The transcriptional repressor Brinker antagonizes Wingless signaling

Abstract

In the embryonic midgut of Drosophila, Wingless (Wg) signaling elicits threshold-specific transcriptional response, that is, low-signaling levels activate target genes, whereas high-signaling levels repress them. Wg-mediated repression of the HOX gene Ultrabithorax (Ubx) is conferred by a response sequence within the Ubx B midgut enhancer, called WRS-R. It further depends on the Teashirt (Tsh) repressor, which acts through the WRS-R without binding to it. Here, we show that Wg-mediated repression of Ubx B depends on Brinker, which binds to the WRS-R. Furthermore, Brinker blocks transcriptional activation by ubiquitous Wg signaling. Brinker binds to Tsh in vitro, recruits Tsh to the WRS-R, and we find mutual physical interactions between Brinker, Tsh, and the corepressor dCtBP. This suggests that the three proteins may form a ternary repressor complex at the WRS-R to quench the activity of the nearby-bound dTCF/Armadillo transcription complex. Finally, brinker and tsh produce similar mutant phenotypes in the ventral epidermis, and double mutants mimic overactive Wg signaling in this tissue. This suggests that Brinker may have a widespread function in antagonizing Wg signaling.

Opposing inputs by Hedgehog and Brinker define a stripe ofhairyexpression in theDrosophilaleg imaginal disc

“…Two elements, one which is Wg responsive (the WRS) and another Dpp responsive (the DRS) function synergistically to activate Ubx expression in ps 7 expression (Yu et al, 1998). In the adjacent ps8, however, Brk binds to the DRS and blocks the activity of the WRS (Saller et al, 2002). Curiously, the D-h-CMB and the Ubx-DRS are similarly organized in that each consists of overlapping CRE/Mad and Brk sites.…”

“…Brk was originally identified as a repressor of Dpp-target genes (Campbell and Tomlinson, 1999;Jazwinska et al, 1999;Minami et al, 1999) and a recent study indicates that Brk can block Wg-mediated transcription as well (Saller et al, 2002). Brk was shown to antagonize function of a Wg-responsive element in the midgut enhancer of the Ultrabithorax (Ubx).…”

“…In the D-h enhancer, however, the CMB is positioned at least 1 kb from the HHRE, implying a longrange effect for this element. Furthermore, Brk repression of the WRS depends on Teashirt (Tsh), which binds Brk and acts as a co-repressor (Saller et al, 2002). Tsh is unlikely to be required for D-h repression because it is only expressed in proximal leg segments (Erkner et al, 1999).…”

“…Active repressors can act either at short range, by inhibiting activity of activators bound to nearby elements (150 bp away or less), or at long range by interfering with activators bound at a greater distances (Cai et al, 1996). Brk can mediate active repression (Kirkpatrick et al, 2001), and binds the co-repressors dCtBP and Groucho (Gro), which mediate short-and long-range repression, respectively (Saller et al, 2002;Zhang et al, 2001). Brk requires Gro and/or dCtBP function for repression of a subset of its target genes, whereas neither is required for repression of others (Hasson et al, 2001).…”

Zinc‐finger paralogues tsh and tio are functionally equivalent during imaginal development in Drosophila and maintain their expression levels through auto‐ and cross‐negative feedback loops

“…During embryogenesis, tsh has been shown to participate in the patterning of the epidermis (Fasano et al, 1991;Roder et al, 1992;Andrew et al, 1994;de Zulueta et al, 1994;Gallet et al, 1998Gallet et al, , 2000Robertson et al, 2004) and the development of the salivary glands (Henderson et al, 1999) and the gut (Mathies et al, 1994;Waltzer et al, 2001;Saller et al, 2002), and tsh null mutants are embryonic lethal (Fasano et al, 1991). The Drosophila genome contains also a tsh paralogue, tiptop (tio; Laugier et al, 2005).…”