The importance of cumulus cells for oocyte maturation and embryo development

Abstract

During oocyte maturation, cumulus cells play an important role. For example, they are important to convert glucose to pyruvate, the preferred energy substrate of the oocyte. Cumulus cells are closely connected to each other and to the oocyte; because of these cumulus cells may be useful to predict the developmental competence of the enclosed oocyte. The developmental competence is defined as the ability of the oocyte to develop into a living progeny. Small-RNA sequencing was used to investigate if microRNA expression in cumulus cells can predict the developmental competence of an oocyte. MicroRNAs are small, non-coding RNAs, which are able to regulate mRNA levels posttranscriptionally. It was found that miRNA expression varied between individual cumulus complexes. Further examinations were done with the highly expressed miRNAs miR-21, miR-25, miR-125, and miR-155. It was further investigated whether these miRNAs could regulate regulate in silicopredicted mRNA targets, like CD44, SLC38A2, SMAD2or WEE1. A luciferase-based reporter gene assay was used but did not show regulation of the target mRNAs by the miRNAs. Furthermore, it was found that the expression of the investigated mRNAs and miRNAs can change over time and also between the inner and outer layer of cumulus cells. Metabolites secreted into the maturation medium are important for the proper maturation of an oocyte. If cumulus cells were removed after 8h of cumulus-oocyte-complex maturation and the denuded oocytes were placed back into their conditioned medium, the percentage of embryo development was higher compared to denuded oocytes placed into fresh medium after 8h of maturation. Conditioned maturation medium was used for a metabolomic analysis, to identify metabolites which might have a beneficial effect on oocyte maturation. From the identified biochemicals carnitine and creatine were further investigated. The presence of carnitine, but not creatine during oocyte maturation improved oocyte developmental competence. A maternal diabetes influences the metabolic activity of a developing embryo and can cause a delay in development. The metabolomic profiles of early embryos cultured in 3 mM glucose, to mimic a diabetic environment, were investigated through analysing the metabolomic profile with UPLC-MS/MS. Embryos cultured in 3 mM glucose showed an increase in glycolysis and activity of the hexosamine pathway compared to embryos cultured in control conditions, without glucose.