357 MOLECULAR AND SUBCELLULAR CHARACTERIZATION OF BOVINE OOCYTES AND THEIR SURROUNDING FOLLICULAR CELLS IN SUBJECT TO THEIR DEVELOPMENTAL COMPETENCE

Abstract

Though many factors have been shown to affect the oocyte developmental potential, it remains difficult to draw clear and reliable criteria for oocyte selection. With the urgent need for establishing non-invasive means for oocyte selection, the brilliant cresyl blue (BCB) staining test based on glucose- 6-phosphate dehydrogenase (G6PDH) activity has been successfully used to differentiate competent and non-competent bovine oocytes (Alm et al. 2005 Theriogenology 63, 2194-2205). Also it has been hypothesized that there is a correlation between the appearance of light atretic granulosa cells (GC) in the follicle and an increased developmental competence of the oocyte. Here we aim to investigate whether different developmental competent oocytes show differences concerning the degree of apoptosis or in the gene expression pattern of their follicular environment [GC and cumulus cells (CC)]. After follicular aspiration, the immature COCs were separately stained with 26 μM BCB for 90 minutes. Based on their colouration, oocytes were grouped into BCB- (colourless cytoplasm, low developmental competence) and BCB+ (coloured cytoplasm, high developmental competence). The corresponding CC and GC were also grouped according to the colouration of the enclosed oocytes. BCB+ oocytes were found to result in a higher blastocyst rate at Day 8 of in vitro culture (34.1%) compared to BCB- ones (3.9%) (n = 601 COCs). Apoptosis in GC was determined either by terminal deoxynucleotidyl transferase mediated dUTP nick end labelling (TUNEL) or by Annexin-V-staining followed by flow cytometric measurement. The degree of apoptosis in GC of BCB+ oocytes was slightly increased in contrast to the BCB- group (17.0 v. 11.0%; P < 0.05). The abundance and activity of protein kinases Akt, MAP kinase, and Caspase-3 were estimated by western blot analysis. CC, GC, and oocytes from the BCB+ group showed a higher ratio of cleaved Caspase-3/Caspase-3 in contrast to all compartments of the BCB- group (P < 0.05). Moreover, a bovine Affymetrix microarray plate form (Affymetrix Inc., Santa Clara, CA, USA) was used to analyze the gene expression profiles in oocytes, CC, and GC. The BCB+ oocytes were found to be enriched with genes regulating the oocyte maturation (EIF3F, PRKCSH) and the transition from maternal to embryonic genome activation (HMG2L1). Also BCB+ derived follicular compartments showed elevated expression of genes related to steroidgenesis, cumulus expansion and gonadotropins. In conclusion, the results demonstrate that the developmental competence of oocytes is associated with the apoptotic level and altered expression of genes in cells of their follicular environment.

This work was supported financially by Deutsche Forschungsgemeinschaft (To 138/5-1).