The Bric-à-Brac BTB/POZ transcription factors are necessary in niche cells for germline stem cells establishment and homeostasis through control of BMP/DPP signaling in the Drosophila melanogaster ovary

Abstract

Many studies have focused on the mechanisms of stem cell maintenance via their interaction with a particular niche or microenvironment in adult tissues, but how formation of a functional niche is initiated, including how stem cells within a niche are established, is less well understood. Adult Drosophila melanogaster ovary Germline Stem Cell (GSC) niches are comprised of somatic cells forming a stack called a Terminal Filament (TF) and associated Cap and Escort Cells (CCs and ECs, respectively), which are in direct contact with GSCs. In the adult ovary, the transcription factor Engrailed is specifically expressed in niche cells where it directly controls expression of the decapentaplegic (dpp) gene encoding a member of the Bone Morphogenetic Protein (BMP) family of secreted signaling molecules, which are key factors for GSC maintenance. In larval ovaries, in response to BMP signaling from newly formed niches, adjacent primordial germ cells become GSCs. The bric-à-brac paralogs (bab1 and bab2) encode BTB/POZ domain-containing transcription factors that are expressed in developing niches of larval ovaries. We show here that their functions are necessary specifically within precursor cells for TF formation during these stages. We also identify a new function for Bab1 and Bab2 within developing niches for GSC establishment in the larval ovary and for robust GSC maintenance in the adult. Moreover, we show that the presence of Bab proteins in niche cells is necessary for activation of transgenes reporting dpp expression as of larval stages in otherwise correctly specified Cap Cells, independently of Engrailed and its paralog Invected (En/Inv). Moreover, strong reduction of engrailed/invected expression during larval stages does not impair TF formation and only partially reduces GSC numbers. In the adult ovary, Bab proteins are also required for dpp reporter expression in CCs. Finally, when bab2 was overexpressed at this stage in somatic cells outside of the niche, there were no detectable levels of ectopic En/Inv, but ectopic expression of a dpp transgene was found in these cells and BMP signaling activation was induced in adjacent germ cells, which produced GSC-like tumors. Together, these results indicate that Bab transcription factors are positive regulators of BMP signaling in niche cells for establishment and homeostasis of GSCs in the Drosophila ovary.

A dual role of lola in Drosophila ovary development: regulating stem cell niche establishment and repressing apoptosis

“…Drosophila ovary is a model system for understanding the stem cell niche, yet the genetic mechanisms underlying establishment of the niche remain largely unknown. To date, only three transcription factors, Bab, Lmx1a and En/Inv, have been identified as essential factors for the formation or proper stacking of TFs [ 18 , 25 , 26 ]. In this study, we characterized the function of Lola in regulation of ovary development in Drosophila .…”

“…By using two independent lola RNAi lines ( lola 12574 and lola 12052R-1 ) driven by somatic drivers c587-Gal4 and Tj-Gal4 , we showed that lola is required for the formation of TFs at the time when ovarian stem cell niche is established. It has been shown that Bab is involved in the formation of TFs [ 18 , 22 ]. Similar to Bab protein, Lola also contains the BTB domain.…”

“…1A ) [ 15 – 17 ]. These cells provide extrinsic signals to GSCs to maintain stem cell identity [ 18 – 20 ]. Terminal filament (TF) formation occurs during the larval stage, and is the beginning point of GSC niche establishment [ 21 ].…”

“…The number of GSC niches or ovarioles in adult flies is equal to the number of TFs that form in the larval ovary [ 24 ]. At present, only a few genes, including bric-à-brac ( bab1 / bab2 ), Lmx1a and engrailed / invected ( en / inv ), have been reported to be expressed in TFCs and CCs and are involved in the formation of TFs [ 18 , 25 , 26 ]. The genetic events coordinating TF formation and function are still largely unknown.…”

Niche formation and function in developing tissue: studies from the Drosophila ovary

“…In addition to these signaling pathways, a few genes, including twinstar (tsr), bric-à-brac (bab1/bab2), LIM homeobox transcription factor 1 alpha (Lmx1a) and longitudinals lacking (lola), have been reported to be involved in TFC formation [46][47][48][49][50]. Chen et al found that in tsr mutant ovaries, the TFC number decreased and all TFCs have a rounded appearance.…”