Many previous studies have reported that melatonin could regulate different specific gene expression in animals, such as quail (Ubuka et al., Reference Ubuka, Bentley, Ukena, Wingfield and Tsutsui2005), sheep (Johnston et al., Reference Johnston, Tournier, Andersson, Masson-Pévet, Lincoln and Hazlerigg2006) and rat (Park et al., Reference Park, Choi and Lee2007). In our experiment, melatonin was found to increase the quality-marker genes of oocytes and their parthenogenetic blastocysts. It may be that melatonin, having a highly lipophilic nature, can cross the membrane of the target cells and bind to an orphan nuclear receptor to modulate gene expression of certain proteins (Becker-André et al., Reference Becker-André, Wiesenberg, Schaeren-Wiemers, André, Missbach, Saurat and Carlberg1994; Carlberg &Wiesenberg, Reference Carlberg and Wiesenberg1995; Vanecek, Reference Vanecek1995; Ubuka et al., Reference Ubuka, Bentley, Ukena, Wingfield and Tsutsui2005).
In conclusion, this study demonstrated that melatonin could improve the nuclear maturation of bovine DOs, increase the expression of oocyte quality-marker genes, decrease ROS levels and reduce the apoptotic rates of MII oocytes in absence of cumulus cells, which contributed to an increased parthenogenetic blastocyst formation rate and quality. These results might enable researchers to more clearly understand the mechanisms by which melatonin could improve the IVM and development potential of oocytes. In future, more information will be obtained when COCs instead of DOs are chosen as the model and IVF instead of parthenogenetic activation is selected to detect the developmental potential of MT-treated oocytes.
Declaration of interest
None
Acknowledgements
The work was supported by the National Dairy Industry and Technology System Fund (CARS-37), the Basic Research Fund of IAS, CAAS (2010jc-3–1), the Key Projects in the National Science & Technology Pillar Program during the Twelfth Five-year Plan Period of China (2011BAD19B02 and 2011BAD19B04).
