Role of co-repressor genomic landscapes in shaping the Notch response

Abstract

Repressors are frequently deployed to limit the transcriptional response to signalling pathways. For example, several co-repressors interact directly with the DNA-binding protein CSL and are proposed to keep target genes silenced in the absence of Notch activity. However, the scope of their contributions remains unclear. To investigate co-repressor activity in the context of this well defined signalling pathway, we have analysed the genome-wide binding profile of the best-characterized CSL co-repressor in Drosophila, Hairless, and of a second CSL interacting repressor, SMRTER. As predicted there was significant overlap between Hairless and its CSL DNA-binding partner, both in Kc cells and in wing discs, where they were predominantly found in chromatin with active enhancer marks. However, while the Hairless complex was widely present at some Notch regulated enhancers in the wing disc, no binding was detected at others, indicating that it is not essential for silencing per se. Further analysis of target enhancers confirmed differential requirements for Hairless. SMRTER binding significantly overlapped with Hairless, rather than complementing it, and many enhancers were apparently co-bound by both factors. Our analysis indicates that the actions of Hairless and SMRTER gate enhancers to Notch activity and to Ecdysone signalling respectively, to ensure that the appropriate levels and timing of target gene expression are achieved.

Structural and Functional Studies of the RBPJ-SHARP Complex Reveal a Conserved Corepressor Binding Site

“…Moreover, rescue of repression by WT RBPJ, but not a SHARP-binding mutant, strongly suggests that SHARP is the primary core-pressor that mediates repression at Notch target genes in lymphocytes and, likely, other cells and tissues, which is consistent with previous in vivo studies of SHARP in mice ( Surendran et al, 2010 ; Tsuji et al, 2007 ; Kuroda et al, 2003 ). These data are also consistent with research in other experimental systems showing that loss of RBPJ results in transcriptional derepression at some, but not all, Notch target genes ( Chan et al, 2017 ; Castel et al, 2013 ). Why RBPJ-SHARP corepressor complexes are recruited to some Notch targets but not others remains an open question.…”

Structural and functional studies of the RBPJ-SHARP complex reveal conserved corepressor binding site

“…Moreover, reconstitution of repression by wild-type RBPJ, but not a SHARP-binding mutant, strongly suggests that SHARP is the primary corepressor that mediates repression via RBPJ at Notch target genes in lymphocytes, and likely other cells and tissues, which is consistent with previous in vivo studies of SHARP in mice 8,39,40 . These data are also consistent with research in other experimental systems that show that loss of CSL results in transcriptional derepression at some, but not all, Notch target genes 41,42 . Why RBPJ-SHARP corepressor complexes are recruited to some Notch targets, but not others, remains an open question.…”

Direct and widespread role for the nuclear receptor EcR in mediating the response to ecdysone in Drosophila

“…Examination of +6hAPF wings revealed no changes in the spatial pattern of Dl enhancer activity relative to −6hAPF, indicating that ectopic enhancer activation in usp clones does not reflect incipient changes in enhancer activity. Recently, EcR binding sites were shown to overlap with those for the Notch regulator, Hairless, supporting a potential role of EcR in regulating spatial patterns of gene expression (45). We conclude that EcR regulates both temporal and spatial patterns of gene expression.…”