Frazzled/Dcc acts independently of Netrin to promote germline survival during Drosophila oogenesis.

The Netrin receptor Frazzled/Dcc (Fra in Drosophila) functions in diverse tissue contexts to regulate cell migration, axon guidance and cell survival. Fra signals in response to Netrin to regulate the cytoskeleton and also acts independently of Netrin to directly regulate transcription during axon guidance in Drosophila. In other contexts, Dcc acts as a tumor suppressor by directly promoting apoptosis. In this study, we report that Fra is required in the Drosophila female germline for the progression of egg chambers through mid-oogenesis. Loss of Fra in the germline, but not the somatic cells of the ovary, results in the degeneration of egg chambers. Although a failure in nutrient sensing and disruptions in egg chamber polarity can result in degeneration at mid-oogenesis, these factors do not appear to be affected in fra germline mutants. However, similar to the degeneration that occurs in those contexts, the cell death effector Dcp-1 is activated in fra germline mutants. The function of Fra in the female germline is independent of Netrin and requires the transcriptional activation domain of Fra. In contrast to the role of Dcc in promoting cell death, our observations reveal a role for Fra in regulating germline survival by inhibiting apoptosis.

AMP-activated protein kinase has diet-dependent and -independent roles in Drosophila oogenesis.

Multiple aspects of organismal physiology influence the number and activity of stem cells and their progeny, including nutritional status. Previous studies demonstrated that Drosophila germline stem cells (GSCs), follicle stem cells (FSCs), and their progeny sense and respond to diet via complex mechanisms involving many systemic and local signals. AMP-activated protein kinase, or AMPK, is a highly conserved regulator of energy homeostasis known to be activated under low cellular energy conditions; however, its role in the ovarian response to diet has not been investigated. Here, we describe nutrient-dependent and -independent requirements for AMPK in Drosophila oogenesis. We found that AMPK is cell autonomously required for the slow down in GSC and follicle cell proliferation that occurs on a poor diet. Similarly, AMPK activity is necessary in the germline for the degeneration of vitellogenic stages in response to nutrient deprivation. In contrast, AMPK activity is not required within the germline to modulate its growth. Instead, AMPK acts in follicle cells to negatively regulate their growth and proliferation, thereby indirectly limiting the size of the underlying germline cyst within developing follicles. Paradoxically, AMPK is required for GSC maintenance in well-fed flies (when AMPK activity is presumably at its lowest), suggesting potentially important roles for basal AMPK activity in specific cell types. Finally, we identified a nutrient-independent, developmental role for AMPK in cyst encapsulation by follicle cells. These results uncover specific AMPK requirements in multiple cell types in the ovary and suggest that AMPK can function outside of its canonical nutrient-sensing role in specific developmental contexts.

Adipocyte amino acid sensing controls adult germline stem cell number via the amino acid response pathway and independently of Target of Rapamycin signaling in Drosophila.

How adipocytes contribute to the physiological control of stem cells is a critical question towards understanding the link between obesity and multiple diseases, including cancers. Previous studies have revealed that adult stem cells are influenced by whole-body physiology through multiple diet-dependent factors. For example, nutrient-dependent pathways acting within the Drosophila ovary control the number and proliferation of germline stem cells (GSCs). The potential role of nutrient sensing by adipocytes in modulating stem cells in other organs, however, remains largely unexplored. Here, we report that amino acid sensing by adult adipocytes specifically modulates the maintenance of GSCs through a Target of Rapamycin-independent mechanism. Instead, reduced amino acid levels and the consequent increase in uncoupled tRNAs trigger activation of the GCN2-dependent amino acid response pathway within adipocytes, causing increased rates of GSC loss. These studies reveal a new step in adipocyte-stem cell crosstalk.

Insulin-independent role of adiponectin receptor signaling in Drosophila germline stem cell maintenance.

Adipocytes have key endocrine roles, mediated in large part by secreted protein hormones termed adipokines. The adipokine adiponectin is well known for its role in sensitizing peripheral tissues to insulin, and several lines of evidence suggest that adiponectin might also modulate stem cells/precursors. It remains unclear, however, how adiponectin signaling controls stem cells and whether this role is secondary to its insulin-sensitizing effects or distinct. Drosophila adipocytes also function as an endocrine organ and, although no obvious adiponectin homolog has been identified, Drosophila AdipoR encodes a well-conserved homolog of mammalian adiponectin receptors. Here, we generate a null AdipoR allele and use clonal analysis to demonstrate an intrinsic requirement for AdipoR in germline stem cell (GSC) maintenance in the Drosophila ovary. AdipoR null GSCs are not fully responsive to bone morphogenetic protein ligands from the niche and have a slight reduction in E-cadherin levels at the GSC-niche junction. Conversely, germline-specific overexpression of AdipoR inhibits natural GSC loss, suggesting that reduction in adiponectin signaling might contribute to the normal decline in GSC numbers observed over time in wild-type females. Surprisingly, AdipoR is not required for insulin sensitization of the germline, leading us to speculate that insulin sensitization is a more recently acquired function than stem cell regulation in the evolutionary history of adiponectin signaling. Our findings establish Drosophila female GSCs as a new system for future studies addressing the molecular mechanisms whereby adiponectin receptor signaling modulates stem cell fate.

Superoxide triggers an acid burst in Saccharomyces cerevisiae to condition the environment of glucose-starved cells.

Although yeast cells grown in abundant glucose tend to acidify their extracellular environment, they raise the pH of the environment when starved for glucose or when grown strictly with non-fermentable carbon sources. Following prolonged periods in this alkaline phase, Saccharomyces cerevisiae cells will switch to producing acid. The mechanisms and rationale for this "acid burst" were unknown. Herein we provide strong evidence for the role of mitochondrial superoxide in initiating the acid burst. Yeast mutants lacking the mitochondrial matrix superoxide dismutase (SOD2) enzyme, but not the cytosolic Cu,Zn-SOD1 enzyme, exhibited marked acceleration in production of acid on non-fermentable carbon sources. Acid production is also dramatically enhanced by the superoxide-producing agent, paraquat. Conversely, the acid burst is eliminated by boosting cellular levels of Mn-antioxidant mimics of SOD. We demonstrate that the acid burst is dependent on the mitochondrial aldehyde dehydrogenase Ald4p. Our data are consistent with a model in which mitochondrial superoxide damage to Fe-S enzymes in the tricarboxylic acid (TCA) cycle leads to acetate buildup by Ald4p. The resultant expulsion of acetate into the extracellular environment can provide a new carbon source to glucose-starved cells and enhance growth of yeast. By triggering production of organic acids, mitochondrial superoxide has the potential to promote cell population growth under nutrient depravation stress.

DrosoPHILA: A Partnership between Scientists and Teachers That Begins in the Lab and Continues into City Schools.

Here, we describe the development, structure, and effectiveness of an outreach program, DrosoPHILA, that leverages the tools of our fly neurodevelopmental research program at the University of Pennsylvania to reinforce the biology curriculum in local public schools. DrosoPHILA was developed and is sustained by a continued collaboration between members of the Bashaw lab, experienced outreach educators, and teachers in the School District of Philadelphia. Since the program's inception, we have collaborated with 18 teachers and over 2400 students. Student outcome data indicates significant positive attitude shifts around science identity and grade-appropriate knowledge gains.

Diverse roles for axon guidance pathways in adult tissue architecture and function.

Classical axon guidance ligands and their neuronal receptors were first identified due to their fundamental roles in regulating connectivity in the developing nervous system. Since their initial discovery, it has become clear that these signaling molecules play important roles in the development of a broad array of tissue and organ systems across phylogeny. In addition to these diverse developmental roles, there is a growing appreciation that guidance signaling pathways have important functions in adult organisms, including the regulation of tissue integrity and homeostasis. These roles in adult organisms include both tissue-intrinsic activities of guidance molecules, as well as systemic effects on tissue maintenance and function mediated by the nervous and vascular systems. While many of these adult functions depend on mechanisms that mirror developmental activities, such as regulating adhesion and cell motility, there are also examples of adult roles that may reflect signaling activities that are distinct from known developmental mechanisms, including the contributions of guidance signaling pathways to lineage commitment in the intestinal epithelium and bone remodeling in vertebrates. In this review, we highlight studies of guidance receptors and their ligands in adult tissues outside of the nervous system, focusing on in vivo experimental contexts. Together, these studies lay the groundwork for future investigation into the conserved and tissue-specific mechanisms of guidance receptor signaling in adult tissues.

Control of Germline Stem Cell Lineages by Diet and Physiology.

Tight coupling of reproduction to environmental factors and physiological status is key to long-term species survival. In particular, highly conserved pathways modulate germline stem cell lineages according to nutrient availability. This chapter focuses on recent in vivo studies in genetic model organisms that shed light on how diet-dependent signals control the proliferation, maintenance, and survival of adult germline stem cells and their progeny. These signaling pathways can operate intrinsically in the germ line, modulate the niche, or act through intermediate organs to influence stem cells and their differentiating progeny. In addition to illustrating the extent of dietary regulation of reproduction, findings from these studies have implications for fertility during aging or disease states.

Adipocyte Metabolic Pathways Regulated by Diet Control the Female Germline Stem Cell Lineage in Drosophila melanogaster.

Nutrients affect adult stem cells through complex mechanisms involving multiple organs. Adipocytes are highly sensitive to diet and have key metabolic roles, and obesity increases the risk for many cancers. How diet-regulated adipocyte metabolic pathways influence normal stem cell lineages, however, remains unclear. Drosophila melanogaster has highly conserved adipocyte metabolism and a well-characterized female germline stem cell (GSC) lineage response to diet. Here, we conducted an isobaric tags for relative and absolute quantification (iTRAQ) proteomic analysis to identify diet-regulated adipocyte metabolic pathways that control the female GSC lineage. On a rich (relative to poor) diet, adipocyte Hexokinase-C and metabolic enzymes involved in pyruvate/acetyl-CoA production are upregulated, promoting a shift of glucose metabolism toward macromolecule biosynthesis. Adipocyte-specific knockdown shows that these enzymes support early GSC progeny survival. Further, enzymes catalyzing fatty acid oxidation and phosphatidylethanolamine synthesis in adipocytes promote GSC maintenance, whereas lipid and iron transport from adipocytes controls vitellogenesis and GSC number, respectively. These results show a functional relationship between specific metabolic pathways in adipocytes and distinct processes in the GSC lineage, suggesting the adipocyte metabolism-stem cell link as an important area of investigation in other stem cell systems.

Genetic Mosaic Analysis of Stem Cell Lineages in the Drosophila Ovary.

Genetic mosaic analyses represent an invaluable approach for the study of stem cell lineages in the Drosophila ovary. The generation of readily identifiable, homozygous mutant cells in the context of wild-type ovarian tissues within intact organisms allows the pinpointing of cellular requirements for gene function, which is particularly important for understanding the physiological control of stem cells and their progeny. Here, we provide a step-by-step guide to the generation and analysis of genetically mosaic ovaries using flippase (FLP)/FLP recognition target (FRT)-mediated recombination in adult Drosophila melanogaster, with a focus on the processes of oogenesis that are controlled by diet-dependent factors.

Control of adult stem cells in vivo by a dynamic physiological environment: diet-dependent systemic factors in Drosophila and beyond.

Adult stem cells are inextricably linked to whole-body physiology and nutrient availability through complex systemic signaling networks. A full understanding of how stem cells sense and respond to dietary fluctuations will require identifying key systemic mediators, as well as elucidating how they are regulated and integrated with local and intrinsic factors across multiple tissues. Studies focused on the Drosophila germline have generated valuable insights into how stem cells are controlled by diet-dependent pathways, and increasing evidence suggests that diverse adult stem cell populations respond to nutrients through similar mechanisms. Systemic signals, including nutrients themselves and diet-regulated hormones such as Insulin/Insulin-like growth factor or steroid hormones, can directly or indirectly affect stem cell behavior by modifying local cell-cell communication or intrinsic factors. The physiological regulation of stem cells in response to nutritional status not only is a fascinating biological problem, but also has clinical implications, as research in this field holds the key to noninvasive approaches for manipulating stem cells in vivo. In addition, given the known associations between diet, stem cells, and cancer risk, this research may inspire novel anticancer therapies.