Stem cell maintenance depends on local signals provided by specialized microenvironments, or niches, in which they reside. The potential role of systemic factors in stem cell maintenance, however, has remained largely unexplored. Here, we show that insulin signaling integrates the effects of diet and age on germline stem cell (GSC) maintenance through the dual regulation of cap cell number (via Notch signaling) and cap cell-GSC interaction (via E-cadherin) and that the normal process of GSC and niche cell loss that occurs with age can be suppressed by increased levels of insulin-like peptides. These results underscore the importance of systemic factors for the regulation of stem cell niches and, thereby, of stem cell numbers.diet ͉ oogenesis ͉ Notch ͉ E-cadherin ͉ aging T he stem cell microenvironment (niche) controls stem cells (1, 2), and niche aging leads to stem cell decline (3-5). The Drosophila germline stem cell (GSC) niche includes terminal filament cells, cap cells, and escort stem cells, and GSC fate and activity require direct contact with cap cells and exposure to niche-derived signals (6). GSCs also respond to systemic signals, such as Drosophila insulin-like peptides (DILPs) (7,8), which directly modulate their proliferation (9). Increased age leads to decreased niche size and signaling and GSC loss (3). The molecular basis for age-dependent changes in the niche, however, remains poorly understood. Results and DiscussionBecause diet influences aging (10), we examined its effects on GSC maintenance, exploiting the fact that GSCs can be unambiguously identified by their anteriorly anchored fusome (a membranous cytoskeletal structure) and by their juxtaposition to cap cells (11). As previously reported (3, 11, 12), we also observed a decrease in GSC numbers in well-fed females over time. In females on a poor diet, however, the rate of GSC loss was significantly increased (Fig. 1 A, B, and E and supporting information (SI) Table S1).Insulin secretion and signaling respond to diet (13) and diminish in aging humans (14). Using a phosphoinositide 3-kinase reporter (15), we found reduced insulin signaling in older ovaries (Fig. S1). To address if GSC maintenance requires insulin signaling, we measured GSC numbers in Drosophila insulin receptor (dinr) mutants (Fig. 1 C-E and Table S1). The dinr 339 /dinr E19 females contain slightly fewer GSCs at eclosion and lose them significantly faster than controls. We did not detect GSC death in dinr 339 /dinr E19 (n ϭ 65) or control (n ϭ 15) germaria, suggesting that GSC loss results from differentiation. The chico 1 homozygotes, which lack insulin receptor substrate, a major insulin pathway component, also show increased GSC loss (Table S1). Thus, insulin signaling controls GSC maintenance.We next tested if DILP expression in germarial somatic cells could counteract the wild-type age-dependent GSC loss. We used the c587-GAL4 driver (see Materials and Methods) to express a UAS-dilp2 transgene, encoding the DILP most closely related to human insulin (16), and thereby ...

The external environment influences stem cells, but this process is poorly understood. Our previous work showed that germline stem cells (GSCs) respond to diet via neural insulin-like peptides (DILPs) that act directly on the germ line to upregulate stem cell division and cyst growth under a protein-rich diet in Drosophila. Here, we report that DILPs specifically control the G2 phase of the GSC cell cycle via phosphoinositide-3 kinase (PI3K) and dFOXO, and that a separate diet mediator regulates the G1 phase. Furthermore, GSC tumors, which escape the normal stem cell regulatory microenvironment, or niche, still respond to diet via both mechanisms, indicating that niche signals are not required for GSCs to sense or respond to diet. Our results document the effects of diet and insulin-like signals on the cell cycle of stem cells within an intact organism and demonstrate that the response to diet requires multiple signals. Moreover, the retained ability of GSC tumors to respond to diet parallels the long known connections between diet, insulin signaling, and cancer risk in humans.

Stem cells depend on intrinsic and local factors to maintain their identity and activity, but they also sense and respond to changing external conditions. We previously showed that germline stem cells (GSCs) and follicle stem cells (FSCs) in the Drosophila ovary respond to diet via insulin signals. Insulin signals directly modulate the GSC cell cycle at the G2 phase, but additional unknown dietary mediators control both G1 and G2. Target of rapamycin, or TOR, is part of a highly conserved nutrient-sensing pathway affecting growth, proliferation, survival and fertility. Here, we show that optimal TOR activity maintains GSCs but does not play a major role in FSC maintenance, suggesting differential regulation of GSCs versus FSCs. TOR promotes GSC proliferation via G2 but independently of insulin signaling, and TOR is required for the proliferation, growth and survival of differentiating germ cells. We also report that TOR controls the proliferation of FSCs but not of their differentiating progeny. Instead, TOR controls follicle cell number by promoting survival, independently of either the apoptotic or autophagic pathways. These results uncover specific TOR functions in the control of stem cells versus their differentiating progeny, and reveal parallels between Drosophila and mammalian follicle growth.

Adult stem cells reside in specialized microenvironments, or niches, that are essential for their function in vivo. Stem cells are physically attached to the niche, which provides secreted factors that promote their self-renewal and proliferation. Despite intense research on the role of the niche in regulating stem cell function, much less is known about how the niche itself is controlled. We previously showed that insulin signals directly stimulate germline stem cell (GSC) division and indirectly promote GSC maintenance via the niche in Drosophila. Insulin-like peptides are required for maintenance of cap cells (a major component of the niche) via modulation of Notch signaling, and they also control attachment of GSCs to cap cells and E-cadherin levels at the cap cell-GSC junction. Here, we further dissect the molecular and cellular mechanisms underlying these processes. We show that insulin and Notch ligands directly stimulate cap cells to maintain their numbers and indirectly promote GSC maintenance. We also report that insulin signaling, via phosphoinositide 3-kinase and FOXO, intrinsically controls the competence of cap cells to respond to Notch ligands and thereby be maintained. Contrary to a previous report, we also find that Notch ligands originated in GSCs are not required either for Notch activation in the GSC niche, or for cap cell or GSC maintenance. Instead, the niche itself produces ligands that activate Notch signaling within cap cells, promoting stability of the GSC niche. Finally, insulin signals control cap cell-GSC attachment independently of their role in Notch signaling. These results are potentially relevant to many systems in which Notch signaling modulates stem cells and demonstrate that complex interactions between local and systemic signals are required for proper stem cell niche function.