Turnover of embryonic messenger RNA in preimplantation mouse embryos

We have estimated the average half-life of embryonic messenger RNA in mouse embryos at two stages of preimplantation development. Embryos were collected at 48 and 75 h post-hCG and cultured overnight in the presence of [3H]uridine. Beginning at 65--68 h (morulae) or 92--94 h (early blastocysts), the label was withdrawn and replaced with unlabelled uridine, and samples were taken at intervals thereafter for RNA isolation. Label in cytoplasmic, poly(A)-containing RNA was measured after binding to oligo(dT)-cellulose, and was normalized to label in 28S and 18S ribosomal RNA, separated on sucrose gradients. The stability of rRNA in both stages was verified directly, as was the integrity and purity of the isolated mRNA. With morulae, the mRNA decay curve was monophasic, with an average half-life of 9.5 +/- 0.9 h. In three experiments with early blastocysts the decay curve appeared to be biphasic, consisting of short-lived (less than 6 h) and long-lived (30--50 h) components; in two other experiments a short-lived component was not evident. In all cases, however, the overall average half-life of mRNA in early blastocysts, determined by linear regression assuming monophasic kinetics, was greater than that in morulae. Our data indicate that the stability of embryonic mRNA increases by at least twofold during the morula-to-blastocyst transition. The results are considered in terms of the transcriptional dependency of early mouse embryos and the regulation of maternal and embryonic mRNA.