What makes a good quality oocyte?
On average female fertility declines rapidly past 30 years of age. Contributing to this decline is a decreased oocyte pool and a deterioration in oocyte quality which is linked to higher incidences of aneuploidy, organelle dysfunction due to oxidative stress, and transcriptomic changes in the oocyte associated with compromised fertility outcomes. Yet, little is known of the mechanisms by which these transcriptomic and physiologic changes arise and how they contribute to cellular and metabolic health of oocytes. These changes are particularly important when transcription becomes inactive during the oocyte-to-embryo transition (OET, i.e., oocyte maturation and fertilization), and the maternal-to-zygotic transition (MZT, i.e., first rounds of mitotic cell division and the zygotic genome activation). The RNA required to resume meiosis and initiate embryogenesis is accumulated during oocyte growth when transcription is active and is post-transcriptionally regulated during the next stages of the OET and MZT. RNA tailing is one such regulator of RNA metabolism. Historically, long poly(A) tails have been associated with stable and highly translated transcripts while short poly(A) tails have been associated with unstable and silent transcripts. Tailing is the post-transcriptional addition of nucleotides to the 3′ end of RNA molecules in a untemplated manner by terminal nucleotidyl transferases (TNTs). Conversely, the enzymes responsible for the shortening of RNA tails are known as exonucleases. mRNA tails are very dynamic in oogenesis and early embryogenesis, and often correlate with waves of maternal transcript translation or degradation.
What are the mediators, regulators, and targets of tailing in the oocyte?
The disruption of several TNTs and exonucleases severely or completely impairs fertility in C. elegans. Yet, our understanding of their function, their targets, and the requirement for tailing during C. elegans oogenesis is incomplete. I hypothesize that the activity of a diverse array of TNTs and exonucleases is critical to regulate and coordinate the expression of essential genes in oocytes. C. elegans are very amenable to genetic manipulations and have a highly characterized development, anatomy and behavior where physiological changes are easy to track. Specifically, they have a highly organized germ line that provides easy access to specific stages of gametogenesis and early embryogenesis. Moreover, the OET and MZT are an ideal context to study regulatory mechanisms that impact RNA stability and translation. In summary, C. elegans are an ideal model to address these very important question and insights gained can be applied to mammalian cells. Work done in our lab fall under three major research goals:
AIM 1: Catalogue the expression and the physiological functions of TNTs and exonucleases in C. elegans.
AIM 2: Identify essential RNA targets involved in oogenesis and early embryogenesis.
AIM 3: Identify and characterize mechanisms that recruit and regulate TNT and exonuclease activity.
With these aims, we hope to identify novel molecular pathways essential for oogenesis, dissect evolutionarily conserved post-transcriptional mechanisms regulating gene expression during the OET and the MZT, and shed light on the physiological implications for RNA tailing in the oocyte. These results will also shed light on the impact of oocyte-donor age and other stressors on RNA tailing and its relationship to oocyte quality. In the long term, this could provide a comprehensive RNA perspective to our understanding of infertility and the development of assisted reproductive technologies and other therapies.