An investigation of insulin-like molecule(s) and their receptor in the locust, Locusta migratoria

Insulin-like molecules have been identified within invertebrates primarily by immunocytochemistry and in some cases the material has been partially or completely purified. In the locust Locusta migratoria the complete primary structure of an insulin-related molecule referred to as Locusta insulin-related peptide (LIRP) has been reported. The function of this molecule has remained elusive. There have been reports of other locust "insulins" from the gut and brain that are thought to play a role in regulating haemolymph carbohydrate or lipid levels. The primary structure of these molecules is not known. The first objective of the current study was to investigate the function of insulin-like molecules(s) within the locust using fat body glycogen phosphorylase activity as the bioassay. Receptor homologues to the vertebrate insulin receptor have been characterized in several invertebrates. To date no receptor homologue has been identified for LIRP or any other insulin-like molecules identified. The second objective of the current study was to identify a locust fat body insulin receptor homologue by immunoblot with antiserum to the human insulin receptor and characterize its binding properties using both a crude membrane preparation and a partially purified receptor preparation obtained by wheat germ agglutinin (WGA) chromatography. There have been some reports of the signal transduction mechanism(s) involved in the pathway of action of invertebrate "insulins". The third objective of the current study was an attempt to elucidate the pathway involved in the signal transduction mechanism for locust insulin-like molecule(s) at the respective fat body membrane receptor.;The current study has found that an acid soluble molecule(s) is present within extracts from the brain, storage lobe of the corpus cardiacum and the ventral nerve cord by its ability to decrease fat body glycogen phosphorylase activity in vitro. The source of this activity is heat stable and protease sensitive. Incubation of the storage lobe extract with anti-bombyxin but not anti-porcine or anti-bovine insulin reduces the inhibitory activity of this extract. When vertebrate insulins were tested, porcine, bovine or equine insulin but not sheep insulin were able to reduce fat body glycogen phosphorylase activity in vitro.;Analysis of 125I-porcine insulin binding using either a crude fat body membrane preparation or a partially purified receptor preparation has shown that binding is saturable. Results of the immunoblot identified two specific bands corresponding to MWs of 125 and 80 KDa. The signal transduction mechanism for inhibition of fat body glycogen phosphorylase by locust insulin-like molecule(s) appears not to involve tyrosine kinase activity or phosphoinositide 3-kinase, a distinction from its vertebrate counterpart. Although fat body glycogen phosphorylaase activity was affected by the presence of calcium in the incubation medium, calcium did not appear to be involved in the signal transduction mechanism for the locust "insulin" homologue.;Results of the current study suggest that within the locust an endogenous insulin-like molecule(s) is present that has the ability to regulate fat body glycogen phosphorylase activity by inhibiting it. It is envisioned that this action of the molecule(s) would be mediated via an insulin receptor homologue present on fat body membrane.