Because both dearth and overabundance of histones result in cellular defects, histone synthesis and demand are typically tightly coupled. In Drosophila embryos, histones H2B/H2A/H2Av accumulate on lipid droplets (LDs), cytoplasmic fat storage organelles. Without LD-binding, maternally provided H2B/H2A/H2Av are absent, but how LDs ensure histone storage is unclear. Using quantitative imaging, we uncover when during oogenesis these histones accumulate, and which step of accumulation is LD-dependent. LDs originate in nurse cells (NCs) and are transported to the oocyte. Although H2Av accumulates on LDs in NCs, the majority of the final H2Av pool is synthesized in oocytes. LDs promote intercellular transport of the histone-anchor Jabba and thus its presence in the ooplasm. Ooplasmic Jabba then prevents H2Av degradation, safeguarding the H2Av stockpile. Our findings provide insight into the mechanism for establishing histone stores during Drosophila oogenesis and shed light on the function of LDs as protein-sequestration sites.

Animal embryos are provisioned by their mothers with a diverse nutrient supply critical for development. In Drosophila, the three most abundant nutrients (triglycerides, proteins, and glycogen) are sequestered in distinct storage structures, lipid droplets (LDs), yolk vesicles (YVs) and glycogen granules (GGs). Using transmission electron microscopy as well as live and fixed-sample fluorescence imaging, we find that all three storage structures are dispersed throughout the egg but are then spatially allocated to distinct tissues by gastrulation: LDs largely to the peripheral epithelium, YVs and GGs to the central yolk cell. To confound the embryo's ability to sort its nutrients, we employ mutants in Jabba and Mauve to generate LD:GG or LD:YV compound structures. In these mutants, LDs are missorted to the yolk cell and their turnover is delayed. Our observations demonstrate dramatic spatial nutrient sorting in early embryos and provide the first evidence for its functional importance.

To ensure fertility, it is paramount to understand the factors controlling oocyte quality. One incompletely characterized factor contributing to oocyte quality is lipids. In somatic cells, a key regulator of lipid metabolism is lipid droplets (LDs), the sites of intracellular fat storage. Yet the role of LDs in fertility is poorly understood. Here we use Drosophila  oogenesis as a model for uncovering if and how LDs promote egg development. LD accumulation in nurse cells coincides with dynamic actin remodeling necessary for late-stage follicle morphogenesis and fertility. Loss of major LD proteins, including PLIN2, Jabba, and ATGL, disrupts both actin bundle formation and cortical actin integrity; this unusual phenotype is also seen when Pxt, the enzyme responsible for prostaglandin (PG) synthesis, is missing. Further, both pharmacologic and genetic loss of PG synthesis or loss of PLIN2 or Jabba impairs intracellular LD dispersal. These similar phenotypes suggest that PGs and LD proteins act in the same pathway. Dominant genetic interaction studies indicate that there are three actin regulatory pathways: PLIN2 regulates actin remodeling independent of PG signaling, whereas Jabba and ATGL act in two separate PG-dependent pathways to regulate actin remodeling. We find that neither Jabba nor ATGL modulate the levels of Pxt or its localization to the endoplasmic reticulum. As ATGL is a triglyceride lipase, we hypothesize that it may release arachidonic acid (AA), the substrate for PG production, from triglycerides stored in LDs. Indeed, lipidomic analysis reveals the presence of AA-containing triglycerides in ovaries. In addition, exogenous AA is toxic and reduction of ATGL ameliorates toxicity; these observations suggest that ATGL indeed generates free AA. Our studies provide the first evidence that LDs and their associated proteins regulate PG signaling to control actin remodeling. In particular, we propose that ATGL releases AA from LDs to drive PG synthesis necessary for follicle development. We speculate that the same pathways are conserved across organisms to regulate oocyte development and promote fertility.

Because dearth and overabundance of histones result in cellular defects, histone synthesis and demand are typically tightly coupled. In Drosophila embryos, histones H2B/H2A/H2Av accumulate on lipid droplets (LDs), cytoplasmic fat storage organelles. Without this binding, maternally provided H2B/H2A/H2Av are absent; however, the molecular basis of how LDs ensure histone storage is unclear. Using quantitative imaging, we uncover when during oogenesis these histones accumulate, and which step of accumulation is LD-dependent. LDs originate in nurse cells and are transported to the oocyte. Although H2Av accumulates on LDs in nurse cells, the majority of the final H2Av pool is synthesized in oocytes. LDs promote intercellular transport of the histone-anchor Jabba and thus its presence in the ooplasm. Jabba prevents ooplasmic H2Av from degradation, safeguarding the H2Av stockpile. Our findings provide insight into the mechanism for establishing histone stores during Drosophila oogenesis and shed light on the function of LDs as protein-sequestration sites.

Lipid droplets (LDs), crucial regulators of lipid metabolism, accumulate during oocyte development. However, their roles in fertility remain largely unknown. During Drosophila oogenesis, LD accumulation coincides with actin remodeling necessary for follicle development. Loss of the LD-associated Adipose Triglyceride Lipase (ATGL) disrupts both actin bundle formation and cortical actin integrity, an unusual phenotype also seen when the prostaglandin (PG) synthase Pxt is missing. Dominant genetic interactions and PG treatment of follicles indicate ATGL acts upstream of Pxt to regulate actin remodeling. Our data suggest ATGL releases arachidonic acid (AA) from LDs to serve as the substrate for PG synthesis. Lipidomic analysis detects AA-containing triglycerides in ovaries, and these are increased when ATGL is lost. High levels of exogenous AA block follicle development; this is enhanced by impairing LD formation and suppressed by reducing ATGL. Together these data support the model that AA stored in LD triglycerides is released by ATGL to drive the production of PGs, which promote actin remodeling necessary for follicle development. We speculate this pathway is conserved across organisms to regulate oocyte development and promote fertility.