Cullin1 orchestrates insulin/mTOR signaling to drive endocycle progression and ecdysteroid production in Drosophila prothoracic glands during critical weight attainment

Critical weight (CW) is a key developmental threshold in insects, beyond which larvae initiate the transformation into pupae. The prothoracic gland (PG), responsible for producing ecdysteroids, plays a crucial role in controlling the timing of this transition. The nutrition dependent endocycle, a modified cell cycle that omits mitosis, coordinates the PG size and activity to influence the timing of CW attainment. However, the molecular mechanisms underlying how nutrient signals determine the endocycle process in PG cells are still not fully uncovered. In this study, we found a conserved SCF (SkpA-Cullin1-Slmb) E3 ligase complex that plays a critical role in regulating endocycle events in Drosophila melanogaster PG cells. Functional disruption of cullin1 (cul1), a core component of this complex, could cause endocycle inhibition, decrease the biosynthesis of ecdysteriod and developmental arrest. The phenotype can be rescued by overexpression cyclin E which may induce rounds of endocycles in the steroidogenic tissue. Remarkably, Cul1 was highly expressed during the CW checkpoint. Starvation before the CW period could repress its expression. In line with this, loss of insulin or target of rapamycin (TOR) signaling could significantly decrease the Cul1 signal during around CW indicating the gene is a nutrient responsive gene. Taken together, our data revealed that Cul1 could serve as downstream regulator of insulin/mTOR signaling pathway to determine the endocycling process around CW in PG cells.