Adipocyte-Derived Amino Acid Storage Proteins are Required for Germline Stem Cell Maintenance in Adult Drosophila Females

Tissue homeostasis is dependent on precise coordination between endocrine organs in response to changes in organism physiology. Secreted circulating factors from adipocytes (called adipokines) regulate the behavior of stem cell lineages in peripheral tissues in multiple organisms. In addition to their endocrine roles, Drosophila adipocytes store and secrete amino acid storage proteins throughout development. During the larval feeding period, adipocytes secrete storage proteins into the hemolymph, which are reabsorbed by the adipose tissue during metamorphosis to control adult organ size and fertility. Despite the known functions for storage proteins during the larval stages, their requirement during Drosophila adulthood and reproduction are uncharacterized. We discover that adipocyte-specific knockdown of the storage proteins Larval serum protein 1 (Lsp1) alpha/beta/gamma and Larval serum protein 2 (Lsp2) results in a decrease in GSC maintenance. We further reveal that decreased GSC number is due to downregulation of Target of Rapamycin (TOR) signaling in GSCs, suggesting compromised amino acid sensing directly in GSCs. We also find that the proteins that mediate storage protein adipocyte reabsorption, Fat body protein 1 (Fbp1) and Fat body protein 2 (Fbp2), are expressed in ovarian follicle cells. Intriguingly, Fbp1 nor Fbp2 appear to be required in follicle cells for GSC maintenance, suggesting undiscovered requirements for amino acid storage proteins in oogenesis. Our results highlight a novel role for Drosophila amino acid storage proteins during adulthood and in regulating tissue stem cell lineages.