Novel Oocyte and Cumulus Cell-Derived Markers of Egg Quality: Potential Diagnostic and Therapeutic Implications.

Abstract

A growing body of evidence suggests oocyte developmental competence is a limiting factor in pregnancy success in multiple species, including humans. However, the inherent phenotypic characteristics of competent oocytes are not well understood. We have conducted fundamental studies using the bovine model to elucidate differences in oocyte and cumulus cell transcriptome composition associated with poor oocyte competence and the potential diagnostic and therapeutic significance of such results. The bovine model was chosen because ovarian physiology and several aspects of embryo development are similar between the two single-ovulating species and because of existence of multiple, well established bovine models of poor oocyte competence. Of particular interest from microarray studies is the increased transcript abundance for members of the cathepsin family of lysosomal cysteine proteinases (cathepsin Z, S and B) in cumulus cells surrounding oocytes collected from prepubertal (model of poor oocyte competence) versus adult animals. Variation in cumulus cell expression of mRNA for these genes for oocytes collected from adults is also associated with success of blastocyst development. Treatment of bovine oocytes with a cathepsin inhibitor during meiotic maturation enhances rates of blastocyst development following in vitro fertilization (IVF) demonstrating therapeutic potential of inhibition of cathepsin activity. Stimulatory effects of cathepsin inhibitor treatment on subsequent blastocyst development following IVF are both dependent on presence of cumulus cells during meiotic maturation and associated with reduced rates of cumulus cell apoptosis. We have also observed a positive relationship between oocyte expression of follistatin and oocyte quality in two distinct models of poor oocyte competence. Follistatin supplementation and ablation experiments in early embryos were performed to determine the functional significance of such findings. Treatment of presumptive zygotes with follistatin during embryo culture in vitro (prior to embryonic genome activation) enhances blastocyst development and blastocyst cell allocation in favor of trophectoderm. In contrast, follistatin ablation via microinjection of follistatin siRNA into presumptive zygotes results in reduced rates of blastocyst development and decreased trophectoderm cell numbers. These effects of follistatin ablation can be rescued by treatment of siRNA injected embryos with exogenous follistatin. To begin to address the potential translational relevance of our results, expression of above markers of oocyte competence in the rhesus monkey model is under investigation using the Primate Embryo Gene Expression Resource (PREGER). Results to date indicate that cathepsin expression is increased in rhesus monkey blastocysts in response to in vitro culture, suggesting that cathepsin expression may also be negatively associated with embryo competence in primates. Collectively, results indicate that cathepsins and follistatin are potential markers to identify oocytes with enhanced developmental competence and that manipulation of cathepsin activity and follistatin levels during in vitro culture has therapeutic potential to improve success of embryo development.