Characterization of preovulatory follicular fluid secretome and its effects on equine oocytes during in vitro maturation.

The research aimed to understand the composition of the substance produced by horse follicular fluid (FF) before ovulation and test its effects on the maturation of horse oocytes (eggs) in the lab. The results showed the presence of numerous proteins and metabolites in the secretome, as well as extracellular vesicles (small particles). While the maturation of the oocytes was not affected, some changes were observed in gene expression.

Composition Analysis of the Follicular Fluid Secretome

• The liquid part of preovulatory equine follicular fluid, called the secretome, was collected and preserved at extremely low temperatures for further study.
• The metabolic and proteomic (relating to proteins) compositions of the secretome were analyzed. Ultrastructural composition was viewed under a special type of microscopy, which uses very cold temperatures for viewing preserved samples.
• Overall, the proteomic analysis identified 255 proteins, with most of them related to the complement cascade (part of the immune response) and cholesterol metabolism (breakdown process). The metabolomic analysis found 14 metabolites (products of metabolism).
• The researchers also observed the presence of extracellular vesicles (EVs) in the FF. EVs are tiny structures that transfer information between cells.

Effects on Oocyte Maturation and Gene Expression

• After the secretome analysis, horse eggs (oocytes) were collected either post-mortem or during life (by a method known as ‘ovum pick up’ or OPU) and were matured in the lab. This process, known as in vitro maturation (IVM), is generally used to produce early embryos (blastocysts).
• The IVM process was carried out in the absence (control) or presence of concentrated secretome (20 or 40 μg/ml).
• The oocytes were then evaluated for any changes in their chromatin configuration (genetic material structure) or were preserved quickly for gene expression analysis.
• The maturation rates of the oocytes did not vary significantly as a result of the different treatments. However, the expression levels of two genes (GDF9 and BMP15) significantly increased in OPU-derived oocytes compared to those from post-mortem sources.
• Furthermore, the presence of concentrated secretome led to increased expression of another gene (TNFAIP6), regardless of where the oocytes were sourced from.

Conclusion

• This research provides valuable insights into the composition of the horse follicular fluid secretome and potential effects on oocyte maturation and gene expression.
• Despite the absence of significant impact on maturation rates, the changes in gene expression raise questions on how the secretome might influence the developmental competence of horse oocytes. Consequently, these findings warrant more research to fully understand this potential relationship.