Abstract
Wing development in Drosophila requires the activation of Wingless (Wg) in a small stripe along the boundary of Fringe (Fng) expressing and non-expressing cells (FB), which coincides with the dorso-ventral (D/V) boundary of the wing imaginal disc. The expression of Wg is induced by interactions between dorsal and ventral cells mediated by the Notch signalling pathway. It appears that mutual signalling from dorsal to ventral and ventral to dorsal cells by the Notch ligands Serrate (Ser) and Delta (Dl) respectively establishes a symmetric domain of Wg that straddles the D/V boundary. The directional signalling of these ligands requires the modification of Notch in dorsal cells by the glycosyltransferase Fng and is based on the restricted expression of the ligands with Ser expression to the dorsal and that of Dl to the ventral side of the wing anlage. In order to further investigate the mechanism of Notch signalling at the FB, we analysed the function of Fng, Ser and Dl during wing development at an ectopic FB and at the D/V boundary. We find that Notch signalling is initiated in an asymmetric fashion on only one side of the FB. During this initial asymmetric phase, only one ligand is required, with Ser initiating Notch-signalling at the D/V and Dl at the ectopic FB. Furthermore, our analysis suggests that Fng has also a positive effect on Ser signalling. Because of these additional properties, differential expression of the ligands, which has been a prerequisite to restrict Notch activation to the FB in the current model, is not required to restrict Notch signalling to the FB.
Highlights
The Notch signalling pathway is an evolutionary conserved short-range signalling pathway that is involved in numerous developmental processes and diseases [1,2,3]
The activity of the Notch pathway is required to establish a stripe-like domain of expression of several genes along the D/V boundary that control wing growth and patterning, chief among them are wg and vg [11]
It is essential for this model to work at the D/V boundary that induction of expression of Ser is restricted to dorsal and that of Dl to ventral cells
Summary
The Notch signalling pathway is an evolutionary conserved short-range signalling pathway that is involved in numerous developmental processes and diseases [1,2,3]. The pathway consists of three core elements, a DSL (Delta/Serrate/Lag 2) ligand, the Notch receptor itself and a transcription factor of the CSL (CBF1/ Su(H)/Lag1) family. In Drosophila two ligands, Delta (Dl) and Serrate (Ser), exist together with one DNA binding CSL factor, Suppressor of Hairless (Su(H)). The binding of a ligand to Notch elicits a proteolytic cascade that results in the release of the intracellular domain of Notch (NICD). The released NICD associates with Su(H) and activates the transcription of the target genes. In the absence of Notch signalling Su(H) is part of a repressor complex that silences the target genes. Loss of H or Su(H) function results in the weak de-repression of several target genes in the absence of Notch signalling in Drosophila [8,9,10]
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