Progesterone acts on oocytes and cleavage-stage embryos by increasing cellular respiration, thereby providing energy for key cellular processes such as mitosis, according to new research from the Division of Reproductive Endocrinology and Infertility at Duke University Medical Center. These findings provide insight into the essential role of progesterone in early embryonic development.

Thomas Price, MD, presented results from the preclinical study at the Scientific Congress & Expo of the American Society for Reproductive Medicine held October 15 to 19, 2016, in Salt Lake City, UT.

Oocytes and embryos are naturally exposed to high levels of progesterone in the fallopian tube and uterus of humans and non-human primates. The human embryo begins to undergo gene activation approximately 3 days after fertilization. Prior research has suggested that progesterone acts on the oocyte and early embryo through mechanisms that are independent of transcriptional regulation.

The present study was designed to evaluate the effects of progesterone on oocytes and cleavage-stage embryos. Specifically, Price and colleagues focused on the interaction between progesterone and the mitochondrial progesterone receptor (PR-M), which has been shown to modulate intracellular respiration and energy production across a variety of cell types.

For this preclinical study, the research team examined oocytes from rhesus monkeys, which carry the same gene sequence for PR-M as human oocytes. The rhesus oocytes were collected after controlled ovarian hyperstimulation and stripped of granulosa cells for analysis or intracytoplasmic sperm injection with cryopreserved rhesus sperm. Rhesus embryos were cultured to the cleavage stage using standard protocols for 3 to 5 days.

Mouse oocytes, which lack the PR-M gene sequence, were chosen for the control group. Cryopreserved, 2-cell mouse embryos were thawed and cultured for 48 hours.

The rhesus oocytes, rhesus embryos, and mouse embryos were analyzed using a range of assays designed to characterize the function and activity of PR-M. Immunohistochemistry showed non-nuclear staining of anti–PR-M antibodies (anti-C19 and anti-P636) that partially co-localized with standard mitochondrial staining in rhesus oocytes and embryos. However, no staining with anti–PR-M antibodies was observed in the control embryos. In addition, neither rhesus nor mouse oocytes nor rhesus embryos expressed the nuclear progesterone receptor.

Real-time reverse transcription polymerase chain reaction assays detected PR-M transcripts in the rhesus oocytes and embryos. The PR-M transcripts were expressed at the same level in the rhesus oocytes and rhesus embryos at day 3. By comparison, PR-M transcripts were not detected in the control embryos.

The researchers then used a standard mitochondrial membrane potential assay to evaluate the function of PR-M (Table). Oocytes and embryos were challenged with R5020, a progesterone receptor agonist, or a dimethyl sulfoxide control.

Table. Mitochondrial Responses to Progesterone vs DMSO Control 

DMSO = dimethyl sulfoxide, NS = nonsignificant.

Treatment with the progesterone receptor agonist resulted in a significant increase in mitochondrial membrane potential in rhesus oocytes and in rhesus cleavage-stage embryos (P < .005 for both; see Table). No change in mitochondrial membrane potential was observed in the control embryos following progesterone treatment.

Results from these assays confirm that PR-M is expressed in rhesus oocytes and embryos. Furthermore, the findings show that progesterone treatment leads to significant increases in mitochondrial membrane potential, indicating increased cellular respiration. By comparison, mouse embryos neither express PR-M nor demonstrate a mitochondrial response to progesterone treatment. These observations suggest a mechanism by which progesterone enhances mitochondrial activity in non-human primate oocytes and embryos to provide the energy needed for early embryonic development.

Future research in this area may examine the role of progesterone treatment as a tool for increasing the rate of conversion from cleavage- to blastocyst-stage embryos. “It’s a delicate balance between increasing cellular respiration enough to facilitate embryo development, but without increasing the production of reactive-oxygen species that may lead to apoptosis,” Price said.

Source: Provost MP, Raburn DJ, Dai Q, Price TM. The effect of progesterone on early embryo metabolism. Presented at: American Society for Reproductive Medicine 2016 Scientific Congress & Expo; October 15-19, 2016; Salt Lake City, UT. Abstract O-262.