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

Abstract

22Drosophila retinal patterning requires the expression of Atonal (Ato) through coordinated 23 regulation of 5' and 3' enhancer modules. ato-3' directs initial expression of Ato which then 24 directs autoregulation via 5'-ato. Notch (N) signaling also regulates 5'-ato, first enhancing Ato 25 expression and later repressing Ato by inducing E(spl) bHLHs. N signaling balances these 26 opposing functions by directing its obligate nuclear transcription factor, Suppressor of Hairless 27 (Su(H)), only in repressing 5'-ato. In this study, we reveal a novel and more nuanced role for 28 Su(H) in its regulation of 5'-ato. During retinal patterning, Su(H) is required for the expression 29 Anterior open (Aop), which, in turn, promotes 5'-ato activity. We demonstrate that Aop is 30 induced early in retinal patterning via N pathway activity, wherein Aop is required cell-31 autonomously for robust Ato expression during photoreceptor specification. In aop mutants, 32 expression from both ato enhancers is perturbed, suggesting that Aop promotes the Ato 33 autoregulation through maintenance of ato-3' activity. Clonal analysis indicates that Aop 34 indirectly opposes E(spl)-mediated repression of Ato. In the absence of both Aop and E(spl), 35 Ato expression is restored and the founding ommatidial photoreceptors, R8s, are specified. 36 These findings suggest that N signaling, through a potentially conserved relationship with Aop, 37 imposes a delay on ato repression, thus permitting autoregulation and retinogenesis. 38 3 39 Author Summary: 40The eye of the fruit fly has served as a paradigm to understand tissue patterning.41 Complex intercellular signaling networks cooperate during retinal development to allow cells to 42 become specialized visual-system precursor neurons at a specific time and place. These 43 neurons are precisely spaced within the developing retina and later recruit other cells to form 44 the repeated units that comprise insect eyes. The exact placement of each precursor cell 45 precipitates from the precise regulation of the atonal gene, which is first expressed in a cluster 46 of (10-20) cells before becoming restricted to only one cell from each cluster. The Notch 47 signaling pathway is required for both aspects of atonal regulation, first permitting up-regulation 48 within each cluster, and then the subsequent down-regulation to a single cell. However, the 49 connection between these two modes of Notch signaling had remained unclear. In this report, 50 we have identified that the anterior open gene is required to impose a delay on the restrictive 51 mode of Notch signaling, permitting the initial up-regulation of atonal to occur freely. In flies 52 mutant for anterior open, atonal bypasses its own up-regulation and proceeds directly to its 53 singled-out pattern but with significantly diminished robustness than occurs normally.