Drosophila Nepl15 controls glycogen and lipid storage by modulating insulin signaling
Deletion of the Drosophila-specific gene Neprilysin-like 15 (Nepl15) results in significant reductions in glycogen and glycerolipid storage in mutant males and increased glycogen storage in mutant females without affecting food intake. Previous studies also indicated downregulation of insulin/mTOR signaling, the central pathway regulating nutrient homeostasis, growth, fertility, locomotor activity, and lifespan. Consistent with these metabolic alterations, Nepl15 mutants exhibit several anti-obesity and healthy-aging phenotypes but display markedly reduced survival under starvation, suggesting impaired nutrient reserve utilization. The present study investigated the intracellular mechanisms underlying these phenotypes by examining insulin signaling and the expression of genes involved in carbohydrate and lipid metabolism. Although transcript levels of the insulin-like peptides (Dilp2, Dilp5, and Dilp6) and the insulin receptor (InR) remained unchanged, Nepl15 mutants exhibited reduced Akt phosphorylation, increased dFoxo abundance, reduced Glut1 expression, and previously reported suppression of mTOR signaling, indicating attenuation of insulin signaling downstream of the insulin receptor. Consistent with impaired anabolic signaling, male mutants showed reduced expression of glycogen metabolic genes (GlyS and GlyP), whereas genes involved in lipid metabolism exhibited predominantly male-specific reductions, including Lipin, Acc, Fasn1, and Fasn2, while Midway and Brummer remained unchanged. In addition, expression of the metabolic regulator PGC1 (spargel) was significantly reduced in males, consistent with previously reported reductions in AMPK expression. Together, these findings demonstrate that Nepl15 functions as an upstream regulator of insulin-dependent anabolic signaling and nutrient partitioning, linking intracellular signaling to glycogen and lipid storage independently of nutrient intake. These results identify Nepl15 as a previously unrecognized regulator of metabolic homeostasis in Drosophila and provide new insight into the metabolic functions of the evolutionarily conserved neprilysin family.