The relaxin-like secreted peptide, Drosophila insulin-like peptide 8 (Dilp8), was first identified as a regulator of the delayed pupariation induced by neoplastic or damaged imaginal discs. Through its receptor leucine-rich repeat-containing G protein-coupled receptor 3 (Lgr3) in a pair of bilateral central brain neurons, Dilp8 represses the activity of PTTH neurons, leading to a delay in the surge of the steroid hormone Ecdysone that stimulates pupariation. In addition, Dilp8 and Lgr3 are required for intra- and inter-organ growth regulation, in which damage to discs can slow the growth of undamaged regions and other discs.
To deconstruct how Dilp8 contributes to wing disc growth during normal disc development we examined its expression and that of lgr3 in whole larvae and wing discs during the 2nd and 3rd instars. We find that relative to whole larvae, dilp8 is very highly expressed in early 3rd instar wing discs. However, by late 3rd instar dilp8 expression is considerably decreased in both, in a temporal pattern that is reciprocal to Ecdysone receptor activity in the wing disc. We compared wing discs from wildtype (WT) and dilp8 mutant larvae across the 2nd and 3rd instars, and find that in dilp8 mutants wing discs are initially smaller than WT discs at the early 3rd instar, but reach a similar size as WT discs by late 3rd instar. These data suggest that Dilp8 regulates the kinetics of wing disc growth during normal development. Dilp8 overexpression has been shown to modulate Ecdysone, Juvenile Hormone and insulin/insulin-like growth factor signaling, suggesting Dilp8 may regulate these pathways under physiological growth conditions. We aim to determine whether and how normal developmental changes in hormonal signaling mediate Dilp8’s role in tissue growth.