Abstract
Argonaute proteins (AGOs), which play an essential role in cytosolic post-transcriptional gene silencing, have been also reported to function in nuclear processes like transcriptional activation or repression, alternative splicing and, chromatin organization. As most of these studies have been conducted in human cancer cell lines, the relevance of AGOs nuclear functions in the context of mouse early embryonic development remains uninvestigated. Here, we examined a possible role of the AGO1 protein on the distribution of constitutive heterochromatin in mouse embryonic stem cells (mESCs). We observed a specific redistribution of the repressive histone mark H3K9me3 and the heterochromatin protein HP1α, away from pericentromeric regions upon Ago1 depletion. Furthermore, we demonstrated that major satellite transcripts are strongly up-regulated in Ago1_KO mESCs and that their levels are partially restored upon AGO1 rescue. We also observed a similar redistribution of H3K9me3 and HP1α in Drosha_KO mESCs, suggesting a role for microRNAs (miRNAs) in the regulation of heterochromatin distribution in mESCs. Finally, we showed that specific miRNAs with complementarity to major satellites can partially regulate the expression of these transcripts.
Highlights
The miRNA pathway is crucial in regulating early embryonic development and differentiation in vivo and in vitro (DeVeale et al, 2021)
AGO1 had previously been reported to interact with RNA Polymerase II in human cells, where AGO1 was linked to chromatin and active promoters (Huang et al, 2013; Alló et al, 2014; Shuaib et al, 2019)
We decided to conduct a genetic approach by depleting Ago1 from WT mouse embryonic stem cells (mESCs) (Fig S1A and B) and assessed by immunofluorescence the localization of the repressive histone mark H3K9me3 and the heterochromatin protein HP1α in the mutant cell lines compared with WT mESCs (Fig 1)
Summary
The miRNA pathway is crucial in regulating early embryonic development and differentiation in vivo and in vitro (DeVeale et al, 2021). MiRNAs can fine-tune gene expression throughout early embryonic development at the post-transcriptional level. Mature miRNAs are loaded into Argonaute (AGO) proteins, which are key components of the RNA-induced silencing complex. They guide the RNA-induced silencing complex to partially complementary target sequences leading to the translational inhibition of these targets (Bartel, 2018). There are four AGO proteins (AGO1-4), but only AGO1 and AGO2 are detectably expressed during early embryonic development, with AGO2 being substantially more abundant (Lykke-Andersen et al, 2008; Boroviak et al, 2018; Müller et al, 2020). Whereas Ago2deficient mice die at a post-implantation stage, because of severe developmental defects (Liu et al, 2004; Alisch et al, 2007; Morita et al, 2007; Cheloufi et al, 2010), Ago1,3,4-deficient mice are viable (Modzelewski et al, 2012; Van Stry et al, 2012)
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