Brinker autoregulation and gradient formation in the Drosophila wing

Establishing patterns of differentiation is an important theme in developmental biology. A key mechanism involved in creating these patterns of differentiation is the establishment and interpretation of transcription factor gradients. The Drosophila transcriptional repressor Brinker (Brk) is expressed in lateral-to-medial gradients across the anterioposterior axis of the wing imaginal disc where it negatively regulates the spatial patterns of expression of genes including spalt (sal) and optomotor-blind (omb); the precise pattern of expression of these targets is determined by their sensitivity to repression by Brk so that the sal domain is narrower than that of omb largely because it is repressed by lower levels of Brk than omb. The brk gradient is established by an inverse gradient of BMP signaling through the secreted BMP homolog Decapentaplegic (Dpp): the intracellular Smad effectors of Dpp signaling, pMad and Medea, bind together with the repressor protein Schnurri (Shn) to silencer elements at the brk locus and repress the activity of a constitutive enhancer. My studies have revealed that the generation of the brk gradient is not simply a precise negative read-out of the dpp gradient; Brk must also negatively autoregulate its own expression by interacting with the pMad/Medea/Shn repressor complex. Additionally, I have demonstrated that this Brk/pMad/Medea/Shn repressor complex alone cannot establish the graded profile of brk but that an additional positive cis-regulatory element that is activated by pMad is required. This may provide the first example of a transcription factor both activating and repressing the same gene, brk, in the same cells at the same time via two different response elements. In conclusion, generating the brk gradient requires at least two positive and two negative inputs: constitutive activation by an activator, Brk negative autoregulation, and both activation and repression by pMad. Generating a stable expression gradient appears to be much more complex than previously thought and may reflect the importance of multiple inputs in generating intermediate levels of gene expression rather than a simple on/off threshold response.