BMP and Hh Signaling Affects Primordial Germ Cell Division in Drosophila

Abstract

The germline is segregated from the remainder of the soma during early embryonic development in metazoan species. In Drosophila, female primordial germ cells (PGCs) continue to proliferate during larval development, and become germline stem cells at the early pupal stage. To elucidate the roles of growth factors in larval PGC division, we examined expression patterns of a bone morphogenetic protein (BMP) growth factor, Decapentaplegic (Dpp), and Hedgehog (Hh), along with factors downstream of each, in the ovary during larval development. Dpp signaling appeared in the ovarian soma from early larval development, and was prominent in the terminal filament cells at late larval stage, whereas Hh appeared in the ovarian soma and PGCs from the third instar larval stage. The number of PGCs decreased when components of these signal transduction pathways were abrogated by RNAi in the PGCs, indicating that both Dpp and Hh signals directly regulate PGC proliferation. Experiments on the up- and down-regulation of Dpp and Hh with a tissue-specific Gal4 driver indicated that Dpp and Hh act as extrinsic and autocrine growth factors. Furthermore, heat-pulse experiments with hs-Gal4 showed that Dpp is active in PGC proliferation throughout larval development, whereas Hh has effects only during late larval development. In addition to Dpp, the reduction of Glass bottom boat (Gbb), another BMP molecule, caused a decrease in the number of PGCs and initiation of larval PGCs differentiation into cystocytes, indicating that Gbb functions to promote PGC division and repress differentiation.

Somatic gonadal cells: The supporting cast for the germline

“…Studies in the developing ovary have shown that dpp is expressed in the somatic cells of the ovary throughout larval development and becomes prominent in the terminal filament and cap cells at late L3 (Sato et al, 2010). This expression continues into the early pupal stage, when GSCs are first established in the ovary (Xie and Spradling, 2000;Zhu and Xie, 2003).…”

“…This expression continues into the early pupal stage, when GSCs are first established in the ovary (Xie and Spradling, 2000;Zhu and Xie, 2003). Expression of a reporter of dpp signaling, dad-lacZ, is observed in all PGCs during larval stages and in the anterior PGCs at early pupal stages (Sato et al, 2010;Xie and Spradling, 2000;Zhu and Xie, 2003). Overexpression of dpp in late embryogenesis and during larval development causes an increase in PGC number, while reduction of DPP or a second TGFb ligand, GBB, results in a reduction in PGC number (Sato et al, 2008(Sato et al, , 2010.…”

“…Expression of a reporter of dpp signaling, dad-lacZ, is observed in all PGCs during larval stages and in the anterior PGCs at early pupal stages (Sato et al, 2010;Xie and Spradling, 2000;Zhu and Xie, 2003). Overexpression of dpp in late embryogenesis and during larval development causes an increase in PGC number, while reduction of DPP or a second TGFb ligand, GBB, results in a reduction in PGC number (Sato et al, 2008(Sato et al, , 2010. During early pupal stages, TGFb signaling regulates clonal expansion of GSCs for population of the niche, as mutation of the DPP receptor, Thickveins (TKV), reduces the mitotic potential of GSCs and their ability to clonally populate the niche (Zhu and Xie, 2003).…”

“…Finally, both the Wingless (WG) and Hedgehog (HH) signaling pathways regulate PGC mitosis in females (Sato et al, 2008(Sato et al, , 2010. WG is present in PGCs by stage 16 of embryogenesis, and a reduction in WG signaling results in the delayed entry of PGCs into mitosis (Sato et al, 2008).…”

Position matters: Variability in the spatial pattern of BMP modulators generates functional diversity

“…Several labs have used fluorescently tagged forms of Dpp (Dpp-GFP) to investigate behavior of Dpp protein in fixed and live tissue (Entchev et al, 2000;Gibson et al, 2002;Teleman and Cohen, 2000;Wang and Ferguson, 2005). There have been reports using antibodies against Dpp (Gibson et al, 2002;Sato et al, 2010) and high levels of BMP activity can be detected with antibodies against phosphorylated Mad (pMad; Dorfman and Shilo, 2001;Eldar et al, 2002;Mizutani et al, 2005;Ross et al, 2001;Shimmi and O'Connor, 2003;Tanimoto et al, 2000;Teleman and Cohen, 2000), a SMAD protein activated by phosphorylation upon binding of BMPs to transmembrane receptors (Fig. 1a).…”