Gamma-aminobutyric acid (GABA) can affect physiological processes in preimplantation embryos via GABAA and GABAB receptors

Purpose

Several widely used substances (e.g., some therapeutics or food supplements) can act on gamma-aminobutyric acid (GABA) receptors, and we investigated whether the activation of these receptors could affect the preimplantation embryo.

Methods

Transcripts of all GABA receptor subunits and selected proteins were examined using quantitative RT-PCR and immunohistochemistry. To analyze the effects of receptor activation, in vitro culture of mouse preimplantation embryos with natural and synthetic GABA receptor ligands was used.

Results

We detected nine GABA receptor transcripts in mouse blastocysts and 14 GABA receptor transcripts in ovulated oocytes. The results of this study indicate that ionotropic GABA_A receptors can be formed from α5, β3, and γ3 (or δ, π) subunits, GABA_A-ρ receptors can be formed from ρ2 subunits and metabotropic GABA receptors can be formed from GABA_B1b and GABA_B2 subunits in mouse blastocysts. Supplementing the culture medium with GABA at concentrations of 2-10 mM or with specific GABA_A and GABA_B receptor agonists (at concentrations of 10-100 μM) significantly increased the proportion of dead cells in blastocysts. The GABA-induced effects were prevented by pretreatment of embryos with GABA_A and GABA_B receptor antagonists.

Conclusion

The results of this study indicate that GABA and synthetic GABA receptor ligands can negatively affect preimplantation embryos via GABA_A and GABA_B receptors.

Glutamate can act as a signaling molecule in mouse preimplantation embryos

Free amino acids are present in the natural environment of the preimplantation embryo, and their availability can influence early embryo development. Glutamic acid is one of the amino acids with highest concentrations in female reproductive fluids, and we investigated whether glutamic acid/glutamate can affect preimplantation embryo development by acting through cell membrane receptors. Using RT-PCR, we detected 15 ionotropic glutamate receptor transcripts and 8 metabotropic glutamate receptor transcripts in mouse ovulated oocytes and/or in vivo developed blastocysts. Using immunohistochemistry, we detected expression of two AMPA receptor subunits, three kainate receptor subunits and member 5 metabotropic glutamate receptor protein in blastocysts. Extracellular concentrations of glutamic acid starting at 5 mM impaired mouse blastocyst development, and this fact may be of great practical importance since glutamic acid and its salts (mainly monosodium glutamate) are widely used as food additives. Experiments with glutamate receptor agonists (in combination with gene expression analysis) revealed that specific AMPA receptors (formed from GRIA3 and/or GRIA4 subunits), kainate receptors (formed from GRIK 3 and GRIK 4 or GRIK 5 subunits) and GRM5 glutamate receptor were involved in this effect. The glutamic acid-induced effects were prevented or reduced by pre-treatment of blastocysts with AMPA, kainate and GRM5 receptor antagonists, further confirming the involvement of these receptor types. Our results show that glutamic acid can act as a signaling molecule in preimplantation embryos, exerting its effects through activation of cell membrane receptors.

Apoptotic cells in mouse blastocysts are eliminated by neighbouring blastomeres

Apoptosis is a physiological process that occurs commonly during the development of the preimplantation embryo. The present work examines the ability of apoptotic embryonic cells to express a signal promoting their phagocytosis, and quantifies the ability of neighbouring, normal embryonic cells to perform that task. Microscopic analysis of mouse blastocysts revealed phosphatidylserine externalization to be 10 times less common than incidence of apoptotic cells (as detected by TUNEL). In spite of the low frequency of phosphatidylserine-flipping (in inner cell mass, no annexin V staining was recorded), fluorescence staining of the plasma membrane showed more than 20% of apoptotic cells to have been engulfed by neighbouring blastomeres. The mean frequency of apoptotic cells escaping phagocytosis by their extrusion into blastocyst cavities did not exceed 10%. Immunochemically visualised RAC1 (an enzyme important in actin cytoskeleton rearrangement) was seen in phagosome-like structures containing a nucleus with a condensed morphology. Gene transcript analysis showed that the embryonic cells expressed 12 receptors likely involved in phagocytic process (Scarf1, Msr1, Cd36, Itgav, Itgb3, Cd14, Scarb1, Cd44, Stab1, Adgrb1, Cd300lf, Cd93). In conclusion, embryonic cells possess all the necessary mechanisms for recognising, engulfing and digesting apoptotic cells, ensuring the clearance of most dying blastomeres.

Different response of embryos originating from control and obese mice to insulin in vitro

The aim of the present work was to investigate the impact of maternal obesity on DNA methylation in ovulated oocytes, and to compare the response of in vitro-developing preimplantation embryos originating from control and obese mice to insulin. An intergenerational, diet-induced obesity model was used to produce outbred mice with an increased body weight and body fat. Two-cell and eight-cell embryos recovered from obese and control mice were cultured in a medium supplemented with 1 or 10 ng/ml insulin until blastocyst formation. In the derived blastocysts, cell proliferation, differentiation, and death rates were determined. The results of immunochemical visualization of 5-methylcytosine indicated a slightly higher DNA methylation in ovulated metaphase II oocytes recovered from obese females; however, the difference between groups did not reach statistical significance. Expanded blastocysts developed from embryos provided by control dams showed increased mean cell numbers (two and eight-cell embryos exposed to 10 ng/ml), an increased inner-cell-mass/trophectoderm ratio (two-cell embryos exposed to 1 ng/ml and eight-cell embryos exposed to 10 ng/ml), and a reduced level of apoptosis (two and eight-cell embryos exposed to 10 ng/ml). In contrast, embryos originating from obese mice were significantly less sensitive to insulin; indeed, no difference was recorded in any tested variable between the embryos exposed to insulin and those cultured in insulin-free medium. Real-time RT-PCR analysis showed a significant increase in the amount of insulin receptor transcripts in blastocysts recovered from obese dams. These results suggest that maternal obesity might modulate the mitogenic and antiapoptotic responses of preimplantation embryos to insulin.