Role of INO80 and SRCAP nucleosome remodellers in transcriptome amplification during B-lymphocyte activation

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Transcriptome amplification is an important process in the immune system, which produces immune response against invading pathogens. This process begins with global increase in mRNA synthesis resulting in rapid shift of quiescent B-cell from low transcription to full gene expression. However, the early epigenetic events that initiate this process are partially understood. Chromatin remodelling is a prerequisite for gene transcription as it renders the DNA to be accessible to the binding factors to facilitate transcription. Chromatin architecture is modulated by changing nucleosome dynamics and incorporating the histone variant H2A.Z. These changes rely on the activity of ATP- dependent remodeling complexes including SRCAP and INO80. It has been reported that ß-actin and Ruvb1, subunits of SRCAP and INO80, play crucial role in the activation process of B-cells as their binding is lost with activation. In contrast, Nfrkb, a subunit of INO80, remains bound to gene promoters after induction. These data suggest the possibility of involving these remodelers in the process of transcriptome amplification in B-cell. Therefore, this study aimed to further investigate the role of these remodellers in this process by studying their binding pattern in quiescent and activated B-cell. ChIP analysis of the binding pattern of these remodellers in quiescent and in vitro activated B-cells showed that gene promoters were highly enriched in SRCAP, INO80 and H2A.Z. These proteins localized primarily at regulatory elements, and they were less abundant at coding regions. Furthermore, while SRCAP was evicted from regulatory elements upon activation, INO80 was still maintained. This suggests an antagonist activity of SRCAP and INO80. SRCAP could represent an epigenetic signature for 3 repressed gene expression while INO80 operates to ensure efficient transcription. H2A.Z accumulated at induced gene promoters representing an epigenetic mark for the inducible state. Furthermore, upon SRCAP clearance, H2A.Z was also evicted from coding regions after activation ensuring prompt lymphocyte response against pathogen invasion. This study reinforces a model where H2A.Z at coding region represents a repressive role on transcription and propose a role for SRCAP in the mechanism of transcriptome amplification that is distinct from its paralogue INO80.
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