Cell-cell signaling mediated by Notch is critical during many different developmental processes for the specification or restriction of cell fates. Currently, the only known transduction pathway involves a DNA binding protein, Suppressor of Hairless [Su(H)] in Drosophila and CBF1 in mammals, and results in the direct activation of target genes. It has been proposed that in the absence of Notch, Su(H)/CBF1 acts as a repressor and is converted into an activator through interactions with the Notch intracellular domain [1--4]. Recently, we have also suggested that the activation of specific target genes requires synergy between Su(H) and other transcriptional activators [5]. Here we have designed an assay that allows us to directly test these hypotheses in vivo. Our results clearly demonstrate that Su(H) is able to function as the core of a molecular switch, repressing transcription in the absence of Notch and activating in the presence of Notch. In its capacity as an activator, Su(H) can cooperate synergistically with a DNA-bound transcription factor, Grainyhead. These interactions indicate a simple model for Notch target-gene regulation that could explain the precision of gene activation elicited by Notch signaling in different developmental fate decisions.
Expression of the Drosophila Enhancer of split [E(spl)] genes, and their homologues in other species, is dependent on Notch activation. The seven E(spl) genes are clustered in a single complex and their functions overlap significantly; however, the individual genes have distinct patterns of expression. To investigate how this regulation is achieved and to find out whether there is shared or cross regulation between E(spl) genes, we have analysed the enhancer activity of sequences from the adjacent E(spl)mbeta, E(spl)mgamma and E(spl)mdelta genes and made comparisons to E(spl)m8. We find that although regulatory elements can be shared, most aspects of the expression of each individual gene are recapitulated by small (400-500 bp) evolutionarily conserved enhancers. Activated Notch or a Suppressor of Hairless-VP16 fusion are only sufficient to elicit transcription from the E(spl) enhancers in a subset of locations, indicating a requirement for other factors. In tissue culture cells, proneural proteins synergise with Suppressor of Hairless and Notch to promote expression from E(spl)mgamma and E(spl)m8, but this synergy is only observed in vivo with E(spl)m8. We conclude that additional factors besides the proneural proteins limit the response of E(spl)mgamma in vivo. In contrast to the other genes, E(spl)mbeta exhibits little response to proneural proteins and its high level of activity in the wing imaginal disc suggests that wing-specific factors cooperate with Notch to activate the E(spl)mbeta enhancer. These results demonstrate that Notch activity must be integrated with other transcriptional regulators and, since the activation of target genes is critical in determining the developmental consequences of Notch activity, provide a framework for understanding Notch function in different developmental contexts.
Suppressor of Hairless (Su(H)) is a DNA-binding protein component of the Notch signalling pathway, thought to be required, with a fragment of the Notch receptor, for target gene activation. Recent studies show that this is only one side of the story: target gene enhancers may be regulated by Su(H) in a variety of different ways.
Suppressor of Hairless [Su(H)] is a DNA-binding protein that is the main intracellular transducer of the Notch signaling pathway in Drosophila. Several different mechanisms have been proposed to account for the activation of Su(H) by Notch. To further investigate how Su(H) activity is regulated we have used misexpression assays with wild-type Su(H) and with modified forms of Su(H) that contained a nuclear localization signal [Su(H)NLS], a transcriptional activation domain [Su(H)VP16], or a deletion of the domain required for interaction with the antagonist Hairless [Su(H)DeltaH]. Only Su(H)VP16 was able to mimic Notch activation effectively in the Drosophila wing, in agreement with the model that Notch activity normally confers coactivator function on Su(H). Neither nuclear localization nor elimination of Hairless binding was sufficient for activation. The phenotypes produced by overexpression of Su(H)wt and Su(H)NLS indicated a mixture of both increased and reduced Notch pathway activity and point to a role for Su(H) in both activation and repression of gene expression, as has been proposed for the mammalian homologue CBF1. Some phenotypes were equivalent to Notch loss-of-function, with wing-nicks and inhibition of a subset of target genes, which is most consistent with the ectopic proteins displacing a Su(H)-coactivator complex. Conversely, other phenotypes were equivalent to Notch gain-of-function, with wing-overgrowths and ectopic target-gene expression. These effects can be explained by the ectopic Su(H)/Su(H)NLS titrating a repressor complex. The wing-overgrowth phenotype is sensitive to the dose of Hairless and the phenotypes produced by coexpressing Su(H) and Hairless suggest that Hairless could form a component of this repressive complex.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.