Expression of aquaporin water channels in equine endometrium is differentially regulated during the oestrous cycle and early pregnancy.

This research study investigates the expression patterns of different aquaporin (water channel) subtypes in the equine endometrium (uterine lining) during various stages of the estrous cycle and early pregnancy. The findings suggest these water channels contribute to changes in the fluid content in the uterine lining and play a role in its receptiveness during early pregnancy in mares.

Methods and Results

• The research team analyzed the expression of 12 different aquaporin subtypes in the equine endometrium on a molecular (mRNA) and protein level.
• The endometrial samples were obtained at various stages such as during anoestrus (no heat period), oestrus (heat period), 8 days and 14 days after ovulation in non-pregnant mares, and 14 days post ovulation in pregnant mares.
• They used quantitative PCR (polymerase chain reaction), a common method to amplify and quantify targeted DNA molecules.
• All aquaporin subtypes except for AQP10 and 12 showed time-dependent patterns of expression.
• Through this, they noticed that AQP3, 5 and 7 had the highest amount of mRNA 8 days post ovulation, whereas AQP0 and 2 were the most abundant on the 14th day of the cycle in non-pregnant mares.
• By the 14th day of pregnancy, AQP1, 4, 8, 9 and 11 showed the highest expression levels.
• Western blot analysis was used to confirm protein expression of AQP0, 2 and 5. This method allows for the detection and quantification of specific proteins in a sample of tissue homogenate or extract.

Immunohistochemistry and Observations

• Immunohistochemistry, a process to visually localize antigens or proteins in cells of a tissue section, was used to locate the protein expressions to luminal and glandular epithelial and stromal cells.
• The findings suggested that staining intensity was highest in AQP0 in samples obtained on the 14th day of the oestrous cycle.
• Furthermore, AQP2 immunoreactivity appeared to be stronger in samples collected 14 days after ovulation from non-pregnant animals, particularly in the luminal epithelial staining.
• On the 8th day post ovulation from cyclic animals, the samples displayed intense AQP5 staining of the glandular epithelium, predominantly in the deeper glands.
• The team also found out that treatment of anoestrous mares with progesterone did not enhance the expression of aquaporins, suggesting that factors other than progesterone are required for the up-regulation of certain AQP subtypes during dioestrus (quiet stage between heats).

Conclusion

• Based on their findings, the researchers concluded that an equine-specific combination of aquaporin subtypes contributes to the changes in endometrial fluid content throughout the oestrous cycle. This implies that these water channels play a significant role in the endometrium’s receptivity during early pregnancy in mares.