Drosophila Aop imposes a delay on E(spl)-mediated repression of Ato during R8 specification.

Abstract

Drosophila retinal patterning requires the expression of Atonal (Ato) through coordinated regulation of 5 and 3 enhancer modules. ato-3 directs initial expression of Ato which then directs autoregulation via 5-ato. Notch (N) signaling also regulates 5-ato, first enhancing Ato expression and later repressing Ato by inducing E(spl) bHLHs. N signaling balances these opposing functions by directing its obligate nuclear transcription factor, Suppressor of Hairless (Su(H)), only in repressing 5-ato. In this study, we reveal a novel and more nuanced role for Su(H) in its regulation of 5-ato. During retinal patterning, Su(H) is required for the expression Anterior open (Aop), which, in turn, promotes 5-ato activity. We demonstrate that Aop is induced early in retinal patterning via N pathway activity, wherein Aop is required cell-autonomously for robust Ato expression during photoreceptor specification. In aop mutants, expression from both ato enhancers is perturbed, suggesting that Aop promotes the Ato autoregulation through maintenance of ato-3 activity. Clonal analysis indicates that Aop indirectly opposes E(spl)-mediated repression of Ato. In the absence of both Aop and E(spl), Ato expression is restored and the founding ommatidial photoreceptors, R8s, are specified. These findings suggest that N signaling, through a potentially conserved relationship with Aop, imposes a delay on ato repression, thus permitting autoregulation and retinogenesis.\n\nAuthor SummaryThe eye of the fruit fly has served as a paradigm to understand tissue patterning. Complex intercellular signaling networks cooperate during retinal development to allow cells to become specialized visual-system precursor neurons at a specific time and place. These neurons are precisely spaced within the developing retina and later recruit other cells to form the repeated units that comprise insect eyes. The exact placement of each precursor cell precipitates from the precise regulation of the atonal gene, which is first expressed in a cluster of (10-20) cells before becoming restricted to only one cell from each cluster. The Notch signaling pathway is required for both aspects of atonal regulation, first permitting up-regulation within each cluster, and then the subsequent down-regulation to a single cell. However, the connection between these two modes of Notch signaling had remained unclear. In this report, we have identified that the anterior open gene is required to impose a delay on the restrictive mode of Notch signaling, permitting the initial up-regulation of atonal to occur freely. In flies mutant for anterior open, atonal bypasses its own up-regulation and proceeds directly to its singled-out pattern but with significantly diminished robustness than occurs normally.