60 TELOMERE LENGTH DYNAMICS DURING BOVINE PREIMPLANTATION EMBRYO DEVELOPMENT

The establishment of telomere length (TL) during embryogenesis determines telomere reserves in newborn mammals. However, limited information is available on TL dynamics during preimplantation in contrast to the extensive existing data on telomerase activity in germ cells and embryogenesis. Telomerase activity is high in the male germ line, low or absent in oocytes and early stage embryos and high in blastocysts (Bl). Mechanisms allowing early embryos to reset TL remain poorly understood. The documented telomere lengthening at the morula/Bl transition in mice and bovines is dependent on telomerase activity. A recombinant-based mechanism termed alternative lengthening of telomeres (ALT) has been postulated to be responsible for the lengthening of telomeres in early embryos. The aims of the present study were to analyse the telomere dynamics during preimplantation embryo development in 2 species of known different TL: mice and bovines and the relative expression of 2 components of telomerase [telomerase reverse transcriptase (Tert; the key factor that controls the activity of the telomerase) and telomerase RNA component (Terc)]. Twenty samples for each developmental stage with an equivalent number of cells (matured oocytes/zygotes: 8 and 32; 2-cell embryos: 4 and 16; 4-cell embryos: 2 and 8; 8-cell embryos: 1 and 4; 16-cell embryos: 2 only for bovine; morulae: 1 and 1; and Bl: 1 and 1 for mice and bovines, respectively) were analysed for relative TL measurement using the real-time quantitative PCR method described previously (Bermejo-Alvarez et al. 2008 Physiol. Genomics 32, 264272). For measuring the mRNA, 3 groups of 10 oocytes/embryos for each developmental stage were used. Data were analysed by 1-way ANOVA. In mice, matured oocytes had the shortest telomeres of all stages examined (P < 0.01); a slight increase up to the 4-cell stage and a decrease at the 8-cell and morula stages was noted (P < 0.05), while a marked increase was observed in Bl, as expected (P < 0.01). In contrast, bovine matured oocytes had longer telomeres than zygotes and this length gradually decreased up to the 4-cell stage and increased again at the 16-cell stage (P < 0.05). Then, telomeres shortened at the morula stage (P < 0.05) and a significant increase was observed at the Bl stage like in mice (P < 0.01). The relative abundance of mTerc increased throughout development with a marked up-regulation at the morula and Bl stages (P < 0.01). On the other hand, the relative abundance of mTert was significantly higher in the mature oocytes and zygotes compared to later stages (P < 0.01); however, it should be noted that there was a gradual increase from the 2-cell stage up to Bl. In conclusion, in contrast to mice, bovine oocytes have longer telomeres than zygotes. Knowing that the telomerase activity is low or absent until the Bl stage (indicated by the low expression of Tert), the TL increase detected in 16-cell bovine embryos indicates an alternative mechanism for telomere elongation during early development, like that observed in mice. Understanding how telomeres reset during early embryo development has implications for the study of stem cells and regenerative biology.