The study, “Elevated peritoneal fluid ceramides in human endometriosis-associated infertility and their effects on mouse oocyte maturation,” was published in the journal Fertility and Sterility.
Infertility is a well-known complication of endometriosis, but underlying mechanisms of this association remain largely unknown. Besides anatomical alterations, factors such as poor quality of oocytes (immature eggs), impaired uterine implantation due to endometrial dysfunction, and poor ovarian reserve have been linked to EAI.
The peritoneal fluid (PF), which is found in the abdominal cavity and in contact with the ovaries, is essential for proper oocyte development. Studies using PF from women with endometriosis showed impaired oocyte maturation or development. This suggests that PF components could contain important information related to the defective peritoneal environment in EAI, the scientists said.
Ceramides are sphingolipids (a type of fat) important for cell membrane structure and function, and with a clinically important role in inducing apoptosis, which refers to programmed cell death, as opposed to cell death caused by injury.
Both in vivo and in vitro studies showed that ceramides are toxic to oocytes and embryos. Also, glucosylceramide (GlcCer) synthase, the enzyme responsible for the addition of a sugar chemical group — a process called glycosylation — to ceramides, is crucial for oocyte and embryo membrane formation.
Although these data suggest the importance of sphingolipids in fertility, they do not clarify which specific sphingolipids are associated with EAI, the researchers noted.
Aiming to address this gap, the team at National University of Singapore used an analytical technique called mass spectrometry to map the PF sphingolipids and conduct functional studies in cells from adult mice to identify which sphingolipids associate with EAI through impaired oocyte maturation.
The study included 27 infertile endometriosis patients and 20 individuals with infertility, but not endometriosis. Infertility was defined as actively trying to conceive for more than one year, being younger than 40 years, bilaterally patent (open) fallopian tubes, and reasonable sperm parameters in male partners.
Infertile women with endometriosis were trying to conceive for a mean time of 4.6 years, while those without this condition were trying for 2.9 years. Women in the groups had similar and healthy body mass index, age, and menstrual phase characteristics.
Results revealed 13 significantly increased PF sphingolipids, particularly eight different ceramides, in infertile women with severe endometriosis, compared to those without the disorder. Subsequent analysis found three very-long-chain ceramides potentially linked to EAI, one increasing, and the other two reducing its risk.
Then, studies in mouse cells validated these findings, as one of the very-long-chain ceramides that reduced EAI risk improved oocyte maturation rate, while the ceramide shown to increase EAI risk impaired these cells’ maturation.
The investigators further found that the ceramide linked to improved maturation increased the production of superoxide in mitochondria, the cells’ power plants, at lower doses, but suppressed it at the highest concentration. Of note, superoxide is a reactive oxygen species that causes oxidative stress.
In turn, the ceramide impairing oocyte maturation increased superoxide production at all doses. However, the team also found that dropping superoxide levels beyond a threshold reduced oocyte maturation, which suggests that an optimal superoxide level is required for oocyte maturation, the investigators noted.
“EAI is associated with accumulation of PF very-long-chain ceramides,” the scientists wrote. Besides providing improved understanding of EAI, the results also suggest that “future therapies targeted at this sphingolipid metabolism may be harnessed for improved oocyte maturation,” they added.