Endogenous siRNAs from naturally formed dsRNAs regulate transcripts in mouse oocytes

Area of Science:

• Molecular Biology
• Genetics
• Epigenetics

Background:

• RNA interference (RNAi) is a sequence-specific gene silencing mechanism involving double-stranded RNAs (dsRNAs).
• Endogenous small interfering RNAs (siRNAs) typically require RNA-dependent RNA polymerase (RdRP) for their biogenesis and function.
• The role and origin of endogenous siRNAs in mammals, which lack RdRP activity, remain largely uncharacterized.

Purpose of the Study:

• To investigate the biogenesis and function of endogenous siRNAs in mouse oocytes.
• To identify the sources and regulatory roles of small RNAs in mammalian oocytes.
• To elucidate the RNAi pathway's involvement in gene and retrotransposon regulation in the absence of RdRP.

Main Methods:

• Deep sequencing of small RNAs in growing mouse oocytes.
• Analysis of small interfering RNAs (siRNAs) and Piwi-interacting RNAs (piRNAs) sequences and origins.
• Assessment of gene and retrotransposon expression in Dicer and Ago2 knockout mouse oocytes.

Main Results:

• Identification of abundant piRNAs bound to Mili and siRNAs derived from retrotransposons and protein-coding transcripts.
• Demonstration that naturally occurring dsRNAs, from sources like inverted repeats and antisense transcripts, generate siRNAs.
• Evidence that loss of Dicer or Ago2 leads to reduced siRNAs and increased target transcript levels, confirming RNAi pathway function.

Conclusions:

• Endogenous siRNAs are present and functional in mammalian oocytes, regulating both protein-coding genes and retrotransposons.
• Mammalian organisms lacking RdRP can generate functional endogenous siRNAs from endogenous dsRNAs.
• Pseudogenes can serve as precursors for siRNAs, contributing to the regulation of their founding source mRNAs via RNAi.