Proteome, not transcriptome, unveils challenges for superovulation

A collaboration of molecular biologists and bioinformaticians from Germany, Austria and Japan has discovered a limitation in the method long used to evaluate oocytes (immature egg cells) in medical research.

The new research focuses on a technique known as superovulation that has been used in laboratory research on mice for nearly a century and more recently in human medically assisted reproduction.

A preprint of the article was posted July 28 on the website Research Square, which prescreens articles before they have been peer reviewed by a journal.

In studies on mouse reproduction, a hormone known as chorionic gonadotropin, derived from horses or humans, is administered to stimulate egg production.

“The reason for preferring the super-ovulated oocytes is one of convenience,” the authors write. “Fewer donor animals are needed, and the time of oocyte collection can be scheduled.”

It had long been assumed that oocytes produced by superovulation are the same as those produced normally. However, the study showed that superovulation can produce changes in the proteome (the assemblage of proteins that can be produced by a given genome) that may affect the traits of the offspring mice.

These changes do not necessarily show up in the transcriptome (the full range of RNA that acts to transcribe or read out the cell DNA) that has been traditionally evaluated to predict the quality of an oocyte.

Prior to this research effort there had been no definitive comparison of the proteome of oocytes derived by superovulation with those derived by natural ovulation. The authors hypothesized that the proteomic data may provide a more robust answer than data from the transcriptome, “since gene transcription is silenced in late oogenesis, and the initial stages of embryogenesis are controlled by maternal proteins.” 

Analysis of the data showed that superovulation is associated with proteomic changes in mice that result in differences in observable traits (phenotype). The oocytes produced by superovulation showed such differences as  a thinner zona pellucida (the thick, transparent membrane near the outside of the ovum), a smaller oocyte diameter, an increase in the number of embryos that stop dividing (cleavage arrest) and defective formation of the blastocyst, the structure that forms the embryo

All these differences were associated with changes in protein expression, such as folding, that could be detected by analysis of the proteome but not the transcriptome.

The abnormalities seem to be associated with the less mature oocytes used in superovulation. Delaying the time of administration of the gonadotropin hormone from two days to three days resulted in a reduction of the defects and no reduction in the litter size yielded by superovulation.

“These improvements after 72 hours support that the oocytes had stopped prematurely on their way to attain the proper size and quality in the conventional superovulation regime,” the authors write. “It may be noted that a longer stimulation can be beneficial also in human reproduction,” they add.

Lead author Leila Taher of Graz University of Technology (Austria) was joined by six coauthors from the Max Planck Institute for Molecular Biomedicine in Muenster, Germany, the Institute of Bioinformatics at the University of Muenster, the Rostock University Medical Center in Germany, and the Department of Medical Genome Sciences at University of Tokyo.