Retinoic Acid Action in Cumulus Cells: Implications to Oocyte Development and In Vitro Fertilization

In the field of human in vitro fertilization (IVF), selecting the best oocyte for freezing or embryo for transfer remains an important focus of clinical practice. Although several techniques are and have been used for this goal, results have generally not been favorable and/or are invasive such that damage to some embryos occurs, resulting in a reduced number of healthy births. Therefore, the search continues for non-invasive oocyte and embryo quality markers that signal the development of high-quality embryos. Multiple studies indicate the important positive effects of retinoic acid (RA) on oocyte maturation and function. Here, we present evidence that part of these effects are via the ability of RA to regulate the activity of connexin 43 (Cx43), the main subunit of gap junction channels in human cumulus granulosa cells (CGC). These channels play an important role in regulating the micronutrient environment of the oocyte by allowing the transfer of ions, metabolites, and small molecules. Multiple studies have demonstrated the requirement for Cx43 in CGC for the normal progression of folliculogenesis, and that increased expression of this connexin is linked to improved developmental competence of the oocyte. The data has shown that RA can up-regulate gap junction intercellular communication (GJIC) in the cumulus-oocyte complex by a non-genomic mechanism that results in the dephosphorylation of Cx43 and enhanced GJIC. Recognizing the positive role played by gap junctions in CGC on oocyte development and the regulation of Cx43 by RA, the findings have highlighted the possibility that CGC RA levels may serve as a non-invasive indicator for selecting high-quality oocytes for IVF procedures. In addition, the data suggests that manipulation of Cx43 with retinoid compounds could provide new pharmacological approaches to improve IVF outcomes in cases of failed implantation, recurrent miscarriage, or in certain diseases that are characterized by reduced fecundity, such as endometriosis.