P-221 Single-cell transcriptomic profiles reveal the characteristics of oocytes and CCs at GV, MI, and MII in oocytes before ovulation
Authors: Hu Y 
Published: June 2023

Abstract

Study question
what is the mechanism of the interaction between oocytes and CCs supporting oocyte development during oocyte maturation?

Summary answer
Cumulus cells are responsible for the accumulation of lipids and fatty acids to ensure oocytes have enough energy available for later embryogenesis.

What is known already
The oocyte and its surrounding cumulus cells (CCs) exist as an inseparable entity. The maturation of the oocyte is reliant on the materials and energy provided by CCs
Although several reports have addressed the transcriptome of oocytes and granulosa cells during human folliculogenesis, many questions remain unanswered regarding the interaction between oocytes and CCs during the final stages of folliculogenesis before ovulation. Therefore, the mechanism of the interaction between oocytes and CCs supporting oocyte development during oocyte maturation is needed to be studied.

Study design, size, duration
We collected a total of 139 samples from 47 preimplantation genetic testing (PGT) patients for RNA sequencing , including 97 CCs (GV: 35; MI: 30; MII: 32) and 42 oocytes (GV: 22; MI: 11; MII: 9).10 cells were randomly selected from individually cumulus-oocyte complex per CC sample to avoid bias due to the different number of CCs in each follicle. The samples were collected within a time frame of 3 months.

Participants/materials, setting, methods
The oocytes removed from CGCs were quickly transferred to the operating dish for marking, the maturation stage was assessed and recorded by observing the nucleus of oocytes, the CCs were mechanically isolated. Isolated RNA from oocytes and CCs underwent library preparation using an oligo deoxy-thymidine(dT) priming approach followed by deep sequencing. Data processing and bioinformatics analysis were performed using software, mainly including FASTP, STAR, String Tie, ARCANE and DESeq2 along with functional annotation analysis.

Main results and the role of chance
We found that oocyte maturation is a dynamic process and MI oocytes can be subdivided into GV-like-MI oocytes and MII-like-MI oocytes. we revealed unique transcriptional machinery, transcription factor networks, and crosstalk, displaying developmental stage-specific expression patterns at the three stages of oocyte maturation. We also identified that both lipid and cholesterol metabolism in cumulus cells are active during the late stage of oocyte maturation, and lipids may serve as a more efficient energy source for oocytes and even embryogenesis. Moreover, we also found that the metabolic profile of MII CCs, particularly those associated with the synthesis pathway of heparan sulfate synthesis, predicts the quality of DAY 3 blastomeres（P = 0.0001）.

Limitations, reasons for caution
The transcriptional profile of mural granulosa cells during the final stages of follicle maturation has not been fully explored.

Wider implications of the findings
This study provides a relatively systematic and comprehensive overview of the transcriptional features and interactions between oocytes and CCs at different stages of maturation before ovulation and may provide insights into developing a non-invasive method for the clinical assessment of oocyte quality and prediction of DAY3 blastomere quality in ART.

Trial registration number
not applicable