Engrailed acts with Nejire to control decapentaplegic expression in the Drosophila ovarian stem cell niche

Abstract

Homeostasis of adult tissues is maintained by a small number of stem cells, which are sustained by their niches. In the female germline stem cell (GSC) niche, Decapentaplegic (Dpp) is the primary factor that promotes GSC self-renewal. However, the mechanism regulating expression in the niche is largely unknown. Here, we identify a 2.0 kb fragment located in a 5' -regulatory region of the locus containing enhancer activity that drives its expression in the niche. This region is distinct from a previously characterized 3' -regulatory enhancer responsible for expression in imaginal discs. Our data demonstrate that Engrailed, a homeodomain-containing transcription factor that serves as a cap cell marker, binds to this region and regulates expression in cap cells. Further data suggest that En forms a complex with Nejire (Nej), the ortholog of histone acetyltransferase CBP/p300, and directs Nej to this -regulatory region where Nej functions as the co-activator for expression. Therefore, our study defines the molecular pathway controlling expression in the ovarian stem cell niche.

The Bric-à-Brac transcription factors are necessary for formation of functional germline stem cell niches through control ofdppexpression in theDrosophila melanogasterovary

“…We first characterized the adult ovarian expression profile of these two lines. In adult germaria ( Fig S1), we found that the two fusion proteins were detected at high levels in CCs (S1A''',B''' Fig, yellow brackets), sometimes in ECs (S1B Fig, blue brackets) and absent from TF cells (S1A''',B''' Fig, green brackets), and were also detected in prefollicular cells (S1B Fig, orange brackets), reflecting the endogenous dpp expression pattern (Liu et al, 2010Luo et al, 2017;Wang et al, 2008;Xie and Spradling, 2000). For analysis in prepupal ovaries, we used the dpp-nlsGFP transgene since the nuclear Dpp:GFP fusion protein produced was less likely to perturb endogenous Dpp function.…”

“…bab regulates Engrailed/Invected accumulation in TF cells, but not in CCs, during larval stages Like the bab genes, engrailed and its paralog invected (en/inv) have also been shown to be involved in proper TF formation (Bolívar et al, 2006). In addition, evidence strongly suggests that En directly controls dpp expression in adult CCs (Luo et al, 2017). Therefore, we addressed whether bab function may act upstream of en/inv in larval stages, and thereby would impact dpp transcriptional regulation indirectly through en/inv.…”

“…In adult ovary GSC niches, en/inv is known to be necessary for GSC maintenance through its regulation of dpp expression in CCs likely by direct binding of En to dpp regulatory sequences (Luo et al, 2017;Rojas-Ríos et al, 2012). We tested whether en/inv gene functions are also necessary for GSC establishment in ovaries by the prepupal stage.…”

“…CCs are considered to be the key component of GSC niches, their number correlating closely with the number of GSCs (Song et al, 2007;Xie and Spradling, 2000). CCs anchor GSCs to the niche by DE-cadherin-mediated adhesion (Song et al, 2002) and produce two Bone Morphogenetic Proteins (BMPs), Decapentaplegic (Dpp) and Glass Bottom Boat (Gbb), acting as short-range secreted signals required for GSC maintenance (Liu et al, 2010Luo et al, 2017;Song et al, 2004;Wang et al, 2008;Spradling, 1998, 2000). Indeed, Dpp/Gbb signal is transduced in GSCs, via the presence of a specific receptor composed of both Thickveins and Punt, leading to phosphorylation of the transcription factor Mad (pMad), translocation of pMad into the nucleus and transcriptional repression of germline differentiation genes such as bag-of-marbles (bam) (Chen and McKearin, 2003b;Song et al, 2004).…”

“…Indeed, Dpp/Gbb signal is transduced in GSCs, via the presence of a specific receptor composed of both Thickveins and Punt, leading to phosphorylation of the transcription factor Mad (pMad), translocation of pMad into the nucleus and transcriptional repression of germline differentiation genes such as bag-of-marbles (bam) (Chen and McKearin, 2003b;Song et al, 2004). It was shown that the homeobox transcription factor Engrailed (En) binds dpp cisregulatory sequences in vitro and activates dpp transcription in CCs (Luo et al, 2017). Several other signaling pathways, such as the JAK/STAT (Lopez-Onieva et al, 2008;Wang et al, 2008) and Hedgehog Lu et al, 2015;Rojas-Ríos et al, 2012) pathways, are active in niche cells and have also been implicated in regulation of dpp expression in these cells.…”