Cumulus Cell Transcriptome after Cumulus-Oocyte Complex Exposure to Nanomolar Cadmium in an In Vitro Animal Model of Prepubertal and Adult Age

SIMPLE SUMMARY: Cadmium (Cd), a highly toxic environmental contaminant, negatively affects human and animal fertility in females. In sheep, Cd displays age-dependent bioaccumulation at ovarian level. At environmental nanomolar concentrations, it reduces oocyte fertilization by inducing oxidative stress on cumulus cells (CCs), a cell population of the cumulus-oocyte complex (COC) supporting oocyte growth, functional maturation and fertilization. In this study, the modifications induced by Cd exposure to all the genes expressed in CCs of in vitro matured COCs, recovered from the ovaries of adult and prepubertal sheep, were analysed by RNA sequencing. A set of genes significantly dysregulated upon Cd exposure was identified. Effects of Cd were more relevant in CCs from adult than from prepubertal COCs. Most genes were upregulated while a minority of them were downregulated. Some genes were already known as involved in ovarian activity or Cd-induced effects, whereas others were completely new in these fields. These findings identify in the sheep, an important livestock species with translational relevance in human reproduction, the set of genes controlling oocyte functional competence, altered by Cd. These biomarkers will make it possible to identify oocytes that cannot be fertilized to evaluate whether they are to be discarded or recovered with detoxifying treatments. ABSTRACT: Cadmium (Cd), a highly toxic pollutant, impairs oocyte fertilization, through oxidative damage on cumulus cells (CCs). This study analysed the transcriptomic profile of CCs of cumulus-oocyte complexes (COCs) from adult and prepubertal sheep, exposed to Cd nanomolar concentration during in vitro maturation. In both age-groups, CCs of matured oocytes underwent RNA-seq, data analysis and validation. Differentially expressed genes (DEGs) were identified in adult (n = 99 DEGs) and prepubertal (n = 18 DEGs) CCs upon Cd exposure. Transcriptomes of adult CCs clustered separately between Cd-exposed and control samples, whereas prepubertal ones did not as observed by Principal Component Analysis. The transcriptomic signature of Cd-induced CC toxicity was identified by gene annotation and literature search. Genes associated with previous studies on ovarian functions and/or Cd effects were confirmed and new genes were identified, thus implementing the knowledge on their involvement in such processes. Enrichment and validation analysis showed that, in adult CCs, Cd acted as endocrine disruptor on DEGs involved in hormone biosynthesis, cumulus expansion, regulation of cell signalling, growth and differentiation and oocyte maturation, whereas in prepubertal CCs, Cd affected DEGs involved in CC development and viability and CC-oocyte communications. In conclusion, these DEGs could be used as valuable non-invasive biomarkers for oocyte competence.