Towards understanding the ancestral role of insulin-like peptides (ILPs) in Hydra
ILPs and their signalling pathways are evolutionary conserved pathways regulating metabolism, growth, stress resistance, reproduction and lifespan of animals. But the ancestral roles of ILPs in early emerged multicellular organisms remain unknown. The present thesis explores the origin of ILPs among basal metazoans and investigates the function of ILPs in a basal organism Hydra. The in silico search for ILPs revealed the presence of ilp genes in Placozoa, Cnidaria and Bilateria phyla, whereas no ilp genes were detected in Choanozoa, Porifera and Ctenophora. This indicates that ilps emerged after the radiation of major Metazoa groups and consistent with multiple origins of the nervous systems. There are 7 ilps in Nematostella vectensis, 6 ilps in Acropora digitfera, 4 ilps in Aurelia aurita and 3 ilps in Hydra, indicating that intensive radiation of ilps on a genus level. Exploring pattern analysis revealed that 3 ilps from Hydra vulgaris AEP named hyilp-A, hyilp-B and hyilp-C, were highly expressed in the head region, while hyilp-B and hyilp-C were expressed in the foot region. Moreover, hyilp-B exhibited a dynamic expression pattern during bud development. Strikingly, hyilp-A and hyilp-B appears to be expressed in neurons at both transcription and protein levels. Starvation promoted high expression of hyilp-A, revealing the role of HyILP-A in the nutrient sensing. Inhibition of GSK-3 suppressed the expression of hyilp-A and hyilp-C, indicating hyilp-A and hyilp-C may be downstream genes of Wnt signalling, which is known to be crucial for pattern formation. Population growth rate analysis revealed that HyILP-B overexpression delays Hydra population growth, while HyILP-B knockdown does not affect it. In addition, HyILP-B is not proteolytically cleaved during maturation. Taken together, the present study reveals that HyILPs as neuron secreted peptides, most possibly play roles in nutrient regulation and growth control through HyINSR signalling pathways in Hydra.