Mechanisms underlying age-related defects in the mammalian oocyte: potential deleterious effects of methylglyoxal, a reactive dicarbonyl compound

Tatone C., Carbone M., Heizenrieder T., Gualtieri R., Eichenlaub-Ritter U. Mechanisms underlying age-related defects in the mammalian oocyte: potential deleterious effects of methylglyoxal, a reactive dicarbonyl compound University of L’Aquila, Dept of Health Sciences, L’Aquila (Italy) University of Bielefeld, Faculty of Biology, Gene Technolo/Microbiol, Bielefeld (Germany) University of Naples “Federico II”, Dept of Structural and Functional Biology, Naples (Italy) Methylglyoxal (MG) is a reactive dicarbonyl compound physiologically produced by various metabolic pathways. It causes inhibition of proliferation, and mitochondrial respiration,and increases reactive oxygen species, apoptosis and formation of advanced glycation end-products. Recent findings revealed a reduced MG scavenging efficiency in aged mouse ovaries. Therefore, we have investigated potential deleterious effects of MG on female gametes by exposing denuded (DO) and cumulus enclosed (COC) mouse oocytes from adult outbred mice (MF1,CD1) to 50-300M MG during in vitro maturation (IVM) for 16h or 19h. MG negatively affected the rate of polar body formation in both CEO and DO at 16h, with a more pronounced effect in DO. Oocytes with normal metaphase-II spindles decreased from about 73% (control) to 18% after 16h maturation in 75M MG. Moreover, oocytes in ana/telophase-I or with unaligned chromosomes were about three-to four-fold more abundant in the MG-exposed than in the control group. When maturation was prolonged to 19h MG-exposed oocytes exhibited normal appearing spindles and aligned chromosomes and increased PB rate. Time lapse analysis by polarisation microscopy confirmed that MG induced a pronounced meiotic delay. There was no increase in hyperploidy in the MG-exposed oocytes after 19h-IVM, while about 70% of meiosis I arrested oocytes had unaligned chromosomes at 16h-IVM suggesting that young oocytes are capable of dealing with a disturbance by MG by prolonging the spindle assembly checkpoint and progressing to meiosis II only when chromosomes are properly aligned and attached. CEO staining with JC-1 dye showed that MG was capable to induce changes in cytoplasmic localization of high polarized mitochondria. Furthermore, the number of foci of histone gamma H2AX increased in the nuclei of GV-arrested oocytes exposed for 5h to 75M MG, consistent with induction of DNA damage by MG. In accordance, TUNEL assay revealed about 30% apoptosis rate in CEO exposed to 300M MG. In conclusion, the present results indicate that MG may be one of the factors, which act synergistically to cause age-related changes in the ovarian and/or follicle microenvironment resulting in high susceptibility to meiotic errors and reduced developmental potential of aged oocytes.