Treacle controls the nucleolar response to rDNA breaks via TOPBP1 recruitment and ATR activation.

Clémence Mooser,Ioanna-Eleni Symeonidou,Manuel Stucki,Sara C Larsen,Arti Jasrotia,Andrew N Blackford,Colin Leikauf,Katja Knechtle,Pia-Amata Leimbacher,Stephanie Jungmichel,Diana Zurbriggen,Daniel Fink,Ann-Marie K Shorrocks,Michael L Nielsen,Alison Ribeiro,Alain Jeanrenaud

doi:10.1038/s41467-019-13981-x

Abstract

Induction of DNA double-strand breaks (DSBs) in ribosomal DNA (rDNA) repeats is associated with ATM-dependent repression of ribosomal RNA synthesis and large-scale reorganization of nucleolar architecture, but the signaling events that regulate these responses are largely elusive. Here we show that the nucleolar response to rDNA breaks is dependent on both ATM and ATR activity. We further demonstrate that ATM- and NBS1-dependent recruitment of TOPBP1 in the nucleoli is required for inhibition of ribosomal RNA synthesis and nucleolar segregation in response to rDNA breaks. Mechanistically, TOPBP1 recruitment is mediated by phosphorylation-dependent interactions between three of its BRCT domains and conserved phosphorylated Ser/Thr residues at the C-terminus of the nucleolar phosphoprotein Treacle. Our data thus reveal an important cooperation between TOPBP1 and Treacle in the signaling cascade that triggers transcriptional inhibition and nucleolar segregation in response to rDNA breaks.

Highlights

Induction of DNA double-strand breaks (DSBs) in ribosomal DNA repeats is associated with ataxia-telangiectasia mutated (ATM)-dependent repression of ribosomal RNA synthesis and large-scale reorganization of nucleolar architecture, but the signaling events that regulate these responses are largely elusive
We propose that Treacle orchestrates the nucleolar response to ribosomal DNA (rDNA) breaks primarily through recruiting the two key adaptors NBS1 and TOPBP1 in the nucleoli, both of which are implicated in ATR activation
The findings reported here reveal a role of ATR kinase activity in the suppression of polymerase I (Pol I) transcription and nucleolar segregation in response to rDNA breaks

Summary

Introduction

Induction of DNA double-strand breaks (DSBs) in ribosomal DNA (rDNA) repeats is associated with ATM-dependent repression of ribosomal RNA synthesis and large-scale reorganization of nucleolar architecture, but the signaling events that regulate these responses are largely elusive. We further demonstrate that ATM- and NBS1-dependent recruitment of TOPBP1 in the nucleoli is required for inhibition of ribosomal RNA synthesis and nucleolar segregation in response to rDNA breaks. Transcriptional repression is accompanied by a large-scale structural alteration of the nucleoli whereby the rDNA repeats along with associated proteins translocate from inside the nucleoli into the nucleolar periphery, where they accumulate in focal structures that are called nucleolar caps[16,17,18,19] These structural changes—often referred to as nucleolar segregation—are induced by inhibition of Pol I transcription by small-molecule inhibitors, suggesting that transcriptional repression is the underlying cause of nucleolar segregation[20]. While nucleolar segregation has been proposed to facilitate the repair of rDNA breaks[16,17,19], the mechanism by which ATM signaling suppresses rRNA transcription and the functional implication of transcriptional repression and nucleolar segregation have not yet been studied in detail

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Results

Conclusion