Abstract
Maternal lipolytic metabolic disorders result in a lipotoxic microenvironment in the ovarian follicular fluid (FF) which deteriorates oocyte quality. Although cellular stress response mechanisms are well defined in somatic cells, they remain largely unexplored in oocytes, which have distinct organelle structure and nuclear transcription patterns. Here we used shotgun proteomic analyses to study cellular responses of bovine oocytes and cumulus cells (CCs) after in vitro maturation under lipotoxic conditions; in the presence of pathophysiological palmitic acid (PA) concentration as a model. Differentially regulated proteins (DRPs) were mainly localized in the endoplasmic reticulum, mitochondria and nuclei of CCs and oocytes, however the DRPs and their direction of change were cell-type specific. Proteomic changes in PA-exposed CCs were predominantly pro-apoptotic unfolded protein responses (UPRs), mitochondrial and metabolic dysfunctions, and apoptotic pathways. This was also functionally confirmed. Interestingly, although the oocytes were enclosed by CCs during PA exposure, elevated cellular stress levels were also evident. However, pro-survival UPRs, redox regulatory and compensatory metabolic mechanisms were prominent despite evidence of mitochondrial dysfunction, oxidative stress, and reduced subsequent embryo development. The data provides a unique insight that enriches the understanding of the cellular stress responses in metabolically-compromised oocytes and forms a fundamental base to identify new targets for fertility treatments as discussed within.
Highlights
Maternal metabolic disorders such as obesity and type-II diabetes are increasing in prevalence and have been strongly linked with reduced fertility[1] and IVF success rates[2,3]
We have previously observed differential expression of few genes related to lipid and carbohydrate metabolism between cumulus cells (CCs) and oocytes derived from free fatty acid (FFA)-treated bovine COCs22
The average counts of total, Inner cell mass (ICM), and trophectoderm (TE) cells were not affected by the treatment as determined by differential immunostaining with CDX2 and Hoechst
Summary
Maternal metabolic disorders such as obesity and type-II diabetes are increasing in prevalence and have been strongly linked with reduced fertility[1] and IVF success rates[2,3]. (ER) causes ER stress that elicit specific “unfolded protein response” (UPRer)[16] This is evident in FFA-treated bovine COCs in vitro[17] in CCs12 and was associated with reduced embryo development rates. The aim of this study was to analyze and compare cellular responses to PA-induced lipotoxicity during in vitro maturation of bovine COCs using shotgun proteomic analysis of CCs and of the enclosed-oocytes. This was complemented by other functional tests of mitochondrial activity, oxidative stress levels, cellular apoptosis and a follow up of subsequent early embryo development
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