Successful embryo implantation requires a synchronized dialogue between the receptive endometrium and blastocyst via locally produced soluble mediators. At the time of implantation, the blastocyst produces a significant amount of lactate, creating a microenvironment around the embryo characterized by high lactate and low pH. Whilst historically considered a ‘byproduct’ of metabolism, the identification of a lactate specific receptor, GPR81, and recent work in cancer cells has established lactate as an important signalling molecule, with roles in angiogenesis, ECM breakdown and immunosuppression. This study aimed to determine the role of blastocyst derived lactate, with and without pH adjustment, on cellular functions essential for establishment of endometrial receptivity and successful implantation.
Hormonally primed ECC-1 cells were exposed to varying concentrations (0mM, 2.5mM, 5mM, 7.5mM and 10mM) of Lactic acid (LA) or LA with neutralised pH (nLA) to mimic the microenvironment created by the blastocyst at implantation. Cells were analysed for cellular tight junction integrity (TER assessment), and changes in cellular proliferation and migration (xCelligence real-time cell function analysis). Stimulated and unstimulated mouse uterine tissue was analysed for GPR81 gene expression by real-time PCR.
Cellular tight junction integrity and cellular proliferation were significantly downregulated in ECC-1 cells exposed to 2.5mM, 5mM and 7.5mM LA (p<0.01) while cellular migration significantly increased in the presence of 2.5mM (p<0.05) and 5mM (p<0.01) LA. A decrease in cellular tight junction integrity (5mM and 7.5mM nLA) and proliferation (5mM nLA) (p=0.07) was still evident in ECC-1 cells exposed to nLA. Expression of GPR81 was detected in mouse uterine tissue, inferring a non-metabolic signalling role for LA in vivo.
These data therefore demonstrate a synergistic effect of LA, in combination with LA induced pH change, in enhancement of endometrial receptivity. This suggests that, via creation of a specialised microenvironment, the blastocyst acts to facilitate its own implantation.