fused regulates germline cyst mitosis and differentiation during Drosophila oogenesis

Abstract

The fused gene encodes a serine-threonine kinase that functions as a positive regulator of Hedgehog signal transduction in Drosophila embryogenesis, wing morphogenesis, and somatic cell development during oogenesis. Here, we have characterized the germline ovarian tumors present in adult ovaries of fused mutant females, a phenotype not observed upon deregulation of any other component of Hedgehog signaling. In the strongest fused mutant contexts, we found that tumorous ovarian follicles accumulate early spectrosome-containing germ cells corresponding to germline stem cells and/or early cystoblasts as evidenced by activated Dpp signal transduction and transcriptional repression of bag-of-marbles, encoding the cystoblast determination factor. These early germ cells are maintained far from their usual position in a specialized niche of somatic cells in the apical part of the germarium, which appears normal in size in fused mutant ovarioles. Therefore, these results indicate a novel function for fused in downregulation of Dpp signaling which is necessary for de-repression of bag-of-marbles and consequent cystoblast determination. The abnormal accumulation of these early germ cells seems to be due primarily to defects in differentiation since we show that germline stem cell proliferation in the germarium is not affected. A later block in germline development, at the 16-cell cyst stage before significant nurse cell and oocyte differentiation, was also observed in tumorous follicles when fused function was only partially lowered. Finally, fused mutant ovaries exhibit some germline cysts having undergone a supernumerary fifth mitotic division. Through clonal analysis, we provide evidence that fused regulates these cystocyte divisions cell autonomously, while the tumorous phenotype probably reflects both a somatic and germline requirement for fused for cyst and follicle development.

Lgd regulates the activity of the BMP/Dpp signalling pathway during Drosophila oogenesis

“…Hence, ectopic activation occurs cell autonomously, but is induced by the Dpp ligand. The ectopic activity induced by loss of fu function causes a variety of phenotypes, including egg chambers that contain 32 GCs, as we have observed upon loss of lgd function here (Narbonne-Reveau et al, 2006). We have previously shown that loss of function of lgd results in a defect in the degradation of Tkv and other cargo proteins, such as Notch and Dl, in FCs and GCs (Jaekel and Klein, 2006).…”

“…The phenotype of lgd in the germline resembles that of fu mutants (Narbonne-Reveau et al, 2006). Therefore, we tested whether lgd and shrub have a functional relationship with fu.…”

“…The short range of the Dpp signal is achieved through degradation of the activated Tkv receptor in the cystoblast by a specific mechanism that involves the kinase Fused (Fu) and the E3 ligase Smurf (Xia et al, 2010). If the mechanism is disturbed, for example, by inactivation of fu, the number of GSCs increases and an extra round of division of the cyst cells occurs, frequently giving rise to ECs with 32 instead of 16 GCs (Narbonne-Reveau et al, 2006).…”

Smurf-mediated differential proteolysis generates dynamic BMP signaling in germline stem cells during Drosophila testis development

“…In contrast, the number of TA division was not sensitive to defects in fu . Because null fu mH63 / Y mutants failed to survive to adulthood, the numbers of spermatocytes in single cysts were examined in testes from adult male flies carrying fu 50 , a viable allele that exhibits the strongest germline tumor phenotype in ovary that is resulting from hyperactivation of BMP signaling in cystoblasts (Narbonne-Reveau et al, 2006). Consistent with our finding that fu was not essential for downregulation of pMad in early germ cells during testis development, 32-cell cysts were not observed in fu 50 testes (lane 7 in Fig.…”

Transduction of the Hedgehog signal through the dimerization of Fused and the nuclear translocation of Cubitus interruptus

“…Given that previous studies demonstrated that the Hh pathway is not required in germline cells, and the loss-of-function of Fu leads to early germ cell proliferation 43 , the mechanism of Fu activation in the germline might be different and Hh independent. Our data indicate that Fu forms a dimer in response to Hh through Cos2/Smo interactions; the formation of a Fu dimer is Smo- and Cos2-dependent and is regulated by both graded Hh and the graded phosphorylation of the Smo C-tail.…”