Immune Responses and Insulin Signaling in 5th instar Manduca sexta
Authors: Cambron Liz D. , Greenlee Kendra J. 
Published: June 2021

Both vertebrates and invertebrates exhibit sickness‐induced anorexia (SIA), an immune response that causes a cessation of feeding during an immune challenge. The mechanism underlying SIA that ties feeding to immunity has not yet been identified. In vertebrates, insulin signaling regulates many physiological processes, including feeding. Because of its role in regulating feeding, we hypothesize that the insulin signaling pathway also plays a role in SIA. Insulin signaling pathways are conserved between vertebrates and invertebrates, therefore, we used a well‐established insect model of immunity, the tobacco hornworm, Manduca sexta to test this hypothesis in several experiments. First, we investigated the role of insulin signaling on SIA by blocking insulin signaling using wortmannin, a downstream inhibitor of phosphatidylinositol‐3‐kinase, on vehicle‐injected and Escherichia coli‐injected (SIA) caterpillars. Our results showed that blocking insulin signaling increased feeding and body mass in caterpillars that were immune‐challenged. Since blocking insulin recovered the effects of SIA on development, we then tested the effects of adding extra insulin during SIA. Caterpillars were given injections of either a vehicle control (phosphate buffered saline), an insulin signaling inhibitor (wortmannin), or extra insulin (bovine insulin). Our results showed that altering insulin signaling only affected adult emergence. Non‐infected caterpillars had delayed emergence when insulin signaling was blocked, whereas infected caterpillars showed a delayed emergence with extra insulin instead. Because altering insulin signaling only affected adult emergence, we then asked whether altering this pathway causes any changes in the non‐infected larval stage. Fifth instar M. sexta caterpillars were given injections of either a vehicle control or bovine insulin, the dose of which ranged from 0.1 μM to 1mM. We measured body mass daily until the end of the 5th instar. Caterpillars that received injections of insulin did not differ from controls. Our results suggest that there may not be any sensitivity to insulin signaling during the 5th instar. One recent transcriptomic study indicates that M. sexta does not express insulin‐like peptides during the developmental stage that we used for our experiments. This is a possible explanation for the lack of difference between control and insulin manipulations in uninfected caterpillars, but does not explain the results observed with immune‐challenged caterpillars. Discovering why insulin affects immune responses when insulin is normally not expressed would lead to a better understanding of the role of insulin signaling in the immune system. Future studies will shift from organismal to cellular level responses, focusing on immune‐related tissues, such as the fat body, to determine how genes in the insulin signaling pathway change during SIA.Support or Funding InformationNational Science Foundation Graduate Research Fellowship ProgramThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.