P-526 Identification of mouse cumulus cell-specific miRNAs and their role in the acquisition of the oocyte developmental competence

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Abstract Study question Do cumulus cells (CCs), during mouse germinal vesicle (GV)-to-metaphase II (MII) transition, produce miRNAs with a role in the acquisition of the oocyte developmental competence? Summary answer We identified a group of 74 miRNAs with a known role in key processes of oocytes maturation and developmental competence. What is known already We recently developed a co-culture platform whereby CC-free, denuded, GV oocytes (DOs) were matured to MII upon a feeder layer (FL) of CCs derived either from developmentally competent (FL-SN-CCs) or incompetent (FL-NSN-CCs) oocytes. When cultured upon FL-SN-CCs, DOs displayed a developmental competence equal to that showed by control cumulus-enclosed oocytes (COCs). Instead, when cultured upon FL-NSN-CCs, all DOs arrested at the 2-cell stage. We found that CCs released into the medium large amounts of extracellular vesicles (EVs), whose content analysis by NGS led to the identification of candidate miRNAs. Study design, size, duration Five-eight-week-old CD1 mouse females (Charles River) were injected with 10 I.U. Pregnant Mare Serum Gonadotropin (PMSG). After 48 hr, the ovaries were isolated and their surface punctured with a thin sterile needle to collect fully-grown antral oocytes which were separated from the surrounding CCs. Eight-ten DOs were cultured upon FL-SN-CCs or FL-NSN-CCs for 15 hr in 50 µl exosome-free alpha-MEM Glutamax medium for EVs production. Experiments were performed in triplicates. Participants/materials, setting, methods EVs were collected and processed for EVs characterisation by Amnis-Imaging-flow-cytometer and transmission electron microscopy (TEM), or for RNA extraction (miRNeasy Micro Kit, Qiagen), libraries production (Small RNA-Seq Library Prep kit, Lexogen) and NGS (Illumina Genome Analyzer and the NextSeq 500/550 High Output v2.5). To identify miRNA’s target genes, we used “mirPath v.3” server of the DIANA tools website (https://dianalab.e-ce.uth.gr/html/mirpathv3/index.php?r=mirpath). Then, STRING and KEGG were used to highlight the pathways in which these genes are involved. Main results and the role of chance Imaging flow cytometry recorded an amount of released EVs in both FL-SN-CCs or FL-NSN-CCs in the mean range of 5-5.9 x 10(8) particles/ml (p > 0.05), with >96% positive to CD9 exosome surface marker. TEM analysis showed an EVs size in diameter comprised between 18.5-54.9 nm. By comparing FL-SN-CCs vs. FL-NSN-CCs, NGS of EVs’ miRNAs content displayed 74 differentially expressed miRNAs, 43 up- and 31 down-regulated. Of them, 7 resulted involved in the regulation of 71 target genes with a known function in meiosis resumption (N = 24), follicle growth (N = 23), fertilisation (N = 1) and in the acquisition of the oocyte’s developmental competence (N = 23). Limitations, reasons for caution We used an in vitro platform to mimic the cumulus-oocyte-complex environment and, thus, the artificial set-up may differ from that of the natural condition. Also, our hypothesis on the role of these miRNAs, needs to be further confirmed by functional assays (e.g., miRNA inactivation). Wider implications of the findings Overall, our results indicate CC-derived miRNAs as candidates of the CC-to-oocyte bidirectional communication and suggest a group of miRNAs as potential regulative molecules in the acquisition of the oocyte developmental competence. Trial registration number NOT APPLICABLE