Project description:Xist represents a paradigm for long non-coding RNA function in epigenetic regulation, although how it mediates X-chromosome inactivation (XCI) remains largely unexplained. Multiple Xist-RNA binding proteins have recently been identified, including SPEN/SHARP, whose knockdown has been associated with deficient XCI at multiple loci. Here we demonstrate that SPEN is a key orchestrator of XCI in vivo and unravel its mechanism of action. We show that SPEN is essential for initiating gene silencing on the X chromosome in preimplantation mouse embryos and embryonic stem cells. On the other hand, SPEN is dispensable for maintenance of XCI in neural progenitor cells, although it significantly dampens expression of genes that escape from XCI. During initiation of XCI, we show by live-cell imaging and CUT&RUN approaches that SPEN is immediately recruited to the X chromosome upon Xist up-regulation, where it is targeted to enhancers and promoters of actively transcribed genes. SPEN rapidly disengages from chromatin once silencing is accomplished, implying a need for active transcription to tether it to chromatin. We define SPEN’s SPOC (SPEN paralog and ortholog C-terminal) domain as a major effector of SPEN’s gene silencing function, and show that artificial tethering of SPOC to Xist RNA is sufficient to mediate X-linked gene silencing. We identify SPOC’s protein partners which include NCOR/SMRT, the m6A RNA methylation machinery, the NuRD complex, RNA polymerase II and factors involved in regulation of transcription initiation and elongation. We propose that SPEN acts as a molecular integrator for initiation of XCI, bridging Xist RNA with the transcription machinery as well as nucleosome remodelers and histone deacetylases, at active enhancers and promoters.
Project description:X chromosome inactivation (XCI) is mediated by the non-coding RNA Xist which directs chromatin modification and gene silencing in cis. The RNA binding protein SPEN and associated corepressors have a central role in Xist-mediated gene silencing. Other silencing factors, notably the Polycomb system, have been reported to function downstream of SPEN. Making use of a SPEN separation-of-function mutation we show that SPEN and Polycomb pathways in fact function in parallel to establish gene silencing. Additionally, we find that differentiation-dependent recruitment of the chromosomal protein SmcHD1 is required for silencing many X-linked genes.
Project description:We show that Spen, an Xist binding repressor protein essential for XCI, binds to ancient retroviral RNA transcribed genome-wide, performing a surveillance role to recruit chromatin silencing machinery to these parasitic loci. Spen inactivation leads to de-repression of endogenous retroviral (ERV) elements in embryonic stem cells, with gain of chromatin accessibility, active histone modifications, and ERV RNA transcription. Spen binds directly to ERV RNAs that are highly similar to the A-repeat of Xist, a region critical for Xist-mediated gene silencing. ERV RNA and Xist A-repeat bind the RRM3 domain of Spen in a competitive manner and insertion of an ERV into an A-repeat deficient Xist rescues binding of Xist RNA to Spen.
Project description:We show that Spen, an Xist binding repressor protein essential for XCI, binds to ancient retroviral RNA transcribed genome-wide, performing a surveillance role to recruit chromatin silencing machinery to these parasitic loci. Spen inactivation leads to de-repression of endogenous retroviral (ERV) elements in embryonic stem cells, with gain of chromatin accessibility, active histone modifications, and ERV RNA transcription. Spen binds directly to ERV RNAs that are highly similar to the A-repeat of Xist, a region critical for Xist-mediated gene silencing. ERV RNA and Xist A-repeat bind the RRM3 domain of Spen in a competitive manner and insertion of an ERV into an A-repeat deficient Xist rescues binding of Xist RNA to Spen.
Project description:We show that Spen, an Xist binding repressor protein essential for XCI, binds to ancient retroviral RNA transcribed genome-wide, performing a surveillance role to recruit chromatin silencing machinery to these parasitic loci. Spen inactivation leads to de-repression of endogenous retroviral (ERV) elements in embryonic stem cells, with gain of chromatin accessibility, active histone modifications, and ERV RNA transcription. Spen binds directly to ERV RNAs that are highly similar to the A-repeat of Xist, a region critical for Xist-mediated gene silencing. ERV RNA and Xist A-repeat bind the RRM3 domain of Spen in a competitive manner and insertion of an ERV into an A-repeat deficient Xist rescues binding of Xist RNA to Spen.
Project description:We show that Spen, an Xist binding repressor protein essential for XCI, binds to ancient retroviral RNA transcribed genome-wide, performing a surveillance role to recruit chromatin silencing machinery to these parasitic loci. Spen inactivation leads to de-repression of endogenous retroviral (ERV) elements in embryonic stem cells, with gain of chromatin accessibility, active histone modifications, and ERV RNA transcription. Spen binds directly to ERV RNAs that are highly similar to the A-repeat of Xist, a region critical for Xist-mediated gene silencing. ERV RNA and Xist A-repeat bind the RRM3 domain of Spen in a competitive manner and insertion of an ERV into an A-repeat deficient Xist rescues binding of Xist RNA to Spen.