(144) Investigating Ribosome Biogenesis and Translation in the Aging Mammalian Oocyte

Anna Galligos^1, Wei-Ting Yueh¹, Joseph Varberg¹, Francesca Duncan², and Jennifer L. Gerton¹

¹Graduate School of the Stowers Institute, Stowers Institute for Medical Research, Kansas City, United States

²Feinberg School of Medicine, Northwestern University, Chicago, United States




 

Abstract Text: Oocyte quality is the greatest barrier to fertility at advanced maternal age. Though the impact of oocyte quality is appreciated, our understanding of its molecular culprits is limited. One aspect of oocyte quality that remains understudied is translation, the production of proteins by macromolecular machines called ribosomes. Oocytes are unique in that they rely entirely upon ribosomes to modulate gene-regulation during meiotic maturation, fertilization, paternal genome unpacking, and the oocyte-to-embryo transition. Furthermore, the ribosomes made by the oocyte provide for the zygote until its own ribosome biogenesis pathway becomes active. Ribosomes are therefore an important - but poorly characterized - factor of oocyte quality. If ribosome production or activity is perturbed with age, oocyte quality would be significantly compromised.
To address this gap in knowledge, we conducted single-cell RNA sequencing from oocytes isolated from reproductively young and old female mice. The pathways identified to be the most enriched with age were those associated with ribosome biogenesis and translation. We then measured the levels of ribosomal RNA (rRNA), the rate limiting factor of ribosome biogenesis, within oocytes. As expected, we found that rRNA also significantly increases in oocytes with age. This observation compliments the finding that cytoplasmic ribosome populations in oocytes significantly increase with age. To determine the functional output of these ribosomes, global translation assays were conducted in germinal vesicle oocytes from reproductively young and old mice. The results revealed significantly lower translation levels in old oocytes. Dual fluorescent translation stalling assays were conducted to pinpoint the mechanisms that contribute to the observed age-dependent loss of translation. Ribosomes in old oocytes are significantly more susceptible translational stalling at stretches polyproline-encoded RNA. This phenotype is significant not only because it could contribute to the loss of global translation with age but also because it could compromise cellular processes that rely on timely and synchronized protein production, such as meiosis. Indeed, a computational screen identified over 40 meiotic proteins involved in chromosome organization, recombination, checkpoints, and spindle positioning, that include one or more polyproline motifs. 
Taken together, we surmise that ribosome numbers increase while translation activity decreases with age. Ribosome stalling may be one source of age-dependent translation decline, which could have negative ramifications for many cellular processes, including meiosis. We hypothesize that as oocytes age, the regulation of ribosome production and translation efficiency are compromised. Continued investigation of age-associated changes in ribosome production and translation activity in oocytes will provide new insight into oocyte quality and how it changes with age.