Notch and LIM-homeodomain protein Arrowhead regulate each other in a feedback mechanism to play a role in wing and neuronal development in Drosophila.

The Notch pathway is an evolutionarily conserved signalling system that operates to influence an astonishing array of cell fate decisions in different developmental contexts. To identify novel effectors of Notch signalling, we analysed the whole transcriptome of Drosophila wing and eye imaginal discs in which an activated form of Notch was overexpressed. A LIM-homeodomain protein, Arrowhead (Awh), was identified as a novel candidate that plays a crucial role in Notch-mediated developmental events. Awh alleles show strong genetic interaction with Notch pathway components. Awh loss-of-function upregulates Notch targets Cut and Wingless. Awh gain-of-function downregulates Notch targets by reducing the expression of the ligand Delta. Consequently, the expression of the Wingless effector molecule Armadillo and its downstream targets, Senseless and Vestigial, also gets downregulated. Awh overexpression leads to ectopic expression of engrailed, a segment polarity gene in the anterior region of wing disc, leading to patterning defects. Additionally, Notch gain-of-function-mediated neuronal defects get significantly rescued with Awh overexpression. Activated Notch inhibits Awh activity, suggesting a regulatory loop between Awh and Notch. Additionally, the defects caused by Awh gain-of-function were remarkably rescued by Chip, a LIM interaction domain containing transcriptional co-factor. The present study highlights the novel feedback regulation between Awh and Notch.