The Functions of Insulin-like Peptides in Insects

Abstract

The insulin/Igf-like signalling pathway plays key biological roles in diverse organisms, in growth and development, fecundity, stress resistance, metabolism and lifespan. The identity and functions of insulin-like peptides in mammals are well documented. Although genes encoding related peptides are present in the genomes of invertebrates, often in multiple copies, their biological roles are less well understood. Seven such genes are present in the fruit fly Drosophila melanogaster, they show higher sequence homology to insulin than to other mammalian peptides. Evolutionary conservation of different regions of the peptides among Drosophila species suggests that they are cleaved, like insulin. Each Drosophila Insulin Like Peptide (DILP) is expressed in a characteristic tissue-and stage- specific manner, suggesting that they may have unique biological functions. Ablation of neurosecretory cells in the brain that produce 3 of the DILPs results in an array of phenotypes, including developmental delay, reduced body size, stress resistance, metabolic phenotypes, reduced fecundity and increased lifespan, but assignment of these phenotypes to individual DILPs awaits genetic analysis. ILPs are also present in other insects, including the silk moth (38 genes), the honey bee (2 genes), and two mosquito species (7–8 genes), and insulin binding proteins have also been found in insects. Given the diverse and central functions of insulin/Igf-like signalling, deeper understanding of the roles of these invertebrate insulin-like peptides and the mechanisms by which they achieve them will throw deeper light on the functioning of this system in mammals.