RNAi in mouse oocytes and early embryos

Double-strand RNA (dsRNA) induced specific mRNA degradation, also known as RNA interference (RNAi), is a recently recognized phenomenon conserved among eukaryotic species. This work summarizes the results of RNAi studies in mouse oocytes and early embryos, which focused on two aspects of RNAi in mouse oocytes and early embryos: its biology and its experimental use in silencing genes of interest. This work was the first to demonstrate and characterize RNAi in mammalian cells. The analysis of the biology of RNAi further (1) identified and characterized the mouse Dicer homolog, a protein involved in the initial step of RNAi; (2) suggested that the mammalian RNAi pathway, unlike those of Caenorhabditis, Arabidopsis , and Neurospora, most likely lacks RNA-dependent RNA polymerase suggesting that not all components of RNAi are universally conserved; and (3) implicated a possible role of RNAi in silencing of transposable elements during mouse preimplantation development. It was observed that the increase in Dicer activity correlates with zygotic genome activation (ZGA). It is hypothesized that ZGA provides a window of opportunity for expression of parasitic sequences. It was found that transcription of the transposable elements L1, IAP, and MuERV-L is initiated during ZGA, and that inhibition of Dicer correlates with increased mRNA levels of IAP and MuERV-L, supporting the hypothesis that RNAi is involved in repression of parasitic sequences during early mouse development. Studies examining RNAi as an approach (1) demonstrated that RNAi is a very suitable approach to study genes in mouse oocytes and early embryos; (2) generated protocols for RNAi induced by microinjection of dsRNA or plasmid expressing dsRNA; and (3) resulted in the first successful transgenic RNAi experiment in the mouse, which has demonstrated that the expression of dsRNA from a transgene leads to specific degradation of the cognate transcript and silencing of the cognate gene. Taken together, this work represents an extensive analysis of RNAi in mouse oocytes and early embryos that is a firm foundation for further RNAi studies.