Differential amino acid transporter expression in adult Drosophila melanogaster tissues.

Organismal macronutrient intake modulates organ and tissue function. Dietary amino acids play essential roles in metabolic processes that support normal tissue growth, repair, and function. For example, in Drosophila melanogaster , protein-deficient diets lead to reduced overall organismal growth during larval development and severely decreased egg production in adult females. Multiple tissues, therefore, must sense and respond to dietary protein input. Amino acid transporter proteins facilitate the movement of amino acids across cellular membranes. Based on high-throughput expression studies, the Drosophila genome is predicted to encode 58 amino acid transporters. We have set out to determine if there are tissue-specific amino acid requirements for proper tissue function by first assessing the complement of amino acid transporters expressed in several adult tissues. Using RT-PCR to assess transcript levels, we find that most of the 24 amino acid transporters examined are expressed in the head, thorax, abdomen, gut, and ovary, while a subset shows differential transcript expression. This work will serve as the foundation for future studies addressing the impact of physiological factors, like nutrition, on amino acid sensing by individual tissues.

Methods to Analyze Nutritional and Inter-Organ Control of Drosophila Ovarian Germline Stem Cells.

Physiological status, particularly dietary input, has major impacts on the Drosophila melanogaster ovarian germline stem cell lineage. Moreover, several studies have shed light on the role that inter-organ communication plays in coordinating whole-organism responses to changes in physiology. For example, nutrient-sensing signaling pathways function within the fat body to regulate germline stem cells and their progeny in the ovary. Together with its incredible genetic and cell biological toolkits, Drosophila serves as an amenable model organism to use for uncovering molecular mechanisms that underlie physiological control of adult stem cells. In this methods chapter, we describe a general dietary manipulation paradigm, genetic manipulation of adult adipocytes, and whole-mount ovary immunofluorescence to investigate physiological control of germline stem cells.

Antibody development to identify components of IIS and mTOR signaling pathways in lepidopteran species, a set of non-model insects.

Nutritional stress impacts many insect species that have differing reproductive strategies and life histories, yet it is unclear how nutrient-sensing signaling pathways mediate tissue-specific responses to changes in dietary input. In Drosophila melanogaster , insulin/insulin-like growth factor (IIS) and mTOR-mediated signaling within adipocytes regulates oogenesis. To facilitate comparative study of nutrient-sensing pathway activity in the fat body, we developed antibodies to assess IIS (anti-FOXO) and mTOR signaling (anti-TOR) across three nymphalid species (Lepidoptera). By optimizing whole-mount fat body immunostaining, we find FOXO nuclear enrichment in adult adipocytes, like that observed in Drosophila . Additionally, we show a previously uncharacterized TOR localization pattern in the fat body.

Insulin signaling acts in adult adipocytes via GSK-3ß and independently of FOXO to control Drosophila female germline stem cell numbers.

Tissue-specific stem cells are tied to the nutritional and physiological environment of adult organisms. Adipocytes have key endocrine and nutrient-sensing roles and have emerged as major players in relaying dietary information to regulate other organs. For example, previous studies in Drosophila melanogaster revealed that amino acid sensing as well as diet-dependent metabolic pathways function in adipocytes to influence the maintenance of female germline stem cells (GSCs). How nutrient-sensing pathways acting within adipocytes influence adult stem cell lineages, however, is just beginning to be elucidated. Here, we report that insulin/insulin-like growth factor signaling in adipocytes promotes GSC maintenance, early germline cyst survival, and vitellogenesis. Further, adipocytes use distinct mechanisms downstream of insulin receptor activation to control these aspects of oogenesis, all of which are independent of FOXO. We find that GSC maintenance is modulated by Akt1 through GSK-3ß, early germline cyst survival is downstream of adipocyte Akt1 but independent of GSK-3ß, and vitellogenesis is regulated through an Akt1-independent pathway in adipocytes. These results indicate that, in addition to employing different types of nutrient sensing, adipocytes can use distinct axes of a single nutrient-sensing pathway to regulate multiple stages of the GSC lineage in the ovary.

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.

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.