As well as genetic information, the egg and sperm also contribute epigenetic annotations that may influence gene activity after fertilisation. These annotations may be direct modifications of the DNA bases or of the proteins around which the DNA is wrapped into chromatin. Our goal is to understand whether, through epigenetics, factors such as a mother’s age or diet have consequences on the health of a child. We examine how epigenetic states are set up in oocytes – or egg cells – and influence gene expression in the embryo. For example, repressive chromatin marks in oocytes lead to long-term silencing of genes inherited from the mother, particularly in cells that will form the placenta. We are also interested in how variations in DNA methylation come about in oocytes and whether we can use this variation as a marker for oocyte quality and embryo potential. To investigate these questions, we develop methods to profile epigenetic information in very small numbers of cells or even in single cells.
NLRP2 is a subcortical maternal complex (SCMC) protein of mammalian oocytes and preimplantation embryos. SCMC proteins are encoded by maternal effect genes and play a pivotal role in the maternal-to-zygotic transition (MZT), early embryogenesis, and epigenetic (re)programming. Maternal inactivation of genes encoding SCMC proteins has been linked to infertility and subfertility in mice and humans, but the underlying molecular mechanisms for the diverse functions of SCMC proteins, and specifically the role of NLRP2, are incompletely understood.
Biphasic in vitro oocyte maturation (IVM) can be offered as a patient-friendly alternative to conventional ovarian stimulation in in vitro fertilization (IVF) patients predicted to be hyper-responsive to ovarian stimulation. However, cumulative live birth rates after IVM per cycle are lower than after conventional ovarian stimulation for IVF. In different animal species, supplementation of IVM media with oocyte-secreted factors (OSFs) improves oocyte developmental competence through the expression of pro-ovulatory genes in cumulus cells. Whether the addition of OSFs in human biphasic IVM culture impacts the transcriptome of oocytes and cumulus cells retrieved from small antral follicles in minimally stimulated non-hCG-triggered IVM cycles remains to be elucidated. To answer this, human cumulus oocyte complexes (COCs) that were fully surrounded by cumulus cells or partially denuded at the time of retrieval were cultured in a biphasic IVM system either without or with the addition of pro-cumulin, a GDF9: BMP15 heterodimer. Oocytes and their accompanying cumulus cells were collected separately, and single-cell RNA-seq libraries were generated. The transcriptomic profile of cumulus cells revealed that pro-cumulin upregulated the expression of genes involved in cumulus cell expansion and proliferation while downregulating steroidogenesis, luteinization and apoptosis pathways. Moreover, pro-cumulin modulated the immature oocyte transcriptome during the prematuration step, including regulating translation, apoptosis and mitochondria remodeling pathways in the growing germinal vesicle (GV) oocytes. The addition of pro-cumulin also restored the transcriptomic profile of matured metaphase II (MII) oocytes that were partially denuded at collection. These results suggest that cumulus cell and oocyte transcriptome regulation by pro-cumulin may increase the number of developmentally competent oocytes after biphasic IVM treatment. Future studies should assess the effects of pro-cumulin addition in human biphasic IVM at the proteomic level and the embryological outcomes, particularly its potential to enhance outcomes of oocytes that are partially denuded at COC collection.
During the latter stages of their development, mammalian oocytes under dramatic chromatin reconfiguration, transitioning from a non-surrounded nucleolus (NSN) to a surrounded nucleolus (SN) stage, and concomitant transcriptional silencing. Although the NSN-SN transition is known to be essential for developmental competence of the oocyte, less is known about the accompanying molecular changes. Here we examine the changes in the transcriptome and DNA methylation during the NSN to SN transition in mouse oocytes.