Highly sensitive sequencing reveals dynamic modifications and activities of small RNAs in mouse oocytes and early embryos.

Small RNAs play important roles in early embryonic development. However, their expression dynamics and modifications are poorly understood because of the scarcity of RNA that is obtainable for sequencing analysis. Using an improved deep sequencing method that requires as little as 10 ng of total RNA or 50 oocytes, we profile small RNAs in mouse oocytes and early embryos. We find that microRNA (miRNA) expression starts soon after fertilization, and the mature miRNAs carried into the zygote by sperm during fertilization are relatively rare compared to the oocyte miRNAs. Intriguingly, the zygotic miRNAs display a marked increase in 3' mono- and oligoadenylation in one- to two-cell embryos, which may protect the miRNAs from the massive degradation taking place during that time. Moreover, bioinformatics analyses show that the function of miRNA is suppressed from the oocyte to the two-cell stage and appears to be reactivated after the two-cell stage to regulate genes important in embryonic development. Our study thus provides a highly sensitive profiling method and valuable data sets for further examination of small RNAs in early embryos.