Nuclear Argonaute:miRNA complexes recognize target sequences within chromatin and silence gene expression

Area of Science:

• Molecular Biology
• Gene Regulation
• RNA Biology

Background:

• RNA interference (RNAi) is crucial for gene silencing in mammalian cells, primarily occurring in the cytoplasm.
• MicroRNAs (miRNAs) and RNAi factors are also found in the nucleus, but their nuclear functions in gene regulation are not fully understood.

Purpose of the Study:

• To investigate the nuclear roles of RNAi factors and miRNAs in gene expression regulation.
• To identify nuclear miRNA:RNA interactions and their impact on target gene expression using a novel experimental approach.

Main Methods:

• Chimeric enhanced crosslinking immunoprecipitation (eCLIP) was employed to identify Argonaute 2 (AGO2)-miRNA complexes and their associated chromatin targets.
• Quantitative analysis of miRNA binding sites and target mRNA levels in both cytoplasmic and nuclear cellular compartments.

Main Results:

• Chimeric eCLIP identified abundant nuclear miRNAs and their chromatin-associated RNA targets.
• High mobility group AT-Hook A (HMGA2) was identified as a primary target of let-7 miRNA-mediated binding.
• Let-7 miRNA repressed HMGA2 mRNA expression in both the cytoplasm and nucleus, with minimal impact on splicing or transcription.

Conclusions:

• RNAi-mediated gene silencing is not exclusively a cytoplasmic process; it can initiate in the nucleus.
• Nuclear miRNA:RNA interactions contribute to the regulation of endogenous gene expression, influencing RNA levels in both nuclear and cytoplasmic compartments.
• Chimeric eCLIP is a valuable tool for discovering functional miRNA:RNA interactions in vivo.