Monitoring Fatty Acid Trafficking during Drosophila oogenesis Reveals a Role for the Triglyceride Synthase DGAT1 in Protecting Mitochondrial Integrity

Successful oogenesis requires the precise coordination of nutrient uptake, storage, and utilization to meet the high metabolic demands of egg production. In mammals, fatty acid (FA) metabolism has emerged as a key driver of oocyte maturation; however, the mechanisms by which follicles regulate FA trafficking and utilization remain poorly understood across all systems. To address these issues, we leverage the genetic tractability of Drosophila melanogaster oogenesis. We found that nurse cell mitochondria are metabolically active and catabolize FA in a stage-dependent manner, with fatty acid oxidation (FAO) peaking during mid-oogenesis. By exposing explanted follicles to fluorescently labeled FAs, we monitored FA trafficking and found massive enrichment in lipid droplets. Mutants for the triglyceride lipase ATGL exhibited a reduction in both mitochondrial membrane potential and FAO, suggesting that mitochondria utilize FA from triglycerides stored in LDs. To determine the significance of this transient FA storage in LDs, we prevented the formation of nurse cell LDs with mutations in the triglyceride synthase DGAT1. The DGAT1 mutant follicles display excess accumulation of FAs in mitochondria, mitochondrial stress, and developmental arrest. We find that this mitochondrial dysfunction and follicle arrest are consequences of FA toxicity to mitochondria: limiting FA influx into follicles or FA import into mitochondria alleviates these defects. Our findings demonstrate that LD-derived FAs are actively mobilized to fuel the energy demands of oogenesis while LDs buffer against lipotoxicity, revealing a critical balance between FA storage and oxidation. These findings highlight LDs as central hubs regulating energy homeostasis and developmental progression in the follicle.