Schnurri transcription factors fromDrosophilaand vertebrates can mediate Bmp signaling through a phylogenetically conserved mechanism

Abstract

Bone Morphogenetic Proteins (Bmps) are secreted growth factors that play crucial roles in animal development across the phylogenetic spectrum. Bmp signaling results in the phosphorylation and nuclear translocation of Smads, downstream signal transducers that bind DNA. In Drosophila, the zinc finger protein Schnurri (Shn) plays a key role in signaling by the Bmp2/Bmp4 homolog Decapentaplegic (Dpp), by forming a Shn/Smad repression complex on defined promoter elements in the brinker (brk) gene. Brk is a transcriptional repressor that downregulates Dpp target genes. Thus, brk inhibition by Shn results in the upregulation of Dpp-responsive genes. We present evidence that vertebrate Shn homologs can also mediate Bmp responsiveness through a mechanism similar to Drosophila Shn. We find that a Bmp response element (BRE) from the Xenopus Vent2 promoter drives Dppdependent expression in Drosophila. However, in sharp contrast to its activating role in vertebrates, the frog BRE mediates repression in Drosophila. Remarkably, despite these opposite transcriptional polarities, sequence changes that abolish cis-element activity in Drosophila also affect BRE function in Xenopus. These similar cis requirements reflect conservation of trans-acting factors, as human Shn1 (hShn1; HIVEP1) can interact with Smad1/Smad4 and assemble an hShn1/Smad complex on the BRE. Furthermore, both Shn and hShn1 activate the BRE in Xenopus embryos, and both repress brk and rescue embryonic patterning defects in shn mutants. Our results suggest that vertebrate Shn proteins function in Bmp signal transduction, and that Shn proteins recruit coactivators and co-repressors in a context-dependent manner, rather than acting as dedicated activators or repressors.

Finding partners: How BMPs select their targets

“…In addition, through the use of SMA-9 fusions to either the herpes virus VP16 transcriptional activation or engrailed transcriptional repression domains, phenotypic rescue experiments have shown that SMA-9 may act as both a transcriptional activator and repressor (Liang et al, 2007). These findings parallel results showing that fly Schnurri has a dual role in mediating activation and repression (Yao et al, 2006). The analogies to Drosophila may become clearer when the mechanistic details of C. elegans's SMA-9 protein interactions with the BMP RSmads and genomic targets are further elucidated.…”

“…In stark contrast to the repressive function of Schnurri, acting on the BRE (or brkSE) in Drosophila, overexpression of either human Shn1 or fly Shn in frog embryos activates the expression of a vent2 BRE luciferase reporter (Yao et al, 2006). Human Shn1 protein fragments supershift a BRE DNA fragment together with the Smads (Yao et al, 2006) just as in the situation with the Drosophila brkSE (Pyrowolakis et al, 2004).…”

“…The mammalian Schnurri family members (4 -5 zinc fingers organized in 2-3 clusters) are variously known as Shn1/Hivep1/ Mbp1/Zas1/PrdIIBFI (Fan and Maniatis, 1990;Baldwin et al, 1990), Shn2/ Hivep2/Mbp2/Zas2 (Nomura et al, 1991;van't Veer et al, 1992), and Shn3/Hivep3/Krc/Mbp3/Zas3 (Wu et al, 1993). These Shn proteins have mainly been studied for their possible roles in regulating an assortment of genes including those encoding collagen type IIA, ␣-A-crystallin, ␣-1-antitrypsin, ␤ interferon, MHC H2-Kb and HIV genes (see review by Wu, 2002), but have not been implicated in BMP signaling until very recently (Yao et al, 2006;Jin et al, 2006).…”

“…This transgene contained several binding sites for Grainyhead (Grh), a transcriptional activator that drives broad expression in the fly embryo and epithelial sheets of the larval imaginal discs. The transgene surprisingly exhibited patterns of expression during early embryogenesis reminiscent of fly DPP target genes (Yao et al, 2006) suggesting that the DPPinduced repression mechanism might have analogies to the activation mechanism in the frog (Yao et al, 2006).…”

“…Human Shn1 protein fragments supershift a BRE DNA fragment together with the Smads (Yao et al, 2006) just as in the situation with the Drosophila brkSE (Pyrowolakis et al, 2004). In addition, as in the fly brkSE situation, modifications to the frog vent2 BRE that alter the spacing between its putative BMP R-Smad and Smad4 binding sites, without altering the sites themselves, similarly disrupt its ability to function as a BMP response element in frog embryos (Yao et al, 2006). Finally, human Shn1 protein can rescue fly schnurri mutant embryos as effectively as a fly schnurri, suggesting that human Schnurri can function as a repressor in flies just as Drosophila Schnurri could activate a reporter by means of the vent2 BRE in Xenopus ( Fig.…”