Context-Dependent Enhancer Selection Confers Alternate Modes of Notch Regulation on argos

Abstract

Wiring between signaling pathways differs according to context, as exemplified by interactions between Notch and epidermal growth factor receptor (EGFR) pathways, which are cooperative in some contexts but antagonistic in others. To investigate mechanisms that underlie different modes of cross talk, we have focused on argos, an EGFR pathway regulator in Drosophila melanogaster which is upregulated by Notch in adult muscle progenitors but is repressed in the wing. Results show that the alternate modes of cross talk depend on the engagement of enhancers with opposite regulatory logic, which are selected by context-determining factors. This is likely to be a general mechanism for enabling the wiring between these pathways to switch according to context.

Spatial and temporal organization of signaling pathways

“…Further investigation into the regulation of argos by Notch in these two contexts demonstrated that different enhancer sequences are used in each tissue. One mediates direct activation of argos by Su(H) in the AMPs, whereas the other mediates its indirect repression via HLHmβ [14], a direct target of Notch signaling in the wing pouch. These results suggest that enhancers are selected based on the expression of tissue-specific transcription factors that modulate enhancer accessibility.…”

“…These results suggest that enhancers are selected based on the expression of tissue-specific transcription factors that modulate enhancer accessibility. In cultured cells derived from the AMPs, expression of the transcription factor Twist is required for Su(H) binding to the relevant argos enhancer, and over-expression of Twist in the wing pouch can convert the regulation of argos downstream of Notch from repression to activation [14,15]. A similar factor (Vvl) was identified that may play a role analogous to that of Twist in regulating enhancer activity in the wing pouch [14].…”

Notch signaling coordinates ommatidial rotation in the Drosophila eye via transcriptional regulation of the EGF-Receptor ligand Argos

“…Based on previous reports in Drosophila, the effect of Notch signaling on aos expression is context dependent (Housden et al, 2014). Gene expression is often controlled by multiple cisregulatory units that integrate information from various transcriptional inputs.…”

“…Essentially, aos has been reported to exhibit context-dependent enhancer selection in the wing: aos contains three enhancer regions identified, that are differentially responsive to factors that act downstream of Notch or EGFR signaling pathways (Ajuria et al, 2011;Krejci et al, 2009). The presence of context-determining factors will determine whether an enhancer is primed to react to a specific signal, which may result in a gene having different responses towards the same signals depending on which enhancer is accessible (Housden et al, 2014). Notably, the aos1-GFP reporter experiment reported here, and the Su(H) ChIP data, argue that the aos1 enhancer is directly responsive to Notch signaling in R4, indicating that Notch activates aos expression in this context.…”

“…To ask which cells in the developing eye disc are susceptible to this regulatory input, we utilized a GFP reporter construct, encompassing the high confidence "peak" region from the Su(H) ChIP, which had previously been shown to respond to Notch in muscle progenitor cells (aos1-GFP, Fig. 5B; (Housden et al, 2014) and tested its expression in the eye discs. Strikingly, expression of aos1-GFP was detected predominantly in R4 cells, as confirmed by co-expression of the R4 marker mδ0.5−lacZ ( Figure 5C).…”