343 EFFECTS OF EPIDERMAL GROWTH FACTOR SUPPLEMENTATION ON IN VITRO MATURATION AND GENE EXPRESSION OF CANINE OOCYTES

Abstract

Despite many efforts to improve canine in vitro maturation (IVM), the efficiency is still low compared to that of other mammalian species (Marie et al. 2004). Epidermal growth factor (EGF) has stimulatory effects on the resumption of oocyte maturation and cumulus expansion in vitro and on prei-mplantation embryonic development in mammals by either an autocrine or a paracrine pathway, or a combination of both systems (Paria et al. 2001 PNAS 98, 1047-1052). The present study investigated the effects of EGF supplementation on in vitro maturation and gene expression of canine oocytes. Oocytes were recovered by slicing ovaries recovered from 40 bitches after ovariohysterectomy at random stages of the estrous cycle. Cumulus-oocyte complexes (COCs) were matured in TCM-199 containing 10% FBS, 1 ¼g/mL FSH and LH, and EGF (0, 10, or 30 ng/mL) for 48 or 72 h at 39°C in a humidified atmosphere of 5% CO₂ in air. In Experiment I (n = 2520 oocytes), the nuclear maturation status was assessed by fluorescence microscopy after bisbenzimide (Hoechst 33342) staining (10 µg/mL) at 0, 48, and 72 h of incubation. In Experiment II (n = 90 oocytes), expression of transcripts such as EGF receptor (EGFR), luteinizing hormone receptor (LHR), and gap junction protein (GJA5) were determined in 10 intact COCs each at 0, 48, and 72 h, respectively, by reverse transcription-polymerase chain reaction (RT-PCR). At 0 h 10-20% of the oocytes had undergone resumption of meiosis (GVBD<MII). After 48 h of IVM, rate of meiotic resumption for 0, 10, and 30 ng/mL EGF were 28, 35, and 30%, respectively. At 72 h of IVM, oocytes in the 10 ng/mL EGF group had resumed meiosis at a higher frequency (55%; P < 0.05) than in the 30 ng/mL EGF or the control group (39 and 42%, respectively). At 72 h of IVM, the frequency of maturation to the MII stage was significantly higher in the 10 ng/mL EGF group (9.6%) than in the 30 ng/mL EGF or the control group (4.2 and 3.3%, respectively). The expression of EGFR was significantly higher (P < 0.05) in 0 h oocytes than in the 48- or 72-h oocytes. Further EGFR expression levels were decreased in the presence of EGF in a dose dependent manner. Transcripts for LHR were detected at all maturation intervals and its expression patterns were not altered by supplementation with 10 ng/mL EGF. Expression of GJA5 was observed only after 48 h of IVM, and levels of expression were similar in oocytes supplemented with both 10 and 30 ng/mL EGF. In summary, our results indicate that supplementation of canine IVM medium with 10 ng/mL EGF had a positive influence on the progression of maturation to MII at 72 h. The effect may not be related to the alteration of mRNA expression of genes analyzed in the present study, due to the complex patterns regulating meiotic arrest in canine oocytes.

This work was supported by Grant no. 204119-03-1-LG000 from ARPC, Republic of Korea.